Retaining structures are engineered to retain soil and/or rock. They are commonly used to accommodate changes in grade, provide increases in right-of-way and buttress the toe of slopes. In a broad sense, retaining structures can be classified according to their face inclination: if it is greater than 70 degrees, they are typically characterized as retaining walls, while slopes have face inclination flatter than 70 degrees. There are several types of retaining structures, including gravity, sheet pile, cantilever, and anchored earth/ mechanically stabilized earth (reinforced earth) walls and slopes.
Gravity Retaining Walls
Gravity retaining walls use their weight to resist earth pressures. They are usually made from heavy materials such as concrete, rocks, or soil. They are designed to provide resistance against various failure modes, including sliding, overturning, bearing and seismically induced failures. One common type of gravity retaining wall is a crib wall. Crib walls typically consist of interlocking precast concrete structural elements that are filled with free draining material (typically gravelly soil). Due to relatively low construction cost and time required to construct these walls, they are commonly used for supporting roadway and highway cuts. Gabions are another common type of gravity wall. They are used to provide erosion and scour protection along river banks and waterways. Gabions are typically made of wire mesh baskets that are filled with rocks and stacked on top of each other to form a retaining wall. In recent years, one of the most commonly used gravity retaining wall has been Mechanically Stabilized Earth (MSE) walls. These walls are also referred to as reinforced earth and reinforced soil walls. In an MSE wall, a zone of reinforced earth provides retention resistance. Reinforcing elements are typically made of metallic reinforcement (e.g., welded wire mesh, steel strip and bar mat) or polymeric reinforcement (e.g., geogrids). The outer facing is connected to a facing unit or constructed by wrapping the reinforcement at the face. MSE wall systems have gained wide acceptance in highway, commercial, and residential construction, and are now available from a number of manufacturers.
Sheet Pile Walls
Sheet pile walls typically consist of steel sheet piles that are driven into the ground to support earth pressures. Depending on the nature of construction and subsurface material, they can be installed by an impact hammer, vibratory equipment, or they can simply be pushed into the ground if ground conditions permit. In some instances, for sheet piles intended to support deeper excavations, tie-back anchors may be required in conjunction with the piles. Sheet piles may be removed but are commonly sacrificial, meaning that they are left in place after completion of construction activities.
Cantilever walls are typically made from a relatively thin stem, typically made of steel-reinforced, cast-in-place concrete. Often they are shaped like an inverted T. The pressures from the retained soils or rocks are carried through the stem to the structural footing (bottom of the T), then are transferred to the soils below and in front of the wall. Soldier pile and lagging walls are a frequently used type of cantilever wall (soldier piles can also be anchored walls). They derive lateral resistance and movement capacity through embedment of vertical wall elements (soldier piles). The soil behind the wall is retained by lagging, which may be wood, reinforced concrete, precast or CIP concrete panels, or reinforced shotcrete.
Anchored Earth Structures
Anchored and reinforced earth structures (also commonly known as soil nail walls, or tie-back walls) are constructed by the assembly of facing units that are tied to rods or strips that are held in place by friction. The resistance of the ties to movement is controlled by the portion of the anchors/nails that are located behind the theoretical active wedge (a.k.a., failure wedge).
Where right of way is available and the cost of a wall is high, an embankment slope is often considered as a cost-effective alternative to a retaining wall. The most common types of embankment slopes are engineered fill slopes and earth-reinforced slopes. If an engineered fill slope cannot be safely constructed with a particular soil, reinforcement of the fill slope can be implemented as an alternative to retaining walls or to flatter unreinforced slopes. Similar to MSE walls, earth-reinforced slopes (also known as Reinforced Soil Slopes, RSS) are a form of reinforced soil that incorporates soil-reinforcing elements (typically geosynthetics) into the soil-retaining structure.
Exponent has considerable experience in evaluating the performance of numerous types of retaining structures. Exponent’s civil engineers and geologists draw on in-house expertise in the areas of structural, materials, and mechanical engineering, to provide a multi-disciplinary approach that will address any retaining-wall problem.
Our services include:
- Emergency response in the event of imminent failure of a retaining structure
- Analysis of the cause(s) of retaining-wall failure, settlement, lateral movement, or cracking
- Analysis of the nature and extent of damage to real property resulting from the failure of a retaining structure
- Evaluation of differing site condition claims during construction of retaining walls
- Cost analysis, delay impacts, and acceleration evaluation for construction projects involving retaining structures
- Analysis and field evaluation of retaining-wall performance subject to vibrations and/or seismic ground shaking
- Field investigation of retaining structures, including:
- Assessing the as-built condition of retaining structures
- Condition assessment of buildings and structures affected by retaining-structure failure
- Geologic mapping
- Subsurface characterization by means of excavation and logging of test pits, small-diameter borings and rock cores, and downhole logging of large-diameter borings
- In-situ subsurface testing
- Installation and monitoring of instrumentation to measure ground movement and groundwater level
- Geotechnical laboratory testing for measuring rock and soil engineering properties
- Analysis of the interaction of landslides with retaining walls
- Development of repair recommendations