The Origin and Development of Anchor Rod Support Theory

Anchor rod support refers to a reinforcement and support method used in surface engineering such as slopes, deep rock and soil excavations, as well as in the construction of underground chambers such as tunnels and mining areas. Using metal, wood, polymer, or other materials to make poles and columns, they are inserted into pre drilled holes in the surface rock mass or surrounding rock mass of the tunnel. The special structure of the head and pole body, as well as the support plate at the tail (or not required), or relying on the bonding effect to bond the surrounding rock with the stable rock mass to produce suspension effect, composite beam effect, and reinforcement effect, in order to achieve the purpose of support. It has the advantages of low cost, good support effect, easy operation, flexible use, and less occupation of construction clearance.

1. The Origin of Anchor Rod Support Theory

In 1907, Russian scholar Protokyakonov proposed the Pugh collapse arch theory, which believed that after the excavation of the roadway, the upper rock layer of the mined space would gradually collapse, and a parabolic natural equilibrium arch would form above it. The height of the lower collapse arch was related to the strength of the rock layer and the width of the roadway. This theory is applicable to determining the support reaction force of tunnels with low surrounding rock strength and low mining depth. Since the 1950s, people have started to use elastic-plastic mechanics to solve tunnel support problems, among which the famous ones are Fener's formula and Kastner's formula.

The development of roadway roof control theory is very fast both domestically and internationally. China began using anchor rod support in 1956. With the continuous development of anchor rod support practice, a large number of research results have been achieved both domestically and internationally.

2. Traditional theory of anchor rod support

Suspension theory

In 1952, Louis Apnake and others proposed the suspension theory, which believed that the role of anchor rod support is to suspend the weaker rock layer of the roadway roof on the stable upper rock layer. Under the action of pre tensioning force, each anchor rod bears the weight of the surrounding rock mass within a certain range, and the anchoring force of the anchor rod should be greater than the gravity of the rock mass it suspends.

Schematic diagram of the principle of anchor rod suspension

Composite beam theory

The theory of composite beams suggests that the axial force provided by end anchored anchor rods will constrain the separation of rock layers and increase the frictional force between each layer. Together with the shear force provided by the anchor rod body, it will prevent relative sliding between rock layers and improve their self-supporting capacity. Lock several layers of thin rock together to form a thicker rock layer (i.e. composite beam). The large bending stress and strain within the thick rock layer are inversely proportional to the square of the beam thickness. The thicker the integrated rock beam, the smaller the large bending stress and strain. By compressing the original thin rock layer through the pre tension stress of the anchor rod, the frictional force between the rock layers is increased. The strength of the anchor rod itself also provides a certain shear resistance, preventing interlayer displacement.

Schematic diagram of the principle of anchor rod composite beam: a. No anchor rod is set; b. Setting anchor rods

Span reduction theory

On the basis of the theory of suspension action and the theory of composite beam action, the theory of reducing span is proposed. The theoretical task of this theory is to fix the end of the anchor rod in a stable rock layer, pass through a thin layer of roof, and each anchor rod acts as a hinge support point, dividing the roadway roof into small spans, thereby reducing the deflection of the roof.

Schematic diagram of span reduction effect

Combination arch theory

The theory of composite arch suggests that after arranging anchor rods along the perimeter of an arched roadway, each anchor rod has a certain stress range under the action of pre tightening force. As long as a reasonable spacing between anchor rods is taken, their stress range will overlap with each other, forming a continuous compression reinforcement belt - that is, a thicker composite arch. The thickness of this reinforcement belt is several times the thickness of ordinary lining support, which is more effective in resisting the stress of surrounding rock, Reduce deformation.

Schematic diagram of the working principle of composite arch.

3. The Development of Anchor Rod Support Theory at Home and Abroad

In the 1960s, Austrian engineers such as Mueller proposed a new tunnel design and construction method based on the experience of predecessors, known as the New Austrian Method (NATM). The core idea is to mobilize the bearing capacity of the surrounding rock and promote it to become an important component of the support structure, abandoning the traditional approach of using the rock mass as a support structure and load, and adopting a rear lining. In the new Austrian tunneling method, anchor rods are important components that provide active bearing capacity for surrounding rock.

