Rock Mass Strength Characteristics at Manjung Area

ICCBT2008 Rock Mass Strength Characteristics at Manjung Area N. I. Mohd Pauzi*, Universiti Tenaga Nasional, MALAYSIA R. Che Omar, Universiti Tenaga Nasional, MALAYSIA R. Roslan, Universiti Tenaga Nasional, MALAYSIA ABSTRACT The geology at Manjung area mainly consists of Granitic rock overlain with some residual soil. The area largely dominated by biotite granite with texture range from fine to coarse type of texture. Boulders of granite can also be found with size up to 3 meter wide. This paper aims to present the rock strength characteristics to be used for slope remedial measures at Manjung area. There are great importance needs to repair the slope at Manjung area because most of the transmission towers are located on top of the slope which can collapse if left unattended. Thus, study need to be done at the area in order to solve the problems. One of the important criteria to be studied is the rock strength characteristics which are needed for slope analysis and design. Rock strength characteristics involve in the design of rock slope is tested using Direct Shear Strength, Indirect Tensile Strength test, Point Load Index test and Uniaxial Compression (UCS). The friction angle and cohesion value are determined from direct shear test. The value of friction angle at Manjung area is found to be in the range of 32 degrees to 34 degrees whilst cohesion value is in the range of 1.2 MPa to 1.6 MPa. The result from indirect tensile strength shows that the tensile strength is in the range of 7.36 MPa to 15.90 MPa. The point load test is done to be used as an index test for strength classification of rock material. The laboratory testing of point load test gives the values of the strength of rock at Manjung area to be in the range of 7.16MPa to 10.41 MPa. UCS test is used to determine the unconfined compressive strength of rock. The unconfined compressive strength of rock at Manjung area is in the range of 54.4 MPa to 98.8 MPa. These values are important to be used in rock mass rating for determining the cohesion and friction angles of the rock mass. Keywords: Rock mass strength, rock mass classification system, friction angle and cohesion *Correspondence Author: Nur Irfah Mohd Pauzi, Universiti Tenaga Nasional, Malaysia. Tel: +60389212020 ext 6254, Fax: +60389212116. E-mail: irfah@uniten.edu.my

Rock Strength Characteristics at Manjung Area 1. INTRODUCTION Determination of rock mass strength characteristics such as uniaxial compression load, tensile stress, point load index and direct shear stress are crucial in rock slope stability analysis. The area of Manjung consist of moderately to highly weathered granite which are not easy to classified based on the rock weathering classification system [2][10]. There are also boulders up to 3 meters wide which located near the transmission tower are also considered in the analysis of rock slope stability. This paper is intended to give the rock mass strength characteristics at that area where transmission tower are located at the top of the slope. 2. BASIC THEORY Rock engineering has been established for thousand years, and it is likely that this branch of engineering extends back hundreds of thousands years. Rock masses are natural, rock masses are not fabricated according to project requirements, and the properties have to be established by site investigation and laboratory testing. Moreover, rock masses are fractured, often in complex ways [5]. The fractured rock immediately indicates the need for special techniques to account for the fractures in any engineering design for construction on this rock. An approach in laboratory testing of rock takes into account the very special nature of rock as an engineering material, inherently incorporates the current knowledge based, can accommodate any rock engineering projects such as transmission towers objective [7]. There are four ways of determining the rock strength characteristics of the rock such as direct shear testing, indirect tensile testing, point load test and uniaxial compression test. These tests has their own purposes such as to determine the shear strength of the rock, to determine the tensile strength, to determine the point load that is the highest load the rock can bare before it collapse and to determine the uniaxial compressive strength of the rock. These rock strength properties suit the need for the construction of the transmission towers. The transmission tower is at high risk if the slope is left unattended and no assessment has been carried out [8] Rock slope stability analysis based on Hoek and Bray criterion suggested that the factors that might affect the rock slope stability are the groundwater table, geometry of the slope, the fracture or discontinuity pattern on the rock surface, and the rock strength of the slope [3]. Therefore, the selection of sample for strength characteristics are of important since the change in the shear strength can result in significant change in the safe height or angle of slopes [9]. The Manjung area has become the study area for determining the rock strength characteristics since this site has history of various rock slope failure where transmission tower are at risk of tripping. 3. SITE GEOLOGY AND TOPOGRAPHY The geology of site was evaluated based on field reconnaissance survey and on review of geological map. The site is located in the main range granite and underlain by in-situ soil originating from the parent rock itself. Most of the fracture and joint of the rock mass has clay infilling. The area itself is covered by residual soil and the thickness of the soil varies. The inspection reveals that the site is largely dominated by biotite granite with texture range from 404

