Reports: Aldrich & Lambrechts report
Civil Engineering Practice
Journal of the Boston Society of Civil Engineers
Volume 1, Number 2
Back Bay Boston, Part II:
Man-made structures that permanently lower groundwater levels can have adverse effects on
buildings with water table sensitive foundations.
Harl P. Aldrich, Jr. & James R. Lambrechts [Haley & Aldrich]
Reprinted from Civil Engineering Practice:
Journal of the Boston Society of Civil Engineers
Volume 1, Number 2
Copyright @1986 BSCES
CIVIL ENGINEERING PRACTICE: JOURNAL OF THE BOSTON SOCIETY OF CIVIL ENGINEERS SECTION/ASCE
(ISSN: 0886-9685) is published twice yearly by the Boston Society of Civil Engineers
Section/ASCE (founded 1848). Editorial, circulation and advertising activities are
located at: Boston Society of Civil Engineers Section/ASCE, The Engineering Center, 236
Huntington Ave., Boston, MA 02115-4701; (617) 536-2576. Known as The Journal of the
Boston Society of Civil Engineers Section/ASCE until 1985.
Back Bay Boston, Part II: Groundwater Levels
Man-made structures that permanently lower groundwater levels can have adverse effects on
buildings with water table sensitive foundations.
Harl P. Aldrich, Jr. &
James R. Lambrechts
The temporary or permanent lowering of the groundwater table can adversely affect both
natural and constructed environments, causing ground subsidence, flooding and damage to
structures. The Back Bay section of Boston serves as an excellent site for the study of
the causes and effects of groundwater level diminishment, and provides ample reasons for
the need to monitor and maintain groundwater levels to preserve building foundations.
The second in a series of studies on Back Bay, this article summarizes groundwater
levels in Back Bay since the area was filled more than 100 years ago, and traces the
effects of construction of sewers and drains, subways and other transportation corridors,
and buildings on the groundwater table. Part I described the geology of Back Bay as well
as subsurface soil conditions and the topographic development of the area, concluding
with a discussion of building foundation practice through the turn of the century, a
practice based primarily on untreated wood piles.(1) Part III, now in preparation, will
complete the series, documenting foundation design and construction practice from 1900 to
This study focused on the geographical area bounded by the Massachusetts Bay
Transportation Authority's Southwest Corridor Project (south), Charles Street (east),
Massachusetts Avenue (west) and the Charles River Basin (north). This area currently
encompasses the Back Bay Historic District (primarily between Boylston Street and Beacon
Street) and the central spine across Back Bay where major projects have been constructed
during the past 30 years. The South End neighborhood, located to the south of the
Southwest Corridor, was excluded, primarily because little data on groundwater levels
exist for this area.
During the nineteenth century, a tidal estuary of the Charles River known to Boston
residents as the Back Bay (see Figure 1) was filled to create
land for an expanding population. Most of the homes, churches and other buildings
constructed prior to 1900 were founded on wood piles driven through fill materials and
organic soils to bear in the underlying sand and gravel or clay stratum. For the most
part, the tops of these piles were cut off below the water table at the time of
construction with the expectation that they would be preserved if permanently immersed
below the groundwater table.
With construction of sewers, drains, subways and the basements of buildings below the
water table, some of which leak, the groundwater level has dropped in Back Bay. Where
wood piles have been exposed to air for some time, the piles have rotted when attacked by
fungi, borers and other organisms. A few buildings have settled and cracked, requiring
owners to underpin their structures at great cost in order to restore the foundations.
The Groundwater Table
The Webster dictionary defines the groundwater table as the level below which the ground
is saturated with water. In geotechnical engineering, it is the stabilized static water
level in an open excavation, or in a shallow well or piezometer, as illustrated by well A
in Figure 2. In Back Bay, the water table generally occurs
within the fill stratum from 10 to 15 ft. below the ground surface.
Three principal water bearing aquifers occur in Back Bay, separated by impervious
soils. The lowest aquifer, a relatively thin, but apparently continuous stratum of
outwash sand and gravel or glacial till underlying the Boston blue clay, is relatively
pervious. The middle aquifer is a compact gravelly sand stratum up to 20 ft. in thickness
confined between the blue clay and a near continuous stratum of organic silt and peat.
This pervious outwash material occurs primarily over the western and northern sections of
Back Bay. It does not exist in the Copley Square area. The top aquifer is the artificial
fill, commonly a silty coarse to fine sand, placed during the nineteenth century. The
groundwater level in the fill, the top aquifer, is the principal concern in Back Bay.
In the westerly section of Back Bay, where the sand outwash stratum occurs below the
relatively impervious organic soils, a second "water table" may be present - one that may
differ from the water table in the fill. This situation is represented by well B in Figure 2. If the water level in all wells or piezometers in the
figures were equal, then the groundwater would be hydrostatic with depth.
"Normal" Groundwater Levels. If there were no loss of groundwater by pumping
and leakage into sewers and drains, and no additions to the groundwater from leaking
water mains and other sources, the probable "normal" water table in Back Bay would be as
shown in Table 1.
In colonial times, Back Bay was a tidal estuary that had a mean water level
approximating the mean tide in Boston Harbor (el. 5.65 Boston City Base (2)). Following
the completion of the Mill Dam along Beacon Street across Back Bay in 1821, and until
1880 when most of Back Bay was filled, water levels in the receiving basin east of
Massachusetts Avenue were variable and generally below mean tide.
After Back Bay was filled, the groundwater table would have been expected to rise
above mean tide level. The land mass was bounded on the north by the Charles River and on
the south by South Boston Bay, both tidal. Surface water from rainfall and snowmelt that
percolated into the ground would have been expected to raise the water table until a
horizontal gradient in the fill was established to conduct groundwater by seepage toward
the adjacent bodies of open water. In the latter part of the nineteenth century, the
groundwater level in Back Bay was, in fact, approximately el. 8.0 ft.(3)
The construction of the Charles River Dam in 1910 raised the mean water level in the
Charles River Basin to el. 8.0. The Back Bay groundwater table would have then been
expected to rise further, perhaps to el. 9.0 or higher along Boylston Street. Normal
groundwater levels in the area between Charles Street and Storrow Drive would have then
been from el. 10.0 down to 8.5, as a result of groundwater runoff from the west side of
Major sources of groundwater in Back Bay are infiltration of rainfall and
snowmelt, leakage from water mains, and recharge from man-made groundwater recharge
systems. The sand outwash receives water from the fill by seepage downward through the
organic soil and by direct flow from the fill through holes, trenches and other manmade
"openings" excavated through the organic stratum.
Only a fraction of the annual precipitation actually enters the ground because more
than 80 percent of the Back Bay is covered by impervious surfaces such as streets,
sidewalks and buildings. Even in open, unpaved areas, only part of the precipitation
enters the ground. Although most of the precipitation in Back Bay becomes runoff and is
carried away by storm drains and sewers, the seasonal variations in the type, and level,
of precipitation cause an annual fluctuation in groundwater levels up to about 2 ft. in
The Charles River may become a source of groundwater in the Back Bay when the water
table falls. However, seepage through the fill is severely impeded by remnants of the
Mill Dam and the West Side Interceptor along Beacon Street, and by the Boston Marginal
Conduit under Storrow Drive. Because the river level is maintained at el. +7.5 to +8.0,
its effect on groundwater levels is essentially constant. The river's influence on water
levels in the fill decreases rapidly with distance from the river.
The relatively pervious sand outwash stratum also underlies the Charles River. The
Mill Dam and Boston Marginal Conduit would not impede recharging in this stratum.
However, since the river bottom is also blanketed by organic soils, its influence on
piezometric water levels in the Back Bay outwash is uncertain.
Leaky pipes, particularly water mains, can be significant localized sources of
groundwater. Cotton and Delaney provided groundwater contours that indicated several
mounds where water levels were as much as 5 to 10 ft. above surrounding areas.(4) The
overall contribution to the water table from leaking water mains may be about equal to
that from precipitation. Cotton and Delaney reported that Boston Water Department data
from the early 1940s indicated that water main leakage would have provided an equivalent
recharge of 0.73 million gallons per day (gpd) per square mile. This amount is
approximately equal to the recharge from 50 in. of precipitation per year, assuming a 30
percent infiltration rate. Storm and sanitary sewers located above the groundwater table
can also leak and contribute to groundwater. Because they are not under pressure, their
effect is probably minor.
In several areas, permanent recharge systems have been installed to help maintain
groundwater levels. Notable examples are the recharge systems at Copley Square and
Trinity Church. In these systems, surface drainage from precipitation is collected and
directed to drywells or reverse drains. Water then seeps back into the ground through
special piping systems. Temporary recharge systems have been used in areas adjacent to
construction projects, notably the Prudential Center, to prevent or correct lowered
groundwater levels caused by deep excavations and construction dewatering.
