Support of Excavation and Dewatering Beside a 1920s Unreinforced Brick Building

Wedge geometry below the footing base

• Height of soil retained below the footing base: H = 6.1 − 1.8 = 4.3 m
• Wedge angle from vertical: 45° − φ/2 = 45° − 16° = 29°
• Horizontal extent: H·tan29° = 4.3 × 0.554 = 2.4 m

The building face is 4.5 m from the excavation line, about 4.0 m to the near edge of the footing - still roughly 1.6 m outside the wedge. Bearing capacity of the footings is therefore not directly undermined and lateral underpinning is not triggered.

That is not the end of the question, though:

• The wedge calculation assumes no wall movement and no surcharge. Set the allowable wall deflection to 0.2% of the retained height (≈12 mm) so the wedge does not migrate toward the footing.
• The 10 kPa building surcharge must be carried by the shoring over the full retained height above the base; it is the governing lateral load at the top of the wall, not the soil.
• Recompute the wedge with any local over-excavation or utility trench between the wall and the building, since a trench would shorten the horizontal distance.

Head to be controlled: base at 6.1 m, water table at 2.5 m → head difference = 3.6 m.

Boiling / piping check (FS = i_c / i_exit, with i_c = (G_s − 1)/(1 + e) = 1.65/1.65 = 1.0):

• Open sumps, no cut-off: vertical seepage path ≈ 3.6 m, i ≈ 3.6/3.6 = 1.0 → FS ≈ 1.0. Unacceptable; a running/s boiling condition at the subgrade is likely in a 1×10⁻⁴ m/s sand, and the sumps will pump fines.
• Cut-off wall to 12 m (5.9 m below the base): i ≈ 3.6/5.9 = 0.61 → FS ≈ 1.6, and deep wells inside the excavation reduce the driving head further. Acceptable, but not generous next to a sensitive structure - target FS ≥ 1.5 and verify with piezometers.

Settlement exposure: sump pumping cannot lower the piezometric surface outside the wall, so it produces no cut-off benefit while still drawing down the fill. A 3.6 m drawdown through 3 m of loose fill typically yields 15–40 mm of consolidation settlement. With an angular-distortion limit of about 1/500 for URM over a 12 m building width, the tolerable differential is ≈ 24 mm - the drawdown alone can consume or exceed it before any wall deflection is added.

Conclusion: reject open sumps. Use a cut-off plus interior deep wells, and add recharge wells between the wall and the building to hold the outside piezometric level.

Support system

• Sheet-pile cut-off wall (Z-section) driven to about 12 m, providing both retention and the seepage cut-off; install by vibratory hammer with pre-augering through the fill at the corner nearest the building, since impact driving at 4.5 m from URM can generate 10–25 mm/s PPV against a typical 5 mm/s limit.
• Alternative if vibration cannot be controlled: a drilled contiguous or secant pile wall with a CFA rig, with a structural facing to make it watertight.
• Interior deep wells with a standpipe piezometer array; keep the outside water level within 0.3 m of its preconstruction level using recharge.
• Design the wall for the 10 kPa surcharge plus traffic and construction loading; do not rely on the fill for passive resistance.

Monitoring plan

Instrument	Quantity	Alert	Action
Inclinometers in wall	4	12 mm	19 mm
Settlement points on building	8	10 mm	15 mm; stop work, underpin
Crack gauges	4	1 mm growth	3 mm growth
Piezometers (inside/outside)	4	0.3 m outside drawdown	0.5 m
Vibration monitors	2	5 mm/s PPV	12 mm/s PPV

Baseline readings must be taken before any driving, with a condition survey signed by both parties. Any action-level exceedance triggers a hold, a review by the structural engineer of record for the adjacent building, and a pre-planned underpinning option.