The stability of deep underground excavations depends upon the strength of the surrounding rock mass and upon the stresses induced in this rock. After mining ...
ARMA 08-237
Endoscopic method of rock mass quality evaluation – new experiences Malkowski P., Niedbalski Z., Majcherczyk T. AGH UST, Krakow, Poland Copyright 2008, ARMA, American Rock Mechanics Association This paper was prepared for presentation at San Francisco 2008, the 42nd US Rock Mechanics Symposium and 2nd U.S.-Canada Rock Mechanics Symposium, held in San Francisco, June 29July 2, 2008. This paper was selected for presentation by an ARMA Technical Program Committee following review of information contained in an abstract submitted earlier by the author(s). Contents of the paper, as presented, have not been reviewed by ARMA and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented.
ABSTRACT: The experiences in evaluating the rock mass quality with the help of Endoscopic Rock Mass Factor (ERMF) are described. ERMF was worked out by the authors and research was carried out in the Polish underground coal mines. ERMF enables the quantitative and qualitative estimation of fracture number and size occurring around underground roadways. The new method of rock mass quality estimation is correlated with RQD index, but the investigations are done directly in the bore-holes. This is an “in situ” method, which takes into account all natural factors influencing the mine working stability. As the range of the crack zone and joint intensity affect the type of working support used, the ERMF factor was presented in view of support and the stand-up time of an excavation. Evaluations of roof separation and support loading carried out in underground mine roadways in relation to ERMF show that the new method of rock mass quality evaluation is effective in support design for Polish underground hard coal mines.
1. INTRODUCTION The main task for mining engineers is to ensure the stability of a working after drifting. It concerns both chambers and roadways. The appearance of fracture zones in the rocks around the working are caused by the undesirable effects of induced stresses that exceed rock strength. The range of the fracture zone and joint intensity depends on several technological and geological factors, among them: primary stress (the depth of exploitation), type of rock, rock mass physical properties, rock layer inclination, the way of drifting, and previous exploitation influence [1, 2]. The stability of deep underground excavations depends upon the strength of the surrounding rock mass and upon the stresses induced in this rock. After mining in jointed rock, the rock mass is in fact made up of an interlocking mesh of discrete blocks. The strength of the jointed rock mass depends on the type and roughness of the joint surfaces between the blocks [3,4]. The scheme of support for the given roadway is selected for the type of crack zone [1,2,4]. On account of the importance of parameters relating to discontinuities (i.e. number of joints, their distribution, the character of joints), they are applied in a series of geotechnical classifications and they describe rock mass quality. Tunnelling quality index Q, distinguishes the number of fracture systems, roughness of joints and weathering of joint walls.
Grimstad and Barton suggest a relationship between the value of Q and the permanent roof support pressure using joint set number and joint roughness. Palmström describes RQD by the number of joints per cubic meter. Parameters relating to joints are also included in the most well-known geomechanical classifications based on the RMR system (joint spacing, character of joint surfaces and orientation of joints in relation to the load direction) [4,5,6,7]. The authors of this paper present an evaluation method for rock mass quality on the basis of endoscopic observations. Such tests were carried out for several coal mines in Poland using a borehole endoscope and its realistic pictures of discontinuities inside the rock mass [2,8,9].
2. ENDOSCOPIC PARAMETERS DRILL CORE LOGS RESEARCH
VS.
Observations of fracture zones in roadways were carried out using a borehole endoscope. The device is applied to observe the walls of testing boreholes by means of an infra-red camera. The picture is recorded on videotape or CD. Simultaneously the picture is observed on the screen of a monitor. It gives a possibility to make a quick and easy quantitative as well as qualitative evaluation of discontinuities in rock mass (Fig.1). The investigations are most often carried out in the
roadway’s roof, but sometimes in vertical or inclined boreholes as well [8,9].
joint filled with gouges is typical for weak rocks where surfaces of joints are soft and easy to crush.
