The uplift capacity of buried plate anchors depends on the tension sustained beneath the anchor. This study provides a detailed treatment of interface tension and the associated seepage flow and gap formation below the anchor. This behaviour is explored via large deformation finite element analysis using a thin highly compressible porous layer for the gap. The observed seepage effect is captured by a simple model using Hvorslev's intake factor, validated across a wide parameter range. This model for uplift capacity at various pull-out rates provides a simple basis for the effect of seepage on plate anchor capacity under sustained loading.