Basement Underpinning Adjacent to Commercial Excavation

Why this matters

When a developer excavates a deep basement (commercial parking garage, underground retail level, mixed-use building) next to an existing structure with shallow foundations, the existing structure's footing is within the active wedge of the adjacent excavation and is at risk of settlement, lateral movement, or collapse. The foundation repair contractor is typically engaged on behalf of the existing structure's owner to install underpinning that transfers the existing footing load to a depth below the adjacent excavation's base. The sequence has to interleave with the developer's shoring contractor and the geotechnical engineer's monitoring. Errors in the underpinning sequence have collapsed party walls and adjacent buildings; the work is high-consequence and the documentation requirements are correspondingly stringent.

The geometry that matters

The active wedge of an excavation is approximately a 1:1 slope from the toe of the excavation back toward the existing structure (the slope depends on soil type; a geotechnical engineer derives it from soil shear strength). Any existing footing whose base is above the wedge line is at risk. The greater the depth of the adjacent excavation and the closer the existing footing to the excavation face, the deeper the underpinning must extend. Typical commercial excavations are 20 to 40 feet deep; existing residential basement footings are 8 to 10 feet below grade; the underpinning typically extends another 12 to 30 feet to clear the wedge.

Method selection

Three methods dominate for this geometry:

• Traditional pit-and-pour underpinning (sequenced concrete pits below the existing footing, typically in 3 to 4 foot bays, dry-packed under the existing footing). Suitable for stiff cohesive soils and modest underpinning depths up to 10 to 15 feet below the existing footing.
• Micro-pile underpinning (small-diameter drilled piles 7 to 12 inches in diameter, post-grouted, with a bracket capturing the existing footing). Suitable for deeper underpinning and tighter access.
• Jet-grouted soilcrete columns supporting a needle beam under the existing footing. Suitable where the excavation will expose a long stretch of existing wall and conventional underpinning bays would be too disruptive.

The geotechnical engineer and structural engineer of record select the method based on subsurface conditions, existing footing geometry, and the available work zone.

Sequencing rules for pit-and-pour

The classic pit-and-pour sequence under an existing wall is to excavate alternating bays no wider than 3 to 4 feet, separated by un-excavated soil at least twice the bay width. Bay 1 is excavated, formed, and concrete is poured up to a horizontal joint approximately 3 to 4 inches below the existing footing. After the concrete reaches a specified strength (typically 24 to 48 hours at 2,000 psi or more), the gap between the new concrete and the existing footing is dry-packed with stiff non-shrink grout. Once bay 1 is dry-packed, bay 3 is excavated, then bay 5, then bay 2 (after bays 1 and 3 are complete), and so on.

The bay width and the sequence pattern come from the engineer. Excavating too wide a bay or excavating adjacent bays before the prior bay is dry-packed has dropped party walls. The contractor's submittal includes the sequence drawing signed off by the engineer.

Coordination with the adjacent shoring

The developer's shoring contractor (typically soldier piles and lagging, secant pile wall, or sheet piles with tiebacks) is the primary lateral support of the adjacent excavation. The underpinning under the existing structure is the vertical support. Both have to be in place before the adjacent excavation deepens past the existing footing elevation. A typical sequence is:

1. Install shoring across the excavation face down to the existing footing elevation.
2. Install all underpinning bays in the affected length of the existing structure.
3. Excavate the adjacent basement in lifts, with monitoring at each lift.
4. Install permanent foundation walls of the new building.
5. Backfill between new wall and shoring.

Skipping step 2 (excavating past the existing footing before underpinning is complete) puts the existing structure at risk.

Monitoring

The engineer of record specifies monitoring instrumentation: settlement points on the existing structure (read by total station or precise level), tilt-meters on critical walls, crack monitors on existing cracks, and inclinometers in the shoring system to measure lateral wall movement. Threshold values for amber (slow the work, increase monitoring frequency) and red (stop work, engineer review) are set in the monitoring plan. Typical thresholds for the existing structure are 5 to 10 mm vertical movement amber and 15 to 25 mm red, but the engineer's plan governs.

Party-wall and license-to-enter

The existing structure is typically on a different property than the excavation. The developer secures a party-wall agreement or license-to-enter from the existing owner before any work proceeds. The foundation repair contractor receives the agreement as part of the bid package; the document defines the work allowed inside the neighbor's property, the access hours, the insurance requirements, and the indemnification.

References

• ACI 562 Code Requirements for Assessment, Repair, and Rehabilitation of Existing Concrete Structures
• ASCE 41-17 Seismic Evaluation and Retrofit of Existing Buildings
• FHWA-IF-99-015 Geotechnical Engineering Circular No. 4: Ground Anchors and Anchored Systems
• Deep Foundations Institute Micropile Design and Construction Reference Manual
• ICRI 510.1 Guideline for Structural Concrete Repair
• Earth Contact Products Helical and Push Pier Engineering Specifications
• ICC Evaluation Service ESR for proprietary underpinning bracket systems