Foundation and excavation design in New York City
Most New York City lots are built to the lot line, so a new foundation has to carry the building and protect the neighbors at the same time. There is usually more than one way to do both. Grant Engineering lays out the options for your site, compares them on cost, schedule and risk, and designs the one that fits.
The options
- Shallow foundations. Spread and strip footings, mat foundations and slabs on grade, where the soil near the surface can carry the building.
- Deep foundations. Drilled or driven piles, micropiles, caissons and helical piles, where good bearing is deeper or where access and headroom are tight.
- Underpinning. Extending an existing foundation, yours or your neighbor's, down below the new excavation.
- Bench footings and other alternatives to underpinning. Stepping or setting back the new foundation so the excavation does not undermine the neighbor's footing. Where it works, the neighbor's foundation is left untouched.
- Support of excavation. Soldier piles and lagging, sheeting, shoring and bracing to hold the sides of the excavation while the foundation is built.
- Retaining walls and groundwater. Permanent retaining walls, dry wells and groundwater control where the site needs them.
How we choose
- The soil, rock and groundwater found in the geotechnical investigation
- The type, depth and condition of the neighboring foundations, confirmed by test pits
- How deep the excavation goes and how close it comes to the lot line
- Access and headroom for equipment
- Whether the neighbor has agreed to work on their property
- Cost and schedule
What you receive
- The engineer's evaluation of the adjacent buildings and subsurface conditions that the code requires before support is designed
- Signed and sealed drawings and calculations for your DOB filing
- The monitoring plan required with underpinning and excavation support
- Coordination with your architect, structural engineer and contractor
Common questions
Do I have to underpin my neighbor's building?
Not always. The Building Code requires the neighboring foundations to be evaluated and protected wherever an excavation could affect their support. Underpinning is one way to do that. The code also allows other methods of support, and a bench footing or a deep foundation set back from the lot line can sometimes avoid underpinning altogether.
Shallow or deep foundations?
It depends on the loads and on what the borings find. Where the soil near the surface is good, footings or a mat are usually the simplest. Where it is not, or where groundwater is high, piles can cost less than excavating to reach better soil.
Do I need a geotechnical report first?
Yes. The design relies on the soil and groundwater conditions at your site. We can perform the investigation and the design together.
Can Grant also inspect the work?
Yes. Foundations, piles, excavation support and underpinning are subject to special inspection, and we perform them.
Code references
NYC Building Code Chapter 18, Soils and Foundations. Chapter 33, Safeguards During Construction or Demolition, including Section 3309, Protection of Adjoining Property. Chapter 17, Special Inspections and Tests.
Related services
Geotechnical investigations and reports · Structural design and assessments · Construction monitoring · TR1 and TR8 inspections