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Cathedra Mine Geomechanics
KRYSHNEV ARTEM
The theme of the master's scientific work is:
"Research of anchor bolting influence on in-seam development workings stability at A.F.Zasyadko mine"
Superviser : Gavrish Nikolay Nicolaevich
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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 [2].
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 [3] 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 caving.
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 interaction of anchor framing and supported rock is discovered in the combined work of lining and rock theory by A.P.Talpakorev [4]. He shows that anchor lining being flexible can not completely prevent inelastic displacement of roof rock, it can only limit the process speed and eliminate it only under some conditions.
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.
Picture 1 Placing of the stations on a drift
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 is 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.
The analysis of opinions on existing geomechanic processes in a rock massif around the supported roadways shows that there appears the zone of non-elastic deformations where various deformation processes take place.
Actual observations at deep measurement stations carried out by L.K.Neyman and O.I.Melnikov show that in deep mine elastic and plastic deformations make 10%. Basic displacements take place due to rock volume increase during their destruction.
Analysis of mine instrumental observations of roadway outline displacement at 56 measurement stations DonSRCI (DonUGI) and other institutes allowed Y.Z.Zaslavsky [5] to define mining and geological conditions under which supported roadway maintains stability and stimulates the development of non-elastic zone around it.
Picture 2 distributing of displacements
Complex research of deformation and rock destruction around deep mines of Donbass carried out by I.L.Chernyak shows that destructed rock zone size and its structure depends on the extraction depth, contents, rock stability and structure, its interaction with the lining. Research of the massif displacement with benchmark stations allowed to examine the destructed rock zone distribution. The criteria of the rock destruction were values of ultimate relative deformations which bring about rock destruction. For shale and sand slate values of ultimate relative deformations make 0.03 and 0.02 correspondingly.
The carried out analysis of the massif condition change around roadways allows to conclude the following:
1. Sinking a working disturbs the equilibrium condition of the surrounding massif and leads to formation of the non-elastic deformations zone. Deformation type is determined by the correlation of working stress along the working frame and long-term and conditional-instant strengths of the rock.
2. Analysis of the mine observations shows that in deep mines up to 80% of framework displacement happens due to appearance of destructed rock zone, which is 2-6m.
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