N. I. Mohd Pauzi et. al. fine (aphanites) to course (phanerite) type of texture. The locations of transmission tower at Manjung area are shown in Figure 1. The topography of the area consists of 20% hilly area and 80% flat land area. The critical area is the hilly area where the slopes are prone to landslides. There are 35 towers but most of the samples for testing are collected at Tower 5, Tower 6 and Tower 15. Only three towers are considered for rock testing because these three towers have high risk to fail compared to other towers which lies on the flat land area. Figure 1 Transmission tower located at Manjung area. The yellow color show the highest contour and the dark green color show the lowest contour of the site. The point in red color indicates the location of the transmission tower. 4. BOREHOLE DATA The deep boring is carried out at Tower 5, Tower 6 and Tower 15. A total of 4 boreholes have been done at the area. The details of the boreholes are shown in Table 1. The locations of the boreholes are shown in Figure 2. 405

Rock Strength Characteristics at Manjung Area Table 1 Number of boreholes located at the transmission tower Tower No. of Boreholes 5 1 6 1 15 2 (Tower 15A & T15C) Figure 2. The locations of borehole at Tower 5, Tower 6 and Tower 15. The soil at the area consists of mainly silty sand near the surface. As the borehole goes deeper, granite rock are encountered. In the middle of the borehole log, some gravel layer can be identified. The samples collected from the deep boring are tested only for the rock strength characteristics. The depth of granite strata at Tower 5, Tower 6, Tower 15A and Tower 15C are 7.5 meter, 5 meter, 4.5 meter and 5 meter respectively. The granite rock is the fresh rock with RQD ranges from 41% to 86%. The details strata of soil profile from the borehole log data are shown in Figure 3. 406

N. I. Mohd Pauzi et. al. Figure 3. Soil strata from borehole log profiles 5. EXPERIMENTAL PROGRAM Four series of experiment are carried out at Tower 5, Tower 6, Tower 15A and Tower 15C. The experiment involves are direct shear testing, Brazilian test, point load test, and uniaxial compression test. All of the experiment procedure are based on ISRM [4] approach. The sequence of rock test starts with point load test, Brazilian test, uniaxial compression test and lastly is direct shear testing. There are 61 samples tested for various test. The detail number of samples for each test is shown in Table 2. 407

Rock Strength Characteristics at Manjung Area Table 2. Number of samples for each test Borehole No. of Samples (Tower) Brazil PI UCS Direct Shear 5 5 9 4 1 6A 5 6 3 2 15A 4 1 2 2 15C 4 7 3 3 Total 18 23 12 8 5.1 Direct Shear ing Direct shear testing is done to determine the friction angles and cohesion values. These parameters will determine the factor of safety of the slopes and the angle of the slopes. Direct shear testing is done by cutting the rock samples into half. The samples are cast with plaster of Paris, so that it can hold the samples inside the box. Normal stress and shear stress are applied on the sample. The shear displacement data is recorded with different applied normal load. The normal load applied is 20 kn, 30 kn and 40 kn which is the highest load the pressure pump can exert on. 5.2 Brazilian Brazilian test is to determine the indirect tensile strength of the rock by diametrical line compression of a disk form specimen. The tensile load is applied on the samples until the samples breaks into two. This applied load is regarded as the highest load that the samples can bear before failure. The load on the specimen must be applied continuously at a constant rate such that failure occurs within few minutes. The actual load depends on the strength of the material and the size of specimen and may vary. 5.3 Point Load The point load strength test is intended as an index test for the strength classification of rock material. It may also be used to predict other strength parameters with which it is correlated for example uniaxial compressive strength and tensile strength. Rock specimen in the form of core (diametrical test) is broken by a pair of spherical truncated, conical platens. 5.4 Uniaxial Compression The aim of uniaxial compression test is to determine the unconfined (or uniaxial) compressive strength of rock specimens of cylindrical form. The test is mainly intended for use in strength classification and characteristics of rock. The loads on the specimen are applied continuously at a constant stress rate such that failure will occur within 5 to 10 minutes of loading. 408

