Cofferdam Design & Water Diversion Engineering in NYC

What Is a Cofferdam?

A cofferdam is a temporary watertight enclosure built in, around, or adjacent to water so that the area enclosed can be kept dry during construction. Think of it as a temporary dam — a structure that holds water out (and sometimes holds soil in) while workers construct foundations, piers, tunnels, and utilities in the dry.

Cofferdams are inherently temporary: they are constructed, used during a specific construction phase, and then removed (or, in some cases, incorporated into the permanent structure as a sacrificial form). Their design must account for the temporary nature — typically designed for the construction period load cases rather than the full service life environmental loads of permanent structures.

When Are Cofferdams Used in NYC?

• Bridge pier construction and repair (East River, Harlem River, Hudson River crossings)
• Waterfront pile cap and footing concrete placement (piers, bulkheads, seawalls)
• Utility and subway tunnel crossings below waterways
• Foundation construction adjacent to tidal waterways where groundwater is too high for conventional open-cut excavation
• Drydock and marine facility maintenance and construction
• Intake and outfall structure construction for water and wastewater facilities

Cofferdam Types

Structural Design of Sheet Pile Cofferdams

The structural engineer designs a sheet pile cofferdam for the following load cases and failure modes:

• Active earth pressure: Soil pressure on the landward side of the sheet pile pushing inward — calculated using Rankine or Coulomb theory
• Hydrostatic pressure: Water pressure differential across the sheet pile wall — highest when interior is fully dewatered and exterior is at high tide or flood stage
• Passive resistance: Soil resistance at the embedded toe of the sheet pile — provides stability against sliding and overturning
• Seepage forces: Upward seepage beneath the embedded toe (heave) — a critical failure mode in soft clays
• Wave and current loading: Tidal currents and wave action in NYC waterways add dynamic lateral loads

The design output includes: required sheet pile section modulus (selecting minimum pile section); embedment depth; waler beam sizing (horizontal channels or wide-flange beams); strut design (axial compression members spanning across the cofferdam interior); and confirmation of global stability against overturning and base heave.

Dewatering: Process and Discharge Requirements

Once the sheet pile perimeter is complete and joints are sealed (using bentonite or water expansive grout at problem areas), dewatering pumps remove water from the interior. Dewatering considerations in NYC:

• Pump sizing: Must overcome both initial drawdown volume and continuous seepage inflow through sheet pile joints and the soil below the pile toe
• Discharge water quality: Dewatering discharge into NYC waterways or storm drains must meet NYC DEP water quality standards — turbidity, pH, hydrocarbon content. Sedimentation tanks or dewatering bags may be required before discharge
• Groundwater monitoring: Piezometers monitor the groundwater table outside the cofferdam to detect excessive drawdown that could cause settlement of adjacent structures with shallow foundations

NYC & Federal Permit Requirements

Cofferdam work in NYC waterways requires a multi-agency permit package:

Adjacent Structure Settlement Risks

In densely built NYC, cofferdam dewatering poses a significant risk to adjacent buildings with shallow foundations. When the groundwater table is lowered:

• Foundations in sand or silt that previously provided uplift support may lose capacity, causing settlement
• Timber piles from historic NYC construction may be exposed to air and desiccate if the water table drops below their tops — causing rapid timber decay and column settlement

Mitigation measures include: recharge wells outside the cofferdam perimeter to maintain the water table in sensitive zones; pre-construction condition surveys of adjacent buildings; and settlement monitoring with precision survey and vibrating wire settlement plates during the entire dewatering period.

Cofferdam Removal

When permanent construction inside the cofferdam is complete, the cofferdam is removed. Sheet piles are extracted using vibro-hammers (the same equipment used for installation). The extraction process must be engineered: the permanent structure must be able to support itself independently before cofferdam support is removed. Permanent waterfront structures typically include internal bracing or rely on the completed concrete structure's own weight and embedment for stability. Where sheet pile extraction might cause ground disturbance adjacent to existing utilities or structures, the piles may be cut off below grade and left in place.