Story Of Anchor

Ground Anchor

Ground anchor is a system that stabilizes the structure against excessive transformation, or displacement by fixing a highly strong tension member such as the P.C. strand to both sides of the inner ground and the structure and applying Pre-Stress. Ground anchors are widely used to stabilize structures at various domestic and overseas construction and engineering sites with its outstanding economical quality and constructability for the past 50 years since its commercialization.

Application of the ground anchor

- Sheeting when constructing the excavation at a construction site
- Supplementation and stabilization of the cut section for new road construction or additional construction
- Stabilization of slope
- Stabilization and supplementation of stone masonry and revetment
- Lift up resistance for structures under ground water level
- Stabilization and supplementation of tunnel pit mouths
- Stabilize structures and foundations against earthquakes
- Stabilization of tower type structures such as steel power transmission towers
- Stabilization of foundations of bridges, such as suspension bridges

Classification of ground anchors

Ground anchors can be classified by their types of fixation ground, installation direction, grouting method, term of usage, and supporting method. The most common classification is as shown in the table below. Based on the term of use, anchors used to temporarily support temporary walls or grounds are called temporary anchors, and those used to resist load for long terms of two years or more, and that are related to the remaining period of the structure, are called permanent anchors.

Also, anchors can be classified into frictional type anchors that are supported by the friction of the grout and the ground, ground pressure type anchors that acquire anchoring force with the passive resistance of the ground using ground pressure boards or piles, and complex type anchors that are a combination of the above two types, based on the supporting method of the fixation ground. Frictional type anchors can also be classified into tensile type anchors and compressive type anchors based on the load application method to the grout. Lastly, compressive type anchors can be classified into load concentrative type anchors and load distributive type anchors depending on the distribution of the load.

Comparison by the Characteristics of Each Anchor Type
■ Tensile type anchor
The drawbacks of tensile type anchors are that a progressive destruction occurs due to the jacking crack in the grout and creeps due to load concentration, greatly reducing the load. Therefore, as in the vicinity friction distribution graph(Figure A), the load transference distribution is as shown in curve 1) at the initial point when the load is applied, which changes into curve 3) due to the above mentioned reasons with the progress of time, reducing the load.

Also, as shown in the load change diagram(Figure 1), load curve ① is expected in designing by applying jacking force to the grout mass, but in fact, since the load concentration sector exceeds the ultimate pull-out force of the subject ground, the curve is loosened to that load curve ②, resulting in load curve ③. The main cause of this is the decrease of the local friction due to load concentration.

■ Load distributive type anchor
Compressive type anchors use PE coated P.C. strands, generating compressing force on the grout by fastening the strand to a separate fixation object. The load decrease due to creeps is small compared to that of the tensile type anchor, but highly strong grouts should be used, and the drawback is that it cannot acquire the necessary anchoring force in relatively soft grounds. Also, as for the compressing force applied to the grout, the load concentration is generated at the front end as shown in the load change diagram(Figure 2), which may crush the grout. Also, another drawback is that it must be fixed to a rock bed that has high confining pressure, or that the drilling diameter increases when attempting to apply a load that exceeds the ultimate resistance. Load concentrative type anchors may also generate load decrease (load change diagram①=>②=>③) in the case of the jacking type and moreover, cause sudden load decrease due to compression destruction.
As shown in the vicinity friction distribution graph(Figure B), in the case of load concentrative type anchors, the load transference changes from 1) to 3), decreasing the load.

■ Load diffusion type anchor
To compensate for the drawbacks of the tensile type and load concentrative type anchors, an extreme load concentration must not occur at the fixation ground and the grout mass, use a PE coated P.C. strand that does not generate confinement of the free anchor length, and the application of the allowed anchoring force according to the ground conditions be easy. To achieve this, the load-distributive-compressive type anchor is developed recently and is being used. In this case, the load is diffused and applied to the various parts of the anchor body, having less impact on the grout strength, and being able to secure necessary anchoring force in relatively soft grounds. This type can have strong loads in normal and sand grounds as in rock beds