Research Article - (2021) Volume 11, Issue 6
Received: 20-May-2021
Published:
17-Jun-2021
, DOI: 10.37421/2165-784X.2021.11.398
Citation: Fhatuwani Sengani. “A New Criterion on Designing an Effective Cable Bolt Length Subject to Static Loading in Underground Excavations.” Civil Environ Eng 11 (2021): 398.
Copyright: © 2021 Sengani F. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
A simple probability analysis failure criterion for cable bolts in underground excavation is described. The criterion was developed using static loading tests of mechanical anchors (38 tons, 18 mm diameter cable, 2 m length and six strands), and Finite Element Method. Three static loading tests were performed, thereafter, an underground excavation was simulated with several wedges wherein the effectiveness of the bolt was tested in varying spacing, while other parameters remaining constant. A simple probability criterion was therefore developed to bridge the gap on identifying effective bolt length and total effective length of the bolts within the excavation in varying strata control (change in angle of the strata and change in thickness of the strata). The criterion is suitable for predicting the effective bolt length, predicting the total effective bolts length within the excavation provided the orientation and thickness of strata in known and also suitable to classify the excavation as over/under supported. Two practical applications and examples of the criterion are described. These are firstly, identifying of and secondly, predicting the effective bolt length and total effective length of the bolts in strata with varying orientation and thickness in both hanging and side walls.
• Mechanical anchor • Probabilistic criterion • Laboratory tests • Numerical simulation • Safety factor
Underground mining usually creates excavations of varying sizes in the surrounding rock mass. However, the rock mass itself is never homogenous, because it can be of varying rock strength and can be traversed by geological disturbances as well as subjected to changing stress fields. During mining activities, the developed excavations need support to maintain the stability of the excavations. Failure to support them can result in the collapse of the excavations on varying scales, causing injuries to miners as well as disruptions to mining, which in turn can affect the mine’s productivity. The safety and life of such excavations can be maximised if properly designed mining layouts are in place accompanied by the use of correct support systems. Some excavations like tunnels are planned for long time usage and therefore require effective support.
As mining progresses towards ultra-deep mining levels, the stress levels increase rapidly, compromising the stability of the tunnels and excavation intersections. Tunnels driven into highly stressed ground typically suffer from stress-induced damage [1]. Stress-induced damage can form from either creation of new fractures or reactivation of existing fractures in the rock mass. As a result of these challenges, rock support systems of underground excavations have changed significantly over the past decades due to improved technologies, experience gained and rock support requirements for different rock masses. The number of rock reinforcement systems has also been developed and evidence to perform very deep level hard rock mining is abundant. Some of the known studies that have outlined the behaviour of rock bolts in mining practices include [2,3]. The above-mentioned authors have introduced numerous rock bolts of different length and energy absorption as well as other parameters. Nonetheless, the understanding of rock bolts behaviour has been based on laboratory, field tests, analytical methods as well as numerical analysis [4-22] Although the methods indicate that the bolts would perform very well, there have been numerous studies which have reported bolts that have failed dramatically. Nevertheless, the assessments of the failure are most concentrated on the bolt performance rather than on the rock strata onto which it was installed as well as the period within which the bolt was installed.
As a matter of fact, there have been several scholars who strived to address some of the limitations posed by the previously developed bolts. Some of the encouraging studies include those conducted by authors in [23-29]. The previous studies mentioned above were intended to present newly developed rockbolts or cable bolts or modify the existing bolts. Although there has been such extensive work, it is very complex to develop a rock reinforcement system which is perfectly applicable in all ground condition. Due to the fact that there is no perfect rockbolt or cable bolt already existing within the field of study, this provides the motivation to continuously develop and test new cable or rock bolts to ensure the safety of employees and to reduce damages at the vicinity of the mining tunnels.
Despite the existence of numerous studies that have documented the new development in effective rock bolts for underground working, these rock bolts have been reported to perform very well under laboratory, field tests and numerical simulations. These bolts are also evidenced to fail rapidly in most underground excavations. This situation has always challenged geoscientists and rock engineers on whether there is any rock bolt which truly works or not. The proposed study is intended to develop a probability analysis criterion that can quantify the effectiveness of the installed roof bolts, to outline the knowledge gap regarding the common factors that influence the ineffectiveness of roof bolts. The criterion provides the ability to classify an excavation either as an over or under supported excavation.
