Mechanically Stabilized Earth Walls & Reinforced Soil Slopes
Exponent has extensive experience in evaluating the performance of Mechanically Stabilized Earth (MSE) walls and Reinforced Soil Slopes (RSS) with various types of reinforcement, including metallic reinforcement (e.g., welded wire mesh, steel strip and bar mat) and various types of polymeric reinforcement (e.g., polyester or polyethylene geotextiles, geogrids and geosynthetic strips). In addition to evaluating the stability of existing MSE walls and RSS, Exponent has developed repair recommendations, monitored MSE walls prior to, during and after implementation of repairs, observed and documented MSE wall demolitions, and directed material testing of various reinforcement and facing elements.
Exponent provides a multi-disciplinary approach by assembling a team of experienced in geotechnical engineering, engineering geology, structural engineering, material and polymer science, mechanical engineering, and construction management that can address issues associated with MSE walls and RSS and assist in the resolution of related construction claims and disputes.
MSE walls are a relatively new form of earth retaining structure that have gained wide acceptance in highway, commercial, and residential construction because of their relatively low cost, efficiency in construction, flexibility in civil design, and aesthetic appeal. MSE wall systems are now available from a number of manufacturers, who also typically provide design recommendations and capacity information regarding their proprietary components.
The basic concept of the MSE wall is to create a zone of reinforced earth that serves as a deformable buttress, ultimately providing retention through resistance from gravity and friction at the base of this zone and resistance through the buttress. Reinforcing elements are embedded fully into the soil mass along their length to resist deformation and movement, and the outer facing is connected to a facing element or constructed by wrapping the reinforcement at the face. There are several types of facing elements for MSE Walls, including segmental retaining walls (SRWs), segmental concrete panels, modular block units, Welded Wire Mesh (WWM), gabion, and geosynthetic facing. In 2009, the FHWA estimated that more than half of all retaining wall constructed every year for transportation applications in the United States is from MSE walls with precast facing. Use of modular block walls (MBW), with modular block dry cast facing units has become more widespread in recent years, in part due to their lower cost and availability.
When the slope of the face of the mechanically stabilized earth exceeds approximately 70 degrees from the horizontal, the structure is typically characterized as a wall. Mechanically stabilized earth structures with face inclination flatter than 70 degrees are typically referred to as RSS. The reinforcement in a RSS is used to resist outward movement of the reinforced fill mass and thus increase the stability of the slope and limit the likelihood of surface sloughing.
Exponent services include:
- Analysis of the cause and contributing factors of excessive settlement, lateral movement, bulging, cracking of the facing, ground cracking, or failure
- Analysis of the nature and extent of damage to real property from failure
- Evaluation of differing site condition claims during construction
- Standard-of-care evaluations for design professionals
- Cost analysis, delay impacts, and acceleration evaluation for construction projects involving MSE walls and RSS
- Analysis and field evaluation of performance during earthquakes
- Monitoring and evaluation of field instrumentation data, including remote access and downloading of data (e.g., inclinometers, tiltmeters, survey points, LiDAR)
- Laboratory testing of the MSE wall or RSS elements, including:
- Directing geotechnical laboratory testing for measuring rock and soil engineering properties
- Evaluating reinforcement and facing material composition, life prediction analyses, stress analysis, evaluation of long-term corrosion of metallic reinforcements, assessment of chemical resistance and microbial degradation of polymeric reinforcements, visual and microscopic examinations, and other physical and chemical analyses
Field investigation of the MSE walls and RSS, including:
- Determining the as-built condition
- Conducting geologic mapping
- Documenting the condition of buildings and structures affected by retaining structure failure
- Collecting subsurface data by excavation and logging of test pits, small-diameter borings and rock cores, and downhole logging of large-diameter borings
- Conducting in-situ subsurface testing, installing and monitoring geotechnical field instruments to measure ground movement and groundwater levels
- Analysis of the interaction of landslides with MSE walls and RSS
- Developing of repair recommendations
- Making observations and measurements during demolition