Soil slope instability concerning highway infrastructure is an ongoing problem in Iowa, as slope failures endanger public safety and continue to result in costly repair work. Characterization of slope failures is complicated, because the factors affecting slope stability can be difficult to discern and measure, particularly soil shear strength parameters. While in the past extensive research has been conducted on slope stability investigations and analysis, this research consists of field investigations addressing both the characterization and reinforcement of such slope failures. The current research focuses on applying an infrequently-used testing technique comprised of the Borehole Shear Test (BST). This in-situ test rapidly provides effective (i.e., drained) shear strength parameter values of soil. Using the BST device, fifteen Iowa slopes (fourteen failures and one proposed slope) were investigated and documented. Particular attention was paid to highly weathered shale and glacial till soil deposits, which have both been associated with slope failures in the southern Iowa drift region. Conventional laboratory tests including direct shear tests, triaxial compression tests, and ring shear tests were also performed on undisturbed and reconstituted soil samples to supplement BST results. The shear strength measurements were incorporated into complete evaluations of slope stability using both limit equilibrium and probabilistic analyses. The research methods and findings of these investigations are summarized in Volume 1 of this report. Research details of the independent characterization and reinforcement investigations are provided in Volumes 2 and 3, respectively. Combined, the field investigations offer guidance on identifying the factors that affect slope stability at a particular location and also on designing slope reinforcement using pile elements for cases where remedial measures are necessary. The research findings are expected to benefit civil and geotechnical engineers of government transportation agencies, consultants, and contractors dealing with slope stability, slope remediation, and geotechnical testing in Iowa.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1006&context=intrans_reports

Innovative Solutions for Slope Stability Reinforcement and Characterization: Vol. I

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=2783&context=rtd

Soil slope stability investigation and analysis in Iowa

Slope stability of embankments bearing over highly weathered shale encountered at shallow depths is a major concern in roadway design. This condition was encountered at a bridge location in southeastern Iowa, where the proposed 120.5-meter x 12.0-meter creek crossing dual bridges are part of a four-lane state highway. The maximum height of the embankment at the north abutment location is approximately 12 meters and 8 meters at the south abutment location, with 3H:1V slopes in the longitudinal and transverse directions. Conventional soil borings with standard penetration testing indicated the presence of weathered shale at very shallow depths, in some cases as shallow as 0.9 meters. The subsurface profiles indicated the presence of sloping shale at both the north and south abutments overlain by sandy lean clay. The average slope ratio of the top of the shale is approximately 10H:1V at the north abutment location and 3H:1V at the south abutment location. As per Iowa DOT guidelines, a 0.3-meter-thick layer of highly weathered shale was assumed at the clay/weathered shale interface and assigned undrained shear strength of 10kPa. Slope stability analyses indicated global slope instability with failure surfaces propagating through the sloping and highly weathered shale layer. As a result, various ground improvement, retaining wall, and increased bridge length alternatives were evaluated. These alternatives were estimated to cost between $3,000,000 and $5,000,000. In view of these high costs, an additional comprehensive subsurface exploration and testing program was developed and executed at the site to verify that the shear strength parameters, especially with respect to the highly weathered shale, were reasonable. The supplemental program consisted of conducting 35 in situ Iowa borehole shear tests at various depths in the 10 boreholes, particularly at the shale-clay interface. Slope stability analyses were performed using SLIDE software, including probabilistic analyses to evaluate the probability of potential global slope instability. The analyses indicated the slopes to be stable under both short-term (end of construction) and long-term (drained) conditions. The judicious use of in situ testing in addition to conventional laboratory testing resulted in the elimination of potential high-cost alternatives and kept the project within budget limits.

Use of In Situ Testing to Optimize Slope Design in Highly Weathered Shale

Slope stability of embankments in highly weathered shale encountered at shallow depths is a major concern in roadway design. This condition was encoun- tered at a bridge location in southeastern Iowa, where dual bridges are part of a 4-lane state highway. The maximum embankment height is 12 meters at the north abutment and 8 meters at the south, with 3H:1V slopes in the longitudinal and transverse direc- tions. Conventional soil borings with Standard Penetration Testing indicated the pres- ence of highly weathered shale at shallow depths. The subsurface conditions indicated the presence of sloping highly weathered shale at both the north and south abutments. A 0.3-meter-thick layer of highly weathered shale was assumed at the clay/weathered shale interface and assigned undrained shear strength of 10kPa per Iowa Department of Transportation guidelines. Slope stability analyses indicated global slope instabil- ity, with failure surfaces propagating through the sloping and highly weathered shale layer. Various alternatives were evaluated at costs ranging from $3,000,000 to $5,000,000. In view of the high cost, a comprehensive subsurface exploration, labora- tory testing, and monitoring program was implemented to evaluate strength and moni- tor slope performance. In-situ testing consisted of Iowa borehole shear tests in the highly weathered shale. Slope stability analyses indicated the slopes to be stable un- der both short-term (end of construction) and long-term (drained) conditions. Incli- nometers and settlement plates were installed during construction to record slope movements. Monitoring data available thus far indicate insignificant movement. The judicious use of in-situ testing and an instrumentation program resulted in significant reduction in construction cost and kept the project within budget limits.

Bhooshan Karnik

Papers

Use of In Situ Testing to Optimize Slope Design in Highly Weathered Shale

Case study: optimization and monitoring of slope design in highly weathered shale