Short review on Mechanical cable anchor
The Mechanical anchor system is an active reinforcement system, used to stabilise large single blocks or wedges formed on the hanging wall and sidewalls of underground excavations. This reinforcement system tends to provide effective reinforcement of the excavation walls where normal rock bolts are inadequate due to their short embedment lengths. They can be used in a wide range of hole diameters from 25 mm to over 60 mm. Various steel diameters can be used to suite the required strengths . Mechanical bolts do have certain limitations in that if the correct torque is not applied on installation, an inadequate load will be achieved. The load is also lost through ground vibration caused by blasting or other rock movement, rock burst or strain burst. If scaling occurs around the collar of the support hole, load is also lost. As a result, full column grout mechanical anchors have been introduced to eliminate some of the limitations associated with mechanical anchors.
The design of the support system in the mining industry and geotechnical fields has being well established and documented in many studies. However, the major focus is always to ensure that the developed support system has the ability to perform all functions of the support system at the vicinity of the excavations [29] is the known first study to propose the common functions of the support system at the vicinity of the excavations where the authors indicated that there are three functions of the support system which are to; hold, retain and reinforce. Few decades later, Cai Ming, et al. [28] revisited the above mentioned functions and found out that there are four functions rather than three functions. In their study, a function called “connect” was then introduced. Functions such as to hold, retain and reinforce can be observed or tested in the field, while the most important function which is to connect cannot be tested yet. However, although other functions of the bolts are achieved, the last function usually plays a major role. As such, to address some of the limitations associated with support failure, the proposed criterion will include the ability to classify the bolt as fully connected or partially connected. The detailed methodology of the paper is outlined below.
Approaches
The methodologies followed in this study includes, laboratory tests (static loading tests), numerical simulation (Finite Element Method), development of the criterion and case studies on the application of the methods, as well as validating the criterion. The detailed procedure of the laboratory static loading test and numerical details are indicated in the following subsections below.
Static pull tests procedure
The tests were conducted in accordance with the following procedure:
● The anchors are in turn installed vertically between the platens using a suitable dye to provide an anchoring surface for the barrels and wedges on the top position, as shown in Figure 1.
Figure 1. A mechanical anchor of 1.5 m, 18 mm diameter steel cable with 18 ton AMS barrel anchor assembly inserted into a testing machine before test with a barrel and wedge assembly on the top platen.
● One of the cable ends is well gripped with a wedge that is fitted into the hollow side of a steel dye, with no evident slippage, whereas the other cable end is fitted with a barrel and wedge assembly inside a clevis.
● A pre-load of between 5 kN and 10 kN is applied to eliminate any slack in the assembly.
● The anchors are then gradually loaded at a rate of 30 mm/min until tensile failure is achieved.
Numerical simulation procedure
In order to understand the best situation in which the bolt can be installed, numerical simulation with a varying spacing of the bolt was performed. The numerical simulation consisted of several input parameters as indicated in Table 1. Nonetheless, the simulation was also used to validate the effectiveness of the bolts in the varying orientation of the bolt installation. It is important to indicate that the simulation was largely focusing on simulating Factor of Safety (FoS) of wedges revolving around the excavation. Three joint sets were used to simulate underground excavation at a deep level with high stress. A stereonet of the simulated excavation is shown in Figure 2. A horseshoe arch was used as a simulating arch since most of the deep level mines usually follow such a pattern, meanwhile, the dimension of the excavation is indicated in Tables 1 and 2. Note that the bolt parameters used are those presented in static analysis of the mechanical anchor, it is crucial to indicate that three bolt tests presented in this study are representative of more than 20 tests.
