Analytical design method for a truss-bolt system for reinforcement of fractured coal mine roofs—illustrated with a case study

This paper presents an analytical design method for the truss-bolt system in reinforcing underground fractured rock roofs in coal mines. The analytical design method is based on the mechanical analysis of the fractured rock roof with reinforcement by inclined roof bolts and a horizontal tie-rod. The mechanical analysis for the system includes a non-linear bending model for the laterally inclined roof bolts and three upper and lower bounds. The lateral resistance of the inclined roof bolts in a truss-bolt-supported roadway is examined using classical theory of a non-linear beam in bending. The paper analyses the arching action by lateral behavior of the inclined roof bolts in reinforcing the fractured roof. Based on mechanical models, the design formula concerning the lateral bolt forces, tensions in the tie-rod in the truss system, as well as the reinforcement behavior have been derived. In order to ensure that the roof truss-bolt system reinforces the coal roof effectively, a lower bound of pre-tightening forces must be applied on the tie-rod for stabilizing the fractured roof by arching action. The pre-tightening forces exerted via the tie-rod also cannot be greater than its upper bound, since the excessive tightening force will cause localized failure in the rock near the bolt tail at the abutment of the fractured roof beam. The analytical formulas for both lower and upper bounds for truss pre-tightening forces are put forward in this paper. Furthermore, the paper also presents analytical equations for designing the axial forces and dimensions for bolts in this kind of system. Parametric studies and optimization analysis are also carried out on the basis of the proposed approach. A flow chart and procedures are presented to show the analytical design method for this truss-bolt system. Case studies are presented to show how the method can be used in actual coal mining tunnels. The predicted results compare well with those measured in the actual case studies.     

龙江·云山墅边坡支护工程施工技术

通过严密设计计算,精心施工,采用锚杆(预应力索)框架+面板+坡体排水工程等支护形式处理以永久性边坡,是目前较常用且有效的支护方法。本文详细介绍了锚杆(预应力索)框架+面板+坡体排水工程施工技术。     

中空注浆锚杆在加固碎裂边坡中应用试验研究

 河南省省道S246线K60+535～K60+585段位于豫西王屋山脉洛阳市新安县境内。该段山高坡陡,石灰岩裸露,节理发育且贯通性好,对坡体一般碎裂区和严重碎裂区分别采用中空注浆杆和全长黏结锚杆加固处理,从极限拉拔力和位移、注浆压力、注浆量进行对比分析2种锚杆的加固效果,在一般碎裂区2种锚杆的拉力极限值为179～189 kN,注浆压力为1.8～2.3 MPa,注浆量为97～128 L,表明在该岩体下2种锚杆所提供极限拉拔力相差不大,而在严重碎裂区中空注锚杆极限拉拔力为179 kN远大于全黏锚杆的153 kN,第一次注浆压力较小,经二次注浆后压力可达2.1 MPa左右,空锚杆和全黏锚杆的注浆量分别为683和627 L,在严重碎裂区中空锚杆提供极限拉力远大于全黏锚杆。采用UDEC模拟实际岩体的节理和锚杆的布置,分析杆体的轴力和剪力分布特征,进一步验证岩体破碎情况下,中空注浆锚杆加固效果要优于全黏锚杆。     

Utilizing a novel fiber optic technology to capture the axial responses of fully grouted rock bolts

