The prestressed anchor is mainly composed of free section and anchorage section, the free section refers to the part passing through the unstable slope body, and the anchorage section refers to the part located in the stable soil body. The main function of prestressed anchor is to reduce the sliding force of soil and improve the stability of soil. This blog will explain the basic concept and classification, working characteristics, mechanical action and support structure of prestressed anchor.
Basic Types Of Rock Anchor Bolts
Prestressed Anchor
prestressed anchor is also called anchor, anchorage. Prestressed anchor is a kind of tensioned bolt (system) which transmits tension to stable or suitable rock and soil mass. It is generally composed of anchor head, anchor free section and anchor anchorage section.
Low Prestressed Anchor
The low prestressed anchor is a type of prestressed bolt with a tensile capacity of less than 200KN.
Non-tensiled bolt
Non-tensiled bolt is also known as a rock bolt. The non-prestressed rock bolt is the full-length bonded or frictional rock bolt without applying prestress in the stratum.
Working Characteristics of The Prestressed Bolt
After stratum excavation, it can immediately provide active support resistance, effectively control the deformation and development of stratum and anchorage structure;
Improve the stress state of rock and soil mass;
Improve the shear strength of weak structural plane and potential slip plane, and improve other mechanical properties of the stratum;
The position, direction, density and construct timing of the bolt can be conveniently set and adjusted, and the best stability effect can be obtained with the minimum support resistance.
Good ductility can significantly improve the ability of anchorage structures to resist earthquake and dynamic action.
The structure and strata can be closely interlocked together to form a common working system;
Mechanics Effect of Prestressed Anchor
Prevent shear failure of the formation
Control the deformation of underground tunnels and caverns and prevent collapse.
Resist vertical displacement of the structure
Resistance to inclined and collapse
Prestressed Anchor Support Structure
- Tensile Anchorage
The tensile anchorage mainly relies on the fixed section to provide enough pull-out resistence to ensure the application of prestress. Generally, cement mortar is used to consolidate the anchor cable body of anchorage section in the stable part of rock mass. According to whether the tensile section is bonded, it can be divided into full-length bonded type and free type.
1.1. Bond Type
Firstly, air or water drill is used to drill the whole length and grouting is carried out in the fixed section. This is followed by tensioning and tensioning section grouting, also known as secondary grouting bolt. The characteristic of this type of support is that the prestress can be maintained even if the anchor head fails.
1.2. Free Type
The whole length is drilled by air drill or water drill first, and then grouting, or grouting while drilling. It is not necessary to add casing outside the fixed section, but it is necessary to add casing outside the tension section. The tensioned bolt is not directly bonded to the anchor solid or adjoining rock, so it will not affected when it is freely telescopic. The characteristic of this support form is that the local stress caused by the deformation of local rock mass can be distributed on the whole tensioning section. - Pressure Anchorage
The stress mechanism of the pressure anchorage is different from that of the tension anchorage. Through the unbonded hollow bolt body, the tension load is directly transferred to the drill bit, so that the grouting body changes from the traditional tensile stress state to compressive stress, which is suitable for the large head of the thin bars. The characteristic of this support structure is that the whole length of the rod is wrapped with cement slurry, which has a good anti-corrosion effect.
Summary
Above is the answer to the question of what a prestressed bolt is. In general, the anchorage of hard rock is suitable for tension prestressed anchor, while for soft rock anchorage, pressure dispersed or tension dispersed prestressed anchor is recommended. The construction materials needs to be selected according to the specific situation of rock strata.