Pore Pressure Generation and Dissipation Effects on Spudcan Fixity in Normally Consolidated Clay

Spudcan rotational fixity under combined vertical, horizontal and moment loading is often assumed to be invariant with time. In reality, the actual rotational fixity of spudcan footing is likely to change with time as excess pore pressure builds up and dissipates. This paper describes a series of centrifuge experimental tests conducted at 100-g acceleration using a small spudcan model and specimens of normally consolidated reconstituted kaolin clay. Using a servo-motor, belt-driven actuator system, loading episodes comprising one thousand cycles of combined loading were applied to model foundation. The PPTs are installed in soil specimens to measure the excess pore pressure and degree of saturation of soil. One small-rotation test is conducted just after spudcan’s penetration; while another one small-rotation is conducted when the excess pore pressure is fully dissipated after the spudcan’s penetration. The results show bending moments at four locations along the spudcan shaft, which indicates that the lattice confers a significant lateral soil resistance, and the presence of the lattice will also cause the location of maximum bending moment to be up-shifted along the leg, towards to the soil surface. The rotational fixity of the spudcan shows distinct changes over time, which is attributed to consolidation and settlement effects. Comparison of fixity of spudcan with and without lattice leg indicates that the lattice leg can lead to a large increase in lateral resistance of deeply penetrated spudcans. This lattice leg effect has been largely ignored in both academic study and industrial design.