Measurement of amorphous peat shear strength in the direct shear box at high displacement rates

The ASTM standard provides guidelines for the drained direct shear test (DST) and requires the samples to be sheared at rates estimated from time to failure, t_f≥ 50 t₅₀ (where t₅₀ is the time required to achieve 50% consolidation). This paper investigates the potential of estimating the undrained strength of peat in the DST owing to its ease of accessibility and simplicity over other laboratory tests. In this experimental study, peat samples were sheared at various displacement rates at t_f/ t₅₀ values of 0.065–70. The samples tested at the ASTM specified rate of t_f≥ 50 t₅₀ exhibit continuous increase and decrease in shear stress and volume respectively with increasing shear strain. Hence, determining the drained strength properties of peat in the DST apparatus is complicated. However, it was concluded that the shear stress ratio, τ/σ′v of the DST test samples sheared at rates corresponding to t_f≤ 0.2 t₅₀, reasonably corresponds to that measured in undrained direct simple shear (DSS) tests on peat, found in the literature. Therefore, DST conducted at high displacement rates will be adequate for preliminary evaluation of the shear strength of peat at close to undrained conditions, when constant volume DSS apparatus is not available.