One cannot adjust the sharpness of the picture but the distance between the borehole’s wall and the camera allow to distinguish cracks even of the thickness of 0,5mm. The fractures with no observable displacement are named “cracks” and these with opening of 1mm are named “open joints”.
All of the above parameters quantitatively describe the disturbed rock mass. The sample result of endoscopic observation for describing the quality of roof rocks is presented in Fig.2. numbe r of dis continu ities
• • • • •
number of fractures – Is, total separation – Ss, range of fracture zone – fz, number of fracture zones – nss, type of fracture – Rs.
POWER
MONITOR POWER
EJECT
R.FD
RECORD
0426 PLAY
R.FW
STOP
PAUSE
TAPE RECORDER
6V BATTERY
12V BATTERY
Fig.1 Borehole endoscope device
The total separation indicates the advancement of fracture propagation in time. The range of fracture zone is the point with the highest level of fracture. It indicates the distance into the rock body from the excavation surface. The range of fracture zone indicates the size and according to it – the weight of the loose rocks in the roof, which load the support of the roadway. The number of joints characterise the degree of rock mass crush. The number of fracture zones characterise a potential number of roof rock sections that can separate (several thin rock layers in the roof is a typical model of carboniferous rock mass in Polish coal mines). The type of discontinuities describes a character of separating rocks on their contact and potential shear resistance. Hardly visible cracks appear usually in hard rocks and the surface of such a close joint is rough. After bending these cracks can open however the space in between is “clear”. The open
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crack
filled open joint
open joint
soft open joint filled
Fig.2 A sample record of endoscopic observation
The main drill core logs parameters are RQD and the number of core pieces what can be recalculated into spacing of discontinuities. RQD is intended to represent the rock mass quality in situ but that is not in fact true. The triaxial state of stress in the rock mass and primary geological discontinuities (e.g. nearby faults) influence the quality of the drill core. However, if you observe the bore hole inside, the picture of rock mass is different. One can compare the compressive strength of rocks measured in nature and during uniaxial laboratory testing. Uniaxial compressive strength of any rock is usually 20 to 50% per cent higher than that obtained in in situ research. Laboratory tests are always done on rock samples cut from the hardest length of core legs [2]. 800 700 600 The number of core pieces
INFRARED
num ber of discontinuit ies
2 stratification
Legend: CAMERA
nu mber of dis c ontinu ities
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The endoscopic observation allows for the distinction of the following characteristic parameters [2,8,9]:
nu mbe r of dis c ontinuities
11 strat ificatio n
500 400 300 200 100 0 0
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The number of discontinuities in endoscopic research Fig.3 The number of core log pieces vs the number of discontinuities observed during endoscopic research
The correlation between the number of rock pieces of the core vs. the number of discontinuities observed during endoscopic research is shown on Fig.3. It concerns 50 roadways where endoscopic and core investigations were carried out. Generally, the endoscopic boreholes were ca. 7m long and drilled in the center of a roadway. The correlation is not very strong but clear. The drilling technology and core transport technique are the most essential regarding the core log quality. The picture of borehole walls depends in turn on the stresses inside the rock mass.
3. ERMF CLASSIFICATION Endoscopic observations of rocks make it possible to present a description of rock quality [2,6]. The rock mass was divided into six classes. All classes depend on endoscopic characteristic parameters: class I – intact rock mass – excellent quality (separation to several millimetres, range of fracture up to 0.5 m, total number of joints less than 10). class II – block rock mass – very good quality (separation up to 20 millimetres, range of fracture zone up to 1.5 m, total number of joints less than 25), class III – slightly jointed rock mass – good quality (separation up to 80 millimetres, range of fracture zone up to 2.5 m, total number of joints less than 40), class IV – jointed rock mass – average quality (separation from 80 to 150 millimetres, range of fracture zone to 4.0 m, total number of joints less than 70); class V – crushed rock mass – poor quality (separation from 150 to 250 millimetres, range of fracture zone up to 6.0 m, total number of joints less than 100), class VI – completely crushed rock mass – very poor quality (separation more than 250 millimetres, range of fracture zone above 6.0 m, total number of joints more than 100). These classes of rock mass may be increased or decreased in the case of the occurrence of a considerable number of fracture zones or open joints filled with rock waste. From the five distinguished parameters in paragraph 2 – first of all three of them are needed to evaluate the rock mass quality in ERMF classification.
