Uplift Capacity of Anchors in Layered Sand Using Finite-Element Limit Analysis: Formulation and Results

The vertical pullout capacity of strip and circular plate anchors embedded horizontally in a layered sandy medium was computed by using the plane strain and axisymmetric lower-bound limit analyses in combination with finite elements and linear optimization. The soil medium below the anchor plate was assumed to be comprised of loose sand. Two different sand layers were considered above the anchor with different combinations of their internal friction angles. For several embedment ratios $(H/B)$, the variations of the pullout factors $F_{\gamma }$ and $F_q$ due to the components of soil unit weight and surcharge, respectively, were computed as a function of $H_{\mathrm{dense}}/H$ for different cases; here, H and $H_{\mathrm{dense}}$ imply (1) the depth of the anchor plate from ground surface and (2) the thickness of the dense sand layer, respectively. The pullout resistance increased continuously with an increase in $H_{\mathrm{dense}}/H$. For a given $H/B$, with the same value of $H_{\mathrm{dense}}/H$, the uplift resistance became greater for a case when the dense sand layer was kept just above the plate rather than placing it close to ground surface. Compared with strip anchors, the uplift factors became considerably greater for circular anchors. The effect of the variation of the unit weights of the two layers above the anchor plate on its pullout resistance was also explored. The results obtained from the analysis compare well with the available theoretical and experimental data.