EARTH RETENTION AND ANCHORS
Conventional Excavation Support
Driven steel sheetpiling and drilled soldier beam and lagging walls are both extensively used for earth retention and excavation support. In granular soils where the groundwater is below proposed subgrade or can be economically drawn down by dewatering, a soldier beam and lagging system remains the most widely used approach. When soils are not conducive to dewatering, sheetpiling capable of withstanding the hydrostatic pressure may be the more appropriate option. For deep cuts, lateral restraint can be provided by horizontal wales with either raker braces or by tieback anchors. For shallow excavations, a cantilevered wall is generally sufficient to resist earth pressures. Narrower excavations can be braced with cross struts.
Secant Pile Walls
Secant pile walls can be used for temporary support of deep excavations or to form permanent earth retention systems. They can also serve the dual function of excavation support during construction and as part of the permanent foundation system. Since secant pile installation creates minimal disturbance, this technique is suited for dense, urban environments. Walls are constructed by the sequenced construction of overlapping, reinforced concrete piles installed in a top-down process by either drilling or augering methods. Although more costly than sheeted excavation support, this system offers greater alignment flexibility and increased wall stiffness and can be installed through bouldery or cobbly soils.
Tangent Pile Walls
Tangent pile wall construction is similar to that used for secant pile walls except that the piles are designed to touch rather than overlap. Since tangent pile wall construction eliminates the need to drill through previously placed wall elements, construction is faster. However, on sites where groundwater is present above excavation subgrade, dewatering is typically required.
Soil nailing is an economic, in situ method of reinforcing and increasing the overall shear strength of unsupported or unstable soil/rock excavations or slopes. The technique involves the installation of closely spaced reinforcing bars (nails) in the face of the excavation or slope. A structural concrete fascia bridges the nails and supports the soil/rock between them, creating a reinforced zone that acts as a retention system. Soil nails are passive elements that become tensioned when the soil/rock deflects laterally as excavation depth increases.
Soil nail walls can be a temporary measure for support of excavation or structural underpinning, or they can function as permanent retaining walls or slope stabilization systems. In general, soil nailing can be used in any soil where a vertical cut on the order of 4 to 6 feet can remain stable for at least 24 hours.
This technique offers the most advantages when:
- Difficult soil conditions are encountered such as the presence of cobbles, boulders or rock above subgrade.
- Due to space limitations, the foundation wall for the proposed structure needs to be within inches of an adjacent structure or property line.
- The site is characterized by low headroom and/or tight access conditions.
- A permanent wall is required.
Vertical Soil Nails (VNAILs)
A VNAIL is a drilled and grouted vertical element designed to promote temporary arching in marginal ground, increasing the stand-up time of an excavated face sufficiently to allow soil nailing to be performed. VNAILs are installed at closely spaced intervals along the proposed cut line and extend to the depth needed to counteract face instability in soils such as uncontrolled fills, loose sands or soft silts and clays. In conjunction with carefully sequenced construction, VNAILs can permit the use of conventional soil nail wall techniques in marginal geotechnical conditions where there otherwise might be unacceptably high risk levels.
Soil and Rock Anchors
Soil and rock anchors are pre-stressed bar or multi-strand tendons that effectively resist significant vertical and lateral earth pressures and uplift forces. Temporary tieback anchors are frequently installed in conjunction with conventional excavation support systems for deep excavations. Permanent tiebacks are used extensively in marine bulkhead construction or upgrade and are effective in increasing the factor of safety against overturning for structures subject to wind, seismic or buoyant forces. Permanent tieback or vertical tiedown anchors are also effective in increasing dam stability to meet seismic codes. In aggressive soils, the long service life of permanent rock and soil anchors may require double corrosion protection.
Exposed rock is inherently unstable, either because of the composition of the rock formation itself or through weathering. The intent of rock bolting is to transfer loads from the unstable zone at the rock face to the stronger interior mass. Bolts are typically installed on a 15-foot grid pattern, with the exposed face then covered by anchored, protective wire mesh. In some circumstances where the condition of the rock is extremely poor, it may be desirable to use closer bolt spacing and add a shotcrete facing after mesh placement.
Applications for rock bolts include tunnel excavations, support of deep open excavation extending through weathered rock, and protection of a rock cut made to facilitate new building or transportation route construction.