4. APPLYING OF ERMF
CLASSIFICATION The rock mass quality evaluation using ERMF factor is still in the trial method. The authors, however, applied the classification in several selected underground roadways in coal mines. There are usually two boreholes through the roof in the middle of the span in the roadway approximately 100 to 200 m apart. Generally, the pictures of discontinuities in both boreholes are similar. Rock mass heterogeneity and anisotropy, particularly the change of rock layer thickness, caused differences in the range of crack zones and the type of discontinuities around the workings. Table 1 presents the results of endoscopic observations for the six roadways: gateroad B-7, center drift Z3, rise heading III-E1, inclined drift Izn, inclined roadway B-1 and gateroad B-3. Investigations took place in four different coal mines. The roadways were selected in terms of various qualities of rock mass: from very poor to very good. The depth of roadway and the length of the borehole were alsoshown in Table 1. On the basis of the observation analysis, it may be concluded that the best quality is represented by the rock masses surrounding gateroad B-7 and center drift Z3, which were described as block rock masses (very good quality). It is interesting that in gateroad B-7 the dominant type of discontinuitonly was cracks within both fracture zones and for single random cracks. There were cracks and open joints and only a single fracture zone around the center drift Z3 roof. The very low total number of joints (3 and 12) and the similar height of fracture zones (1.1 m and 1.45 m) allowed the evaluation of the quality of the rock mass in the same way. Slightly worse quality for the rocks in the premises of the inclined drift Izn is caused by the very far range of fractured zone, compared to the previous one. The height of 4.4 m notwithstanding only 10 single joints classified the rock mass in this case as slightly jointed. The rocks at the gateroad B-3 were also evaluated as slightly jointed. Comparing the parameters values one can notice that in spite of therange of the fractured zone which is 2.0 m lower, the other parameters increased and the dominant type of fractures were the same (only open joints).
Table 1. Evaluation of rock mass quality on the basis of endoscopic observations Roadway Parameter
gateroad B-7
center drift Z3
rise heading III-E1
inclined drift Izn
inclined roadway B-1
gateroad B-3
860
900
980
1000
780
930
8.0
7.0
8.0
6.0
7.1
7.0
12
3
101
10
19
19
14
11
298
22
38
72
1.45
1.1
7.8
4.4
4.9
2.4
1 zone+ single
single
completely fractured
single
1 zone + single
3 zone + single
Dominant type of fracture
cracks
cracks and open joints
Rock mass description
Block rock mass
Block rock mass
Quality evaluation
very good
very good
Depth of roadway H[m] Length of testing borehole Number of joints Total separation Ss [mm] Range of fracture zone fz [m] Number of fracture zones
The most important factor in determining the rock mass quality as fair or poor is joints filled with gouge (waste rock). They show that the rocks crush easy, implying that their strength and the joint contact roughness are alsolow. In addition to gouge, there may be pieces of rocks between the joint faces. There is a photo of such a case presented in Fig. 4.
