Macro-scale direct shear test device for assessing soil-solid interface friction under low effective normal stresses

Abstract

The paper describes a new macro-scale direct shear test device for assessing the largedisplacement soil/solid interface drained shear strength at low effective normal stresses. The testing method arises from a need to obtain the interface friction between soils and pipelines under low effective normal stress levels, which is an important consideration in the design of pipelines laid on the sea bed. The test device is fundamentally similar to the conventional small-scale direct shear box device except for its large footprint that provides a plan interface shear area of 3m2 (1.72m by 1.75 m). The device is designed to impart displacement-controlled interface-shearing at displacement rates ranging from 0.0001mm/s to 1 mm/s and with the ability to reach a maximum interface shear displacement of 1.2 m. The desired normal stress at the soil/solid interface is obtained using surcharge loads externally applied by means of bulk sand or water masses, or both in certain cases. The device is instrumented with transducers mounted flush with the top of the solid surface for the measurement of pore-water pressure at the shear interface. As a result, the device allows for accurate determination of the effective normal stress at the soil/solid interface during interface shear testing. The key features of this device are described, and the device capabilities are demonstrated using tests conducted on Fraser-River-sand/mild-steel, plastic-silt/epoxy-coated-mild- steel, non-plastic-silt/epoxy-coated-mild-steel, and kaolinite/ epoxy-coated-mild-steel interfaces at effective normal stresses between 3 and 7 kPa. The results can be also used as benchmarks to justify possible modifications of the currently available small-scale apparatus to reliably measure interface shear strength at such low effective normal stresses.