Estimation of Shear Strength of Prototype Rockfill Materials

Transcription

1 International Journal of Engineering Sciences, 2(8) August 13, Pages: TI Journals International Journal of Engineering Sciences ISSN Estimation of Shear Strength of Prototype Rockfill Materials Sharma Pankaj * 1, Mahure NV 2, Gupta SL 3, Dhanote Sandeep 4, Singh Devender Central Soil and Materials Research Station, New Delhi, India. A R T I C L E I N F O Keywords: Rockfill materials Consolidated drained triaxial shear tests Particle size Strength parameters Breakage factor Extrapolation A B S T R A C T Rockfill material is the most readily available and the most flexible material for the construction of dams especially in the seismic prone regions. The material is obtained either by blasting available rock or is collected from the alluvial deposits of the river. The properties of the materials cannot be ascertained directly due to their large size. The sizes are thus modeled down to reduce the sizes suitable for testing in the laboratory. Further these results are extrapolated to the actual sizes available in the field. Present studies have been carried out to ascertain the validity of extrapolation technique. Materials for the studies were collected from the alluvial deposits of the river and also by blasting available rock. Consolidated drained triaxial shear tests have been conducted on the modeled samples with mm, mm, 6 mm and 8 mm maximum particle size. The test data have been analyzed to obtain shear parameters for sample with 8 mm maximum particle size by extrapolating results of shear parameters for sample with, and 6 mm maximum particle size. The extrapolated value is compared with the actual shear parameter value for sample with 8 mm maximum particle size obtained experimentally. 13 Int. j. eng. sci. All rights reserved for TI Journals. 1. Introduction Construction of dams using rockfill (RF) material is becoming increasingly more common because of self realigning capacity of the materials. This makes RF dams more stable even in the seismic zones. The abundant availability of such materials in the vicinity of proposed dam construction sites makes dams construction more economical. The materials consist primarily of angular to sub-angular particles obtained by blasting parent rock (QRF) or rounded / sub-rounded particles obtained from terrace alluvial deposits of the river (RBRF). Testing and understanding the behaviour of the rockfill material have been a challenging task to geotechnical engineers. Some kind of modeling technique is often used to reduce the size of particles so that the samples prepared with smaller size particles can be tested. The material parameters obtained from the tests are generally used to get the parameters for large size prototype material by extrapolation. 2. Review The behaviour of rockfill materials is often studied by performing triaxial tests on modeled rockfill materials because actual rockfill material is large in size. Modeling techniques viz. scalping technique [1], parallel gradation technique [2], quadratic grain size distribution technique [3] and replacement Technique [4] are available for testing of rockfill materials. Investigations conducted on rockfill materials indicate that the magnitude of the particle breakage during loading is directly related to maximum particle size. These empirical factors either cause the variation of grain diameter [5, 6] or the shift of the whole grain size distribution curve [7, 8]. The results of tests conducted by Marachi [9] show a variation of the angle of internal friction with the grain size distribution. High pressure causes considerable particle breakage [6, 8,, 11, 12, 13, 14, 15]. There are several factors that affect the amount of particle breakage in a geological material [5, 8, 16, 17, 18, 19,, 21, 22, 23, 24, 25] 3. Material used for testing QRF and RBRF materials are collected to prepare sample with maximum particle size d(max),, 6 and 8 mm. 3.1 Quarried Material from Kol Dam Project, H.P. The parent rock at this project site is either dolomite or limestone. The QRF materials (Figure 1) possess the following mechanical properties; Uniaxial Compressive Strength : 37.1 MPa to.4 MPa Los Angeles Abrasion Value : 48.1% Aggregate Impact Value : 42.3% Aggregate Crushing Value : 41.2%. * Corresponding author. address: pan2256@gmail.com

