Retaining Wall Types and Selection
Retaining walls are structures designed to hold back soil and resist lateral earth pressures in situations where the ground elevation changes abruptly. The selection of the retaining wall type depends on the height of the retained soil, the soil conditions, the space available for the wall base, the aesthetic requirements, and the project budget. Gravity retaining walls rely on their own weight to resist overturning and sliding. These walls are typically constructed from mass concrete, stone, or concrete masonry units and are economical for heights up to 10 feet. The cross-section of a gravity wall tapers from a wide base to a narrower top, with the base width typically 50 to 70 percent of the wall height. The stability of gravity walls depends on the weight of the wall and the friction between the wall base and the foundation soil. Gravity walls work well when adequate space is available for the wide base and when the foundation soils have sufficient bearing capacity to support the heavy structure.
Cantilever retaining walls use a reinforced concrete stem and base slab to resist lateral forces. The base consists of a heel under the backfill and a toe extending in front of the stem. The weight of the soil on the heel adds to the wall stability, reducing the amount of concrete required compared to gravity walls. Cantilever walls are economical for heights from 10 to 20 feet and are the most common type of cast-in-place retaining wall. The reinforcement in the stem resists the bending moment from the lateral earth pressure, with the main vertical reinforcement placed on the tension face of the stem. The base slab is reinforced in both directions to resist the bending moments from the soil pressure and the stem connection. The design of cantilever walls requires consideration of the earth pressure distribution, the wall stability, and the structural capacity of the stem and base.
Mechanically stabilized earth walls use soil reinforcement to create a composite structure that resists lateral earth pressure through tension in the reinforcement. The MSE wall consists of a wall facing, a reinforced soil mass, and a backfill zone. The reinforcing elements are typically steel strips or geosynthetic sheets placed horizontally within the soil mass at vertical spacings of 12 to 24 inches. The tensile forces in the reinforcement provide the lateral resistance that allows the reinforced soil mass to act as a gravity structure. The wall facing is typically precast concrete panels or a wrapped-face geosynthetic system that provides erosion protection and a finished appearance. MSE walls can be constructed to heights exceeding 50 feet and are economical for a wide range of applications including highway embankments, bridge abutments, and retaining walls. The construction of MSE walls requires careful compaction of the reinforced soil and proper installation of the reinforcing elements to achieve the design performance.
Earth Pressure and Stability Analysis
The calculation of lateral earth pressures is fundamental to retaining wall design. The Rankine and Coulomb theories provide the classical methods for calculating active and passive earth pressures based on the soil properties and the wall geometry. The active earth pressure develops when the wall moves away from the soil, reducing the lateral pressure to the minimum value that can be maintained without failure. The passive earth pressure develops when the wall moves into the soil, increasing the lateral resistance to the maximum value that can be developed. The at-rest earth pressure exists when the wall does not move relative to the soil and is higher than the active pressure but lower than the passive pressure. The selection of the design earth pressure depends on the allowable wall movement and the importance of limiting wall deflection. mechanically stabilized earth wall design principles. rankine earth pressure theory for retaining wall design. tieback anchor installation for sheet pile walls. Walls that can tolerate some movement are designed for active pressure, while walls where movement must be minimized are designed for at-rest pressure.
The stability analysis of retaining walls must verify that the wall is safe against overturning, sliding, bearing capacity failure, and overall slope stability. The factor of safety against overturning is the ratio of the resisting moment to the overturning moment, with a minimum value of 2.0 for cantilever walls and 1.5 for gravity walls. The factor of safety against sliding is the ratio of the sliding resistance to the sliding force, with a minimum value of 1.5. The key to improving sliding resistance is increasing the base width or adding a shear key at the base of the wall that engages the passive resistance of the soil in front of the wall. The maximum bearing pressure under the wall base must not exceed the allowable bearing capacity of the foundation soil, with the resultant of the vertical forces located within the middle third of the base to prevent tension in the soil-structure interface. Overall slope stability analysis evaluates the potential for a deep-seated failure surface that passes beneath the wall and through the foundation soil.
Sheet Pile and Soldier Pile Walls
Sheet pile walls are used for temporary and permanent retaining applications where the wall must be installed before excavation. Steel sheet piles are the most common type, with interlocking joints that create a continuous wall capable of resisting lateral earth and water pressures. The sheet piles are driven into the ground using vibratory or impact hammers, with the installation sequence designed to maintain the vertical alignment and interlock integrity. Cantilever sheet pile walls are used for excavations up to 15 feet deep in competent soils. Anchored sheet pile walls with one or more levels of tieback anchors are used for deeper excavations. The tieback anchors are drilled through the sheet pile wall at an angle, grouted in place behind the active wedge, and tensioned to provide active support. The design of anchored sheet pile walls must consider the anchor capacity, the wall bending moments, and the overall stability of the anchored system.
Soldier pile and lagging walls are used for temporary excavation support in urban areas where space is limited. The soldier piles, typically wide-flange steel beams, are installed at regular intervals of 6 to 10 feet before excavation begins. As excavation proceeds, horizontal lagging of timber or steel plates is placed between the soldier piles to retain the soil. The lagging is not structural but simply retains the soil between the piles, with the soldier piles providing the primary lateral resistance. The soldier piles are supported by one or more levels of tieback anchors or internal bracing as the excavation deepens. The tieback anchors are installed through pre-drilled holes in the soldier pile webs and grouted in place. The anchor installation must be coordinated with the excavation sequence to maintain wall stability at all stages of construction.
Drainage and Waterproofing for Retaining Walls
Adequate drainage is critical for the performance of retaining walls because water pressure behind the wall can exceed the earth pressure and cause wall failure. The drainage system must prevent the buildup of hydrostatic pressure by providing a path for water to escape from the backfill. Granular backfill material with high permeability is placed directly behind the wall to promote drainage and reduce ice lens formation in freezing climates. The drainage zone typically extends 12 to 24 inches from the wall face and uses clean sand or gravel with less than 5 percent fines. A perforated drain pipe at the base of the wall collects water and conveys it to a suitable outlet. The drain pipe must be wrapped in geotextile fabric to prevent clogging by soil particles and must be sloped to drain completely between wet periods. Weep holes through the wall face at 4 to 6 foot spacing provide additional drainage for walls where a drain pipe is not provided.