Loss of groundwater, and the resulting lowered water levels in Back Bay, occur
primarily from leakage into sewers and drains, leakage through walls and floors of subway
tunnels, underpasses, building foundations and other structures below the water table,
and by pumping from sumps. In addition, water levels may be lowered temporarily by
pumping from excavations in order to facilitate construction.
Adverse Effects of Lowered Groundwater Levels
Temporary or permanent lowering of the groundwater table from man-made or natural causes
have been shown to adversely affect buildings, streets, underground utilities and other
structures, as discussed by Aldrich.(5) Potential problems applicable to the Back Bay are
illustrated in Figure 3 and include:
- Deterioration of wood piles
- Ground subsidence
- Negative friction (drag) on piles
Deterioration or decay of wood piles is clearly the most serious potential
problem associated with lowered water levels. As long as the water table remains above
the tops of the piles, and the wood and surrounding soil remain saturated, the wood will
not rot. Under these conditions, untreated wood piles can be considered to be permanent.
However, if the groundwater level drops below the tops of the piles, favorable
conditions may be present for plant growth and insect attack. A greatly increased supply
of oxygen, combined with moisture and moderate temperatures, facilitate the growth of
fungi. Grubs or wood borers, termites and other insects may also attack the "exposed"
The butts of piles that are surrounded by fill, in particular sand and gravel as well
as ashes and cinders, are more prone to rotting than are piles that are embedded in
organic silt, peat and other relatively impervious soils. When the water table drops, the
fine-grained soils remain saturated for a time, thus protecting the piles from immediate
The time required for significant deterioration to occur, following a drop in
groundwater level below the tops of wood piles, is highly variable. It depends on the
species of wood, the type of soil in which the piles are embedded, the amount of
moisture, temperature and other factors. Exposure for a few months is not considered
serious. However, serious deterioration will probably occur after a drawdown period of 3
to 10 years.
Ground Subsidence. When the groundwater level is lowered, the effective stress
on soils that occur below the water table is increased. Buoyancy in the zone of drawdown
is lost. If underlying soils are compressible organic soils or soft clays, these
materials will consolidate as the soil structure adjusts to the increase in the
overburden load. Settlement will also occur if the upper soils dry out and shrink when
the water table is lowered.
Most areas of the Back Bay have experienced one or more significant temporary
groundwater drawdowns for the construction of sewers and drains, subways, foundations for
buildings, and other excavations that have required pumping. For this reason, ground
subsidence due to future temporary or nominal permanent lowering of the water table is
not considered to be a serious concern.
Negative Friction. All buildings in Back Bay that are supported by piles driven
through fill and organic soils - whether they are wood piles bearing in the sand and
gravel outwash or marine clay, or are long piles driven to bear in the glacial till or
bedrock - will experience negative friction or drag loads when the ground surrounding the
piles settles. The building may settle as a result. The potential adverse effects are
most pronounced for wood piles that derive their support by skin friction in the marine
While significant negative friction undoubtedly developed in the nineteenth century
from the compression of organic soils under the weight of overlying fill, and from
temporary groundwater drawdowns, this factor is not likely to be a serious concern in the
Construction in Back Bay
The construction and maintenance of embankments, sewers and drains, transportation
corridors and buildings throughout Back Bay have affected groundwater levels. The impact
of this complex interconnected underground system, shown in Figure 4, on the water table cannot be appreciated without some
knowledge of each component.
Mill Dam. The first significant filling in Back Bay took place in 1820 when the Mill
Dam was constructed along Beacon Street from Charles Street to Sewall's Point in
Brookline, near the present Kenmore Square. From a description given by Howe, a cross
section of the dam can be developed as shown in Figure 5.
As a dam, the structure was relatively impervious to the flow of water from one side
to the other, except where it has been breached locally by construction in the past 100
years. However, in a longitudinal direction along Beacon Street, the structure is
probably very pervious.
Filling of Back Bay began in 1858 at the Public Garden, continuing westward to
Massachusetts Avenue by 1880. From 10 to 20 ft. of sand and gravel fill were placed over
soft organic soils that were underlain by a deep clay stratum. Considerable ground
subsidence occurred over a long period of time from the compression of the organic soils
and, to some extent, the clay.
Concurrent with the filling, a sea wall was constructed along the Charles River to
create Back Street that parallels Beacon Street on the water side. The top of this wall
is clearly visible from Storrow Drive. A similar wall was constructed in about 1865
behind homes on the water side of Brimmer Street on Beacon Hill. Both walls were composed
of dry-laid granite placed on a timber platform and supported on wood piles (see Figure 6). It is probable the walls were ballasted with stone or
gravel, similar to the Mill Dam walls.
Construction of buildings followed closely behind the Back Bay filling. All of
these buildings were founded on untreated wood piles cut off typically at el. 5.0,
approximately 2 to 3 ft. below the groundwater table.(1)
Sewers and drains in Back Bay have contributed to at least localized
depressions in the groundwater table. Furthermore, dewatering for sewer construction
undoubtedly caused extensive temporary lowering of the water table in some areas. Plans
of the principal existing sewers and conduits in the Back Bay are shown in figures in a
report by Camp, Dresser & McKee.(7)
The earliest sewers and drains in Boston discharged by gravity from the hills to
adjacent tidal areas. Flow velocities were high and there were few problems. With the
development of the low filled-land areas like the Back Bay, the extension of the sewer
system created serious drainage problems in Back Bay because of the area's flat gradients
and ground settlement.
Most house drains and sewers were below basement level, and when minimum slopes to
street sewers and interceptors were provided, the outfalls were rarely above low tide. As
a result, the contents of the sewers were dammed up by the tide during the greater part
of every day. (Tide gates were commonly adopted to prevent salt water from flooding the
lower reaches of the sewers.) Settlement of the filled land caused numerous breaks in
sewer connections and reversals of slope. Deposits of sludge and debris within the sewers
and in tidal areas accumulated rapidly, with their attendant health and odor problems.
By 1868, the State Board of Health recognized a serious public health problem and, in
1875, the City Council authorized the Mayor to appoint a commission to study the sewage
system and to plan for future needs of the city. The plan adopted became the Boston Main
The Boston Main Drainage System, was constructed from 1877 to 1884. The
principal feature of these works was a system of intercepting sewers along the margins of
the city to receive the flow from the already existing sewers. These intercepting sewers
drained to a pumping station located at Old Harbor Point on Dorchester Bay (Calf Pasture
on Columbia Point) where sewage was pumped to Moon Island and discharged into Boston
Harbor on outgoing tides.
Existing combined sewers (storm water and domestic sewage) in the northerly section of
the Back Bay that formerly discharged into the Charles River at Beaver, Berkeley,
Dartmouth, Fairfield and Hereford Streets were connected to the West Side Interceptor
that was constructed along Beacon Street. Other sewers locates south of the railroads
drained into the East Side Interceptor that follows Albany Street.
Design and construction of the West Side Interceptor is of articular interest (see Figure 4). It travels down Charles Street to Beacon Street,
where it turns westerly down Beacon to Hereford Street, then turns southerly down
Hereford and Dalton Streets to Falmouth Street, and then westerly to Gainsborough Street.
In the Beacon Street area, the invert grade varies from approximatel el. 0 at Beacon and
Arlington Streets, to el. -2.4 at Beacon and Hereford Streets and to el. -4.7 at
Huntington Avenue and Gainsborough Street.
Excavation and dewatering for the construction would have been required to at least 2
ft. below these grades, into the fill and organic soil on Beacon Street; and to
approximately el. -6.0 in the sand outwash stratum in Dalton and Falmouth Streets. So,
over 100 years ago, if not before, the outwash stratum experienced its first significant
temporary drawdown. Significant ground subsidence and negative friction on wood piles
The intercepting sewers and the main sewer, from the upper reaches to the pumping
station at Calf Pasture, varied in size fro 3 to 10.5 ft. in diameter. The larger ones
were circular and the smaller ones were generally egg-shaped. The West Side Interceptor
was egg-shaped, 57 in. wide and 66 in. high (see Figure 7).
Sewers were constructed with double or triple rows of mortared brick, and where piles
were required, a timber platform was constructed and the sewer was cradled on mortared
granite masonry. It is of considerable importance to note that the intercepting sewers
were constructed with an
underdrain pipe varying from 8 to 12 in. in diameter that was placed below the sewer to
control groundwater during construction.
Observation wells in Back Bay at that time indicated water levels similar to those
measured before sewer construction, but within 10 years, in 1894, areas were found where
the groundwater was as low as el. 5 or lower, indicating that there was leakage into
low-level sewers or that groundwater was being pumped.
