Effect of Trapped Cavity Mechanism on Interpretation of T-Bar Penetrometer Data in Uniform Clay

This paper describes large deformation finite element (LDFE) analysis of the penetration of the T-bar penetrometer in uniform clay, identifying soil flow mechanisms around the T-bar, the extent of any cavity above the T-bar and the evolving penetration resistance profile. A trapped cavity above the advancing T-bar penetrometer and its influence on the corresponding bearing capacity factor are the crucial findings of this paper. The formation and evolution of the trapped cavity mechanism are studied extensively, exploring a large range of normalized undrained shear strength of soil and surface roughness of the T-bar. It is shown that the depths of forming a trapped cavity and being fully filled with soil increase with increasing normalized undrained shear strength of soil and roughness of the T-bar. The trapped cavity results in a reduction (up to 13%) in the commonly used bearing capacity factors based on plasticity solutions and a flow-round failure mechanism. According to the depth span of an existing trapped cavity, there are three scenarios: (1) for clay deposits with $s_{\mathrm{u}}/{\gamma }^{\prime }D\le 1$, a shallow failure mechanism is directly followed by a flow-round mechanism since the trapped cavity span is negligible; (2) for clay deposits with $1<s_{\mathrm{u}}/{\gamma }^{\prime }D\le 8.3$, all three stages—shallow failure mechanism, trapped cavity mechanism, and flow-round mechanism—can be observed; and (3) for clay deposits with $s_{\mathrm{u}}/{\gamma }^{\prime }D>8.3$, the trapped cavity is not fully closed up to a penetration of $30D$, leading to a lower bearing capacity factor profile compared to the stabilized factors for the other two scenarios. A systematic interpretation procedure is therefore proposed to account for the effect of a trapped cavity for more accurate interpretation of soil undrained shear strength from the T-bar penetration resistance.