General characteristic of activity
INTRODUCTION
Operation of deep mines shows that one of the problems which needs prompt solution is mine roadways stability. To support workings at Donbass mines lining of nearly 100 types are used. The most widely used are metal arch yielding supports, the least – anchor supports.
In-seam workings are in especially poor condition. Loss of their cross-section area is 60-70%. This results in the fact that 30% of workings are repaired or re-supported every year. Share of expenses on roadways sinking, lining and support in Donbass makes 25% of coal cost price. Its increase is impossible.
Experience of the use of bigger dimension size profiles and increase of support density shows that increase of steel intensity and labour-intensiveness general effect is little.
Thus, constructive modernization of the lining itself and its installation process cannot ensure considerable increase of roadways stability.
One of the main directions in mine support technology is the working out of resource-saving technology of development workings sinking with anchor support combined with metal arch lining, and with anchor support with rectangular and trapeziform cross-section in shallow depth roadways.
Experience of anchor bolting use combined with metal support shows that anchors decrease roof rock displacement and load on framework. But the absence of common method of anchors parameters calculation as well as simple and efficient constructions of yielding anchor support does not allow to ground the area of its rational application.
The research aim is to examine the deformation peculiarities of the rock massif containing roadways with anchor support at different bed attitudes and anchoring depths.
Variety of mining-geological and technical conditions of the workings exploitation resulted in appearance of a number of various mechanical models of the massif conditions around the roadways.
Chernyak I.L. suggested to assess the rock massif condition on the basis of ultimate stress and strength correlation. According to him there are 3 types of deformations:
1. In the rock around the shaft there appears the zone of inelastic tight deformations less than long-term strength.
2. The second type of deformations appears at tension along the outline more than long-term strength but less than instantaneous rock steadiness.
3. The third type of deformation around the shaft appears when the tension along the outline is more than instantaneous rock steadiness.
Opening strength means its ability to keep the definite parameters and cross-section form during the given lifetime.
To achieve the strength of the opening it is necessary to increase the rock bearing capacity by any type of support. The most efficient way is to increase flexural and tensile strength affecting the rock. It is necessary to strengthen the rock against tensile stress. It is achieved by bolting steel and polymeric rods – anchors.
In contrast to framework anchor bolting is constructed being pre-stressed, that is why immediately after the construction the cohesion between the rock layers or gullets.
Experimental and analytical research has not led to working out of widely-used anchor bolting work and single method of its parameters calculation.
However analysis of the present calculation methods shows that most scientists name as the main factor influencing the stability of bolting bearing capacity of anchor fastener or its starting tension.
There are five basic theories on mine roadways anchor bolting: fastening of direct roof to the main one; forming of load-carrying construction; compression of supporting rock; combined work of lining and rock; energetic theories.
The theory of direct roof fastening was originally introduces in 1950 by F.Bakky. Its supporters are L.Rabtsevich and A.Shirokov.
The theory presupposes that weak, caved or inclined to layer separation rocks of the direct roof are bolted to firm rocks.
The main design value is the roof area per one anchor, which is defined as its lock bearing capacity function and its starting tension.
Theory of load-carrying construction was formed by O.Yakobi. It presupposes that while anchoring mine rocks are hardened artificially and in rock massif load-carrying construction is formed. Strengthened rocks are mainly compressed and anchors receive tensile force. According to this theory anchor length is defined taking into account different considerations – roof area per one anchor depends on tension or stress-strain properties of bolting materials.
Followers of the supporting rocks compression Z.Talobor and A.Yugon suppose that anchor bolting function is to compress the bolted rock and to prevent tensile stress in it. If immediately after extraction anchor lining is installed compressing the rock, there will be no development of tensile stress and rock distruction.
These theories of anchor support operation agree with the practical data in cases for which they are suggested.
However suggested methods of calculation based on these theories simplify the situation and have a number of admissions that decrease the reliability and practical value of the calculated results.
Thus, interaction between anchor lining and supported rock these theories view as a statistical task. But it is known that after installation mine rock continue for some time shifting towards the roadway.
The main factor determining the density of anchors installation is the anchor locks bearing capacity or their starting tension. At that it is presupposed that the load-bearing capacity and anchor tension are constant values. But the experience shows that these values can vary significantly from the original ones.
In this connection more attention has been recently paid to the anchor bolting work in the conditions of their prolonged exploitation.
The mechanism of anchor bolting and supported rock interaction is described in the theory by A.T.Talpakorev. He shows that anchor bolting due to its yielding cannot completely prevent the process of inflexible roof rock displacement – it can only limit its speed and under certain conditions to stop.
Soon after installation anchors’ tension on rock is defined by starting tension. Then under the influence of force applied to grabs reactance of lining increases and anchors in the result of metal elastic extension displace towards the roadway. During displacement stress condition decreases and anchors’ capacity to react a load increases.
This process will continue until there is the balance in the system “lining - rock”, i.e. until anchors’ reactance equals rock pressure. If anchoring is installed behind the rock outcrop and the needed stress is created, rock layers will be in natural connection and their stratification is eliminated.
This also increases the rock bearing capacity. Due to anchor lining influence rock strata displacement is decreased. Some layers are fastened by anchor bolting, thus decreasing vertical tension and providing roof loading capacity.
That is why in thin-layer rock it is possible to fasten rock strata in the roof thus preventing their separation from the massif, mutual approach and downwarping. Roof stability will depend on ultimate flexural strength and number of strata fastened.
Supporters of energetic theory of rock and anchor lining interaction consider that while driving a roadway potential energy is released. Its value is determined by initial stress condition. In the fixed workings released energy is used not only for deformed rock destruction, but also for overcoming support resistance. This approach is universal, but its practical application is made harder by the initial data definition.
It is also possible to support the roadway by anchor lining when its roof is not flat but arch, rock is partially caved or various characters of bed attitudes are supposed.
With arched roof anchors are bolted in the rock where there is no equilibrium stress condition and where stress does not exceed rock elastic limit.
To resist tensile stress anchors have to be installed to planes of incipient cracks. In layers with different bed attitudes anchors have to be fan-shaped.
Conclusions:
1. Analysis of the rock massif condition change shows that in deep mines framework displacement happens due to appearance of destructed rock zone, its size is 2-6m.
2. There is no common opinion on the mechanism of anchor support and surrounding rock interaction.
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