The Boston Main Drainage System was designed with sufficient capacity to carry the
estimated dry weather flow of sanitary sewage and a small volume of storm water. Excess
storm flow and diluted sewage from the West Side Interceptor were discharged into the
Charles River at numerous overflow outlets.
Boston Marginal Conduit. With the construction of expensive homes along the
Charles River, there were increasing demands to eliminate the odors and nuisance of the
tidal basin. Under the Acts of 1903, a half-tide dam was completed in 1910 at the
location of the former Craigie's bridge, where the Museum of Science is now located. The
dam was constructed with gates and a lock to maintain the water level in the Charles
River basin at approximately el. 8.0.
As part of the dam project, the Boston Marginal Conduit was constructed along the
Boston side of the basin to collect flows from Stony Brook, and mixed sewage and storm
water overflows from the West Side Interceptor that formerly discharged into the river at
the sea wall (see Figure 4). Water was to be maintained at a
low level in the conduit by means of tide gates constructed at the outfall below the
Charles River dam.
The marginal conduit was constructed in a 100-ft. wide earth fill embankment placed
immediately north of Back Street, beyond the old dry rubble retaining wall. Presently,
the conduit lies beneath Storrow Drive. Over most of its length, it is a reinforced
concrete horseshoe-shaped section 76-in. wide by 92 in. high, supported on wood piles, as
shown in Figure 8.
The structure was constructed level with an invert grade estimated at el. -1.5.
Drawings indicated that it was built within a double row of tongue and groove wood
sheeting that was driven into the organic silt and left in place. Again, a large diameter
underdrain pipe was placed just below the marginal conduit to facilitate dewatering
When the Storrow Drive underpass was built in 1951, a portion of the conduit was
relocated inland, away from the river. The relocated section, from Dartmouth Street to
Mt. Vernon Street, was an 8-ft. diameter reinforced concrete pipe with an invert grade at
el. -1.5. An underdrain pipe was placed beneath this new pipe, and it was apparently
connected to the old underdrain when the relocated section was tied in.
The Mill Dam, West Side Interceptor and the Boston Marginal Conduit act as dams
impeding the flow of groundwater from the Charles River basin into the Back Bay.
Furthermore, while relatively impervious perpendicular to their axes, they can conduct
groundwater with relative ease in a longitudinal direction.
The present system of low level sewers was constructed throughout Back Bay
between 1910 and 1912. Underdrain pipes again were commonly used as shown in Figure 9, which presents a section through the St. James Avenue
storm and sanitary sewers. By that time, nearly 75 years ago, there was little doubt that
groundwater leaked into sewers, that the problem was widespread and that groundwater
levels in Back Bay were controlled primarily by this leakage.
Two major subways have been constructed across Back Bay by the Boston Transit
Commission (now known as the Massachusetts Bay Transportation Authority). Between 1912
and 1914, the BoyIston Street subway tunnel was built, and from 1937 to 1940 the
Huntington Avenue subway was added. Their locations are shown on Figure 4 on page 37.
The Boylston Street subway crosses Back Bay from Massachusetts Avenue to Charles
Street. Within this area, the bottom of the subway varies from approximately el. +3.0 at
Massachusetts Avenue, to el. -19.0 between Arlington Street and Hadassah Way (its lowest
point) to el. -10.0 at Charles Street. Table 2 presents elevation and soil condition data
for the subway. A cross-section through the structure between Berkeley and Clarendon
Streets is shown in Figure 10.
The structure was supported on a wide variety of soils including the fill, organic
silt, and natural sand and gravel outwash. Where peat was encountered, approximately
between Hadassah Way and Charles Street (a distance of 460 ft.), wood piles were driven
to support the structure.
L.B. Manley, Asst. Engineer for the Transit Commission at the time, reported on soil
"As is well known, the land reclaimed from the Back Bay consists of sand and gravel
filling resting on a bed of silt whose upper surface lies at about grade 0, Boston City
base, or grade 100, Boston Transit Commission base. This layer of silt is continuous
throughout the length of the subway, and attains a thickness of about 17 ft. at Dartmouth
Street, and over 20 ft. in the Fens. Between Exeter Street and Charlesgate East and
between Clarendon Street and Charles Street, where it finally disappears, it averages
about 8 ft. in thickness. Below the silt between Massachusetts Avenue and Hereford
Street, and at Exeter Street, are pockets of peat from 2 to 4 ft. in thickness. Another
extensive body of peat occurs between Arlington and Charles Streets, where it attains a
"Below the silt and peat is a stratum of sand and gravel which also extends throughout
the length of the subway excavation except for a length of about 1,600 ft. between Exeter
and Clarendon Streets. This sand and gravel carries large quantities of water laden with
sulphurated hydrogen, which has been offensive to passersby and injurious to the health
of those working in it. This gas, as it leaves the surface of the water, is particularly
destructive to metal, and copper floats in several of the temporary pump wells have been
corroded through at the surface of the water in a few weeks' time by the action of this
gas. It is supposed that this layer of gravel is the same as that which appears in the
bed of the Charles River and affords an underground water course which tends to equalize
the level of the groundwater in the Back Bay."
During construction, a temporary drawdown of water levels both in the fill and in the
sand-gravel stratum would have occurred. Where the subway route passed opposite to what
is now the Prudential site, drawdown in the sand stratum to el. -10.0 is estimated.
Constructed between October 1937 and 1940, the Huntington Avenue subway crosses under
Massachusetts Avenue as it enters Back Bay and joins the BoyIston Street subway at Exeter
Street (see Figure 4). Within this area, the bottom grade of
the subway structure varies from el. -10.0 at Massachusetts Avenue down to el. -19.0
where the structure passes below the railroad tracks (and under the Massachusetts
Turnpike Extension). Table 3 presents elevation and soil condition data.
The subway was founded on the outwash stratum that extends from 5 to 12 ft. below the
bottom of concrete from Massachusetts Avenue to the Turnpike. North of the Turnpike to
Boylston Street, the structure bears on clay and organic soils, without piling. During
construction, the outwash stratum was dewatered for the entire length of the subway along
Huntington Avenue to grades as low as, or even below, el. -20.0. A very significant
drawdown of water level occurred over a wide area, for a period of 2 to 3 years. An
observation well at Massachusetts and Commonwealth Avenues, 0.4 miles away, was reported
to have dropped from el. 7.0 to el. 0 in 1939.
Construction for the Huntington Avenue subway required extensive and prolonged
dewatering to levels below any known construction before or since. In addition, drains
installed in the tunnels of both subway lines have undoubtedly collected groundwater that
leaked into the structure.
Construction of Storrow Drive in the early 1950s included an underpass and
traffic interchange in the Berkeley Street area. This underpass is approximately 1,300
ft. long between portals, with 300-ft. long approach ramps at either end. The road
surface descends as low as about el. -4.0, about 15 to 17 ft. below the ground surface.
The underpass was designed to prevent groundwater lowering by the extensive use of
waterstops. The structure was designed as a boat with sufficient weight to resist
hydrostatic uplift pressure. Invert slabs were up to 2-ft. 8-in. thick. Precipitation and
other surface water is collected in catch basins and cross drains that feed into pipes
below the slab. These pipes transport water to wet wells near each portal where the water
is then pumped into the Charles River.
Soon after completion, leaks were reported in the reinforced concrete walls. In order
to collect the infiltrating groundwater and improve the appearance, gutters and false
walls were installed. The leaks were evidently never repaired. A significant volume of
groundwater is apparently infiltrating into the underpass as recent dry weather pumping
volumes have been reported to be about 20,000 gallons per day from each wet well.
The Massachusetts Turnpike Extension, a six-lane limited access highway,
crosses the Back Bay. The highway was constructed between 1963 and 1966, and is located
just north of the Conrail (formerly Boston and Albany) railroad alignment (see Figure 4). The roadway was depressed 15 to 20 ft. below adjacent
city streets and developed areas. The road surface descends from about el. 11.0 at
Massachusetts Avenue down to el. 6.0 at Tremont Street.
The turnpike was designed to prevent a permanent lowering of groundwater levels below
about el. 6.5 to 8.5, depending on the location. West of Huntington Avenue, an underdrain
system was used to limit uplift pressures on the slab, Through the Prudential Center
site, two lines of steel sheetpiling driven 5 ft. into the clay inhibit the flow of
groundwater to the turnpike underdrain.
Because the road surface east of Huntington Avenue was lower, underdrains were not
used. The turnpikestructure was designed for uplift as a boat section, using a thick
concrete slab to prevent flotation. A drain was provided along the north wall to prevent
groundwater levels from exceeding el. 8.5. Existing drains in the railroad alignment to
the south maintain water levels at about el. 7.0.