2 422 Sharma Pankaj et al. International Journal of Engi neering Sciences, 2(8) August 13 Figure 1. Quarried material Figure 2. River bed material 3.2 River Bed Material from Kol Dam Project, H.P., (RBRF) The material mainly composed of quartzite and dolomite/ lime stone. The RBRF materials (Figure 2) possess the following mechanical properties; Uniaxial Compressive Strength : 42.7 MPa to 5.1 MPa Los Angeles Abrasion Value : 44.2% Aggregate Impact Value : 39.8% Aggregate Crushing Value : 37.9%. 4. Methodology and experimental investigations Out of all the modeling techniques, parallel gradation technique is considered to be more appropriate and as such it is widely used [26]. Four modeled gradation curves (Figure 3 and Figure 4) having a maximum particle size (d max ) of,, 6 and 8 mm both for QRF as well as RBRF are derived using John Lowe s parallel gradation modeling technique. Using these modeled grain size distribution curves, the required quantities of various fractions of rockfill materials have been calculated. Consolidated drained triaxial tests have been conducted using a large size triaxial shear apparatus at 3, 6, 9 and 12 kg/cm 2 confining pressures on the modeled materials. Consolidated drained (CD) triaxial tests have been conducted on the modeled rockfill materials. A specimen size of 381 mm diameter by 813 mm long was used for testing. For testing, a dry density corresponding to 87% of relative density was adopted. Samples were isotropically consolidated under four different confining pressures (σ 3 ), kg/cm 2 in the range between 3 kg/cm 2 to 12 kg/cm 2 for each modeled rockfill material. In accordance with the modeled gradation curves the total quantity of various fractions of rockfill materials required to achieve the specified density was determined by weight. The computed quantity of fractions was thoroughly mixed and moistened with 3 to 4% water by weight for maintaining reasonably uniform composition as per physical observations based on pilot test. The mixed sample was compacted in a split mould in six equal layers by vibratory compaction. The sample was saturated by allowing water to pass through the base of the triaxial cell and using a top drainage system for removing air from the voids. CD tests were conducted. Kol H.E. Project, H.P. Quarried Material 6 8 Percent finer (%) Grain size (mm) Figure 3. Modeled gradation curves for QRF material

3 Estimation of Shear Strength of Prototype Rockfill Materials Internati onal Journal of Engineeri ng Sciences, 2(8) August Kol H.E. Project, H.P. River Bed Material 6 8 Percent finer (%) Grain size (mm) Figure 4. Modeled gradation curves for RBRF material 4.1 The Apparatus The tests are conducted using triaxial cell of 381 mm dia and 813 mm height. The sectional elevation of triaxial cell is shown in Figure 5. The test assembly includes axial Loading Unit, Confining Pressure Unit to maintain and control the applied confining pressure through an air-water pressure system and strain measurement unit. To isolate the sample from cell water two sleeves (1 mm thick inner sleeve and 3 mm thick outer sleeve) are used. Figure 5. Sectional elevation of triaxial cell

4 424 Sharma Pankaj et al. International Journal of Engi neering Sciences, 2(8) August 13 Drained Triaxial Tests Preparation and Setting up of Specimen In accordance with the model gradation curves, the total dry weight required for achieving 87% of relative density are computed for each of the specimens to be tested. The quantity of individual fractions is moistened with 4-5% moisture, thoroughly mixed and packed in 6 layers inside a split mould using vibrating compactor. The specimen is saturated by allowing the water to pass through from the base connection of the cell and using the top drainage system for removing the air voids till full saturation is achieved. The cell is filled with water by connecting it to the water supply source. The bottom water outlet of the specimen is connected to burette for volume change measurement. Consolidation and Shearing the specimen Constant confining pressure is generated using air-water pressure system. Sample is then consolidated at desired confining pressure. The deviatoric load is applied on the specimen using hydraulic pressure unit under drained condition at a low constant rate of deformation of 1 mm per minute till failure. 5. Experimental Results The p q plots for QRF and RBRF materials for the samples with d(max),, 6 and 8 mm for are presented in Figure 5 and 6 respectively. 6. Discussion of Results The angle of internal friction decreases with increasing particle size for quarried material while increases with particle size in case of river bed material [23, 27, 28]. From figures 5 and 6 it is observed that the value of the angle of internal friction (Ø), in degrees for QRF materials is found to decrease from 45.8 o to o as the d(max) increased from to 8 mm while for RBRF material it is found to increase from 44.9 o to o as the d(max) increased from to 8 mm. 6.1 Angle of Internal Friction for Prototype Material by Extrapolating the Experimental Results The values of angle of internal friction obtained for tested samples with d(max), and 6 mm for QRF and RBRF are plotted on a loglog scale and are presented in figure 8 and 9 respectively. Equation for the power line for both QRF and RBRF is obtained which is applied to predict Ø value for sample with d(max) f 8 mm. The same are presented in figure 8 and Comparison of Experimental and Predicted Value of Ø From table 1 it is evident that the difference in the predicted and experimental value of Ø is less than 1% in both cases. 5 3 d(max) = mm y = Ø = 45.8 degree Average Normal Stress, 5 3 d(max) = mm y =.6949x Ø = 44.2 degree Average Normal Stress, 5 3 d(max) = 6 mm y =.698 x Ø = degree Average Normal Stress, 5 3 d(max) D(max) = 8 mm y =.6819x Ø = degree Average Normal Stress, Figure 6. p q plots for samples with d(max),, 6 and 8 mm QRF material

