A determination of the shear strength parameters of a fine-grained cemented alluvium in the US Desert Southwest

Abstract

Abstract Laboratory determination of shear strength parameters for alluvial soils in the US Desert Southwest is imprecise at best. These soils contain varying amounts of carbonate cementation and it is extremely difficult to acquire undisturbed samples using conventional soil sampling techniques. Furthermore, even when samples can be retrieved, they are usually not representative of the in situ soil which can demonstrate a wide range of cementation. Therefore, the results of laboratory strength testing may not be representative of the actual in situ behaviour. For these reasons, a simple field test is developed which may be used in conjunction with conventional laboratory testing and analytical procedures to estimate the in situ strength properties of such alluvial deposits. The in situ testing involves a series of plate load tests performed at the crest of freshly cut slopes. A steel plate of known dimensions is loaded by an 89 kN (10 tons) hydraulic jack until the slope fails. The counter weight of a power shovel is used as a reaction support. The failure load and slip surface geometry are measured and recorded. A two-dimensional slope stability procedure (Bishop’s Modified Method) is used to back calculate the value of cohesion associated with failure of the field slopes. The analytical procedure developed by Hovland is used to account for the three-dimensional ‘end effects’ present in the field load tests and to correct the value of cohesion obtained from the Bishop solution. The values of c and φ obtained in this way are used to evaluate the stability of the trench slopes at the State of Arizona’s site for the US Department of Energy’s proposed Superconducting Super Collider (SSC) Project. The determination of a realistic factor of safety during construction is critical to the economical use of open-cut construction techniques for approximately 50 per cent of the 83.7 km (52 miles) circumference of the proposed SSC ring.