Vertical Uplift Resistance of Two Interfering Horizontal Anchors in Clay

The vertical uplift resistance of two interfering rigid strip plate anchors embedded horizontally at the same level in clay has been examined. The lower and upper bound theorems of the limit analysis in combination with finite-elements and linear optimization have been employed to compute the failure load in a bound form. The analysis is meant for an undrained condition and it incorporates the increase of cohesion with depth. For different clear spacing ($S$) between the anchors, the magnitude of the efficiency factor (${\eta }_{c\gamma }$) resulting from the combined components of soil cohesion ($c$) and soil unit weight ($\gamma$), has been computed for different values of embedment ratio ($H\text{/}B$), the rate of linear increase of cohesion with depth ($m$) and normalized unit weight ($\gamma H\text{/}c$). The magnitude of ${\eta }_{c\gamma }$ has been found to reduce continuously with a decrease in the spacing between the anchors, and the uplift resistance becomes minimum for $S\text{/}B=0$. It has been noted that the critical spacing between the anchors required to eliminate the interference effect increases continuously with (1) an increase in $H\text{/}B$, and (2) a decrease in $m$.