Mechanics and stress analysis of the Pine Cobble landslide, Williamstown, Massachusetts

The Pine Cobble development in Williamstown, MA, is plagued by a well known, but poorly understood slump failure. The slumping was initiated by a road cut that oversteepened glacial sediments which cover the hillside. This investigation estimates the location and geometry of the failure surface for this slump, and quantitatively identifies the factors contributing to motion. Cross sections of three transects across the major slump were constructed, using data from drill logs and surface measurements. Under the assumption that the slump moves as a cohesive block, the dip angle of the failure surface below several measurement stations on each transect was determined using data from surface motion. To estimate the failure plane's location, the assumption of a circular arc rotational slump was adopted, and an axis of rotation was constrained by the geometry of surface motion and the subsurface bedrock profile. From cross sections, shear stress at the failure plane was calculated using the Fellinius method, by dividing each transect into several linear sliding blocks. Combining this data with the geometry of the failure surface and estimates of shear strength from drill blow counts, the relative importance of variables that contribute to slope motion was evaluated with a spreadsheet program. This investigation provides probable values for cohesion and internal friction, and suggests that pore pressure plays a negligible role in causing instability. Instead, the principal factors which cause slumping appear to be the removal of substantial amounts of material from the toe of the slope, the presence of weak zones in the soil mass with angles of internal friction of 22°- 24°, and decreases in friction angle caused by water. In addition, this study offers a quantitative explanation for the slump's present boundaries.