Investigation, Design and Mitigation of a Landslide in Newport, Vermont

Shortly after new embankment construction for Route 191 in Newport, Vermont in 1971, a slow-moving landslide developed, requiring the Vermont Agency of Transportation (VTrans) to periodically maintain the roadway with pavement shimming, guard rail repair, and culvert replacement. Initial investigations and mitigation in the 1970’s included borings and drains, but slope movement was not reduced. Removal of 4 ft of pavement shim in 1986 indicated 3.2 in/yr of vertical movement from 1971-1986. VTrans installed piezometers and inclinometers in the 1980’s to further delineate the landslide depth and extent, leading to installation of a stability berm near the suspected landslide toe to slow movement. After movement continued, additional deep inclinometers installed in 2007-2008 further downslope indicated the slide extent was much greater than suspected, and that artesian groundwater pressures exist deep in the slide mass. The deep inclinometer installations relieved deep groundwater pressure and slowed movement, indicating hydrogeology plays an important factor in landslide movement and hence mitigation. VTrans conducted a comprehensive geologic, hydrogeologic and geotechnical investigation in 2012-2013 to collect data and evaluate several mitigation approaches, with the goal of selecting a suitable remedy to slow or stop landslide movement. The investigation included field reconnaissance geologic mapping; an extensive subsurface investigation program including sonic and conventionally drilled test borings; extensive geotechnical laboratory testing; well, piezometer and automated inclinometer instrumentation; and hydrogeologic testing. Field and laboratory data were used to refine the site geologic and hydrogeologic 3-D models; and to develop a calibrated numerical groundwater model to support geotechnical stability analyses, remedial design development, and construction cost estimating. Basal glacial sediments of the slide (alternating clayey silts, sandy silts and silty sands), which are key in evaluating remedial alternatives, exhibited high overconsolidation ratios, very stiff to hard consistency when undisturbed, high plasticity, and folded varves. One geologic interpretation of the origin of these sediments is deposition by a pre-Pleistocene glacial advance, and subsequent burial by the last ice sheet. Sonic cores indicate several previous slip planes/zones exist, defined by slickensides and folded varves, indicating slumping or ice grounding deformed the sediments. The slip planes contributing to the current movement may occupy some of these historical failure planes, and residual shear strengths at these zones likely govern behavior. Pumping tests indicate hydrogeologic connectivity exists between the lowermost coarser sediments and overlying coarser sediments separated by clayey silts, suggesting the silty clay units are discontinuous, but act as semi-confining units. Groundwater and slope stability models simulating several groundwater withdrawal options including passive and active groundwater extraction indicate landslide movement can be slowed or even halted. However physical constraints, such as right-of-way, groundwater chemistry and treatment, and permitting, must be evaluated to design, construct and operate a permanent groundwater extraction system.

• Record URL:
  http://www.highwaygeologysymposium.org/wp-content/uploads/66_HGS-OPT.pdf
• 
• Supplemental Notes:
  • Copyright © 2015 Highway Geology Symposium (HGS). Abstract reprinted with permission of the HGS.
• Corporate Authors:

  Highway Geology Symposium

  ,   United States 
• Authors:
  • Smerekanicz, Jay R
  • Lloyd, Jeffrey D
  • Peterson, Mark S
  • Ingraham, Peter C
  • Benda, Christopher C
• Conference:
  • 66th Highway Geology Symposium
  • Location: Sturbridge Massachusetts, United States
  • Date: 2015-9-14 to 2015-9-17
• Publication Date: 2015

Language

• English

Media Info

• Media Type: Digital/other
• Features: Figures; Photos; References; Tables;
• Pagination: pp 471-499
• Monograph Title: Proceedings of the 66th Highway Geology Symposium (HGS 2015)

Subject/Index Terms

• TRT Terms: Geotechnical engineering; Groundwater; Hydrogeology; Landslides; Slope failure; Slope stability; Soil stabilization; Stability analysis
• Geographic Terms: Newport (Vermont)
• Subject Areas: Construction; Design; Geotechnology; Highways; Hydraulics and Hydrology; Maintenance and Preservation;

Filing Info

• Accession Number: 01638279
• Record Type: Publication
• Files: TRIS
• Created Date: Jun 21 2017 5:16PM