What are Ground Screws?
Ground screws are also referred to as soil screws or earth screws. The physical structure consists of an AB Chance helical anchor, where a helical end-bearing plate is welded to a central steel shaft acting as a large-scale ‘earth screw’ that is drilled into lateral soil walls for support. Retention systems using ground screws aid in lateral stabilization for temporary or permanent walls, much like tieback walls.
However, the design of a ground screw system differs from tieback walls in that it does not call for tension with the added ‘H’ wall construction. This is because load is transferred instantly to the bearing plates of ground screws. Furthermore, specialized equipment is not required for the installation. Any drill motor with sufficient torque output can be attached to a backhoe or skid loader.
Soil Screw Installation
Central steel shafts are available in two different types: Square Shaft (SS) series or Round Shaft (RS) series. The Type SS series are available in 1-1/4″ to 2-1/4″ square sizes. The Type RS series are available in 2-7/8″ to 7-5/8″ diameter sizes. Type SS-RS combinations are also available for compression applications in soil conditions where dense/hard soils must be penetrated with softer/loose soils above the bearing strata.
Segments or sections are joined with bolted couplings. Installation depth is limited only by soil density and practicality based on economics. A helical bearing plate or helix is one pitch of a screw thread. All helixes, regardless of their diameter, have a standard 3″ pitch. Being a true helical shape, the helices do not auger into the soil but rather screw into it with minimal soil disturbance. Helical plates are spaced at distances far enough apart that they function independently as individual bearing elements; consequently, the capacity of a particular helix on a helical anchor/pile shaft is not influenced by the helix above or below it. Load is transferred instantly per bearing plate.
The first section or lead section contains the helical plates. This lead section can consist of a single helix or up to four helices. Additional helices can be added, if required, with the use of helical extensions. The helices are arranged on the shaft such that their diameters increase as they get farther from the pilot point. The practical limit on the number of helices per anchor/pile is four to five if placed in a cohesive soil and six if placed in a cohesion-less or granular soil.
Plain extensions are then added in standard lengths of 3, 5, 7 and 10 feet until the lead section penetrates into the bearing strata.
Ground Screw Applications
The use of ground screws is versatile. Early technology of ground screws or soil screw system began as temporary excavation support in the 1960s. After proper research of the design in the late 70s and considering different soil densities, effective use was established, enabling permanent earth retention needed for deep foundation construction.
- Earth retention: When installed in a closely aligned grid pattern, ground screws, are able to hold retention in one solid mass (depending on soil densities).
- Alternative to a tieback wall for temporary excavation: A tieback wall design differs considerably compared to the use of ground screws in that ground screws are not tensioned. Compared to helical anchors used for tieback walls, there is no need to build pile and lag.
- Repair and reconstruction of a previous retaining wall: Instead of having to replace a failing retaining wall, the introduction of a ground screw wall design allows for less material that would otherwise be needed to replace construction.
- Slope stabilization or Landslide remediation: Ground screws applications are used to reinforce failed slopes and walls in-situ. Effective use calls for drilling AB Chance helical anchors beyond the reach of the failed slope. Efficiently, since the use of this stabilization is created in-situ, there is no need for removal and/or replacement of the failed soil.
- Roadway Widening under Existing Bridges: The use of ground screws or earth screws can eliminate construction steps needed when widening roadways under an existing highway bridge. When combining ground screws or earth screws with a shored permanent wall, there is no need for shoring.
Ground Screw Jobs and Considerations
Construction of a ground screw retention system application is created in vertical steps onto an excavated slope. The earth screw reinforcement is started at the top of the slope in vertical lines and then proceeds down. Once internal stabilization is met through the use of soil screws or earth screws, then a permanent or temporary facing is applied to retain the soil.
Components of a Ground Screw System
Each part of a ground screw system works together for reinforcement when applied for earth retention.
- Standard lead section: This is the deepest section which contains the first helix.
- Helices or bearing plates: Used to withstand the load, added helices are needed based on soil densities.
- Central Steel Shaft: Available in the square series (SS) or the round shaft series
The ground screw system’s objective is to stabilize in one solid mass. The helical bearing plates are spaced closely together causing a tight juncture. The idea of using ground screws is that by forming a solid mass, there will not be disturbance due to sliding or global instability.
Earth Screw Benefits
- No grouting: Ground screws do not need the use of grouting. This calls for less mess or spill. Ground stabilization is immediate without waiting for grout to cure.
- Cost effective and economical: The design of a ground screw grid to stabilize the soil does not need an ‘H’ wall construction made out of timber or steel. This reduces the manpower to build a lateral wall since the load transfer is one solid mass that is instantly stabilized.
- Quick Installation: Compared to traditional tieback systems for retention, ground screws can be applied at a faster rate. One ground screw can be drilled in a matter of minutes. Load is instantly transferred to the helical plates. Furthermore, since the helices are spaced evenly throughout the beam, there is minimal soil disturbance during installation.
- Predicable Load Testing Results: Monitoring accurate torque is constant. This allows for accurate soil holding capacities during installation.
- No Specialized Equipment: Installation of ground screws consists of a mini-excavator and load skidder to rotate the screws in place.
Why MDTI Ground Screws?
Make sure that ground screws are the correct application that you need when designing your project. There are limitations concerning soil densities, permanent walls and adaptability, and location concerning adjacent structures. Discuss with your MDTI representative which application is right for your job.