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The Reinforcement Mechanics of Fully Grouted Rock Bolt in Underground Engineering

Author: ZhuXunGuo
Tutor: YangQing;ZuoMaoTian
School: Dalian University of Technology
Course: Geotechnical Engineering
Keywords: Rock mechanics Underground Engineering Fully Grouted Rock Bolt Anchoring Mechanics Load transfer Mechanics
CLC: TU470
Type: PhD thesis
Year: 2007
Downloads: 951
Quote: 5
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Abstract


The anchoring technology is an important branch of the Geotechnical engineering domain. The anchoring technology can greatly improve and enhance the rock & soil mass own strength and the steady ability, improve rock &soil mass stress condition, greatly reduces the structure volume and weight, obviously save the engineering material, safely enhance the construction process. The anchor technology has already became the most economically and effectively methods to enhance the Geotechnical engineering stability and solve complex Geotechnical engineering stable question in the domestic and foreign multitudinousGeotechnical engineering domain, like slope engineering, underground engineering and so on building foundation ditch. So that it is obtained the widespread application, especially in underground engineering. But, it is not completely clear for the anchoring mechanics of rock bolt, and exit many faults. For example, regards all type bolt anchoring mechanics as same in all Geotechnical Engineering domains, and how aspect also exists not completely makes the anchor mechanism and the stress transfer mechanism. The paper takes underground engineering as studying background, to study the anchoring mechanics of tunneling engineering utilizing rock bolt from different aspects and different viewpoints, for obtaining the deeply level rock bolt’s anchoring mechanics, and providing theory proofs for anchoring design. Below therefore, the paper content mainly has several parts:1. The mechanics model is established by the anchor convention pullout experiment. According to the load - displacement functional relation as well as the anchor’s force balance, the anchor stress distribution function has been established, and analyzed the factors affecting the stress distribution tendency and size. At the same time, it has been studied the relations between the limit supporting capacity and the factors. From the studying, it is obtained that the anchor stress distribution function is one kind of exponential distribution function, and has a stronger stress concentration phenomenon in the anchor head part, about in 1/3 length probably is undertakes about anchor supporting capacity 80%. At the same time, it has studied the influence factors affecting anchor stress distribution, through the analysis, it has been obtained that the different anchor radius its stress distribution function form is invariable, but it provides the stress quantity is changing. Increasing the anchor diameter may be effectively improve the anchor effect, and makes the anchor acting range to increase. Therefore, in the installing anchor construction process, it is should choose a bigger diameter anchor. It has been shown that the function relation is a kind of linear function between bolt diameter and its limited bearing capacity; the anchor solid’s elastic modulus to the anchoring effect is obvious, the elastic modulus is bigger, the anchoring effect is greater. The bigger elastic modulus can effectively reduce the bolt shearing stress intensity, and it can increase the reacting force of country rock mass, improving the anchoring effect of the anchoring system, and enhancing the adjacent rock mass stability. Through analyzing the tentative data, it may be showed that the interface average supporting capacity and the logarithm of the elastic modulus is a kind of linear relations; between the anchor length and the anchor limited supporting capacity also is a kind of linear function relations. At the same time, it also has analyzed the functional relation between grouting mass mechanics characteristic and the anchor solid mass shearing strength, for further improving present the anchor design to provide the certain rationale base.2. According to bolt’s mechanics model characteristic, the anchor stress distribution function is established under the elastic condition in the basis theory of the point’s displacement question’s Mindlin solution in infinite spatial space for a concentrating force acting the boundary. Then it has analyzed bolt’s stress distribution characteristic, it has obtained through the analyzing, at the hole head place, the shear stress of the anchor is zero, then increasing along the anchor direction, and reaching the max, after maximizing, then weaken to zero value; The maximum value of anchor’s axial force is at hole head place, from here along the anchor direction decreases progressively, in certain distance to zero; Anchor under elastic condition stress distribution and anchor actual stressful condition although also has the certain difference, but with the actual stressful condition in the anchor restraint section stress situation is same, the result also has been confined by many actual tests result, therefore, it may be saying , the established anchor elastic condition mechanics model is reasonable; Under the elastic condition, the anchor stress scope quite is small, after the interface shear stress maximizing, if