Southwest Corridor Project. This new transportation structure was constructed
between 1981 and 1985. It has two tracks for the relocated Massachusetts Bay
Transportation Authority Orange Line subway and three tracks for commuter rail and Amtrak
service. Through Back Bay, the alignment followed parts of two original railroad
embankments that were constructed across the Receiving Basin in the mid-1830s (see Figure 4 on page 37). From Massachusetts Avenue to Dartmouth
Street, the new concrete structure was below ground in a 3,000-ft. Icing cut-and-cover
tunnel that required excavations as deep as 38 ft.- East of Dartmouth Street, the
structure extended about 10 ft. below former grade. Depths of excavations and other data
are summarized in Table 4 on page 46.
Reinforced concrete slurry walls were used for lateral support of the sides for about
2,100 ft. of the tunnel excavation (see Figure 11). The
concrete walls were 3-ft. thick and penetrated 8 to 15 ft. into the clay stratum. They
were used as the tunnel's permanent outside walls. Although water leakage did occur
through some of the vertical joints between wall panels, there was no appreciable
lowering of groundwater levels in adjacent areas.
In other deep excavation areas where adjacent structures were further away from the
excavation or absent, steel sheet-piling was used for temporary lateral support of the
excavation. East of Dartmouth Street, excavations were shallower and soldier piles with
wood lagging were used. Water seepage into these excavations temporarily lowered
groundwater levels in adjacent areas as much as 12 ft.
Where concrete slurry walls were used, the tunnel was supported on a thick concrete
invert slab bearing on
compacted sand and gravel fill that was used to replace
unsuitable organic soils. East of this portion of the tunnel, the structure was supported
on precast-prestressed concrete piles driven through the clay to end bearing on glacial
till or bedrock.
In order to allow groundwater movement across the corridor structure, a groundwater
system was installed. This system consisted of longitudinal drains placed 2 to 4 ft.
below the pre-construction groundwater level on either side of the structure. Where
slurry walls formed the tunnel walls, 8-in. diameter header
pipes surrounded by crushed stone were connected to 8-in.
galvanized steel pipes cast into the walls and connected
beneath the invert slab. In other areas, rectangular drains of crushed stone wrapped in
filter fabric were constructed
beneath the invert slab and up the outside of each wall in order to allow water to flow
between longitudinal drains on
Major Buildings. The first major buildings with deep basements in Back Bay were
the Liberty Mutual and New England Life buildings, constructed in the late 1930s. Since
that time, other buildings requiring excavation well below the groundwater table have
Temporary Effects of Building Construction Dewatering
Where excavations have been carried below the water table for building construction in
Back Bay, the water table in nearby areas has been lowered, in some cases by a
significant amount. Table 5 on page 48 summarizes pertinent information - dates of
construction, location, foundation type, elevation of the deepest excavation, dewatering
and drawdown - for major construction projects gathered from the
available literature, reports and construction records. The locations of major deep
excavations for both buildings and sewer and transportation projects, and the
approximate elevations of the bottoms of these excavations are shown in Figure 12 on page 50.
Copley Square Area. The sand Outwash is shown in Figure
4 on page 37 to be absent around and for some distance north, east and south of
Copley Square, which is generally near the intersection of Dartmouth and Boylston
Streets. Therefore, the principal source of water to excavations in the area is by
seepage from the fill. Drawdown in the fill is limited to the distance between normal
groundwater level and the top Of tile underlying organic silt, generally less than 7 to
10 ft. Drawdowns of this magnitude were usually confined to areas near the excavation.
The volume of water entering excavations has been small and temporary recharging has
generally not been practiced.
Excavations for deep foundations have advanced from the early use of unsupported
slopes, often combined with deeper laterally supported soldier piles and wood lagging, to
the more recent use of steel sheetpiling and concrete diaphragm walls installed in slurry
In the case of the New England Mutual Life Insurance Co. building, a nearly 40-ft.
deep excavation was opened in 1939 Over the western two-thirds of the block bounded by
Clarendon and Berkeley Streets, and Boylston and Newbury Streets. Steel soldier piles and
wood lagging were used to support the sides of the excavation in the organic silt
stratum, and the overlying fill was cut back to a stable slope. Surface water and
groundwater were collected in troughs cut into the top of the organic silt stratum at the
toe of slope behind the sheeting, and sumps were used to dewater the excavation, The
building was founded on large spread footings and mats bearing on the stiff crust of the
clay at el. -17.0 to -22.0, in early classic example of a "floating" foundation. Adjacent
to the excavation, groundwater levels in the fill would have been lowered to the top of
the underlying organic silt, about el. 0. Water
levels in observation wells located immediately west of the
site, at Clarendon Street, dropped 4 to 6 ft. to about el.
2.0. There were no reports of adverse effects to surrounding
The excavation, in 1947, for the John Hancock Berkeley building was very similar to
that for the New England Mutual building. The excavation was opened on Berkeley Street
between St. James Avenue and Stuart Street. Again, soldier piles and wood lagging were
used to support the sides of the excavation in the organic silt, while the overlying fill
was cut to a slope of about 1.5 horizontal to 1 vertical. In order to intercept
groundwater and surface runoff, an 8-in. pipe was installed around the sides of the
excavation in a sand-filled trench located just above the organic silt. The excavation
was dewatered using three caisson wells.
During construction, groundwater levels in the fill adjacent to the excavation would
have been lowered to the top of the organic silt, as they were during the construction at
the New England Mutual building. A 10-ft. drawdown, to about el. -2.0 on St. James
Avenue, would have occurred. Casagrande reported a 10-ft. drawdown in 1947 across
Berkeley Street at the Liberty Mutual Building.(9) To the west of the excavation,
groundwater levels in the fill were also lowered by 4.5 and 2.5 ft. at distances of 125
and 300 ft., respectively.
An increase in the rate of settlement of the Liberty Mutual building, located across
from the site at Berkeley Street, was attributed to an increase in the effective stress
in the clay stratum caused by lowered groundwater levels, and to the effects of
disturbance to the structure of the clay from the driving of steel H-piles.(10) The
building settled an additional 0.5 in., about half of which was recovered in rebound when
the groundwater returned to pre-construction levels.
Construction of the John Hancock Tower, begun in late 1968, required excavation to el.
-28.0 (45 ft. below ground surface).(11) Interlocking steel sheetpiling was driven
approximately 5 ft. into the clay to form a cofferdam around the site. The sheeting
extended to ground surface, unlike excavations at the New England Mutual and the Hancock
Berkeley buildings where the fill was sloped down to the laterally-supported organic
silt. Prior to construction, during October and November 1967, water levels at the site
varied between el. 4.5 and 6.0, with an average of el. 5.0.
Contrary to experience at the Prudential Center, there was no significant drawdown of
the water table beneath the streets surrounding the site. Water levels measured in the
fill during construction were generally from el. 4.0 to 5.0. Immediately behind the steel
sheeting, a local drawdown exceeding 10 ft. was measured in a piezometer installed in the
organic silt. Very little pumping was required inside the sheeting and no recharging was
performed. Drawdown at the John Hancock Tower was insignificant because steel sheet
piling was used and the sand outwash stratum was absent.
Foundation construction for Copley Place began in 1981 on a 10-acre site bounded by
Huntington Avenue, the Southwest Corridor, Harcourt and Dartmouth Streets. The lowest
floor level was established at el. 6.6, somewhat above the pre-construction groundwater
table. Dewatering was required only for the construction of pile caps and a deep water
main. Excavation and dewatering for one large pile cap below the Westin Hotel was carried
to approximately el. -5.0, using steel sheeting for lateral support. Elsewhere, the sides
of the excavation were either unsupported slopes or supported with soldier piles and wood
The impact of dewatering on groundwater levels adjacent to the site was minor. An
observation well on Blagden Street adjacent to the Boston Public Library dropped
temporarily about 3 ft. However, there was no observable drawdown at Trinity Church.
A quarter-mile north of Copley Square, at the corner of Beacon and Clarendon Streets,
significant construction was required for a 17-story apartment building constructed in
1964-1966 at 180 Beacon Street. Here, soil conditions were much like those in Copley
Square. Concrete diaphragm walls installed in slurry trenches were used both for lateral
support of the sides of excavation and as permanent basement walls, the first such use in
Boston. The 2-ft. thick reinforced concrete walls were internally braced and surrounded
the site. These walls penetrated about seven feet into the clay stratum, which is
overlain at this location by 5 ft. of sand outwash. The site was excavated down to about
el. -20.0 for the 3-1/2 basement levels required (12 to 14 ft. above the outwash
Leakage through the concrete wall was apparently the cause of a 12 to 15-ft. draw down
in observation wells installed in the outwash on adjacent property. Water from city mains
was pumped into the outwash through five 2-inch diameter recharge wells, but with only
moderate success in raising the piezometric head. The extent to which the perched water
level in the fill was affected is not known. Some minor settlement of an adjacent wood
pile-supported 10-story apartment building was attributed to the construction, but the
cause was never clearly established.
