Often existing retaining walls for a variety of reasons can no longer resist the lateral loads of earth pressure against it. Once it is realized that more resistance is needed, tiebacks can be installed to supplement the existing structure. Tiebacks typically consist of micropiles or helical piles.
These piles are spaced horizontally and vertically to minimize deflection between the piles. Most retaining walls are not initially designed to accommodate point loads resisting the earth pressures creating positive and negative loading points along the walls. Sometimes the outer surface of the walls can be reinforced with steel or composites to help manage those moments between the spans.
Helical piles have the advantage of requiring smaller equipment to install, making it easier in limited space requirements. Usually smaller helixes are used in multiples, often with square shafts since they accommodate smaller helixes better and are more aggressive. The reason aggression is needed it that with retaining walls, the design requires penetration some distance past the slip zone.... A zone that is determined to be the plane in using the angle of repose where one side stays with the global surrounding, and the other closer to the wall that slips with it. If the pile does not reach significantly beyond that slip plane, it will be at risk for failure.
Helical piles have the disadvantage in tieback applications where they may reach their maximum torque capacity (where beyond that capacity it is as risk for falling apart) before it can be driven to reach that point beyond the slip plane needed to resist the earth loads.
Micropiles have the advantage of driving past stiff soils, rocks or other obstructions to the depth required. Reaching sufficient depth past the slip plane is not typically a problem. Since they are percussively driven, they can usually penetrate through rocks, stiff soil, or concrete that may lie in the soils.
The disadvantages of micropiles are that they require more expensive equipment, larger equipment making it more expensive and difficult in tight or difficult access locations. The other chief disadvantage is that they are messy. They typically require grout mix to be pumped through them while drilling requiring a grout return out of the hole that must be collected and disposed.
Often these products can be used for the construction of new retaining walls, basement walls or other below grade structures that require tieback resistance as a part of the design to install the new walls.
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