Rock bolts are one of the primary support systems utilized in underground excavations within the civil and mining engineering industries. Rock bolts support the weakened rock mass adjacent to the opening of an excavation by fastening to the more stable, undisturbed formations further from the excavation. The overall response of such a support element has been determined under varying loading conditions in the laboratory and in situ experiments in the past four decades; however, due to the limitations with conventional monitoring methods of capturing strain, there still exists a gap in knowledge associated with an understanding of the geomechanical responses of rock bolts at the microscale. In this paper, we try to address this current gap in scientific knowledge by utilizing a newly developed distributed optical strain sensing (DOS) technology that provides an exceptional spatial resolution of 0.65 mm to capture the strain along the rock bolt. This DOS technology utilizes Rayleigh optical frequency domain reflectometry (ROFDR) which provides unprecedented insight into various mechanisms associated with axially loaded rebar specimens of different embedment lengths, grouting materials, borehole annulus conditions, and borehole diameters. The embedment length of the specimens was found to be the factor that significantly affected the loading of the rebar. The critical embedment length for the fully grouted rock bolts (FGRBs) was systematically determined to be 430 mm. The results herein highlight the effects of the variation of these individual parameters on the geomechanical responses FGRBs.     

螺纹钢与圆钢锚杆工作机理对比试验研究

为深入研究以螺纹钢为杆体的全长粘结砂浆锚杆锚固机理,进行了螺纹钢锚杆与圆钢锚杆对比试验研究。试验结果表明拉拔条件下螺纹钢受力范围相对圆钢小且衰减快;螺纹钢周边浅部粘结物应力水平高于圆钢周边粘结物应力,且其变形破坏更显著：螺纹钢以屈服形式破坏而圆钢则整体拔出。通过试验分析了螺纹钢由于存在螺纹起伏使其与粘结物之间存在明显挤压、剪胀、剪断等作用从而较大地提高了锚固强度。     

Observational design of underground cable bolt support systems utilizing instrumentation

 The observational approach to cable bolt design uses visual observations as well as data collected from rock mass and cable bolt instruments to gain increased understanding of the response of the rock mass to the creation of mining openings and the interaction between cable bolts and the deforming or failing rock mass. Predictions of rock mass and cable bolt behaviour, made from models, can be used to determine the type of instruments required in the programme and then to compare the predicted to the measured response. Increased understanding of the interaction between the cable bolts and the rock mass can lead to changes in the type or pattern of cable bolts specified in design. A case history based on a cable bolting instrumentation programme carried out at Ansil Mine illustrates many of these principles in practice. As a result of the findings of this case history, the cable bolt design for Ansil Mine was modified to take account of the anticipated conditions of mining-induced stress change. Received: 4 September 1999 · Accepted: 8 December 1999     

Corrosion susceptibility of potential rock bolts in aerated multi-ionic simulated concentrated water

Rock bolts used for the reinforcement of underground mines, tunnels and nuclear waste repositories are made up of low and medium carbon steels, and high strength low alloy steels. Typical rock bolt systems used for rock reinforcement are mechanically anchored bolts, grout anchored, frictional rock stabilizers and strand anchors. For nuclear waste repository sites such as Yucca Mountain (YM) as well as for mines and tunnels, in addition to mechanical properties, corrosion properties are also important due to potential seepage of water through the fractures or pores in the rock. During temporary rock support period of 50-100 years, the temperature of the tunnel at YM should be maintained at ambient conditions. For any reason if the rock bolts are exposed to YM waters and high temperatures in the tunnel then there is a chance of corrosion of steel rock bolts. In this study an attempt was made to study the corrosion properties of various potential rock bolts for YM tunnel support via the aid of electrochemical corrosion testing. At ambient temperature (25 °C) all the rock bolts that were studied showed good corrosion resistance in these waters. At higher temperatures, 60 °C and 90 °C, corrosion resistance of rock bolts decreased, but due to special stress relief heat treatment of one of the frictional rock stabilizers (Swellex Mn 24) the corrosion rates were lower than all other tested rock bolts. Note: Swellex, Split set and Williams are the proprietary names of Atlas Copco, International Roll Forms, Inc. and Williams Form Engineering Corp, respectively.     

