The south slope of the west bound roadway near Mile Post 21.7 of I-195 had suffered erosion and slope instability. Portions of the approximately 20-ft embankment slope had been eroded to the point where a portion of the guardrail was compromised and a portion of the roadway’s base course was exposed on the opposite side of the roadway. Apparently the erosion had resulted from uncontrolled surface water discharged at the top of the slope.
YU & Associates, Inc. (YU) developed and performed a subsurface investigation and subsequent laboratory testing program. Field investigation disclosed that medium dense to dense sandy fill was placed over 5 to 10 ft of soft clay and silt overlying medium dense sand. Slope stability analyses indicated that the factor of safety associated with global stability may be below engineering standard, particular under seismic condition. Various mitigation approaches were considered, including slope geometry, light weight fill, erosion protection, slope reinforcement, and retaining structures. A combination of engineering control measures and a sheet pile wall or soldier pile and lagging wall constructed near the toe of the slope were preliminarily recommended to mitigate both slope stability and erosion control issues.