00 · Start here

The whole idea in one picture

Every building stands on a foundation buried in the ground. Newer buildings sit on concrete footings. Many older NYC buildings, from the 1800s and early 1900s, sit on rubble foundations instead: rough stone laid in lime mortar, sometimes on large flat stones, with no concrete at all. Either way, the soil under and beside the foundation is what holds the building up. A deep hole dug right next door removes some of that soil. Tap the buttons to see what happens.

InteractiveTap a button to run the dig

Before digging. The existing foundation sits on undisturbed ground about 10 ft down. We need to dig 20 ft.

Think of a chair at the edge of a sandbox. Dig a hole beside the chair, deeper than its legs, and the sand under the leg slides into the hole. The chair tips. Underpinning makes the leg longer, down past the bottom of the hole. Shoring is a wall that holds the sand back. Bracing is the steel that keeps that wall from leaning in.

Three jobs on every site

1 · The problem

Digging takes away support

Loss of bearing. Digging below an existing footing removes the soil that carries the weight of the building above it.

Ground movement. Soil beside the footing can slide or bulge into the open excavation, and the building settles with it.

Water loss. Pumping groundwater out of the excavation also lowers the water under the existing building. The soil compacts as it dries and the building can settle.

2 · The fix

Hold it up, hold it back

Underpinning holds the neighbor up. New concrete is built under the neighbor footing, one short section at a time, until the building rests on ground deeper than the bottom of our hole. Digging next to it no longer removes anything the building stands on.

Shoring holds the soil back. Where there is no neighbor footing, such as the street side or a yard, steel posts with timber boards between them, or a wall of concrete columns, line the sides of the hole so the soil cannot slide in.

Bracing keeps the walls from leaning in. Soil pushes sideways on both the underpinning and the shoring. Steel beams set across the hole, added as it gets deeper, push back so neither wall moves.

3 · The process

Survey, design, plan the work

Survey existing conditions. Document the existing building, its foundation and any cracks before work starts.

Design. Engineers design the underpinning, shoring and bracing for the soil and the building.

Method of procedure. A written, step-by-step sequence for the work, with monitoring and inspection at each step.

How to read the drawings on this page

Builders draw the same thing from different directions. Once you know which view you are looking at, every drawing below makes sense.

Plan view is used in section 02. Sections in 03 and 04. Elevation in 05. The model in 01 is 3D.

Tap any word with a dotted underline for a plain-English definition. The full list is in the glossary at the bottom.

01 · 3D site model

A typical 25 x 100 midblock lot

Five story walk-up on the left, four story on the right, row houses behind, street in front. The new building needs a 20 ft deep cellar and sub-cellar, about 10 ft below the neighbor footings. Drag to orbit, pinch to zoom, then step through construction.

In plain EnglishYou are looking down at a city block from above the street. Our lot is the empty one in the middle. Drag the stage bar to watch the job happen in order, one step at a time.

InteractiveDrag to orbit, pinch to zoom, then play the job

Drag to orbit · Pinch to zoom

View

0 / 8

Existing condition

Underpinning pinsPins being placedSoldier piles, strutsLaggingMonitoring points1:1 influence zone

Illustrative only. Dimensions are typical, not design values. Every site is designed by the SOE and underpinning engineer of record.

02 · Plan view

Who gets what along each lot line

The system changes by edge. Where a neighbor footing sits above our subgrade, it gets underpinned. Where there is no building (street, yards), soldier piles and lagging hold the soil. Cross-lot struts tie the two party walls together across the narrow lot.

In plain EnglishThis is the site seen from directly above, like a map. The small numbered blocks along each side are concrete pins under the neighbor walls. The number tells you which round of digging that pin belongs to.

InteractiveSwitch layers on and off

Pin, number = dig setSoldier pileLaggingCross-lot strutSurvey targetSeismographxCrack gaugeGroundwater well

Note the pins sit under the neighbor footings, on the neighbor lot. That is a physical encroachment, which is why access and license agreements drive the schedule. Where the neighbor buildings stop (rear of A and B), the side walls switch to soldier piles and lagging.

