Ground Anchors and Soil Nailing
A ground anchor is a structural member, which can transmit an applied tensile force to a load bearing stratum. The main application on existing structures is to prevent horizontal movement of retaining walls, bridge abutments, etc. In new construction, anchors are used to retain sheet pile, bored pile walls steel tubes. Anchors may be inclined or vertical, passive, prestressed, temporary or permanent.
There are several advantages to using ground anchors:
- Economical and practical alternative to propping
- Allows for open excavation
- Versatile form of earth retention
- High loads can be obtained in relatively poor ground
- Can be used in a wide variety of soil conditions
- Used to install 'active' forces into structures.
Some typical examples of where ground anchors may be used include:
- Tie backs to river and canal walls
- Supports to bored pile walls
- Dam upgrading
- Strengthening of dock structures
- Stabilising marine structures
- Rock face stabilisation
- To resist hydrostatic uplift pressures
- Anti-flotation for deep structures
Anchor capacities generally range from 100kN to over 3,150kN, with lengths of up to 70m. The capacity achievable depends upon the quality of the anchoring material and the tendon used. The anchor hole may be straight shafted or under-reamed with bar or strand tendon.
Various different types of anchor are available to suit a particular application:
Permanent anchors incorporate full double corrosion protection provided by two concentric corrugated UPVC ducts.
Temporary anchors may incorporate a single duct or no corrosion protection at all.
Removable anchors can be used where there are wayleave issues beneath adjacent properties.
A recent development has been the introduction of multistage anchors. These can be used to improve
efficiency and achieve high loads in poor ground through the use of multiple discrete unit bond lengths.
Tendons can be made of steel bar, wire strand or GRP and are fabricated under factory conditions. This is usually done by an external supplier, but for large projects Quinn Piling can set up dedicated manufacturing facilities.
Anchor holes can be bored using a variety of drilling techniques including rotary, top-drive rotary percussive, down-the-hole hammer, and augering. In difficult ground conditions simultaneous drilling and casing systems can be employed to temporarily support the boreholes prior to installation of the anchors. The type of drill rig used will be dependent upon the access, working room and ground conditions. have a wide variety of ground engineering drill rigs ranging in weight from 1.5 to 12.0 tonnes. When the bore hole is complete, the anchor is either lowered into a pre-grouted hole or post-grouted after installation using an integral primary grout tube. When the grout has achieved full compressive strength the anchor is stressed using a portable hydraulic jack and locked off against a head plate in accordance with BS8081. Restressable head blocks with load cells can also be provided for long term monitoring and adjustment if required.