N. I. Mohd Pauzi et. al. 6. EXPERIMENTAL RESULT The experimental result from the direct shear testing, Brazilian test, point load test and uniaxial compression test are described based on the location of the samples taken from deep boring with respective depth. 6.1 Tower 5 The quality of the rock at tower 5 is good quality with grade of weathering as grade II. The Brazilian (indirect tensile) test shows the highest value which is 13.5 MPa at the depth of 14.00 to 15.50 meter. Whilst the lowest value of Brazilian test is 7.36 MPa at the depth of 15.50 to 17.00 meter. The point load test gives the result of 7.16 MPa the lowest and 10.41 MPa the highest. The highest point load value is at the depth of 12.50 to 14.00 meters. The loading of 54.4 MPa to 98.8 MPa are the uniaxial compression load the rock could retain before it fails. This indicates a good sign of a good quality of rock. The direct shear test result gives the values of friction angles and cohesion values of 11 degrees and 1.2 MPa respectively. The details of the test are shown in Table 3. Table 3. Rock test result at Tower 5 Depth Weathering (m) Grade Brazilian Type of test PI UCS Direct Shear (MPa) (MPa) (MPa) (%) 12.50-14.00 II 7.59 8.88 98.8 c = 1.2MPa 89.3 Good 8.35 7.16 35.6 Φ = 11 9.58 10.41 9.06 7.24 7.71 14.00-15.50 II 3.90 6.77 83.7 86.7 Good 13.50 7.32 54.4 15.50-17.00 II 7.36 80.0 Good 6.2 Tower 6 RQD Description (RQD) The rock quality at tower 6 is rated from poor to excellent based on the RQD description. Although the grade of weathering of the rock at the area are the same which is grade II (slightly weathered) but the testing result shows different values. The Brazilian (indirect tensile) test gives the highest values of 15.9 MPa for the rock sample taken at the depth of 11.50 meter to 13.00 meter. The lowest value is 9.66 MPa which is also obtained at the same level of depth. The point load test gives the result of 3.68 MPa to 8.83 MPa. The cohesion and friction angle values obtained from direct shear test are 0 MPa and 20 degrees respectively. The Uniaxial compression strength is 89.9 MPa at the depth level of 14.5 meter to 16 meter and 62.8 MPa at the depth level of 13 meter to 14.5 meter. The details of the rock test result are shown in Table 4. 409