The Christian Science Church Center is southwest of Copley Square. In this
area, the pervious sand outwash stratum is fully developed to a thickness of 12 to 20 ft.
Most of the structures built in the Christian Science complex have been founded, in one
way or another, on this outwash. The Mother Church was founded on untreated wood piles.
Dewatering of the confined outwash aquifer had been required during construction of
several buildings constructed in the last 55 years in this complex.
In 1932, excavation for the 100 by 630 ft. Christian Science Publishing House building
on the former Norway Street was carried as low as el. -6.0 for spread footing
construction on the sand outwash. Dewatering by wellpoints in the outwash dropped the
piezometric head an estimated 10 to 15 ft. for approximately eight months. The lateral
earth support system used for this excavation is not known.
Water levels in the confined outwash aquifer responded quickly to the dewatering over
a large area. The water level in an observation well located 1,200 ft. Southwest of the
site dropped 4 to 5 ft. in two weeks, while at the Mother Church, the water level was
lowered 8 ft. The effects of construction dewatering in the fill were much less, with the
water table dropping only 2.5 ft. at the Mother Church.
Drawdown in 1958 for the excavation of the Christian Science Publishing House
Underground Equipment Room adjacent to the Mother Church was similar to that experienced
in 1932 during Publishing House construction. Excavation extended to el -3.0 in the sand
outwash. Lateral earth support was provided by steel sheet piling that penetrated part
way through the sand. Wellpoints were used inside the sheeting to dewater the outwash to
approximately el. -4.0. Water levels in observation wells in the outwash 500 and 1,200
ft. from the site were lowered to el. -1.0 and 3.0, respectively.
As part of a major construction program at the Christian Science Church between 1969
and 1972, an approximately 1,000-ft. long section of the West Side Interceptor was
relocated into a gallery. Construction was carried out between two rows of steel sheet
piling, 16 ft. apart, and driven 5 ft. into the clay stratum. The bottom of the
foundation for the gallery was el. -7.5 in the sand outwash. Dewatering was accomplished
by wellpoints installed inside the cofferdam. Drawdown in the sand stratum at the nearby
Mother Church was initially to el. -2.0. Six large-diameter recharge wells installed
around the Mother Church raised piezometric levels to about el. 3.0. Later, as recharge
became ineffective, outwash water levels fell back to el. -2.0 and -3.0. In the fill
stratum, the initial drawdown to el. 3.0 was successfully recharged to el. 6.0.
For the 1973 construction of a new portico for the Mother Church, the last major
project at the Christian Science complex, foundations at the front of the Mother Church
were underpinned with six concrete piers bearing on the outwash at el. -3.5. Dewatering
by wellpoints lowered water levels in the outwash by 8 and -5.5 ft. at distances of 40
and 230 ft. from the excavation, respectively. There was no observable drawdown in the
The Prudential Center is immediately west of Copley Square. Construction began in 1959
with a 52-story tower. The entire development, the largest in Boston at the time, was
enclosed within a wall of interlocking steel sheeting that was reported to have been
driven 5 ft. into the clay stratum to form a relatively impermeable cofferdam.
Internally, the area was divided by sheet pile walls into several cells.
A parking garage was constructed beneath most of the Prudential Center, with the
lowest floor level at el. 3.0. A portion of the slab was supported on compacted sand and
gravel fill that was placed after the organic soils were excavated. This excavation and
backfill operation, and other excavations requiring dewatering, were accomplished with
Drawdown in the outwash sand just outside the sheet piles was reported to have been to
as low as el. -12.0. Construction specifications required recharging outside the sheeting
to maintain groundwater levels at or above el. 5.0. There were considerable problems with
recharging the sand outwash, and it was only moderately successful. However, there were
no significant problems in maintaining water levels outside the sheeting in the fill.
In 1969 and 1970, an area at the edge of the Prudential Center was dewatered for the
construction of a new entrance to the Huntington Avenue subway. Excavation and dewatering
were carried out to el. -15.0 in the sand outwash. Dewatering lowered water levels in
wells at the Mother Church and Massachusetts Avenue to el. -7.0 and 0.0 (1,000 and 1,200
ft. away, respectively). Drawdown in the sand outwash was reported to have caused the
Prudential Center garage to settle 0.5 in. Here again, most of the subsidence was
recovered after dewatering when water levels returned to pre-construction levels.(9,12)
Effects from Outside Back Bay. In 1957, the Boston Herald Traveler Corporation
started construction of a new building at 300 Harrison Avenue, well outside of the fill
area to the east of Copley Square. Dewatering of the glacial till/outwash strata
occurring below the clay was required for construction of deep caissons. Because these
materials are relatively pervious, piezometric levels in the till and outwash were
lowered significantly in Back Bay. Water levels in deep observation wells at the
Prudential Center, approximately one mile away, dropped as much as 30 ft. within a month
of the start of pumping.(9) This drawdown over a period of five months caused about 0.1
and 0.3 in. of settlement at the New England Mutual Building and the Liberty Mutual
Building, respectively, located about 0.8 and 0.6 miles from the Herald Traveler
Building.(10) Most of the subsidence was recovered by rebound after dewatering.
Overall, for sites where the sand outwash stratum was absent, for example in the
Copley Square area, deep excavations have been successfully accomplished within steel
sheet pile cofferdams without significant groundwater drawdown in the fill and without
having to install recharge systems. Where the outwash occurred to the west, even the use
of steel sheeting had not prevented drawdown in the pervious sand stratum because of
leakage through untensioned interlocks.
Drawdown in the sand outwash stratum generally extended 5 to 10 times further from a
deep excavation than groundwater drawdown in the fill. Water levels in the outwash were
lowered significantly at distances of 1,000 ft. or more from an excavation. In the fill,
however, drawdown greater than one foot did not usually occur at distances beyond 400 ft.
Drawdown in the fill was limited by the depth to the organic silt stratum. Groundwater
recharge systems have been successfully used to limit lowered water levels in the fill.
Similar systems have not been effective in the outwash stratum.
Historical Groundwater Levels in Back Bay
Groundwater levels in Back Bay are influenced by the natural process of precipitation and
infiltration, and by water levels in adjacent bodies of water. If there were no man-made
structures, the water table would be relatively uniform across Back Bay and would vary
little with time, being affected only by the amount of precipitation and local
Construction over the past 100 years - sewers and drains, dams, transportation
corridors and building foundations described above - have diverted or withdrawn
groundwater, have impeded its flow and, in other respects, have influenced the water
table. Groundwater levels are non-uniform, complex and have varied substantially in
localized areas over time. Therefore, the interpretation of groundwater data can be
confusing, frustrating and misleading.
Concern for groundwater levels in Back Bay has prompted sporadic action during the
past 100 years. Area-wide studies were made before and after construction of the Boston
Main Drainage System in the late 1800s, during the 1890s for the Charles River Dam, in
the late 1930s under a WPA program, by the USGS in 1967 and 1968, and in 1985 for the
BRA." In the past 25 years, numerous studies have been undertaken in local areas for
building construction, most recently in 1985. 18 However, there has been no long-term
1880s Study. Stearns reported that groundwater levels in wells installed in
1878 before construction of the main drainage system were practically the same in 1885,
one year after construction, with water levels "nearly level at Grade 7.7 over the whole
district" (P. 26)."(3) The data indicated levels between el. 6.7 and 8.5. Engineers of
that time realized the importance of maintaining groundwater levels and were concerned
about the effects of the new main drainage works on the water table.
1894 Charles River Dam Study. Water levels were measured in wells installed for
an 1890s study of the proposed Charles River Dam as reported by Stearns.(3) Generally,
groundwater levels were similar to those measured between 1878 and 1885. Stearns blamed
leaky sewers for some levels below el. 5.0, but considered these instances to be local
and isolated. He recommended that el. 8.0 be established as the water level for the new
Charles River Basin. In discussions to Worcester's 1914 paper, Gow and Stearns cited
leaky sewers as a cause of local groundwater depressions.(8)
1930s Copley Square Study. In 1929, public officials and residents noticed
several alarming cracks in the Boston Public Library Building, and settlement of the
stone platform in front of the library facing Copley Square on Dartmouth Street.
Investigations by the Building Department and consulting engineers found that the tops of
many wood piles supporting the building were completely rotted away or badly
decayed.(14,15) Piles were originally cut off at approximately el. 5.0 and groundwater
was found to average el. 4.0 at the time of underpinning.
Rotted piles below approximately 40 percent of the building area were cut off to sound
wood and were posted with 6-in. steel H-sections bearing on steel plates and wedged
against the underside of the stone foundation. The cost of this underpinning in 1929-30
was reported to be "nearly $200,000."
