APPLIED GEOLOGY FOR INVESTIGATION, DESIGN AND MITIGATION OF A LANDSLIDE IN NEWPORT, VERMONT

Following construction of Rt 191 in Newport, VT in 1971, a slow-moving landslide developed, requiring pavement shimming, guard rail repair, and culvert replacement. Initial investigation and mitigation included borings and drains, but slope movement continued. Removal of 4 ft of pavement in 1986 indicated 3.2 in/yr of vertical movement occurred in 1971-1986. Additional investigations to further delineate landslide depth and extent led to installation of a stability berm near the suspected landslide toe to slow movement. After movement continued, additional investigations in 2007-2008 further downslope indicated a much greater slide mass exists, containing artesian groundwater pressures. Deep inclinometers relieved groundwater pressure, slowing movement, indicating hydrogeology plays an important factor in landslide movement.

VTrans conducted studies in 2012-2013 to collect additional data and evaluate mitigation options to slow or stop landslide movement. The studies included field mapping; subsurface drilling (sonic and conventional borings); geotechnical analysis; well, piezometer and automated inclinometer instrumentation; and hydrogeologic testing. Field data were used to refine site geologic and hydrogeologic models, develop a calibrated numerical groundwater model, analyze geotechnical stability, develop remedial design, and estimate construction costs.

Basal glacial sediments (alternating clayey silts, sandy silts and silty sands), exhibit high overconsolidation ratios, and contain very stiff to hard, high plasticity silty clays, and folded varves. These sediments, key to evaluating remedial alternatives, were likely deposited by a pre-Pleistocene glacial advance, and subsequently buried by the last ice sheet. Sonic cores identified several slip planes/zones, defined by slickensides and folded varves, indicating slumping or ice grounding deformed the sediments. The failure planes causing current movement may occupy some of these historical slip planes. Pumping tests indicate hydrogeologic connectivity exists between coarser sediments separated by clayey silts, suggesting the silty clay units are discontinuous and act as semiconfining units. Evaluations indicate deep groundwater extraction is one potential remedy to further slow and stop landslide movement.