5 Estimation of Shear Strength of Prototype Rockfill Materials Internati onal Journal of Engineeri ng Sciences, 2(8) August d(max) = mm y =.759 x Ø = 44.9 degree Average Normal Stress, 5 3 d(max) = mm y =.7312 x +.7 Ø = degree Average Normal Stress, 5 3 d(max) = 6 mm y =.7444 x Ø = degree Average Normal Stress, 5 3 d(max) = 8 mm y =.7565 x Ø = degree Figure 7. p q plots for samples with d(max),, 6 and 8 mm RBRF material Average Normal Stress, Angle of Internal Friction, degree Ø Experimental [for d(max) = 8 mm] = degree Ø Predicted [for d(max) = 8 mm] = degree y = x Particle Size, mm Figure 8. Extrapolation of Ø values for modeled QRF material Angle of Internal Friction, degree Ø Experimental [for d(max) = 8 mm] = degree Ø Predicted [for d(max) = 8 mm] = 49.8 degree y = x Particle Size, mm Figure 9. Extrapolation of Ø values for modeled RBRF material Table 1. Percent drift in the experimental and predicted value of Ø Material D(max) = 8 mm Ø (experimental) Ø (predicted) % drift QRF o o.63 RBRF o 49.8 o.47

6 426 Sharma Pankaj et al. International Journal of Engi neering Sciences, 2(8) August Conclusion Prototype rockfill material generally contain particle of large dimensions. There is no direct method to obtain strength parameters as the laboratory testing facilities are inadequate to test for such materials. As such the approach of extrapolation for predicting strength parameters is being widely followed. Above study clearly shows that the difference in the predicted and actual experimental value obtained for sample with d(max) of 8 mm is less than 1%. As such the extrapolation technique provides a promising method for prediction of values of Ø for prototype rockfill materials. Acknowledgements The authors extend sincere thanks to Director, Central Soil and Materials Research Station, New Delhi for the constant encouragement extended by him. We gratefully acknowledge and thank the Rockfill Division of CSMRS, New Delhi and the authorities of Kol Dam Project, Himachal Pradesh, India for the support extended by them. We extend our sincere thanks to Dr. N.P. Honkanadavar, Scientist D CSMRS and Dr. Sameer Vyas, Assistant Research Officer, CSMRS for extending valuable suggestions from time to time. Sincere gratitude are extended to all the authors whose publications provided us directional information from time to time. References [1] Zeller, J. and Wullimann, R. (1957). The Shear Strength of the Shell Materials for the Go-Schenenalp Dam, Switzerland. Proc. Fourth Int. Conf. on SMFE, London, 2, [2] Lowe, J. (1964) Shear Strength of Coarse Embankment Dam Materials, Proc. 8th International Congress on Large Dams, 3, [3] Fumagalli, E. (1969). Tests on Cohesionless Materials for Rockfill Dams. J. of SMFE, ASCE, 95, SM 1, [4] Frost, R.J. (1973). Some Testing Experiences and Characteristics of Boulder-Gravel Fills in Earth Dams. 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