Development of a soil boundary friction meter: application to scale model testing

The development and application of a boundary friction meter is presented. The friction meter was designed to allow for the measurement of a resultant shear stress on a planar interface between a structural boundary and a soil mass. Calibrations illustrate the repeatability of the device and its moderate level of parasitic interaction due to the presence of normal pressure. The friction meter was used to consider the development of shear stresses along a vertical, non-yielding model retaining wall, subjected to uniform vertical pressure at K0conditions. Testing was performed with a uniform, fine-grained dune sand, placed at loose and medium dense conditions. The surface of the model wall was configured to have three distinct levels of roughness; from very smooth to the geometrical roughness of the sand itself. Measurement of normal soil pressures was performed in tandem to the shear stress measurements. The results illustrate that interface stress paths are unique to surface roughness. Angles of interface friction are defined and independent of soil density. The development of a shear stress along the vertical wall during unloading is significantly different than that measured during loading and the direction of the vertical shear stress reverses as vertical pressures approach zero.