A numerical model of fully grouted bolts considering the tri-linear shear bond–slip model

A numerical model of fully grouted bolts is proposed in this study by implementing the tri-linear bond–slip relationship of bolts into the numerical framework presented by Hyett et al. (1996). The bond–slip relationship of bolt–rock interface is simplified and represented by the tri-linear model. The proposed numerical model is characterized by (1) its ability in modeling decoupling mechanism of bolt–rock interface, i.e. the degradation of the interfacial shear bond stress along bolts; (2) its easy implementation in a numerical code. The proposed numerical model is verified with the pullout tests, and good agreements with the experimental results can be observed in terms of axial stress distributions in the bolt, shear stress distributions along the bolt and load–displacement relationship. The proposed model also gives a reasonable prediction on the behavior of bolts installed in the field.     

轴心受拉柱脚节点受力性能有限元分析

轴心受拉柱脚节点在实际工程中应用比较普遍,本文对该节点受力性能进行有限元分析,研究锚栓所受拉力、底板位移和应变,得出底板下应力分布与底板抗弯刚度有关,在底板绝对刚性时,底板保持平面,底板下应力均匀分布;在此基础上又提出该节点的有关设计公式,并进一步对该节点受力性能进行参数分析,讨论底板厚度和加劲肋设置对其受力性能影响,为实际工程设计提供参考。     

Non-destructive testing of rock bolts using guided ultrasonic waves

The research outlined in this paper is aimed at the development of a portable non-destructive testing instrument for evaluating the condition of rock bolts. In applications such as coal mine roof reinforcement, the opportunities for rock bolt inspection are currently limited to destructive techniques such as the pull-out test. It is proposed that guided ultrasonic waves can be used to solve this inspection problem, using a pulse-echo test carried out from the free end of the bolt. Suitable test frequencies have been identified through the use of modelling software, and successful laboratory and site trials have been undertaken. The research has shown that the proposed approach is capable of determining the bolt length, and of identifying major defects such as necking, deformation, and loss of resin encapsulation.     

3D Behaviour of bolted rock joints: experimental and numerical study

Rock bolting is the most effective and also the most economical means of supporting excavations in rock. Various types of bolts are used today, and an understanding of the way in which these bolts work is essential for an optimal, safe, and economical use. Fully grouted, untensioned bolts have been commonly used in rock mechanics (i.e., mines, rock fall stabilisation, underground works) for many years. In the 1980s a new type of bolt, called Swellex, was developed, becoming more and more widespread because of their easy and fast installation. However, regardless of the type, the mechanical behaviour of the bolted rock joint is not fully understood, and only the experience accumulated on rock bolting gives the know-how for the reinforcement calculation and execution. In this paper the different mechanical responses of full steel bars as opposed to the frictional Swellex are discussed. The study was done through experimental tests coupled with numerical simulations. The analysis of the results obtained both from finite element (FEM) modelling, and from large-scale (1:1) shear tests on rock joints, reinforced with fully grouted rods and Swellex bolts, clearly shows that the two bolt types deform in dissimilar ways, responding very differently to shear load.     

An analytical model for estimating rock strength parameters from small-scale drilling data

The small-scale drilling technique can be a fast and reliable method to estimate rock strength parameters. It needs to link the operational drilling parameters and strength properties of rock. The parameters such as bit geometry, bit movement, contact frictions and crushed zone affect the estimated parameters. An analytical model considering operational drilling data and effective parameters can be used for these purposes. In this research, an analytical model was developed based on limit equilibrium of forces in a T-shaped drag bit considering the effective parameters such as bit geometry, crushed zone and contact frictions in drilling process. Based on the model, a method was used to estimate rock strength parameters such as cohesion, internal friction angle and uniaxial compressive strength of different rock types from operational drilling data. Some drilling tests were conducted by a portable and powerful drilling machine which was developed for this work. The obtained results for strength properties of different rock types from the drilling experiments based on the proposed model are in good agreement with the results of standard tests. Experimental results show that the contact friction between the cutting face and rock is close to that between bit end wearing face and rock due to the same bit material. In this case, the strength parameters, especially internal friction angle and cohesion, are estimated only by using a blunt bit drilling data and the bit bluntness does not affect the estimated results.     