| Excavation Dimensions | Joints Properties | Bolts spacing | Principal stress | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Joint sets | Dip (0) | Dip Direction (0) | In plane (m) | Out of plane (m) | Magnitude (MPa) | ||||
| Height (m) | 5.7 | Joint 1 | 70 | 37 | Stage 1 | 1 | 1 | σ1 | 89 |
| Stage 2 | 1.5 | 1.5 | |||||||
| Width (m) | 5 | Joint 2 | 85 | 101 | Stage 3 | 2 | 2 | σ 2 | 47 |
| Type of Arch | Horse Shoe | Joint 3 | 48 | 262 | Stage 4 | 2.5 | 2.5 | σ3 | 27 |
| Test number | Yield load (kN) | Deformation at Yield load (mm) | Maximum Deformation at Yield load (mm) | Peak load (kN) | Stress (MPa) |
|---|---|---|---|---|---|
| 1 | 344.1 | 47 | 79 | 380 | 1495 |
| 2 | 332.4 | 41 | 48 | 357.5 | 1405 |
| 3 | 342.0 | 42 | 69 | 357.5 | 1474 |
| Mean | 339.5 | 43 | 65 | 371.0 | 1458 |
| Standard Dev | 6.2 | 3 | 16 | 12.0 | 47 |
| Maximum | 344.1 | 47 | 79 | 380.4 | 1495 |
| Minimum | 332.4 | 41 | 48 | 357.5 | 1405 |
Figure 2. Stereonet of the simulated underground situation using Unwedge (Finite Element Method).
The experimental investigations on the performance of the 38 tons Mechanical Anchor of 18 mm diameter, 4.5 m length with six strands has shown that the bolt has the ability to withstand the initial minimum yielding load of 332.4 kN with a displacement of 47 mm and a maximum peak load of 380 kN with a displacement of 79 mm. The laboratory tests were found to differ with the desired performance of the bolts as per manufacturing specifications. Nevertheless, some tests have shown a slightly different performance of the bolts as compared to the desired performance. It can then have concluded that the slight difference can be due to slipping or manufacturing error when designing different strands.
In order to understand the actual performance of the bolts in some real situations, an underground situation was simulated using Finite Element Method (Unwedge). The purpose of the simulation was to identify best bolt spacing that could be used to stabilise the excavation. Likewise, the common support installation principles were followed where it was noted that when the bolts are spaced at 1 m by 1 m in plane and out plane respectively, the Safety Factor among the wedges increases, meanwhile the stability of the excavation increases as well. A further step was undertaken to identify the distribution of tensile failure of bolts along wedges as the spacing of the bolts vary. The model has revealed that the number of bolts that fail under tensile failure mode reduces with an increase in bolt spacing. Although the performance of the bolts appeared to be excellent, the common change that concerns geoscientists and rock mechanic engineers is that, every now and then new bolts have been installed but still fail rapidly. This question has posed a serious challenge to the study prompting the researcher to further suggest a probabilistic analysis of failure of the cables based on the effective lengths of the installed bolts with varying orientation and thickness of strata. The developed criterion has the ability to evaluate one of the important functions of the bolts which has been ignored in many studies. However, it appears that three functions of the bolt have been well established in terms of assessment while connecting them remains unestablished.
The developed criterion has proven that indeed if the understanding of strata orientation and thickness is not well established, the performance of the installed bolts cannot be justified because there is one missing function of the bolts. Furthermore, the criterion also revealed that it is possible to identify the total effective lengths of the installed bolts along the ring. As a result, one could easily deduce if the ring is over supported or under supported. In the meantime, the criterion emphasises the view that the installation of the bolts should be based on the strata conditions (orientation and thickness). This clearly means that the excavation might not be supported by bolts of the same length or bolts that are installed at the same orientation to ensure the stability of the excavation. It is believed that this criterion can reduce rapid failure of well performing bolts (based on their laboratory, numerical and underground tests (pull tests and torque tests). Furthermore, it is important to indicate that so far the criterion seems to be more suitable in layered rock mass. However, further investigation is in progress to develop a specific criterion that could be suitable for any rock mass composition and implementation of other sophisticated methods to improve the criterion.
The authors wish to confirm that there are no known conflicts of interest associated with this publication. Furthermore, there has been no financial support given to influence the outcome of this work.
The author would like to say rest in peace to all mine workers who lost their life as a result of rockburst accidents.
Journal of Civil and Environmental Engineering received 1798 citations as per Google Scholar report