03 · Section · why it is required

Five ways an excavation damages the building next door

A footing does not just press straight down. It spreads its load into a zone of soil below and beside it. Anything that removes, loosens, drains or shakes that zone moves the building. Tap each mode.

In plain EnglishThis is a slice cut straight down through the neighbor and our hole, like cutting a cake to see the layers. Tan is soil. White is the hole we dig. Grey is concrete.

InteractiveTap a mode, or swipe the drawing

The influence line. Draw a line up from the bottom edge of the excavation toward the neighbor. Any footing above that line is inside the zone the excavation will disturb. The slope depends on the soil and groundwater: 1H:1V is the common starting point, flatter in weak or wet soil. The code uses the same geometry in three places: monitoring of buildings within a distance equal to the excavation depth (BC 3309.4.4), a 2H:1V setback that waives adjoining owner notice (BC 3304.3.2), and maximum temporary slopes of 1:1 in Type A and B soil and 1.5:1 in Type C (Table 3304.4.2). MTA uses 1H:1V to 2H:1V for transit structures.

04 · Detail section

Anatomy of one underpinning pin

A pin is a short reinforced or mass concrete pier cast in a hand-dug pit directly under the existing footing. It transfers the building load down past our future subgrade to new bearing.

In plain EnglishA zoomed-in slice of a single pin. In the sandbox picture, this is the longer chair leg: new concrete under the old footing, reaching below the bottom of our hole.

The pin is stopped short of the footing, then the gap is packed with stiff, low-water mortar rammed in from the side. That closes the gap without relying on concrete shrinking against the footing. BC 1817.7 caps dry-pack at 3 in, requires preloading by wedging, permanent jacks or another engineered method, hand tools only in the pit, and lagging no more than 2 ft below the lowest board.

What the pin stops

Loss of vertical support. Once the pin is in, removing soil beside it no longer removes the soil the footing stands on.

What it does not stop

Lateral soil load. When the site is dug out, the row of pins acts as a wall and has to be braced with struts, rakers or tiebacks, or designed as a gravity wall.

05 · Elevation · sequence

Why pins go in as 1, 2, 3, 4

Looking at the neighbor wall from inside our site. A footing can span a short gap, never a long one. With 4 ft piers and the 12 ft minimum clear distance between open pits in BC 1817.7.1.2, only every fourth position can be open at once. Each set waits for the previous piers to be packed and cured.

In plain EnglishNow we stand in our lot and look straight at the neighbor wall. The crew never digs under the whole wall at once. They dig short pieces with solid ground on both sides, fill them with concrete, then move on.

InteractiveTap a step, swipe the drawing, or play it

06 · Methods

Underpinning vs. shoring, side by side

Underpinning moves the load down. Shoring, called support of excavation or SOE, keeps the soil from moving. Most NYC sites use both, edge by edge.

In plain EnglishThere are two jobs: hold the building up (underpinning) and hold the dirt back (shoring). Bracing is the steel that pushes the walls apart so they do not lean into the hole. Most sites mix all three.

Pit underpinning

Vertical supportYes
Lateral supportNeeds bracing
Crosses lot lineYes, under footing
Best fitShallow footings, rowhouse walls

Soldier piles and lagging

Vertical supportNo
Lateral supportYes
Crosses lot lineOnly with tiebacks
Best fitStreet, yards, dry soil

Secant or tangent pile wall

Vertical supportIndirect
Lateral supportYes, stiff
Crosses lot lineNo, fully in lot
Best fitHigh groundwater, no access

Bracing: struts, rakers, tiebacks

Cross-lot strutsNarrow lots
RakersWide lots, need kicker
TiebacksClear hole, need license
TradeoffAccess vs. clutter

Underpinning is not only hand-dug pits. BC 1817.4.1 also covers drilled piles, jacked piles, bracket piles and similar systems that take the existing load below the excavation. Non-encroaching alternates include interior rakers, soil improvement, and secant or tangent pile walls kept fully inside the lot. Where the neighbor already bears on Class 1a, 1b or 1c rock, BC 1817.4.3 allows rock support in lieu of underpinning, provided the rock is verified per BC 1817.3.4. Common in parts of Manhattan.