The discovery of rotted wood piles under the Library sparked renewed interest in
groundwater levels, especially among the Trustees of the nearby Trinity Church that was
constructed in 1876 on 4,500 wood piles. Numerous observation wells were installed in the
Copley Square area, showing water levels as low as el. 2.0.
When contours of equal water level were analyzed (see Figure
13), the loss of groundwater was traced to leakage into a 30-in. diameter sewer on
St. James Avenue (see Figure 9). Construction of a partial
dam in the sewer on Dartmouth Street, where it joins the Boylston Street sewer in front
of the Public Library, caused observation wells to rise immediately, proving without a
doubt the source of the lost groundwater.
Fortunately, Trinity Church was spared. Excavations to examine the condition of wood
piles, originally cut off from el. 5.0 to 5.5, showed no significant deterioration. The
structure had settled nearly one foot in 50 years and pile butts were now lower. This
case history was documented by Robert Treat Paine in 1935.11, In addition, Snow traced
the loss of groundwater in the area.(14)
1936-1940 WPA Surveys. The city of Boston measured groundwater levels
throughout the Boston Peninsula between 1936 and 1940. The project was funded by the
Works Progress Administration under projects No. 5325 and No. 188868. The impetus for
this study was the growing concern about groundwater levels in the city during the 1920s
and 1930s, heightened by the discovery of the rotted wood piles at the Boston Public
Observation wells installed for the WPA project and wells previously installed by the
Boston Sewer Department were monitored. Throughout the Boston Peninsula, a total of
approximately 700 observation wells were used in the WPA survey. Approximately 300 of
these wells were located in Back Bay. A report prepared for the Boston Redevelopment
Authority (BRA) contains tables and plans that describe the location of each well, and
the highest and lowest water levels recorded during the four-year monitoring period."
Unfortunately, complete records of all water levels recorded during the program are not
available, since they were destroyed in a fire at Boston City Hall. Figure 14, on page 56, prepared from contour plans by Cotton and
Delaney, shows areas in Back Bay having water levels below el. 5.0 during that 4-year
Most wells in Back Bay experienced a water level below el. 5.0, and, in seven wells,
the highest water level measured was also below el 5.0. Local drawdowns from
leaking sewers are the most probable cause for the low water levels in the seven wells.
Precipitation during the period was about average for Boston, with yearly deviations up
to 6 in.
Significant construction projects during 1936-1940 included the Huntington Avenue
subway that dewatered the sand outwash to el. -20.0 or below; the Liberty Mutual
building, with excavation below el. 0; and the New England Mutual building where
excavation extended to el. -21.0. These projects cannot, however, account for the broad
extent of low water levels in Back Bay. Leaking sewers and pumping from sumps in
basements of buildings undoubtedly were major contributors.
Dewatering of the pervious outwash stratum for subway construction was probably
responsible for the lowered groundwater levels north of the Southwest Corridor alignment
from Massachusetts Avenue to Clarendon Street. Groundwater drawdown immediately adjacent
to the Huntington Avenue excavation is not known because data are not available for wells
there during the construction period.(4)
Throughout much of this area, the outwash stratum is particularly well developed and
is separated from the fill by a relatively thin layer (as little as 3 ft.) of organic
silt and/or peat. However, in many locations, where trenches and holes have been
excavated, the outwash and fill strata are connected and lowered water levels in the
outwash can directly affect water level in the fill. Some of the WPA observation wells
may have been installed into the outwash stratum. Water levels observed in some wells
may, therefore, have been lower than groundwater levels in the fill.
In the Copley Square area, south of Boylston Street between Dartmouth and Berkeley
Streets, the low groundwater levels may have been caused by leakage into sewers and
drains and pumping from sumps in building basements. Because the outwash stratum
generally does not extend into this area and the Boylston Street subway structure forms a
barrier to groundwater seepage from across the street, low groundwater levels in this
area were probably not related to construction dewatering and drawdown.
The St. James Avenue sewer has a history of causing local groundwater lowering.
Groundwater levels have also been lowered by drainage in a crawl space along the easterly
side of the John Hancock Clarendon building that reduces hydrostatic pressures on
basement floors and walls. Until 1984, the water level had been held at el. -0.5. It has
been raised somewhat with recent renovations to the structure. Sump pumping has also been
performed at the YWCA building at Stuart and Clarendon Streets.
In other areas, low groundwater levels were probably due to leakage into sewers. The
West Side Interceptor beneath Beacon and Charles Streets may have been responsible for
low groundwater levels in that area. Low groundwater levels along Tremont Street in the
South End were probably caused by leakage into major sewers that join in that area. Some
water levels there were as low as el. 0 to -3.0. In this area, there were also several
groundwater mounds, probably due to water main leaks. These local recharges
intermittently interrupt the drawdown pattern toward Tremont Street.
1967-1968 USGS Measurements. On two occasions, in September 1967 and March
1968, the United States Geological Survey (USGS) measured groundwater levels throughout
the Boston Peninsula. This study was made in response to a request by the Massachusetts
Department of Public Works which was concerned about the potential adverse effects of
construction for the then proposed Inner Belt expressway on groundwater levels. The USGS
used observation wells extant from the WPA survey completed in 1940. Less than half of
the original wells were found to be usable. Results of the USGS study were published in
1975 as Hydrogeologic Investigation Atlas HA-513.(4) Figure 15 has been prepared from the groundwater contours
Areas in Back Bay where the September 20-21, 1967, water levels were below el. 5.0 are
shown in Figure 15. Areas where both readings, the second on
March 20-22, 1968, were below el. 5.0 are also indicated. In addition to the areas shown,
low levels were observed in wells around the Christian Science Center; however, those
data may not have been available to the USGS.
It would appear, by comparison of Figure 15 with Figure 14, that water levels throughout Back Bay were higher in
the 1960s than in the 1930s. Note, however, that Figure 15
was based on two isolated readings while the 1930s data represent extreme lows from
numerous readings over a four-year period. Furthermore, the 1960s readings were taken
during a wet period; precipitation in 1967 was 6 in. above normal and 5 in. of rain fell
on March 17-18, 1968. On the other hand, looking at the high water level readings, two of
the seven wells that were never above el. 5.0 in the 1930s, were above that level in the
1960s. No significant construction dewatering during the 1967-68 period has been
reported. Groundwater levels were again below el. 5.0 in the John Hancock area and along
Tremont Street in the South End.
Construction within the study area between 1940 and 1967, which includes the
Prudential Center, does not appear to have permanently lowered groundwater levels below
el. 5.0 by 1967.
1970-1985 Groundwater Levels. The BRA report summarized available groundwater
data from numerous building projects.(13) Water level observations were generally made at
the building sites and at immediately adjacent areas before, during and shortly after
construction. Table 5 lists major projects constructed both before and during the period.
In addition, data were collected from other sources where monitoring is ongoing, for
example, at the Christian Science Church, Prudential Center, Boston Public Library,
Trinity Church, Massachusetts Turnpike Extension and Church of the Advent. Primarily,
data were available for the area along the central spine across Back Bay. Essentially, no
recent groundwater data are available for the Back Bay Historic District and other
important areas having buildings founded on wood piles.
Figure 16 shows the location of most of the observation
wells monitored at some point during the 15-year period from 1970 to 1985 and the area
where the lowest water level observed was below el. 5.0. The monitoring period often
lasted less than a year, since the purpose of the monitoring was to monitor the effects
of construction and dewatering that caused the temporary lowering of water levels in
areas adjacent to sites. Some data in Figure 16 were affected
by construction dewatering while the two readings in 1967-68 (see Figure 15) were not affected.
Groundwater levels substantially below el. 5.0 in the area of the Christian Science
Center and in the Park Square area south of the Public Garden were due to
construction-related dewatering. Observed low groundwater levels around Hadassah Way were
due to sump pumping from a basement in that area. Groundwater levels of approximately el.
3.0 were observed within the Prudential Center, probably caused by leakage into the
underground parking garage. The effect of these low levels on groundwater in adjacent
areas is mitigated by a wall of steel sheetpiling that encloses the Prudential Center
site. (Note that the USGS observations shown in Figure 15 did
not include the Prudential Center.)
Other areas of low groundwater include the area bounded by the Prudential Center,
Dartmouth Street, Boylston Street, and the Massachusetts Turnpike, and the block occupied
by the John Hancock Clarendon and Berkeley buildings. The drain between the two older
Hancock buildings continues to cause lowered groundwater levels in that area. During
subsurface investigations for the Copley Place project, low groundwater levels between
the Boston Public Library and the Massachusetts Turnpike were concluded to have largely
been due to leakage into the St. James Avenue sewer. The Prudential Center, the subway
tunnel beneath Exeter Street and the Conrail alignment may also be lowering groundwater
levels in this area.
