Question 3 of 7: Short Answers — Foundation Failure, Shear Strength, and Preload Fill

Question 3: Short Answers — Foundation Failure, Shear Strength, and Preload Fill (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Why a rising water table can tip an intact silo. A tall, narrow structure like a silo carries a large overturning moment (from wind, eccentric filling, or seismic load) relative to its footprint, so its stability against tipping depends on the bearing/shear capacity the soil can mobilize under the leeward edge of the foundation, not merely on total settlement. While the water table sat 15 m below surface, the near-surface soil was unsaturated: negative pore pressure (matric suction) contributed an "apparent cohesion" that boosted the soil's effective shear strength beyond what saturated conditions alone would give. When the water table rises to just below the foundation, that apparent cohesion is lost and, more importantly, the effective stress in the bearing zone falls sharply (σ′ = σ − u, and u rises from strongly negative toward zero/positive), which by the Mohr–Coulomb criterion (τ_f = c′ + σ′tan φ′) directly reduces the mobilizable shear strength. Under an eccentric or lateral load the bearing capacity on the loaded edge can then be exceeded locally — a bearing-capacity/rotational failure of the shallow foundation — which tips the rigid, structurally sound silo without ever cracking its walls. A secondary or contributing mechanism is loss of frictional/adhesive resistance to sliding or uplift along the base, and, if the near-surface soil is loose and cohesionless, a partial loss of effective confining stress can also soften it enough to allow differential settlement under the eccentric load.

b) Shear strength parameters and how they are measured. Soil shear strength is described by the Mohr–Coulomb failure criterion in terms of the effective cohesion intercept c′ and effective friction angle φ′, applied to the effective normal stress on the failure plane: $$\tau_f = c' + \sigma'\tan\phi' = c' + (\sigma-u)\tan\phi'$$ Because pore pressure u often cannot be measured or controlled in the field, an equivalent total-stress parameter, the undrained shear strength s_u (with φ_u ≈ 0 for saturated clay under undrained loading), is used for short-term stability problems. Common laboratory methods are the direct shear test (quick, but forces the failure plane and cannot control drainage precisely), the triaxial test in its unconsolidated-undrained (UU), consolidated-undrained (CU, with pore pressure measurement), and consolidated-drained (CD) variants (the most versatile, allows controlled drainage and any stress path), and the unconfined compression test (a quick s_u estimate for saturated clay). Field methods include the vane shear test (in-situ s_u for soft clay) and correlations from the standard penetration test (SPT) and cone penetration test (CPT).

c) Purpose of the temporary sand fill. Placing 10 m of sand fill over the footprint two years before construction, then removing it just before building, is a classic surcharge preload: the temporary fill applies a large overburden stress to any soft, compressible (typically clayey) soil beneath the site, driving most of the primary consolidation settlement — and squeezing pore water out through the two years of drainage — before the permanent structure exists to be damaged by it. Removing the fill immediately before construction leaves the underlying soil pre-consolidated (its effective stress history now exceeds what the lighter permanent building alone will apply), so the actual structure experiences much smaller post-construction settlement and, being over-consolidated, a stiffer, less compressible response to its own load. This is the same consolidation/effective-stress logic tested quantitatively in Question 7.