Rock Strength Characteristics at Manjung Area Table 4. Rock test result at Tower 6 Depth Weathering Grade Brazilian (m) (MPa) 11.50-13.00 II 13.07 15.90 9.66 Type of PI UCS Direct Shear (MPa) (MPa) 7.88 - c = 0 MPa Φ = 20 RQD Description (RQD) (%) 69.3 Fair 13.00-14.50 II 10.28 8.83 62.8 100 Excellent 14.50-16.00 II - 3.76 6.60 3.68 8.71 89.9 80.7 Good 16.00-16.50 II 13.10-63.5 44 Poor 6.3 Tower 15 A At tower 15A, the rock can be described as poor to fair quality based on the RQD description. The qualities of rock are poor because of the grade of weathering which is grade III (moderately weathered). The Brazilian (indirect tensile) test is at the highest values at 13.07 MPa and the lowest value of 6.45 MPa. The point load test gives the result of 8.42 MPa. The uniaxial compression strength value is the highest at the depth 4 meter to 5.5 meter with the value of 75.3 MPa. The cohesion and friction angle values obtained from direct shear test are 1.6 MPa and 5 degrees respectively. The detail of the rock test result is tabulated in Table 5. Table 5 Rock test result at Tower 15 A Depth Weathering Grade Type of PI UCS Brazilian Direct Shear (m) (MPa) (MPa) (MPa) (%) 1.00-2.50 III 13.07 8.42 - c = 1.6 MPa 41.3 Poor 2.50-4.00 III 6.45-44.8 Φ = 5 54.7 Fair 6.82 4.00-5.50 II-I 8.81-75.3 67.7 Fair RQD Description (RQD) 6.4 Tower 15 C The rock at tower 15C can be described as fair to good quality. The percentage of RQD ranges from 74.7% to 86%. The grade of weathering is grade II (slightly weathered) to grade III (moderately weathered). The Brazilian test shows the highest value which is 6.29 MPa at the level of depth of 7.80 meter to 8.30 meter. The point load test gives the result which does not differ so much that is in between 2 MPa to 3 MPa. The uniaxial test which in the range of 19.3 MPa to 22.3 MPa. The cohesion and friction angle values obtained from direct shear test are 3.1 MPa and 5 degrees respectively. The detail of the rock result is tabulated in Table 6. 410

Table 6 Rock test result at Tower 15 C Depth Weathering Grade Brazilian (m) (MPa) (MPa) 3.30-4.80 III 5.48 3.29 3.13 2.37 2.08 Type of PI UCS Direct Shear N. I. Mohd Pauzi et. al. RQD Description (RQD) (MPa) (%) 19.3 c = 3.1 MPa 86 Good Φ = 5 4.80-6.30 III - - - 74.7 Fair 6.30-7.80 III 2.78 2.68 22.3 74.7 Fair 2.03 7.80-8.30 II 2.52 6.29 2.71 21.5 86 Good 7. ROCK MASS CLASSIFICATION OF ROCK TEST RESULT BASED ON BENIAWSKI (1989) The rock test result is classified whether the rock is good quality or poor quality based on the rating developed by [1]. The ratings are based on 5 parameters which are uniaxial compressive strength in MPa, drill core quality (RQD) in percentage, spacing of discontinuities, condition of discontinuities and groundwater. The parameters for rock mass classification come from laboratory testing and site investigation data at Tower 5, Tower 6 and Tower 15. The details of the result from laboratory testing and site investigation that are considered for classification are shown in Table 7. Table 7 Rock Mass classification based on [1] Parameter Tower 5 Tower 6 Tower 15 Uniaxial Compression 50-100 50-100 25-50 Strength (MPa) Rating 7 7 4 Drill core quality (RQD) 75-90 50-75 25-50 (%) Rating 17 13 8 Spacing Discontinuities 200-600 mm 200-600 mm 200-600 mm Rating 10 10 10 Condition of Discontinuities Slightly rough Separation < 1 mm Slightly weathered wall rock Slightly rough Separation < 1 mm Slightly weathered wall rock Slightly rough Separation < 1 mm Slightly weathered wall rock Rating 25 25 25 Groundwater Wet Wet Wet Rating 7 7 7 Total rating 66 62 54 Description Good rock Good Rock Fair Rock Conclusion The cohesion of the rock mass (kpa) = 300-400 kpa The friction angle of rock mass = 35-45 degrees The cohesion of the rock mass (kpa) = 300-400 kpa The friction angle of rock mass = 35-45 degrees The cohesion of the rock mass (kpa) = 200-300 kpa The friction angle of rock mass = 25-35 degrees 411

