Earth Support Corp

Cambridge & Somerville

Shoring Contractor for Cambridge & Somerville Infill

Two cities, one kind of job: infill on lots that were built out long ago, where the excavation often sits 4 to 8 feet from the neighbor's foundation. Under it, the ground runs from filled river flats and marine clay in Cambridge to thin till over shallow argillite on Somerville's hills.

Send us the address and drawingsThe local numbers

The ground

What's under Cambridge & Somerville

The strip of Cambridge along the Charles where MIT now stands was filled in the second half of the nineteenth century (Kaye, 1976), and 30% of the city is now mapped as artificial fill. Another 28% is glaciomarine clay and silt and much of the rest is glacial sand and gravel: around Harvard Square, excavations have exposed 10 feet or more of sand and gravel over silts, clay and till (Kaye, 1976). The Fresh Pond moraine crosses the west of the city (Stone and others, 2018).

Somerville is higher and rockier: thin till, where rock is usually within 10 to 15 feet of the surface, covers 45% of the city, and thick till in drumlins such as Prospect and Spring Hills (Kaye, 1976) another 19%. Both cities sit on Cambridge Argillite, 95% of their mapped bedrock.

Public logs that reached rock found it at a median of 57 ft in Cambridge and 27 ft in Somerville. Neither city has a groundwater overlay like Boston's Article 32, though Cambridge's building-permit records note whether a job involves construction dewatering.

Cambridge, toward the river
  1. Made landFill on the old Charles River flats and marshes, including the MIT campus area, filled in the second half of the 1800s (Kaye, 1976).
  2. Marine clay and siltGlaciomarine clay under the flats and much of the low ground; soft to stiff, compressible under new load.
  3. Sand, gravel and tillGlacial sand and gravel over dense till; around Harvard Square the sand and gravel is 10 feet or more thick (Kaye, 1976).
  4. Cambridge ArgilliteMedian depth 57 ft in 130 public logs across Cambridge; the middle half fell between 21 ft and 77 ft.
Somerville's hills
  1. Thin tillStony glacial soil, usually less than 10 to 15 feet thick over rock (Stone and others, 2018).
  2. Thick till (drumlins)Compact till in hills such as Prospect and Spring Hills (Kaye, 1976).
  3. Cambridge ArgilliteMedian depth 27 ft in 54 public logs across Somerville; the middle half fell between 14 ft and 56 ft.

Typical, from published geology and public logs. The site’s own borings and the engineer of record govern any design.

The work

Typical below-grade jobs in Cambridge & Somerville

Nearly every job here is next to an occupied house. The distance to the neighbor's foundation decides the shoring system before the soil does, and on Somerville's hills the rock decides how it goes in.

  • Infill a few feet from the neighbor

    New two- and three-family houses and small apartment buildings go onto long-built-out lots: 446 new-building permits across the two cities in 2021–2025. With 4 to 8 feet to the next foundation, the support is often a cantilevered or braced soldier-pile wall, or the neighbor's footing is underpinned, because tiebacks would cross the lot line.

    • The neighbor's foundation within a few feet
    • Tiebacks need written permission to cross the line
    • Monitoring the house next door while the dig is open

    Shoring & support of excavationYour neighbor is excavating

  • Basement conversions

    In 2021–2025, 212 Cambridge and 64 Somerville building permits described excavating, underpinning or digging out a basement, many of them to turn a cellar into living space in a two- or three-family house.

    • Old stone or brick foundation walls
    • Groundwater in the sand-and-gravel plains
    • Ledge on the hills

    UnderpinningWhat underpinning costs

  • Shallow rock on Somerville's hills

    Where thin till covers the argillite, soldier piles and anchors are drilled into rock rather than driven, and a basement can hit ledge partway across the footprint.

    • Rock usually within 10 to 15 feet
    • Drilled rather than driven elements beside occupied houses
    • Rock anchors in place of soil tiebacks

    Tiebacks & rock anchorsLimited-access drilling

  • Soft ground in East Cambridge

    On the filled flats and marine clay near the river, excavations go below the water table in soft ground, and new loads usually go down through the clay to till or rock on drilled elements.

    • Watertight excavation support
    • Construction dewatering flagged on the permit
    • Micropiles or drilled shafts through the clay

    Micropiles & pin pilesDrilled shafts

The local numbers

Cambridge & Somerville in public records

Public records for the two cities, counted with the same rules. Cambridge's open permit data covers full years from 2019 and Somerville's from 2015, so each city is compared with itself.

Permits that describe digging below grade
276
2021–2025
New-building permits
446
2021–2025
Depth to rock, median
52 ft
Middle half 16 ft–75 ft, from 184 public logs
How these numbers are counted
Permits that describe digging below grade
Building permits whose applicant narrative uses, at the start of a word, excavate/excavation, underpin/underpinning, dig out, or lowering the basement or cellar. Negated phrases (“no excavation”) are removed first; driveway, parking-lot and paving jobs that name nothing structural are dropped. Boston amendments and trade permits are not counted. A floor, not a census: plenty of basement jobs never use these words.
New-building permits
Permits in the city's Building Permits: New Construction dataset.
Depth to bedrock
Public borings and well-driller logs that reached rock, placed inside the boundary by their coordinates. Median and middle half (25th to 75th percentile). Fewer than 10 logs: no typical depth is quoted. Holes that stopped above rock are counted separately.
Inside the Groundwater Conservation Overlay District
Share of the land area inside the overlay's mapped areas, measured on a flat (Massachusetts State Plane) projection.
Groundwater
Active Boston Groundwater Trust observation wells with at least two readings in the last twelve months. Each well's median reading, then the median across wells: as an elevation (Boston City Base) and as a depth below the well's rim, which sits at sidewalk level. Fewer than 10 wells: not quoted.

Data as of 2026-09-18. Sources: Building Permits: New Construction, Addition/Alteration and Short Form (City of Cambridge Open Data (ViewPoint permitting system)); Applications for Permits and Licenses (City of Somerville Open Data (Citizenserve)); Bedrock Data Points (borings and well-driller logs that reached rock) (MassGIS / Massachusetts Geological Survey).

Read next

Guides and services for this ground

Scope

What we install in Cambridge & Somerville

The below-grade scope a builder or general contractor can hand to one subcontractor.

Projects

Representative Cambridge & Somerville work

Representative deep-foundation projects in and around Cambridge & Somerville. References available on request.

FAQ

Cambridge & Somerville: questions builders ask

References

Building below grade in Cambridge & Somerville?

Send the address, the drawings and the distance to the neighbor's foundation. We'll scope the shoring or underpinning for the Cambridge or Somerville lot and quote it.

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Or email plans to bids@earthsupportcorp.com

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