Fast of the Back Bay railroad station, groundwater levels 1 to 2 ft. below el. 5.0
have been observed on both sides of the right-of-way occupied by the Massachusetts
Turnpike and the Southwest Corridor Project. Groundwater levels in this area were
observed to be below el. 5.0 for several years before Southwest Corridor construction
began and therefore do not reflect construction-related groundwater lowering. Drains in
the former railroad right-of-way were probably responsible for the lowered water levels.
Massachusetts Turnpike drains were probably not the cause since they are located above
the observed low water levels.
Along Tremont Street in the South End, where groundwater levels had been below el. 5.0
in the two previous monitoring periods, data were available for only one observation
well. Low water levels in this well, near the intersection of Berkeley and Tremont
Streets, were below el. 5.0. These findings could indicate that lowered groundwater
levels along Tremont Street still exist.
Recent Lower Beacon Hill Study In early 1984, attention was focused once again
on foundation problems caused by lowered groundwater levels, on this occasion in the
lower Beacon Hill area from Charles Street to Embankment Road, bounded to the south by
Beacon Street. Residents along the waterside of Brimmer Street, between Pinckney and Mt.
Vernon Streets, became alarmed when cracks developed in interior and exterior walls and
when other evidence of differential settlement appeared. Test pits were excavated to
enable visual examination of the wood piles. In most cases, the wood in the top 1 to 3
ft. of the piles was severely decayed. Groundwater levels were found to be several feet
below the pile tops and as much as 6 ft. below the water level in the Charles River
The principal cause of lowered groundwater has been determined to be leakage through
cracks and joints in combined sewer overflows, where they join the Boston Marginal
Conduit at the foot of Pinckney and Mt. Vernon Streets. The Metropolitan District
Commission (MDC), Massachusetts Water Resources Authority (MWRA) and Boston Water and
Sewer Commission have investigated the problem and are taking steps to correct the loss
Occurrences of Rotted Wood Piles
Except for well-publicized cases, records of wood pile deterioration are buried in Boston
Building Department files, in the files of building owners, their architects, engineers
and contractors, or they do not exist. Owners are understandingly reluctant to talk about
Six thousand applications for building permits filed with the building department
between 1979 and 1984, and representative samples of permit data from 1967-1972 and
1976-1979 were examined in the preparation of the 1985 BRA report.(13), Only two of the
permits issued were for repairs to wood piles, suggesting that the problem in recent
years (to 1984) has been minor or not reported.
With the exception of the lower Beacon Hill area, other cases throughout Back Bay
appear to have been isolated and infrequent. In addition to the Boston Public Library
problem in 1929-30, J.R. Worcester identified two occurrences of rotted wood piles:(15)
"Such extensive repair work has been necessary in other localities in the Back Bay; e.g.,
the Fire Insurance Protective Headquarters at 4 Appleton Street (South End) in July 1921
had to be underpinned where piles cut as low as elev. 3.96 were rotted off above ground
water level found at the time at elev. 3.30; again at 12 Hereford Street, corner of
Beacon Street, in June 1933, piles cut at elev. 8.13 were rotted to within 3" of ground
water level found to be at elev. 6.50."
Additionally, four buildings between Boylston and Beacon Streets are known to have had
deteriorated wood piles that required repair.
The lower Beacon Hill area from Charles Street to Embankment Road has had a history of
problems related to rotted wood piles dating back to 1927. The Boston Inspectional
Services Department (formerly the Building Department) reported that repairs to wood
piles had been made at 38 of the 188 houses and buildings in this 10-block area, some
having been made in each decade since the 1920s.(17) The Brimmer Street problem described
earlier is the most recent example.
Problems in the lower Beacon Hill area appear to be related to both a lowered water
table caused primarily by leakage into sewers and drains, and an original wood pile
cutoff level at el. 7.0, 2 ft. above the el. 5.0 that was commonly used throughout Back
Bay. The area is outside of the Mill Dam and West Side Interceptor and, until 1910,
groundwater was readily recharged by the nearby and then tidal Charles River.
Construction of the Boston Marginal Conduit and embankment appeared to have impeded
Effect of Contemporary Buildings on the Water Table
From the available groundwater data, it is difficult to assess long-term changes in
groundwater levels that may have resulted from the construction of buildings in Back Bay.
While temporary drawdown has occurred during construction, groundwater has shown that it
will return to pre-construction levels unless there is continued pumping from foundation
drains or leakage into basements. There is no evidence that buildings constructed in Back
Bay within the last 50 years have caused significant permanent adverse effects to
groundwater levels. However, older buildings are known to have foundation walls and
floors that leak, requiring sump pumping.
Recent heightened public interest in Back Bay groundwater levels prompted an extensive
study of existing and probable post-construction groundwater levels around the proposed
Hines/New England Mutual Life development at 500 Boylston Street.(18) The study concluded
that the building's proposed deep basements would have little impact on long-term
off-site groundwater levels.
Preserving Groundwater Levels
The importance of maintaining groundwater levels in Back Bay has been recognized since
the late 1800s. Pipes were placed to act as siphons beneath an early sewer and subway
tunnel to mitigate their impact on groundwater movement and levels. In several areas,
permanent recharge systems have been installed to replenish groundwater, particularly
around historic structures founded on wood piles. Temporary recharging around excavations
for building construction projects has also been used.
Siphons. In order to lessen the damming effect of the 8-ft. high Boston
Marginal Conduit and the wood sheeting that was left in place, Worcester reported that
siphon pipes were "placed under the conduit from the Basin to the Back Bay intended to
carry groundwater from one side to the other."(15) Worcester questioned the long-term
effectiveness of these siphons because they would probably have filled with silt and
would have been only locally effective.
Four 12-inch diameter tile siphon pipes were placed under the Boylston Street subway
tunnel in the vicinity of Copley Square to transport groundwater from one side to the
other. These pipes were probably considered necessary because the bottom of the tunnel
was in the clay and its top was at about el. 6.0, thus forming a virtual dam along
Boylston Street. The distinct difference in groundwater levels observed on opposite sides
of Boylston Street since 1930, confirmed by studies for several projects in recent years,
raises doubts about the effectiveness of these siphons.
Siphon pipes were also used in the groundwater equalization system of the recently
constructed Southwest Corridor structure to connect perforated header pipes placed on
either side of the tunnel.
Recharging. Early inadvertent recharging was undoubtedly performed at many
locations by drywells that were used to dispose of precipitation from roofs. These
systems were probably not installed frequently enough to have a significant impact on
In 1930, the first reported recharge system intended to raise groundwater levels in
order to protect wood piles was installed at Trinity Church. Only a year before, severely
rotted wood piles were found at the nearby Boston Public Library. Large conductors from
the Church's roof gutters were connected into long, stone-filled drywells outside the
Church and into a brick-lined pit in the basement. Although dry weather groundwater
levels around Trinity Church were below el. 4.0 to 5.0, the intermittent rise in water
level and wetting of wood piles due to the recharge system were probably responsible for
the preservation of the Church's foundations.
Other recharge systems have since been installed in Copley Square. In the mid-1950s, a
recharge system was constructed at a triangular grass plot across Dartmouth Street from
the Boston Public Library. In 1968, when Copley Square was redeveloped with its current
sunken plaza and fountain, another recharge system was installed below the plaza. Both
systems conveyed surface water runoff into the fill through perforated pipes laid in 3 to
5-ft. thick beds of gravel or screened stone.
An underdrain system was provided below the slab-on-grade floor of the Christian
Science Church Center parking garage. This system was designed to function only when
water levels rose above approximately el. 6.7. The underdrains could be reversed to
recharge groundwater should water levels in the fill fall to levels that would threaten
wood piles that support the Mother Church. Again, a system of perforated pipes in a thick
granular drainage blanket were used.
Recharging to minimize temporary drawdown outside of construction sites had been
undertaken for several building projects where there was particular concern for wood
piles supporting nearby structures. In these instances, recharging usually involved
injecting water, under pressure, into the fill or sand outwash stratum through
wellpoints. In some cases, water was pumped into open ditches and large diameter recharge
wells and then allowed to percolate into the ground.
Sewer Dams. In the early 1930s, the St. James Avenue sewer was found to be the
cause of lowered groundwater levels along most of its length. It was found that when
sewage was backed up behind a dam installed in the sewer, groundwater levels rose to
"normal" levels, thereby mitigating the effects of leakage into the sewer. Over the
years, the original butterfly valves deteriorated and were replaced by a sand bag dam
that requires periodic repair.
Since 1985 the MWRA has maintained raised water levels in the Boston Marginal Conduit
in order to minimize the impact of local leaking sewers that lower groundwater levels. A
permanent solution is still being sought.
Pumping from water supply wells, accompanied by lowered groundwater levels, has caused
subsidence of major cities around the world - including Mexico City, Venice, Taipei and
Bangkok. While Boston has not experienced a comparable problem, areas of the Back Bay
have suffered damage from lowered groundwater levels. The groundwater table should be
restored to levels that preserve the integrity of foundations for the city's historic
nineteenth century buildings.
