Stratigraphic profiling, slip surface detection, and assessment of remolding in sensitive clay landslides using the CPT

Abstract

A multi-year cone penetration testing program was conducted at a landslide subject to episodic retrogression in Mud Creek, Ottawa, to assess whether a hand-operated mobile cone penetration test (CPT) could yield new insights into the current degree of remolding under progressive failure in metastable areas of a landslide where conventional tracked rigs are unable to gain access. The mobile CPT rig permitted tests to be performed through the entire thickness of the Champlain Sea deposit at a penetration rate of 0.5 cm/s, with similar results to tests performed at the standard 2 cm/s. Measurements of pore pressure varied considerably with cone size, with the magnitude of pore pressure response decreasing with cone size. The elevation of the slip surface was identified in the tip resistance as the point of transition between the remolded soil above the slip surface and the intact soil below the slip surface, whereas a further 0.5 m of penetration was required to elevate pore pressures to values indicative of the intact soil behaviour. In situ measurements of shear strength of corresponding layers between the intact and remolded profiles to be compared indicated that the soil above the slip surface had remolded to 50% of its fully remolded strength.