Ground anchors and anchored systems or soil anchorage method, like the soil nailing method, is used to earth retention, trenches, etc. In ground anchorage method or tieback construction, after drilling the borehole and place ground anchors, the end part of the borehole is injected and after trapping the cement slurry, the ground anchors are pulled with a jack and prestressed. Applying pre-stressing force ensures that the ground anchors function correctly. Ground anchoring operations are often performed top down construction on a sloping or vertical surface.
Ground Anchorage is a relatively new and economical method for stabilizing slopes, trenches adjacent to roads and widening roads, excavation near structures, strengthening old retaining structures, increasing bearing capacity, limiting deformation with minimal interference and so on.
The length of the borehole, the length of the ground anchor trapping part (injected part or bond length) and the length of the free soil anchorage part (non-injected part or that bond) depend on soil characteristics, excavation depth, anchorage tensile tonnage, overhead above excavation, presence or absence of groundwater and so on.
Applications of the soil anchorage method
The soil anchoring method has several applications, some of which are mentioned below:
- Retaining wall along highways
- Slope stabilization
- Preventing Uplift Pressure
- Consolidation of the layer of concrete dams
Differences between soil nailing and ground anchorage
- The main difference between soil nailing and ground anchoring is the pre-stressing of the reinforcing member in the anchorage method.
- The Soil Nailing Technique is passive and the ground anchorage method is active.
- In soil nailing operation, activation of force in soil nails requires deformation in the soil mass of the excavation wall, but in the ground anchoring method, due to the pre-stressing of the ground anchors, before the movement of the soil mass, the force in the ground anchors causes a significant reduction in deformation.
components of the ground anchorage method
- Strand or steel mono bar: Cable strand consists of 3 to 6 high-strength steel strands (about 1860 MPa). Depending on the project conditions, steel mono bar with lower strength (about 400 MPa) can be used instead of the strand. A bar that stretches like a strand is called a mono bar.
- Spacer: It is used to observe the appropriate distance between strands.
- Centralizer: It is used to place the strand in the middle of the borehole.
- Grout tube: One or more tubes are used to inject grout into the boreholes.
- Packer: Packer is sometimes used for jet grouting. The packer must withstand the jet grouting and be properly dewatered to prevent leakage from the beginning and end of the anchorage trapping length.
- Sheath: For two reasons, the strands are placed inside the sheath while they are free:
- Preventing strands from sticking to the grout in free length
- No transfer of anchorage force to the soil in free length
- Wedge Plate: To transfer the tensile force of the soil anchor, a cylinder with several anchor head holes is installed.
- Wedge: In order to lock the strand, hollow tapered steel parts are used inside the ground anchor head holes.
- Bearing Plate: It is used to evenly distribute the strand pre-stressing force on a larger surface and thus reduce the stress on the wall. This prevents perforation of the wall (punch cu).
*** If you use a bar instead of the strand, some of the above components are not needed.
Ground Anchors and Anchored Systems Design
The purpose of earth retention by soil anchorage method, like the soil nailing method, is to create a stable soil mass against external instability. Unlike the soil nailing method, the ground anchor is active and the pre-stress force in the soil anchorages before the deformation of the soil mass limits the deformation of the wall. Therefore, the use of ground anchors in the vicinity of displacement-sensitive structures such as older structures is recommended.
According to FHWA regulations, the tensile capacity of strand and mono bar is considered to be 60% of their tensile capacity. In addition, in temporary loads performed in the form of control tests, this number reaches 80% of the tensile capacity.
The instabilities that may occur in the soil anchoring method and should be considered when designing are as follows:
- Tensile failure of strands (tendons)
- Strand (trapping part) bond failure
- Failure due to the protrusion of the strands
- Bending excavation wall failure
- Wall failure due to lack of resistance wall capacity
- Failure due to the rotation of the wall vertically
- Wall failure due to lack of axial capacity of the wall
- Failure due to wall overturning
- Sliding wall failure
- Rotational failure of the soil mass
Ground anchors and anchored Systems execution
The proposed method for executing Ground anchor includes several main stages that are performed according to soil conditions, and so on:
1) Excavation:
Initial excavation is done to a maximum depth so that the soil can maintain its temporary stability to capture the cement slurry injected in the borehole and perform soil anchorage tension. After anchorage tension, the next stage of excavation is done in the same way. For better stability and less wall deformation, excavation of each stage is done one by one. The width of each part should also be suitable for the placement of ground anchoring machines.
- Steel or concrete piles can also be used to prevent landslides.
- Steel piles: To execute these piles before starting the excavation, well drilling operations are performed at intervals provided by the design engineer. The depth of these wells is equal to the depth of the excavation plus some value for the execution of the pile root. The root of the pile is to secure the profiles and its length is about 20% of the depth of the excavation. After concreting at the end of the drilled well, steel piles are placed in the well, thus forming a steel pile with a trapping end. Then the excavation can be started step by step.