Fig.4 Open joint with gouges
open joints and filled open joints Considerably crushed rock mass
open joints
cracks and filled open joints
open joints
Slightly jointed rock mass
Jointed rock mass
Slightly jointed rock mass
very poor
good
average
good
Rock mass around the rise heading III-E1 is typically crushed considerably. Nearly 30 cm of separation on the 7.0 m run with 101 joints and completely fractured rock structure indicates very poor quality of the rock mass. The endoscopic observation examples shown above suggest that the ratio of rock damage in vicinity of the roadway can be precisely determined using this method. Overall if the number of joints and number of fracture zones are relatively small, the total separation of roof layers does not exceed 20 mm. However, if there are dozens or more than a hundred joints, the total separation increases to as high as 200-300 mm. The high range of fracture zones doesn’t have to influence the number of joints and total separation of discontinuities. The description of rock mass quality is rather easy after the short time of experience.
5. ROCK MASS QUALITY AROUND THE ROADWAYS AND THEIR SUPPORT
evaluating RMR index. The core log is often crushed too much during its transportation.
From the engineering point of view the quality of the rock mass should be relative to the support of the working. The higher the quality the lower capacity of load bearing support one can build.
The interesting example is the gateroad B-7, where the bore core analysis pointed only 24 % RQD (poor rock mass), Q index as average, RMR system as fair and ERMF factor as very good. In this case perhaps the borehole drilling technology or improper transport of the bore core were the reasons of its very poor quality. It influences not only the RQD index but most of all the value of RMR.
There were different schemes of mining support in the chosen workings. Because the scheme of support is sometimes not suitable to geologicalmining conditions, we compared the rock mass quality and the types of support in the above mentioned roadways. For verification purposes, the endoscopic method was compared to the wellknown rock mass classifications of Bieniawski’s classification (RMR system), Barton’s classification (index Q) and Deere’s classification (index RQD). Table 2 present the results of the evaluation of rock quality obtained from the application of the four selected evaluative methods in the analysed workings. The additional line shows the type of support in the working and the next line the influence of the longwall face for the working and its support. One difficulty in comparing the different techniques is that each has a different number of rating categories: Q index has nine, RQD has five, RMR has six and ERMF has six. For the purpose of easy comparison, it was indicated in brackets which category in the particular scale was corresponding to the rock mass around the roadway. It was assumed that the highest values of the number in the bracket correspond to the best quality. It could be assumed that applying these classifications for the sake of evaluation of rock mass quality in the same area of testing should give similar results, but it was not true [8,9]. On the basis of the above comparison, it may be concluded that in general the lowest quality ratings are given by the RQD method. Rock mass quality estimated by RQD is usually one class lower than this evaluated by RMR. The reason is that it is only one out of six factors influencing RMR’s final value Generally the ERMF index corresponds with both the RMR and Q systems well. The evaluation of rock quality according to the Q index is quite similar to the endoscopic observations more than of the RMR. After the authors’ experience, the number of rock pieces of the core logs is the most essential factor in
The factors connected with drilling technology and sometimes with the picture sharpness in endoscopic research show that carrying out the evaluation of rock mass quality using only one selected method may be unsatisfactory for a proper determination of the rock mass class, as well as for launching the mining or geomechanical works in the analyzed area. Taking into consideration the type of support used in above mentioned workings one can say that it was designed without rock mass quality analysis but in the other ways. There was roof bolting in rise heading III-E1 where one obtained the worst results. Gateroad B-7 and center drift Z3 used special supports using wire-strand cable bolts (there were shales and mudstones in gateroad B-7 roof and sandstones in center drift Z3 roof). The rock mass surrounding these workings were evaluated as good whereas that around III-E1 was fair. If the ERMF evaluations were accurate, it seems that support in the second working was overdesigned. The same conclusion may be reached after analyzing the rock mass quality around inclined drift Izn and inclined drift B-1. The type of support - arch yielding steel sets with the roof bar reinforced by resin bolts, is designed for good rock mass (inclined drift Izn) and with no roof bolting for fair to average rocks (inclined drift B1). Taking into consideration that no drift was influenced by the longwall face, one can assume that the schemes of the support were not planned correctly. Looking at the support scheme in gateroad B-3, which is the same as for inclined drift Izn but with longwall face influence, it seems that the support parameters are improper to the mining conditions. It is worth remarking that the evaluation of rock mass quality in both workings were exactly the same.
