An excavation support system (ESS) or shoring is a system designed to support the ground behind an excavation. The shoring may also be designed to resist groundwater pressure, building loads, construction traffic, roadways, railroads, and many other types of surcharge loads. The shoring may be used to temporarily support the excavation until a new structure is built or be designed as a permanent earth retaining structure. Permanent excavation support systems differ from temporary systems in that many of the components in a permanent wall incorporate protection from long term corrosion.
The best type of shoring to use is dependent on a number of factors including soil conditions, required stiffness, and whether or not a groundwater cut-off is required. Shoring system’s provided by Schnabel include soldier piles and lagging, soil nailing, secant/tangent piles, deep soil mix walls, and sheet pile walls. Lateral support for the excavation support system is provided by tiebacks, soil nails, braces or by designing the vertical elements of the shoring to cantilever. .
Jet grouting is a procedure for mixing soil with injected cement grout insitu to create a cement stabilized soil. With proper quality control, the stabilized soil exhibits increased strength and decreased permeability. These modified properties allow the stabilized soil to be used as an engineered product for foundation support and groundwater control. Underpinning of existing structures and cutoff walls for excavation support are two typical applications.
The jet grout process involves the drilling or jetting of a small diameter pipe, typically around 4 inches in diameter, to the bottom of the desired treatment depth. Once that depth is reached, a neat cement grout is injected under high pressure, typically around 6,000 psi, through ports at the bottom end of the drill pipe as the pipe is rotated and withdrawn to create a circular column of treated soil. The rate of withdraw can be varied to increase or decrease the diameter of the jet grout columns. These columns are located as required for vertical support or overlapped to create a wall or barrier to the horizontal movement of weak soil and groundwater. Jet grouting is particularly effective when the subsurface to be treated contains existing utilities or other buried obstructions since the small diameter pipe can fit between them and create a larger diameter column in the soil below.
landslide occurs when the driving forces on a soil mass exceed the internal resisting forces of that soil mass. Some of the most commonly used techniques to stop or prevent a landslide include constructing a buttress at the toe, mass excavation to unload the head, draining the slide surface to increase internal strength, and installing tieback anchors, soldier piles, and/or micropiles to provide additional external resisting force.
Tieback anchors provide the most positive and cost effective solution in many circumstances. If the site cannot accommodate mass excavation, or if the soil along the failure surface is difficult to drain, tieback anchors may be the only practical solution. At other times, tiebacks in combination with one or more of the other methods may work best.
Tieback walls constructed to control landslides are designed to increase the resisting forces to a predetermined factor of safety. The wall can be located within the slope to provide the resisting forces in an optimal direction and they can be installed with minimal site disturbance. The tiebacks through the wall are anchored in competent soil or rock behind the slope failure surface. In some situations, the tieback loads are distributed directly to the soil with discrete concrete elements rather than a wall. Often these walls or elements are buried within the regarded slope and the slope is re-vegetated to its original condition. .
Micropiles are small diameter, typically 12 inches or less, deep foundation piles. Because they can be installed with relatively small drilling equipment, they are a cost effective foundation system in low overhead conditions such as inside existing buildings or under existing bridges. Schnabel has specialized drills that can even travel through a standard doorway and drill under an 8-foot ceiling height.
Micropiles are often used to underpin existing structures because they can be installed inside the structure and their installation produces less vibration than other deep foundation systems. Micropiles are also used is to construct deep foundations where there are natural or man-made obstructions since the drill systems developed for these smaller diameter holes are better able to penetrate cobbles, boulders and other obstructions than conventional drilled or driven pile systems. In the most difficult drilling conditions, such as within landslide debris, we have drilled micropiles and used them as soldier beams in permanent tieback wall construction. In difficult subsurface conditions and restricted access, micropiles can be a more cost effective solution for new structure construction. Schnabel has successfully installed and tested micropiles in compression to over 1,200 kips.
Secant piles are drilled shafts constructed in such a way that the shafts overlap each other to form a continuous wall. The construction sequence involves drilling every other shaft (primary shafts) and then returning after the concrete has partially set to drill and pour the overlapping shafts between the primary shafts (secondary shafts). The concrete in the shafts can be all low strength, all high strength, or a combination of low strength primary and high strength secondary shafts. Steel piles or reinforcing steel is typically placed in the secondary shafts. Unreinforced high strength shafts can also be constructed in a self supporting circular ring such as those frequently used for access shafts to tunnels.
Permanent tieback and soil nail retaining walls are a practical and economical solution when a permanent wall is to be built in a cut or partial cut and fill situation, particularly when it is desirable to leave a road, structure, slope or other feature behind the wall location undisturbed. While excavation behind the wall face is not required to construct a permanent tieback retaining wall, an easement for the tiebacks and/or soil nails is required.
Permanent walls are typically designed to last for a specified period of time (often 50 to 100 years). For this reason, the wall environment including the soil and groundwater are evaluated to determine their potential for corrosion, and the tiebacks, soil nails and facing connections are designed so that they will last for the lifetime of the wall.
To ensure the longevity of the wall, the facing is usually concrete or shotcrete. The wall facing can have any number of aesthetic treatments including form lined precast, cast in place concrete, and sculpted shotcrete.
Secant piles are relatively impermeable and can be used to control the flow of groundwater into an excavation and limit draw down outside the excavation. Secant pile walls are also very stiff and are often used when there are sensitive structures behind the wall.