Shared party walls. The model shows two separate walls, one per lot. Many NYC rowhouses share a single party wall on the lot line, with a footing that straddles it. Where a party wall is affected by excavation, at any depth, the person excavating must preserve it at their own expense. Where the new building will use the party wall, it must be supported by proper foundations so it stays practically as safe as before (BC 3309.4.2). Underpinning then sits partly under our own lot, and the wall condition, load sharing and future connection all go into the evaluation report.

E1 · Engineer of record · design basis

Pit-pier check per BC 1817.5

A first-pass check of one party wall pier line, per foot of wall. It follows the code basis: at-rest earth pressure, water and surcharge (1817.5.4.2), a retaining wall analysis with and without the building vertical load, and FS 1.5 against sliding and overturning (1817.5.4.1.1 and 1817.5.6). Bearing includes the overturning moment (1817.4.1). Change the inputs to see when bracing becomes mandatory and what it carries.

InteractivePick a scenario or edit any value. Results update as you type

Excavation depth H, ft Neighbor cellar slab z0, ft Footing bottom zf, ft Pier base zb, ft Groundwater depth, ft Soil unit weight, pcf Friction angle, deg Surcharge at z0, psf Wall line load P, k/ft P arm from excavation face, ft Pier thickness t, ft Pier width along wall, ft Base friction coefficient Allowable bearing, ksf Upper strut depth, ft Lower strut depth, ft Strut spacing, ft

At-rest earth plus surchargeWaterPier

Assumptions. Per foot of wall. Earth pressure from the neighbor cellar slab down, at K0 = 1 minus sin of the friction angle, with the surcharge applied at the slab as a uniform load. Hydrostatic water from the water table, with no seepage reduction. Passive resistance in front of the embedded pier and uplift are ignored. The pier is treated as a rigid block that includes the footing, with P acting at its arm from the excavation face. Strut and toe reactions use tributary heights about each support. Seismic, which BC 1817.5.4.1 brings in through Chapter 16, and wall friction are not included. This is a sanity check that shows the code logic. It is not a substitute for the engineer of record calculations, the soil parameters in the geotechnical report, or a soil-structure analysis of deflection, which BC 1817.5.5 also requires.

E2 · Engineer of record · code criteria

BC 1817 design and drawing checklist

Paraphrased from the 2022 NYC Building Code. Tick items as you review a set. Ticks stay in this browser only.

InteractiveTick items as you review a set

07 · Monitoring

Record and monitor

Monitoring does not hold anything up. It tells you the support is working, early enough to stop. Baselines are taken before demolition or excavation, readings are compared against thresholds the engineer sets, and crossing a limit means stop and notify.

In plain EnglishInstruments work like smoke detectors. They do not stop damage. They warn early so the crew can stop, fix the cause and protect the neighbor.

0.5 in/sPeak particle velocity limit, TPPN 10/88 historic structures

0.25 inMax movement of historic structure, TPPN 10/88

1/8 inOptical survey accuracy, at least twice weekly

2x dailyGroundwater readings during dewatering

Optical survey targets

Prisms on neighbor facades and footings. Settlement, lateral movement and tilt.

Seismographs

Vibration in peak particle velocity, placed at or in the closest sensitive structure.

Crack gauges and telltales

On existing cracks found in the preconstruction survey. For historic buildings, photographed weekly.

Observation wells

Water level outside the site, checked against the max permissible drawdown in the geotechnical report.

