IMPLICATIONS FOR THE MECHANICAL PROPERTIES OF THE MANNING CANYON SHALE FROM EFFECTS OF GROUNDWATER LEVEL ON THE DISPLACEMENT RATE OF THE SHERWOOD HILLS LANDSLIDE, PROVO, UTAH

The Sherwood Hills landslide, located in the foothills of the Wasatch Mountains in Provo, Utah is approximately 150 meters by 200 meters in size and is underlain by the Manning Canyon shale, a clay-rich unit that occurs in numerous landslides along the Wasatch front. The slide has been active since at least 1998 and has caused significant property damage. Utah Valley University students and faculty have been measuring the slide’s displacement one to six times per year since 2004 using carrier-phase GPS, and the groundwater table level in monitoring wells has been tracked by UVU and the UGS during the same time period. Total measured displacement is 0.62 m and the displacement rate has varied from 3.4 mm/yr to 388 mm/yr in response to water table level variations of up to 4.6 m, measured in the central portion of the slide. These rates equate to strain rates along the glide plane of 2.18 x 10^-10 and 2.46 x 10^-8 s^-1, respectively, assuming a shear zone 0.5 m thick, and the slide’s constant motion implies zero cohesive strength on the glide plane. The glide plane is inferred to dip ~24.5^o based on the relative horizontal and vertical motions of the slide. The relationship of water table level (WL) to displacement rate (DR) is well modeled by DR = 0.39*exp(1.66*WL)+27.6 (R²=0.83). This very non-linear relationship implies a maximum DR of ~54 m/yr if the ground were to be saturated with water to the surface. The non-linear relationship of the slide’s displacement rate to ground water level implies that the strain rate in the Manning Canyon shale along the slide’s glide plane varies non-linearly in response to changes in effective shear stress. Water table level can be equated to changes in effective shear stress on the glide plane as follows. A 1 m rise in the water table level increases normal stress on the glide plane by ~10 kPa, which equates to an effective change in shear stress of 2.5 kPa, assuming a friction coefficient of ~0.25 and applying the effective stress law to a Coulomb-Navier rheology. The relationship of shear stress (σ_s) to strain rate (de/dt) [s^-1] can then be modeled by de/dt = 2.48e-30*exp(0.67* σ_s)+1.75e-9 (R²=0.83). This relationship may be refined by better accounting for variations in the dip of the glide plane and may be useful in understanding and predicting the behavior of other landslides that involve the Manning Canyon shale.