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

Every year, Ohio Department of Transportation (ODOT) undertakes numerous pavement patching/resurfacing projects to repair pavement distress and structural failure due to soft and/or organic soils constituting the subgrade. Other than the temporary solution of patching/resurfacing, removal of weak soils and replacement with new suitable engineered fill can be used for permanent remediation. However, when problem soils are relatively deep this method can be too costly. There are various vertical column support methods used for civil engineering structures when soils are not strong enough to support the structure. Although some of these methods are utilized by several state departments of transportation (DOTs) to remediate settlement problems of existing roadways, they were not used in Ohio for this purpose. This study investigated the applicability of various vertical column support systems to improve subgrade and reduce settlements for existing roadways in Ohio. A decision matrix has been developed to identify the feasible methods. Two sites with ongoing subgrade settlements have been investigated through detailed subsurface investigations for possible implementation. Several technically feasible vertical column support methods have been identified for these sites. Lifetime cost-benefit analysis performed show that although there are some upfront costs associated with these remediation methods, the vertical column support ground improvement methods are significantly more cost effective compared to the current practice of patching/resurfacing temporary alternative during the project’s lifetime, and can result in significant cost savings to ODOT and roadway users.

Analysis of Aggregate Pier Systems for Stabilization of Subgrade Settlement

The use of rammed aggregate pier (RAP) foundations for support of retaining walls and earth fill embankments has increased in recent years to become a geotechnical solution for rapid construction of earth structures in soft ground conditions. A nominal 6-m mechanically stabilized earth wall was constructed over piers installed in relatively compressible soil to investigate the performance of RAP foundation elements in terms of stress-deformation and settlement behaviors for such applications. Geotechnical instrumentation consisting of total earth pressure cells, settlement plates, and vibrating wire piezometers was installed within the pier elements and at the foundation surface for short- and long-term monitoring of pier response. Monitoring data indicate: (1) mobilization and concentration of vertical stress on pier elements and matrix soil; and (2) load transfer response for the boundary condition associated with support of geogrid-reinforced earth fill. The practical implications of the experimental research findings are briefly discussed.

Performance Monitoring of a Rammed Aggregate Pier Foundation Supporting a Mechanically Stabilized Earth Wall

Stability berms are commonly constructed where roadway embankments cross soft or unstable ground conditions Under certain circumstances, the construction of stability berms cause unfavorable environmental impacts, either directly or indirectly, through their effect on wetlands, endangered species habitat, stream channelization, longer culvert lengths, larger right-of-way purchases, and construction access limits Due to an ever more restrictive regulatory environment, these impacts are problematic The result is the loss of valuable natural resources to the public, lengthy permitting review processes for the department of transportation and permitting agencies, and the additional expenditures of time and money for all parties The purpose of this project was to review existing stability berm alternatives for potential use in environmentally sensitive areas The project also evaluates how stabilization technologies are made feasible, desirable, and cost-effective for transportation projects and determines which alternatives afford practical solutions for avoiding and minimizing impacts to environmentally sensitive areas An online survey of engineers at state departments of transportation was also conducted to assess the frequency and cost effectiveness of the various stabilization technologies Geotechnical engineers that responded to the survey overwhelmingly use geosynthetic reinforcement as a suitable and cost-effective solution for stabilizing embankments and cut slopes Alternatively, chemical stabilization and installation of lime/cement columns is rarely a remediation measure employed by state departments of transportation

Review of Stability Berm Alternatives for Environmentally Sensitive Areas

Case History — Southlands Orchard Road Retaining Walls

Highway construction often requires the placement of embankments and earth retaining walls to facilitate grade separations. Instability and settlement occur when these structures are placed on top of weak and compressible soils. Historically, the severity of these problems have been reduced using toe berms and surcharging. More recently, Rammed Aggregate Piers have been used to avoid the need for extending large right-of-ways required for toe berm construction or for time-consuming surcharging. The installation of Rammed Aggregate Piers reinforces weak and compressible foundation soils prior to construction of earth embankments and walls. The installation of Rammed Aggregate Piers increases the factor of safety against slope stability as a result of the high angle of internal friction (48 to 52 degrees) achieved during ramming and reduces the magnitude and time of settlement by increasing the overall stiffness of the foundation soils and providing a drainage pathway for dissipation of excess pore water pressure. This paper presents analytical methods used to design Rammed Aggregate Piers to reinforce weak soils and control settlements below highway and railroad retaining walls and embankments. The analytical methods are illustrated by a case history for a Mechanically Stabilized Earth (MSE) wall support project near Houston, Texas. This work is of particular significance because it presents design methodologies and a case history for an effective ground reinforcement technique increasingly used to support highway embankments and walls.

D O Wong

Papers

Stabilization of Retaining Walls and Embankments using Rammed Aggregate Piers TM