隧洞中全长粘结式锚杆的受力分析

以隧洞围岩位移为参量,推导出仅有围岩变形而产生的锚杆界面剪应力,然后根据现场监测资料获得的锚杆界面的剪应力特性,把地下洞室中的全长粘结式锚杆所穿过的围岩依次分为3个部分:变形区、过渡区和稳定区。在每一个围岩区域,锚杆界面剪应力有其特点,根据其特点提出松动区的锚杆界面剪应力模型,进而得到锚杆中性点的位置。以中性点的轴力为集中力,由半无限体受集中力作用的Mindlin解导出中性点以内锚杆界面剪应力的分布。综合中性点前后的剪应力分布,给出了全长粘结式锚杆的轴力分布。     

The ways ahead for rock engineering design methodologies

In this paper we discuss the future of rock engineering design methodologies within the framework of the eight current and potential basic design approaches. We note that many rock engineering projects are on the limit of precedent practice, involving large scales, increasing costs, enhanced environmental awareness and the need to understand and predict the consequences of coupled mechanisms involving stress, water, heat and chemical reactions. At the same time, rock mechanics modeling is also becoming more complex and many more component elements and mechanisms can now be included in the numerical codes. Thus, because both the projects and the modeling are becoming more sophisticated, yet it may not be possible to obtain the necessary site information to support all the required input data for modeling, there is a need to consider how a design approach can be developed that integrates the current methods and includes those methods that are not based on exact 1:1 mapping between the problem geometry/mechanisms and the model. Our emphasis, therefore, is on development of the integrated design approach and the ‘intelligent’ methods. We also discuss how a virtual laboratory and the Internet can support these developments. An illustrative design case history of the shiplock slopes at the Threee Gorges Dam project in China is included. The paper concludes with nine specific points.     

围岩中原始垂直地应力对圆形隧洞全长注浆 岩石锚杆应力分布模式的影响分析

 通过分析原始垂直地应力对圆形地下洞室塑性区半径、围岩体塑性区和弹性区内岩体的位移变化以及对锚杆应力分布的影响,揭示原始垂直地应力对锚杆锚固效果的影响。研究结果表明,弹性区内任一点的应力大小与垂直地应力有关,而塑性区内任一点的应力与原始垂直地应力无关;随着垂直地应力的增大,巷道塑性区半径也在不断增大,二者之间呈明显的非线性关系;塑性区的径向位移及隧洞壁的最大位移也随之增大。同时,根据围岩与锚杆的相互作用,建立全长注浆岩石锚杆在圆形隧洞围岩中的应力分布解析本构方程。在此解析模型的基础上,对不同垂直地应力作用下锚杆的应力分布模式进行较详细分析,得出随着垂直地应力增大,锚杆的摩阻力和轴向载荷也随之增大,并且锚杆端部的应力集中现象更加明显,当锚杆端部的摩阻力超过围岩的容许抗剪强度时,则锚杆与围岩发生开裂破坏,故在设计与施工中应予以考虑。最后,提出提高和改善由于锚头开裂导致锚固效果降低的方法,对全长注浆岩石锚杆施加垫板,能有效改善锚杆的锚固效果。     

Discussion on “Analysis of Bingham fluid radial flow in smooth fractures” [J Rock Mech Geotech Eng 12 (2020) 1112–1118]

Recently, Zou et al. (2020a) published a theoretical analysis on the radial flow of a Bingham fluid, where they argued that the classical analysis by Dai and Bird (1981) violates the mass conservation. The present discussion aims to clarify this conflict between those two studies. It is noted that Zou et al. (2020a) presumed the gap-wise mass flux is negligible in the mass conservation equation, while Dai and Bird (1981) did not require so in their model, and this is found to be the origin of the conflict. In fact, Dai and Bird (1981)’s model is shown to not violate the mass conservation. Therefore, those two models should be viewed as separate models derived from different perspectives. Details of the major difference between the two models are discussed.