E3 · Engineer of record · monitoring program

Designing the monitoring plan

BC 3309.4.4 requires monitoring, under a plan per BC 3309.16, of buildings within a distance from the excavation edge equal to or less than the maximum excavation depth, and of historic structures contiguous to or within 90 ft of the lot. The only exception is excavation 5 ft deep or less that stays more than 5 ft from footings, or does not go below them. Where underpinning or an alternate method of support is used, the engineer designing that work determines the plan (BC 1817.9).

What BC 1817.9 says the plan must define

Location and type of instruments
Frequency and duration of readings
Frequency of reporting, and the maximum time allowed to report readings
Reporting requirements
Permissible movement and vibration criteria, accounting for the buildings monitored and their condition
Actions to address exceedances, including notification of the commissioner
Special requirements for historic and landmarked structures, as set by the department

Instrument matrix

Typical NYC practice for a site like the one modeled above. Frequencies and accuracies are set by the engineer for each project.

Instrument	Measures	Where	Frequency
Optical prisms	3D movement, settlement, tilt	Party walls, facades, footings within influence distance	Baseline before demolition. At least twice weekly during underpinning and excavation (TPPN 10/88 historic: twice weekly, 1/8 in accuracy)
Seismographs	Peak particle velocity	At or in the closest sensitive structure	Continuous with alarms during demolition, drilling, driving, rock removal, compaction
Crack gauges and telltales	Crack width change	Cracks mapped in the preconstruction survey	With survey rounds. Weekly photos for historic buildings
Tiltmeters	Wall rotation	Unreinforced masonry party walls, slender walls	Automated, or with survey rounds
Inclinometers	SOE wall deflection profile	Behind soldier pile or secant walls near structures	Each lift, and before and after strut installation
Observation wells	Water level outside the site	Between site and neighbors	Twice daily during dewatering (TPPN 10/88). Compare to max permissible drawdown
Strut load cells	Brace load vs design	Critical cross-lot struts	Each lift and temperature swing

Setting thresholds from damage criteria

Thresholds should come from what the specific building can tolerate, then be checked against what the instruments can resolve. The common basis is the limiting tensile strain method.

Category	Severity	Typical damage	Limiting tensile strain
0	Negligible	Hairline cracks under about 0.1 mm	0 to 0.05%
1	Very slight	Fine cracks, treated in normal redecoration	0.05 to 0.075%
2	Slight	Cracks easily filled, some repointing, doors may stick	0.075 to 0.15%
3	Moderate	Cracks need opening and patching, pipes may fracture	0.15 to 0.3%
4 to 5	Severe to very severe	Wall repair or rebuilding, loss of bearing	Over 0.3%

After Boscardin and Cording (1989) and Burland (1995). Categories 0 to 2 are aesthetic, 3 to 4 serviceability, 5 stability.

1
PredictEstimate pier settlement, SOE deflection and ground movement for every stage, not only the final one (BC 1817.5.5 and 1817.5.7).
2
TolerateUse the evaluation report to judge the building: masonry type, existing cracking, out of plumb, rubble footings, historic status.
3
Set the limitPick a target damage category, convert to allowable settlement, tilt and lateral movement for that wall geometry, and set the limit at or below it. Apply TPPN 10/88 where historic.
4
Set alert and actionAlert and action levels sit below the limit with enough margin to act. Survey accuracy must be a small fraction of the alert value, or the alert will trigger on noise.
5
Write the responseWho reads, who decides, who stops work, how fast, and when the commissioner is notified.

Exceedance protocol, worked example

Synthetic settlement record for one prism on the party wall of Neighbor A. Threshold values are non-historic examples only. For a historic neighbor, TPPN 10/88 caps movement at 0.25 in. Drag through the job.

InteractivePress play, or drag through the job

Day 0

08 · Code and legal path

From design to sign-off

In plain EnglishThe city rules, in order: study the ground, design the support, tell the neighbor, get permission, record everything before, inspect during, sign off after.