In the 1970s, M D. Salamon et al. proposed the theory of energy support. This theory suggests that the support structure and surrounding rock interact and deform together. During the deformation process, the surrounding rock releases some energy, and the support structure absorbs some energy, but the total energy remains unchanged. Therefore, it is advocated to utilize the characteristics of the support structure to automatically adjust the energy released by the surrounding rock and the energy absorbed by the support system. The support structure has the function of automatically releasing excess energy. This theory mainly considers the rock mass as a homogeneous linear elastic body for analysis, which has certain limitations.

The proposed anchor spraying large arc plate support theory utilizes soft solidification filling behind the wall and compressible pad plates at the joints to give the support a certain degree of compressibility and compression. When the support reaches a certain level, it must be firmly supported to meet the characteristics of soft rock support, which is "supporting while yielding, first soft and then rigid, moderate flexibility and sufficient rigidity".

The proposed theory of surrounding rock loosening circle support suggests that before and after excavation, the rock mass changes from a three-dimensional stress state to a two-dimensional stress state, and the strength of the rock mass sharply decreases. Due to the transfer of stress, stress concentration occurs around the tunnel, causing an increase in the stress on the surrounding rock mass. If the stress exceeds the strength of the rock mass, the rock mass will be damaged, causing its bearing capacity to decrease and the stress will transfer to the deeper part until the stress is lower than the plastic yield stress of the rock mass. A fracture zone, plastic zone, and elastic zone are formed around the tunnel at once. Selecting reasonable support parameters through on-site measurement of the size of the surrounding rock loosening zone.

The theory of primary and secondary bearing area support was proposed, which suggests that after excavation of the roadway, in order to seriously form a tension and compression zone, the compression zone is located in the deep part of the rock layer, in a three-dimensional stress state, with high surrounding rock strength, and is the main bearing area for maintaining the stability of the roadway. The tension zone is located around the roadway, and the surrounding rock strength is relatively low. Through support reinforcement, there is also a certain bearing capacity, which is called the secondary bearing zone. The coordination between the primary and secondary bearing areas determines the final stability of the roadway.

Through in-depth research, the key theories and technologies of coal roadway anchor support have been obtained, especially the theory of strengthening surrounding rock strength, which mainly includes:Anchor rod support is essentially the interaction between the anchor rod and the rock mass in the anchoring area, forming an anchor solid and forming a unified bearing structure.

Anchor rod support can improve the mechanical parameters of the anchor body, including the mechanical parameters before and after the failure of the anchor body, and improve the mechanical properties of the anchored rock mass.

There are fractured, plastic, and elastic zones in the surrounding rock of the tunnel. The peak strength, post peak strength, and residual strength of the rock mass in the anchor bolt anchoring area can all be strengthened.

Anchor rod support can change the stress state of surrounding rock, increase confining pressure, improve the bearing capacity of surrounding rock, and improve the support condition of tunnels.

After the strength of the surrounding rock anchor is increased, the range of fractured and plastic zones around the roadway and the displacement of the roadway surface can be reduced, controlling the development of the fractured zone of the surrounding rock and thus promoting the stability of the roadway surrounding rock.

The theory of large horizontal stress suggests that when the vertical stress increases, the rock layer undergoes lateral deformation due to the Poisson effect, causing relative sliding along the low friction layer between the rock layers, resulting in additional horizontal stress acting on the roof rock layer. Australian scholar W J. Gale obtained the stability effect of horizontal stress on the deformation of tunnel surrounding rock through on-site observation and numerical simulation analysis, and believed that the deformation and stability of tunnel roof are mainly affected by horizontal stress.

It is believed that traditional rigid anchor rods allow the deformation of the surrounding rock of the tunnel to generally be below 200mm, which cannot adapt to the large deformation and failure of the tunnel surrounding rock and be pulled to failure. The constant resistance large deformation anchor rod can provide a deformation of 500-1000mm under dynamic impact and static tension. When the surrounding rock undergoes significant deformation and failure, the constant resistance and large deformation anchor rod can absorb the deformation energy of the rock mass, allowing the energy in the surrounding rock to be released. The constant resistance and large deformation anchor rod provides a reliable support method for soft rock roadway support.