Plastic Flow of Sand and Pullout Capacity of Suction Caissons

A suction caisson has a relatively small length-to-diameter ratio, and the suction-induced seepage forces can liquefy the sand surrounding the caisson, depending on the pullout rate. Houlsby et al. (2005) modeled the seepage forces effect, and its related impact on the effective stresses and the corresponding pullout capacity. However, ignoring potential liquefaction and sand flow that may occur due to the seepage-induced forces can yield an overestimate of pullout capacity as a function of the induced deformation.

A Bingham plastic flow model is used to describe the shear stresses as a function of shear strain rate. The general rheological behavior is expressed as when sand flow occurswhere $\tau$ = shear stress; $G$ = shear modulus; $\gamma$ = shear strain; $\dot{\gamma }$ = shear strain rate; ${\tau }_o$ = yield stress; and $\mu$ = viscosity. Referring to Fig. 1, when the $\tau$ of the sand surrounding the caisson is within the range of ${\tau }_o$, the sand flow velocity will be uniform in the uniform flow zone. The soil in this flow zone remains in the solid state, whereas the rest of the soil will be in a plastic flow state. The velocity and shear stress in domain I and domain II (Fig. 1) were obtained herein using finite element analysis program COMSOL. Results from representing the sand with the Bingham flow model are compared with experimental results by Houlsby et al. (2005). Among the five reported tests, failure in two tests occurred where the core soil-plug moved upward with the caisson, while an external sand flow was generated around the caisson to fill in the space at the base. Fig. 2 shows the computed suction and pullout capacity using the proposed model along with the data by Houlsby et al. (2005). The trend from the proposed model matches well both the measured suction pressure and pullout capacity data. The peak value is achieved soon after the start of caisson pullout, and gradually decreases to a residual value. Ignoring viscous flow of the sand yields significant residual values of suction and capacity at large pullout displacements, as shown in Fig. 2; such behavior was not supported by the experimental results.