Soil and Rock Slope Stabilization for Bridge and Highway Reconstruction, State Routes 9 and 125 Lisbon-Durham, Maine

摘要

The project site has been eroded by the rapidly flowing waters of the Androscoggin River for thousands of years. Over the last 200-plus years, an upstream dam and its tailrace have contributed to the heavy scour along the riverbank and at the bridge. What remains of the riverbank is a 30-plus foot high scoured rock slope with a thin mantle of glacial till soil supporting the approach roadway at the top of the riverbank slope. Stability of the approach roadway is in jeopardy due to the progressive failure of the soil veneer supporting the embankment. At the bridge site, the riverbed consists of irregular, exposed bedrock with eroded seams and unfavorable discontinuities beneath proposed structures. The paper describes subsurface investigations including test borings, angled air-rotary probes, seismic refraction, optical and acoustic televiewer surveys and bedrock exposure mapping completed on foot and using ropes-access; engineering analyses including soil slope stability analyses, soil slope retention alternative evaluations, stereographic projection of bedrock discontinuities, kinematic analysis of the bedrock bearing surface, buttress design for the abutment foundation, and doweling and grouting assessment for pier foundation stabilization. The objective of the exploration program was to provide sufficient, yet still cost-effective investigations and limit the risk associated with uninvestigated subsurface conditions, i.e. surprises. The design solutions were developed in close conjunction with the bridge and highway designers and the owner (Maine DOT) whose input allowed the geotechnical designs to achieve their desired balance between cost and risk.