- Concrete piles: To execute these piles, before starting the excavation, the concrete piles are executed in situ at the intervals provided by the design engineer. The depth of these wells is equal to the depth of the excavation plus some value for the execution of the pile root. The root of the pile is to secure the profiles and its length is about 20% of the depth of the excavation. Then the excavation can be started step by step.
2) Drilling:
One of the most important stabilization stages in the soil anchrage method is borehole drilling. Because there are many variables to consider:
- Bore length: The compressor air production capacity for boreholes above 15 m should be more than short boreholes.
- Soil type: In coarse-grained soils, there is a possibility of localized boreholes and air escape.
- Power and speed of drilling machine: it should be considered appropriate to the soil material.
- Drilling movement capacity: In saturated soils or disturbed soil should be considered more than natural and dry soils.
- Hammer and drill bit size: it should be selected according to the diameter of the borehole.
Drilling equipment is used for drilling in the wall. The specifications of boreholes are often as follows:
- Borehole diameter: 10 to 15 cm
- Borehole length: 10 to 30 cm
- Bore angle to the horizon: 0 to 20 degrees
- Distance of boreholes with each other: 2.5 to 3.5 m
3) Strand anchorage installation and injection of cement slurry:
After drilling the borehole to the length desired by the designer, it is time to place the soil anchor (strands) inside the borehole. To place the soil anchor (strands) in the middle of the borehole and prevent the strand from sticking to the borehole wall, a device called a spacer is used. After placing the ground anchorage inside the borehole, the end of the soil anchorage should be injected to the bond length (specified in the soil nailing drawing).
4) Shotcrete wall, installation of the plate and wedge:
After finishing the soil anchorage and injection operations:
- Execution of drainage strips: Drainage strips (geocomposite) are installed behind the wall to reduce water pressure and conduct water on the wall surface.
- Execution of nail steel mesh: A mesh of welded bars called mesh (WWM: Welded Wire Mesh) that run almost in the middle of the thickness of shotcrete. The length of steel mesh should be such that at least one complete source of the mesh overlaps with the next mesh.
- Shotcrete: After installing steel nail mesh, the wall surface with a layer of 10 to 20 cm, becomes shotcrete to maintain greater integrity between the nails to prevent soil erosion.
- Prestressed ground anchor: Using the prestressed jack, the strands are prestressed to the value specified in the executive drawing. Normally, each strand is prestressed 15 tons. Therefore, the following are prestressed.
- A strand with 3 tendons: 45 tons
- A strand with 4 tendons: 60 tons
- A strand with 5 tendons: 75 tons
- A strand with 6 tendons: 90 tons
- Installing plate and wedge plate and wedge:
- Complete connection of the soil anchorage heads to the wall
- Preventing punching in the wall
One or two steel plates are placed on the wall as an soil anchor head and are connected to the strand by a wedge and wedge plate.
6) Execution of the ground anchorage pad foundation (concrete pad):
Due to the high tensile strength of the soil anchor, a strong concrete pad section should be applied behind the anchors. Of course, in the presence of metal piles, there is no need to perform this durable section.
7) Execution of drainage tubes: In order to drain the water that may fall behind the wall, tubes are installed in the wall that direct the water from behind the wall into the excavation.
Advantages of ground anchorage
- Relative reduction of deformations
- Ability to stabilize deep excavations
- Minimal occupation of project interior space
- Non-interference with the main structural components
- High speed operation
- Possibility of permanent stabilization
- Adaptation to drainage and dewatering methods
- Flexibility and the possibility of strengthening the design during and after execution
- Ability to execute on projects with irregular geometry
- Possibility of use in semi-finished or un-stabilized projects
- Ability to increase the depth of the excavation at the time of the execution or after the completion of excavation
- Better quality control due to anchors tension
Disadvantages of soil anchorage method
- Entering the adjacent ground area
- Requiring special equipment and machinery
- Executive problems and restrictions related to the existence of underground cavities
- Requiring a specialist contractor
- Noise pollution
- Possibility of damaging green space and adjacent wells
A few practical points for performing anchoring operations
- In order to avoid legal problems, before performing the anchorage operation, a consent must be obtained from the adjacent owners in the notary public offices.
- Soil Anchorage operation is not possible under the water surface. Therefore, drainage must be done first.
- Before soil ground anchorage, the project soil studies should be done to determine the soil characteristics.
- Permitted excavation height depends on factors such as the shear strength of the soil, the amount of overhead, the amount of penetration of surface water, etc., which must be calculated.
- Before drilling boreholes, underground features adjacent to the excavation, such as sewage, aqueducts, underground structures, etc. are examined so that they are not encountered during drilling.
Video on how to do soil anchorage
Related contents:
Federal Highway Administration (FHWA)
ES Evolution , A Guide To Ground Anchor Installation
DYWIDAG THREADBAR Anchors , Anchor Design
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