If there were no loss of groundwater by pumping and by leakage into sewers, drains and
foundations, and no additions to groundwater from leaking water mains and other man-made
sources, the probable groundwater table throughout Back Bay would be expected to vary
from el. 8.0 to 10.0. Actual groundwater levels in the fill are lower, except for local
groundwater mounds probably caused by leaking water mains. In some areas, the water table
is below el. 5.0, a common level at which wood piles were cut off in the nineteenth
The principal cause of lowered water levels is leakage into sewers and drains.
Groundwater loss through the walls and floors of the Storrow Drive underpass, into
subways and the basements of older buildings below the water table also occurs.
With the available data, it is virtually impossible to determine if "permanent" water
levels have changed significantly during the past 50 years, except in one or two local
areas - for example, the Brimmer Street area and westward along the Boston Marginal
Conduit where low water levels have been discovered in the past two years. Little or no
water level data have been available over the past 20 years for major sections of Back
Bay, notably the Back Bay Historic District where most buildings are supported on wood
piles. A long-term groundwater monitoring program should be established in this and other
The Charles River Basin cannot effectively recharge the groundwater table in the fill
because the Mill Dam and Boston Marginal Conduit act as dams. However, some recharging to
the sand outwash may occur, but the overall effect throughout the entire Back Bay area is
not very significant.
Of the three principal adverse effects of lowered groundwater levels, temporary or
permanent, the major future concern in Back Bay is for the deterioration, or rotting, of
untreated wood piles. Numerous buildings in Back Bay have suffered damage during the past
60 years from differential settlement caused by rotted wood piles. Problems have been
reported in the lower Beacon Hill area in the past two years. Underpinning is currently
underway to restore foundations. Future ground subsidence and negative friction on pile
foundations are not likely to be significant because of extensive and prolonged
dewatering for construction projects over the past 100 years.
There is no evidence that buildings constructed during the past 50 years have caused
permanently lowered or significant changes in groundwater levels. Future development in
Back Bay would similarly not be expected to cause permanent adverse effects on water
levels, provided foundation walls and basement floors are watertight.
Dewatering for the construction of sewers and drains, subways and other transportation
corridors, and many buildings has temporarily lowered the groundwater table in the fill
and, in particular, the piezometric head in the sand outwash over a large area of Back
Bay, in some cases for a period of several years.
Temporary drawdown of piezometric levels in the sand outwash are not likely to cause
deterioration of wood piles. There is no evidence of failure or settlement attributed to
rotting at the tips of wood piles that commonly bear on the outwash stratum a few feet
below the organic silt. Nevertheless, it must be assumed that the sand outwash and fill
are connected where construction has penetrated the organic soils. Therefore, dewatering
in the sand stratum may affect the groundwater levels in the fill at localized areas that
lie a considerable distance from the source of pumping.
In situations where buildings have been constructed with groundwater level sensitive
foundations, special emphasis must be given to:
- An extensive and continuing water table monitoring plan.
- A thorough review of the temporary and permanent effects of all construction projects
on the water table, with construction plans and/or methods altered to minimize their
effects on, or replenish, the water table.
- Rapid response by appropriate agencies to correct lowered groundwater levels found in
the monitoring program.
The consequences of repairing and improving water distribution and the sewerage
drainage systems throughout Back Bay must be considered. Unless the work is undertaken in
the correct sequence, these improvements may adversely affect groundwater levels. For
example, since leaking water mains recharge the water table and leaking sewers deplete
it, it seems obvious that sewers should be repaired before water mains are fixed.
Furthermore, improvements in the sewer system or changes in operations that facilitate
drainage and lower fluid levels in pipes will exacerbate the groundwater problem unless
leaking pipes are repaired prior to the improvements.
ACKNOWLEDGEMENTS - This article was originally presented at a meeting of the BSCES
Geotechnical Group on March 10, 1986. The authors wish to thank the Boston Redevelopment
Authority and Gerald D. Hines Interests for funding the Haley & Aldrich, Inc., study
on groundwater in Back Bay Boston, for which the authors were principal investigators and
upon which much of this article is based.(13) Local organizations providing data on
groundwater levels and information on construction included: Christian Science Church,
Trinity Church, Church of the Advent, Boston Public Library, Prudential Center, and the
Massachusetts Turnpike Authority. The illustrations were drawn by Ms. Acey Welch and the
text was prepared by Ms. Marion Keegan, whose assistance is gratefully acknowledged.
HARL P. ALDRICH is Chairman of the Board and a founding principal of Haley &
Aldrich, Inc. He received an Sc.D. in civil engineering from MIT in 1951. Dr. Aldrich has
over 30 years of experience in solving foundation problems in Back Bay Boston, including
recent work on the study of "Groundwater in Back Bay Boston "for the Boston Redevelopment
JAMES R. LAMBRECHTS is a Senior Engineer with Haley & Aldrich, Inc. He received
his B.S.C.E. from the University of Maryland in 1973 and an M.S.C.E. from Purdue
University in 1976. His experience with the geotechnical problems in Back Bay Boston have
been principally associated with the MBTA Southwest Corridor Project. He was also primary
investigator for the study of "Groundwater in Back Bay Boston" for the Boston
1. Aldrich, H.P., "Back Bay Boston - Part I," Journal of the Boston Society of Civil
Engineers, Vol. 57, No. 1, January 1970, pp. 1-33.
2. All elevations used herein are referenced to the Boston City Base (BCB) Datum which is
5.65 ft. below the National Geodetic Vertical Datum (NGVD), formerly called the U.S.
Coast and Geodetic Survey Sea Level Datum of 1929. El. 0.0 BCB datum is el. -5.65
3. Stearns, F.P., "Report of the Engineer," Report of the Joint Board Upon the
Improvement of Charles River, House No. 775, April 1894.
4. Cotton, J.E., and Delaney, D.F., "Groundwater Levels on Boston Peninsula,
Massachusetts," Hydrologic Investigations Atlas HA-513, U.S. Geological Survey,
Reston, VA, 1975, 4 sheets.
5. Aldrich, H.P., "Preserving the Foundations of Older Buildings," Technology &
Conservation, Summer 1979.
6. Journal of Engineering Societies, The Boston Society of Civil Engineers, Vol.
7. Camp, Dresser & McKee, Inc., "Report on Improvements to the Boston Main Drainage
System," HUD Project No. P-Mass-3306 for City of Boston, September 1967, 2
8. Worcester, J.R., "Boston Foundations," Journal of the Boston Society of Civil
Engineers, Vol. 1, No. 1, 1914, with discussions pp. 179-248 and 395-417.
9. Casagrande, A., and Casagrande, L., Investigation of Settlement at Prudential
Center, Report to Charles Luckman & Associates, September 1970.
10. Casagrande, A., and Avery, S.B., Jr., Investigation of Building Settlements in
Back Bay Area, Report to Metcalf & Eddy, March 1959.
11. Haley & Aldrich, Inc., Report on Investigation of Settlement and Lateral
Movement, Trinity Church, Boston, MA August 1982.
12. Weber, R.P., "Foundation Response Caused by Disturbance of Clay," Journal of the
Geotechnical Engineering Division, ASCE, Vol. 104, No. GT5, May 1978.
13. Haley & Aldrich, Inc., Report on Groundwater in Back Bay Boston, for
Boston Redevelopment Authority, Boston, MA, March 1985.
14. Snow, B.F., "Tracing Loss of Groundwater," Engineering News-Record, July 2,
15. Worcester, J.R., & Co., Report on Pile Foundations and Ground Water Levels at
Trinity Church - Boston Public Library - S.S. Pierce Bldgs., Copley Square, Boston,
MA December 31, 1945.
16. Paine, R.T., Trinity Church - The Church Endangered by the Low Level of the Ground
Water - How the Danger Has Been Temporarily Averted, April 20, 1935.
17. Folkins, P.M., Report to the Commissioner of Inspectional Services Department -
Structural Report - Lower Beacon Hill, WD 5, October 15, 1984.
18. Haley & Aldrich, Inc., Report on Subsurface Investigations and Foundation
Design Recommendations, Proposed Development 500 Boylston Street Boston,
Massachusetts, for Gerald D. Hines Interests, March 1985.
19. Charles River Basin Commission, Boston Marginal Conduit: Section 2 - Details of
Masonry, Record Plan Sheet No. 2, May 21, 1910.
20. Charles River Basin Commission, Boston Marginal Conduit - Section 3, Boston
Embankment, Section 1, Record Plan Sheet No. 8, May 21, 1910.
21. Boston Transit Commission, Plan No. 10478, for Boylston Street Subway, Section 4,