An advantage of using secant piles for cutoff wall construction is the ability of Schnabel’s high torque drill rigs to penetrate hard drilling conditions while simultaneously casing the drill hole. These drills can advance a cased hole through man-made and natural obstructions including cobbles, boulders, and reinforced concrete. When difficult drilling conditions are anticipated, secant piles can be a very economical method for cutoff wall construction. Another benefit is the relatively low cost of mobilization when compared to other cutoff wall types.
Shotcrete is pneumatically applied low slump concrete that is blown onto the receiving surface with compressed air. Shotcrete has a relatively low water/cement ratio, high cement content, and typically small aggregate not exceeding pea gravel size. High compressive strengths in excess of 5,000 psi are commonly attained. The sticky nature of shotcrete allows it to be placed on vertical and even overhead surfaces without the need for formwork. With proper quality control, a wall surface can be built out to an 8 inch thickness or more in a single pass.
Schnabel typically uses shotcrete to build temporary and permanent froacings for our soil nail retention walls, but it can also be used as a facing for any other type of earth retaining wall. For soil nail walls, a temporary shotcrete layer of 4 to 6 inches is applied directly to the excavated ground surface, often prior to soil nail installation. If the wall is to be permanent an additional 6 to 12 inches of reinforced shotcrete will be applied after the soil nails are installed. The surface of the permanent facing can have a finish that varies from gun finished, to finish troweled, similar to a formed concrete wall, to sculpted and stained.
Soil mixing is a technique that combines cement grout with existing soil in-situ to form a soilcrete mixture that has both increased strength and reduced permeability. Steel piles are typically placed in the soilcrete while it is still wet when soil mixing is used for excavation support. The increased strength allows the soilcrete to span between the piles and the reduced permeability allows the excavation to take place under relatively dry conditions with limited draw down of the water table outside the excavation.
Soil mixing can also be used to create an impermeable barrier below the ground surface where no excavation takes place such as to reduce seepage through a dam or to contain contaminated groundwater. Another use for soil mixing is the improvement of foundation soils to reduce the probability of liquefaction during an earthquake and increase their strength.
Soil nailing is an in-place reinforcement of the soil/rock that utilizes steel tendons grouted into drilled holes to create a stable block. Shotcrete, reinforced with wire mesh and/or reinforcing steel, is applied to the face of the excavation to secure the soil between the nails. Soil nailing is installed from the top down as an excavation is made in 4 to 6-foot lifts.
Soil nailing is used for the temporary support of excavations, construction of permanent retaining walls, and for the control of landslides. Schnabel has successfully constructed soil nail walls in stiff clays and silts, some coarse grain soils, mixed soils with rock and weathered rock. Soil nailing is not recommended in cohesionless soil, organic soil or cuts below the water table.
Soil nail walls are usually built with smaller, easily mobilized equipment and without extensive steel fabrication. This minimizes the start-up time and makes soil nailing the best application for congested sites. Soil nailing can also used where there are overhead restrictions, such as bridge abutments and power lines, as there are no vertical elements to install. Soil nails are typically shorter than tiebacks for the same height of wall and thus may be preferable where there limited right-of-ways or property lines. In the proper soil and groundwater conditions, soil nail walls can be constructed faster and more economically than other retaining wall systems.
Tiebacks are used to provide the lateral resisting force for many of the retaining walls that Schnabel builds. They are constructed by grouting a high strength steel bar or strand bundle into the soil behind the retaining wall face. Tiebacks can be used for both temporary and permanent applications. Permanent tiebacks differ from temporary tiebacks in that the critical components of the tieback tendon and anchor head are protected from corrosion. Tiebacks can be anchored into most types of soil and rock. The capacity of the anchors in soil will vary depending upon soil type and installation method. Denser, granular soils will typically produce higher capacity tiebacks. Schnabel has installed tiebacks with lengths in excess of 200 feet and tested capacities of over 500 kips.
A slight variation of the tieback is the tiedown. The construction is similar, but the direction of the force is primarily vertical rather than horizontal. Tiedowns are used to resist hydrostatic uplift on structures built below the water table and seismic loads created by earthquakes. In addition to their use in the construction of new retaining structures, tiebacks are also used to stabilize existing retaining walls which are moving or deteriorating. Schnabel has patented systems for the tieback connections and facings used for the repair of distressed MSE and cantilevered retaining walls. Tiebacks are also used to stabilize landslides.
Underpinning is used to transfer the load from an exiting structure’s current foundation level to a deeper level in the ground. There are a number of reasons why this may be required. Often an excavation for a new structure will occur within feet or inches of an existing foundation, and the bottom of the new excavation will be lower than the bottom of the existing foundation. To prevent damage to the existing structure, its foundation is extended to an elevation below the bottom elevation of the proposed excavation using underpinning. Another use for underpinning is to mitigate settlement of existing foundations. Underpinning can be used to transfer the foundation load to a deeper and more competent soil stratum. This same method of load transfer can be used if there is a desire to add dead load to an existing structure in excess of the capacity of the existing foundations.
Schnabel has decades of experience in the design and construction of hand-dug piers, bracket piles, micropiles and other systems used to underpin structures. More recently, Schnabel has also used jet grouting for underpinning. Schnabel’s design\build approach to underpinning that includes close communication between our experienced engineers and field staff, combined with regular monitoring, has helped us to keep the movements of some very sensitive structures well within acceptable limits.