Liquefaction and Lateral Spreading at a Pile-Supported Warehouse
A new single-story warehouse is planned on a site with 12 ft of loose silty sand over stiff clay. Groundwater is at 6 ft. The geotechnical report gives SPT (N1)60 values of 6 to 9 in the sand and a design peak ground acceleration of 0.45g. The structural engineer proposes a slab-on-grade with grade beams supported on 40-ft driven piles. The architect wants a 6-in. slab with no special detailing. The geotechnical engineer notes the site is underlain by a buried stream channel and that lateral spreading of up to 8 in. is possible toward the channel.
1. Identify the seismic hazards that govern foundation and slab design and explain how each affects the proposed system.
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Three hazards govern:
- Liquefaction of the loose silty sand. (N1)60 of 6–9 with 0.45g PGA and shallow groundwater is highly susceptible. Liquefaction reduces skin friction on the upper pile length and removes lateral support, increasing unsupported pile length and bending demand.
- Lateral spreading (up to 8 in. toward the channel). Spreading imposes kinematic demands on the piles: the crust moves laterally while the deeper clay does not, so piles must resist the resulting curvature and shear. It also opens the door to ground fissures and differential movement at the slab.
- Slab-on-grade vulnerability. A 6-in. unreinforced slab cannot tolerate 8 in. of differential ground movement; it will crack and lose serviceability.
Implications: piles must be designed for reduced friction plus kinematic spreading demands (typically a nonlinear soil-structure interaction analysis or simplified equivalent-static approach). The slab should be structurally tied to the grade beams and reinforced to span local voids, or the site should be improved (e.g., ground improvement) to reduce spreading.
2. The contractor proposes shortening the piles to 25 ft to bear on the sand. Evaluate.
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Shortening piles to bear in the liquefiable sand is not acceptable.
- The sand is the liquefiable layer. Piles bearing in it lose end bearing and shaft resistance when liquefaction occurs, and they gain no embedment into the stiff clay to provide post-liquefaction support.
- Kinematic demands from lateral spreading are worst for short, stiff piles because the crust displacement is concentrated over a short length, producing high curvature.
- Downdrag and loss of lateral support during liquefaction would leave the structure unsupported.
The piles should extend through the liquefiable layer into the stiff clay for bearing and lateral support, with the upper length designed to accommodate liquefied soil (reduced friction, increased unsupported length). If ground improvement is used to mitigate liquefaction, the pile length could be revisited, but only with a revised geotechnical analysis.