Table 2 Comparison of rock mass quality around the analysed roadways and the type of supports Classification RQD index Q index RMR system ERMF factor
gateroad B-7
center drift Z3
rise heading III-E1
inclined drift Izn
inclined drift B-1
gateroad B-3
poor (2) average (5) fair (3)
average (3) good (6) good (4)
very poor (1) extremely poor (2) poor (2) very poor (1)
average (3) good (6) good (4)
poor (2) poor (4) fair (3)
average (3) good (6) fair (3)
good (4)
average (3)
good (3)
very good (5) very good (5)
Type of support
arch yielding steel sets + wire-strand cable bolt between the arch
arch yielding steel sets + wire-strand cable bolt between every 2nd arch
bolting (resin bolts)
arch yielding steel sets + resin bolts
arch yielding steel sets
arch yielding steel sets + resin bolts
Successful of support stability
yes
yes
no
yes
yes
yes
Longwall face influence
yes
yes
yes
no
no
yes
Thus the next investigations underground should be focused on describing the rock mass quality in different ways and comparing the rock mass quality with the support schemes. Such an analysis will enable the design of more appropriate working supports and help to avoid oversizing them.
6. CONCLUSIONS 1. The classification of rock mass quality introduced on the basis of endoscopic observation (rock mass quality index ERMF) can be applied for the proper evaluation of rock mass quality. The results are comparable to those obtained from the application of the commonly known indexes: RMR, RQD and Q. 2. Rock mass quality evaluation using only one selected method may be unsatisfactory for a proper determination of the rock mass class. 3. In order to appropriately design working supports, two or three rock mass quality indexes should be evaluated. It will help optimize the support schemes. 4. One may assume that schemes of underground workings’ support are designed according to operating rules rather than to engineering experience. It is possible to turn the loadbearing capacity of the support to good account
in nearly any classifications.
cases
using
rock
mass
ACKNOWLEDGEMENT The support of the Ministry of Science and Higher Education under research grant 4 T12A 00229 is greatly appreciated.
REFERENCES Hoek E. 2007. Rock Engineering. Rocscience Inc. www. rocscience.com. Majcherczyk T., Małkowski P., Niedbalski Z. 2006. Ruchy górotworu i reakcje obudowy w procesie niszczenia skał wokół wyrobisk korytarzowych na podstawie badań ,,in situ. Kraków. Monograph AGH UST. Faculty of Mining & Geoengineering. Kuszmaul J. S. 1999. Estimating keyblock sizes in underground excavations: accounting for joint set spacing. International Journal of Rock Mechanics and Mining Sciences 36: 217-232. Hoek E., Kaiser P.K., Bawden W.F. 1995. Support of Underground Excavations in Hard Rock. A.A. Balkema, Rotterdam/Brookfield. Sheorey P.R. 1993. Experience with the Application of Modern Rock Classifications in Coal Mine Roadways. Comprehensive Rock Mechanics, Principles, Practice & Projects vol.5: 411-431, Pergamon Press.
Singh B., Goel R.K. 1999. Rock Mass Classification. A Practical Approach in Civil Engineering. 1st ed. Amsterdam. Elsevier. Milne D., Hadjigeorgiou J., Pakalnis R. 1998. Rock mass characterization for underground hard rock mines. Tunnelling and Underground Space Technology vol. 13, is. 4: 383-391. Elsevier. Majcherczyk T., Małkowski P. 2002. Badania szczelinowatości skał stropowych endoskopem otworowym. Bezpieczeństwo Pracy i Ochrona Środowiska w Górnictwie WUG no. 2: 6-12. Małkowski P. 2003. Badania endoskopowe dla określania jakości skał. Kwartalnik AGH seria Górnictwo no 3-4: 419-425.
