Site Characterization and Evaluation of a Deep-Seated Slope Movement

Deep seated slope movements can manifest as small, slow movements and be defined as slope creep. Over a period of time, enough movement can occur that results in damage to property and structures. Site reconnaissance alone cannot identify slope movement along a deep seated shear plane and can occur at depths greater than explored by test borings, especially for lightly loaded structures. This case history discusses a site in upstate South Carolina that experienced two episodes of slope movement between December 2009 and January 2014. Subsequent to a $3.5 million legal claim that a "slope failure" caused damage to a house, the defendant's expert witness team developed an exploration and laboratory testing program to help determine the cause of the slope movement. Observations made during site reconnaissance are presented and the subsurface conditions revealed by the field exploration programs are described. Evaluation of inclinometer data is shown to disclose a deep seated slope movement that was not contemplated during initial site characterization. The data indicated that the slope moved intermittently and was very slow. Lastly, triggering mechanisms that potentially initiated the slope movement are presented. Inclinometers typically thought of as an instrument for monitoring slope performance, not as an exploration tool, were proved essential for use in site characterization in detecting the deep shear plane. Failing to reach an eleventh hour settlement, the case went to trial. The jury found in favor of the defendant, ruling that they were not negligent during their initial site characterization.