1
Geotechnical reportLateral earth pressures, recommendations for evaluating adjacent properties, and max permissible drawdown outside the site when dewatering. BC 1803.6
2
Adjacent building evaluationPE report on each affected building and the subsurface, naming acceptable methods of support. Often needs early access to the neighbor cellar. BC 1817.3
3
Design of supportUnderpinning designed as a permanent structural element, with sequence, materials and monitoring criteria including movement thresholds and water table. BC 1817 BC 3304.4.1
4
Notice and accessWritten notice to adjoining owners at least 60 days before work, with project details, schedule, monitoring and protection. License to cross the lot line, or a court license under RPAPL 881. Sites within 200 ft of MTA property also need MTA External Partner Program approval before the DOB permit. BC 3309 BC 3309.2
5
Preconstruction survey and baselinesDocument existing conditions of all adjacent buildings, install instruments, take baseline readings. Required before excavating 5 to 10 ft within 10 ft of a building, or over 10 ft anywhere. BC 3309.4.3
6
Excavation noticesWritten notice to adjoining owners at least 10 days before excavation, unless set back 2H:1V from the deepest point. DOB notified 24 to 48 hours before work starts. Demolition of an existing building on the lot has its own 10 day notice and monitoring plan. BC 3304.3 BC 3306.3
7
Special inspectionUnderpinning and alternate methods of support are a TR1 progress inspection category, plus tieback testing when anchors are used. BC 1705.25.4
8
Excavate and monitorFollow the approved sequence. Report readings, act on thresholds. BC 1817.9 BC 3309.16
9
Sign-offSpecial inspector signs off, monitoring ends per plan once permanent structure braces the walls.

Triggers

Below or reducing support of an adjacent foundationSupport required, BC 1804.1
5 to 10 ft within 10 ft of a building, or over 10 ft anywherePreconstruction survey and 10 day notice, BC 3309.4.3 and 3304.3.2
Building within a distance equal to excavation depthMonitoring, BC 3309.4.4
Historic structure within 90 ft of the lotMonitoring, BC 3309.4.4
5 ft or less, more than 5 ft from footings or not below themMonitoring exempt
Excavation over 20 ftSOE by a NYS PE, BC 3304.4.1

Liability

The old 10 ft curb threshold that split responsibility is gone. Whoever causes the excavation must protect the adjoining building at any depth, and courts treat BC 3309.4 as strict liability. If the neighbor refuses access, the duty to protect shifts to the neighbor.

RPAPL 881, amended Dec 2025

Courts can now license permanent encroachments such as underpinning, tiebacks, anchors and wall ties when required by law. No reply within 60 days after written notices counts as refusal. No court license where a state entity such as the MTA owns or occupies the property.

Landmarks

Historic structures within 90 ft of the lot must be monitored, regardless of depth ratio (BC 3309.4.4). TPPN 10/88 criteria apply, and LPC asks for a monitoring plan that meets or exceeds it.

E4 · Engineer of record · approval package

What goes to DOB, and when

The approval package for lot line excavation is several linked documents, each sealed by the responsible engineer. They have to agree with each other. Most objections come from a mismatch, for example a monitoring threshold on the SOE drawings that does not match the evaluation report.

BC 1803.6

Geotechnical report

Sealed by the investigating engineer. Required on all but some 1 and 2 family sites. The items lot line design depends on: design lateral earth pressures, recommendations for evaluating adjacent properties, maximum permissible drawdown outside the site, expected total and differential settlement, parameters for safe temporary slopes, and anticipated special inspections.

BC 1817.3

Evaluation report, each adjacent building

Submitted at foundation plan approval. Vertical and lateral load paths, loads at the foundations, plumb survey, defects with photos, a test pit at each change in foundation type or geometry, allowable bearing now and during installation, earth, surcharge and water pressures, subsurface risks, anticipated settlement, and the movement and vibration thresholds with exceedance protocols. Rubble foundations (1817.3.2) and unreinforced masonry stability (1817.3.3) need specific findings.

