Guidelines for Static Pile Design, 1991-PDF

PDF
Pdf Summary
The guide presents a unified method for designing individual piles and pile groups by treating capacity, settlement, negative skin friction, and structural strength as interconnected rather than separate issues.<br /><br />A pile’s axial capacity is the sum of shaft resistance and toe resistance. Shaft resistance is estimated primarily from effective overburden stress and a soil-dependent beta coefficient; toe resistance uses effective stress and a toe-bearing coefficient. Site investigation, soil layering, groundwater and pore-pressure conditions, and installation effects are essential to selecting these parameters. Pile driving may temporarily reduce resistance through excess pore pressure, while subsequent soil set-up increases it. Residual compression from installation can also affect how static load tests are interpreted.<br /><br />Negative skin friction develops where surrounding soil settles relative to the pile. It adds dragload to the pile and creates a neutral plane—the depth where relative movement changes direction and shaft shear shifts from negative to positive. The maximum pile load generally occurs there and consists of the sustained head load plus dragload. Capacity calculations, neutral-plane location, settlement, and structural checks must therefore be iterated together. Live load is excluded from the long-term neutral-plane load assessment.<br /><br />For a pile group with a stiff cap, the guide estimates settlement using an equivalent footing at the shared neutral-plane elevation. Stress increases below this footing are calculated using the 2:1 method, and soil compression is determined with Janbu’s tangent-modulus approach, which accounts for nonlinear stress–strain behavior. Pile compression is added to the soil settlement.<br /><br />Structural design must check stresses during driving and service. In particular, the pile must safely resist the peak load at the neutral plane, not just the load applied at the pile cap. The examples emphasize realistic soil parameters, consistent safety factors across analysis methods, consideration of excavation and fill, and field testing where uncertainties in capacity or drivability could affect design.
Keywords
pile capacity
shaft resistance
toe resistance
negative skin friction
neutral plane
dragload
pile settlement
pile group design
structural strength
soil set-up