it surpasses the shearing strength of the grouting mass or the rock mass, then the interface will have the plastic deformation. Then it has analyzed influence anchor stress variation many factors, the anchor diameter, the anchor elastic modulus, the rock mass elastic modulus, as well as the experimental pullout load.3. Based on the theory of partially together distorts between rock bolt and country rock mass, the established stress distribution function is a kind of hyperbolic curve function. The model although similar the conventional pullout experiment stress function, but it manifested the grouting mass, country rock mass and rock bolt, and the two interfaces between bolt and grouting and country rock mass, so it is a advanced mechanics model than fore two mechanics models. At the same time, according the transfer load model, it has analyzed the relation between the limited supporting capacity and the anchor length, as well as established one fast determination anchor length method, through the actual project confirmation, it is obtained that the method is very effective.4. According to the interface between the anchoring solid part and the country rock mass following the Mohr Coulomb criterion, the shear stress distribution equation is established. And base on it, following the interaction of country rock mass and grouting mass, so a simple mechanics model is established which the rock bolt shear stress distribution function in uniformly deformed rock mass. Base on it, these have been analyzed of affecting factors to anchoring effect. And according to the general shear stress function, it is established of neutral points. In the analyzing of affecting the anchor stress distribution tendency and the stress quantity, it is studied that the time of rock bolt applying time, and it is few studied by fore researchers. The paper according to the actual shear stress distribution function, and through change the country rock mass situ stress for different rock mass deformed time, and it is indicated that the applying time of rock bolt is a kind of important factor, and different applying time to different shear stress between rock bolt and country rock mass. Through the studying, it is quite appropriate when the country rock mass is the tunnel wall distorts probably 25~50% this stage of the finally distorts5. It is established that the load transfer differential equation of a small section of rock bolt on the base of the load transfer mechanism under the interaction between country rock mass and grouting rock bolt. And according to the different deformed displacement for different stress condition, it is established that the tunnel radial displacement function. On the assumption of the relation is a linear function between the small section of grouting mass and the country rock mass, the actual shear stress distribution function is established. On the base of it, some kinds of rock bolt stress distribution functions are established for different type rock mass. And the transfer mechanism and anchoring mechanics are studied. Through the studying, it is obtained of the rock bolt axial displacement function under the interaction between rock bolt and country rock mass, then base on these, the rock bolt axial force function and shear stress distribution function are established. And then, the function is analyzed of rock bolt stress distribution model with perfectly elastic-plastic rock. And analyzing some factors affecting anchoring effect. Through the analyzing, it is obtained that the main affecting factors are country rock mass elastic modulus, the bolt length and applying time of rock bolt. Following above studying, it is studied that the rock bolts stress distribution in perfectly brittle elastic rock mass. In this part, the radial displacement function of runnel rock mass is established, and combining it and the rock bolt axial force function, and it is established of actual shear stress function in this type rock mass. Base on it, those are analyzed of some affecting factors to anchoring effect. In those factors, the dilatancy angle is analyzed detail. With the dilatancy increase, the rock bolt axial force and shear stress increase. At the same time, the stress concentration phenomenon is weakly with small dilatancy angle. So that, the rock bolt’s anchoring effect is better in the lower dilatancy angle rock mass than higher dilatancy angle rock mass. On the other hand, the dilatancy angle has weakly influence to anchor neutral point displacement, with the dilatancy angle increase, the neutral point position have no change. Afterwards it is studied of the bolt anchoring mechanism in viscoelasticity rock mass, mainly has used two kinds of rock mass model of Kelvin model and Burgers model. After studying, when utilized the Kelvin model research the bolt anchoring mechanics, namely stress distribution function and load transfer mechanics, after the time surpassed a definite time value the country rock mass’s distortion no longer to change, the interaction between rock bolt and rock mass was invariable. When used Burgers model, then it has no such appeared definite time value, with the time change, the rock mass distortion continues to develop, simultaneously the anchor stress also in the unceasing change, only the change speed of rock mass which changed after some time big reduces down, but still had the new action yielding.

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