Rock Strength Characteristics at Manjung Area The rock at Tower 5 and Tower 6 is good rock which is suitable for construction of transmission tower with the cohesion and friction angle range from 300-400 kpa and 35 to 45 degrees respectively. This result is considering the groundwater table at the site and the discontinuity spacing and condition. At tower 15, the rock is fair quality rock. The differences are in the rock quality design and unconfined compression strength of the rock at tower 15 which is slightly lower compared to the other two towers. The rock has been weathered and thus reduced the strength of the rock mass [6] since in Manjung area; the rainfall intensity is quite high. At the site, the soil condition has a history of erosion but there are shrubs and trees have been planted to control the erosion. Therefore, the designer has to be careful in determining the remedial measures for the slope at tower 15 and the friction angle and cohesion values must be in the range of 25 to 35 degrees and 200 to 300 kpa respectively. The picture showing the site condition at tower 15 is shown in Figure 4. Figure 4 Site condition at Tower 15 8. CONCLUSIONS The rock mass strength at tower 5, tower 6 and tower 15 in Manjung area can be concluded to be in fair to good quality based on the rock mass classification system. Rock mass classification system has been widely used for tunneling and rock mining. The rock mass classification is also suitable to be used to determine the quality of rock at the construction area of transmission tower on the rock slope. Rock mass classification method has simplified the work of the slope designer for determining the rock strength parameters such as friction angle and cohesion. This is because the quality of the rock and the shear strength parameters is interrelated with each other. Since the selection of rock mass strength properties are not easy because it has different grade of weathering. The rock mass classification has support the laboratory testing result and site investigation result into a simple method of classification. This will in turn save the cost of the construction. 412

N. I. Mohd Pauzi et. al. Acknowledgments The authors would like to thanks the Management and authorities of Universiti Tenaga Nasional (UNITEN), Tenaga Nasional Berhad Research (TNBR) and Tenaga Nasional Berhad Transmission (TNBT) for their constant support and encouragement. This project research is funded under TNBR and TNBT entitled A slope risk analysis and Assessment for Segari-Ayer Tawar 275 kn Transmission Line. REFERENCES [1]. Beniawski Z.T., 1989, Engineering Rock Mass Classification, Wiley, Witchester [2]. Geological Society of London, 1995, The description and classification of weathered rocks for Engineering Purposes, Geol. Soc. Eng. Group Working Party Report, Q. J. Eng. Geology. 1995: 171: 224-227 [3]. Hoek E. and Bray W.J. 1981, Rock Slope Engineering, 3 rd Edition, Institute of Mining and Metallurgy, London [4]. International Society of Rock Mechanics (ISRM), 1981, Suggested Method for Determining of Uniaxial Compressive Strength of Rock Materials in Rock Characterization, ing and Monitoring, Pergamon Press, London, 1981, 113-120 [5]. Lan, H.X Hut R.L. Yue Z.Q. Lee C.F. Wang S.J. 2003, Engineering and Geological Characteristics of Granite Weathering Profiles in South China, Journal of Asian Earth Sciences 21. [6]. Lumb P., 1962, The properties of decomposed granite, geotechniques, Vol. 12, 226-243 [7]. Muller L. 1959, The European approach to slope stability problems in open pit mines, Proc. Third Symposium on Rock Mechanics, Colorado School of Mines Quarterly, Vol. 54, 3 :116-133 [8]. Omar, H., 2002. Development of Risk Assessment and Expert Systems for Cut Slopes, PhD Thesis, Faculty.of Eng., Uni. Putra Malaysia [9]. Omar, H. Irfah, M.F., Shahril, M.R., Syamsul, M.I., Maail, S. and Azlan, A. Aziz, 2004, Rainfall Induced Landslides at Pos Selim Highway, Proceedings of the Malaysia-Japan Symposium on Geohazards and Geoenvironmental Engineering Recent Advances, Malaysia [10]. Zainuddin Md. Yusoff, Hanisafwa M.H., M. Marzuki, Azlina A.W., Nur Azimah A.H., 2001, Accelerated Weathering of Granite Rocks, Proceedings of 2 nd GEGEU Research Seminar, UPM Publisher, 66-74 413