IMPROVED HAZARD ASSESSMENT, EFFECT OF WATER TABLE HEIGHT ON LANDSLIDE DISPLACEMENT RATE, AND FAILURE PLANE RHEOLOGY FROM NINE YEARS OF MONITORING OF THE SHERWOOD HILLS SLUMP, PROVO, UTAH

The Sherwood Hills landslide, located in the foothills of the Wasatch Mountains in Provo, Utah, is a rotational slump > 300 by 200 m in size that is underlain by the Manning Canyon Shale, a clay-rich unit that also occurs in other landslides on the Wasatch front. The slump has been active since at least 1998 and has caused major damage to 3 homes plus significant damage to others. Its slow motion during dry years and lack of morphological expression reflecting active movement have impeded assessment of its hazard potential. We have measured the slump’s displacement up to 6 times per year since 2004 using carrier-phase GPS, and the groundwater table level in monitoring wells has been tracked by us and the Utah Geological Survey during the same period. The long term monitoring has helped hazard assessment by enabling us to 1) better map the extent of the slide; 2) document the variations in its motion during relatively dry and wet years; 3) quantify with increased refinement the relationship between the slump’s displacement rate (DR) and water table height (WT) and estimate the likely maximum DR of the slump; and 4) determine rheological properties of the glide plane shale that may be applied to other landslides failing on the same unit.

Maximum total displacement since 2004 is 66.4 cm towards the SW, and much of the slump has moved as a nearly rigid block. However, the slump has a sharp headscarp and lateral boundary only on its eastern and southeastern edges. There is a diffuse southern lateral boundary, and to the north the slump is apparently broken into two blocks. The displacement rate (DR) has varied from 3.4 to 388 mm/yr while the WT has varied up to 4.6 m; the relationship of DR to WT is well modeled by DR = 0.21*exp(1.36*WT)+25.2 (R²=0.89). Importantly, this suggests a maximum DR of ~ 22 m/yr would occur if the WT reached the ground surface. The relationship of WT to DR in the slump can be used to infer rheologic properties of Manning Cyn. Shale. Assuming the glide plane is 0.5 m thick, has a friction coefficient of 0.21 (which correlates to an estimated F.o.S. ~1), and that the increased pore pressure from a 1 m rise in the WT equates to an increase in effective shear stress (σ_s) of ~2.1 kPa, then the relationship of σ_s (kPa) to strain rate (de/dt, s^-1) in the Manning Cyn. Shale along the glide plane is given by de/dt = 3.88e-29*exp(0.66*σ_s)+1.60e-9 (R²=0.89).