BC 1817.6 · 1817.7

Underpinning construction documents

The 13 required items in the checklist above, including the pit sequence diagram, timber shoring design for the pits, step-by-step procedure, bearing stratum and allowable pressure, and a load table of gravity and lateral load per pier. Must direct the contractor and special inspector to read the evaluation report first.

BC 3304.4.1 · 107.8

SOE documents and earthwork plans

Support of excavation design for every edge, by a registered design professional with retaining and bracing experience, and by a NYS PE where the excavation exceeds 20 ft. Footing levels of all adjacent structures shown. Must reference the monitoring plan (3304.4.1.1) and incorporate the geotechnical report, the 1817 evaluation and the preconstruction survey (3304.4.1.2).

BC 3303.14.5 · 3304.10

Dewatering plan

Where the water table is drawn down. Includes the program to monitor and control drawdown and to monitor affected structures, tied to the geotechnical drawdown limit.

BC 1817.9 · 3309.16

Monitoring plan

Instruments, frequencies, reporting times, criteria, and exceedance actions including notifying the commissioner. Prepared by the engineer designing the support.

BC 3309 · 3304.3 · 3309.4.3

Notices, access and preconstruction survey

Written notice to adjoining owners at least 60 days before work, and at least 10 days before excavation where 3304.3.2 applies. DOB notified 24 to 48 hours ahead. License or RPAPL 881 order. Existing conditions of all adjacent buildings documented before excavation.

MTA EPP

Transit adjacency

Construction within 200 ft of MTA property goes to the MTA External Partner Program for review before the DOB permit, with archival drawings, the transit influence line, SOE and a construction monitoring plan. Transit criteria are often tighter than DOB criteria.

BC 1705.25.4 · TR1

Special inspection

Underpinning and alternate methods of support as a progress inspection category, plus anchor installation and testing under BC 1815 where tiebacks are used. Peer review under BC 1818 where structural peer review applies, or as the commissioner requires.

Where DOB plan examiners push back

Drawn from DOB training on underpinning and SOE. Check a set against these before filing.

Lateral soil, surcharge, hydrostatic or seismic loads not accounted for on the pier or SOE design.
Dead load of the existing building not quantified, or a lightweight structure assumed without basis.
Eccentrically loaded underpinning: pier not centered under the footing, with no moment check.
Approach pit and box pit construction not detailed, or formwork shown in place of a pit shoring design.
No sequence, or a sequence that leaves a cantilevered footing over a soil ledge.
Footing elevations of adjacent buildings assumed, not confirmed by test pits.
Monitoring thresholds missing, or not tied to the evaluation report and the building condition.
Dewatering shown without a drawdown limit or observation wells outside the site.

09 · Owner and CM checklist

Where projects actually get hurt

InteractiveTick items as your team completes them

Start access talks during design, not after permit. The 1817.3 evaluation often needs the neighbor cellar, and RPAPL 881 adds a 60 day clock.
Get real footing elevations from test pits, not assumptions. Footing depth drives everything downstream.
Take baselines before demolition. Demo vibration counts.
Hold a pre-start meeting with the SOE engineer, underpinning sub, special inspector and monitoring firm. Agree who stops work and how fast.
Keep a pin log: bay, set, dig date, pour date, dry-pack date, inspector initials. No adjacent bay opens until the log says so.
Never leave a pit open overnight unsheeted, or a pin face unbraced during bulk excavation.
Watch water. Leaks through lagging or an SOE gap can cause near-instant settlement. Have a grout and pump plan ready.
Call 811 and locate sidewalk utilities before drilling soldier piles along the street. Gas, water and sewer lines sit inside the street-side influence zone too.
Check the MTA 200 ft radius at site selection. Transit review adds time before the DOB permit.
Read the monitoring reports the day they arrive. A trend is a warning before a threshold is.

10 · Glossary

Words you will hear on site

InteractiveType to search, or tap a dotted word anywhere on the page

Sources