Experimental Study of Basic Road Performance of Weathered Granite Soil

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1 1 st International Conference on Transportation Infrastructure and Materials (ICTIM 216) ISBN: Experimental Study of Basic Road Performance of Weathered Granite Soil Xirong Niu 1,2, Yanfang Sun 3,4, Zekun You 1 and Pengyan Bi 1 1 Department of Civil Engineering, Shanxi University, Taiyuan 313, China; niuxirong@sxu.edu.cn 2 Key Laboratory for Road Structure and Material of Ministry of Transport, Chang an University, Xi an 7164, China; 3 College of Architecture and civil engineering, Taiyuan University of Technology, Taiyuan 324, China; sunyanfang@tyut.edu.cn 4 Key Lab of Highway Construction & Maintenance Technology in Loess Region, Ministry of Transport, Taiyuan 36, China ABSTRACT: Weathered granite soil is abundant in China and it is used extensively as a fill material. However, systematic research on this material is relatively less as compared to sedimentary type of soils like sand and clay. In this study, a series of laboratory tests were performed, including X-ray diffraction test, sieving test, liquid and plastic limit test, compaction test, California bearing ratio (CBR) test and large-scale triaxial test, to study the basic road performance of weathered granite soil. Specially, in order to evaluate the effects of clay content on the mechanical properties of the compacted weathered granite soil, the extra compaction test and CBR test were made by using samples with the four different clay contents of 4%, 8%, 1% and 12% by weight. Results of the laboratory tests indicated that the clay content of 8% could be a good indicator to determine the compaction characteristics of weathered granitic soil with different clay contents. Thus, the clay content of about 8% would be advantageous to the compaction rate of subgrade and the stability of roads. However, The California bearing ratio of weathered granite soil with clay content of about 4% is largest, which is 95%. Though large-scale triaxial test, it is found that the particle breakage leads to the obvious characteristic of strength nonlinearity of the granite weathered soil. Keywords: subgrade engineering, strong weathered granite, road characteristics, test INTRODUCTION Granite is a kind of igneous rocks which is formed by Magma in inside the earth,and the main components of igneous rocks are quartz, feldspar, mica and hornblende (Lu et al., 22). Granite shows a full crystal grain structure, hard texture and uniform property, and high compressive strength (12-2 MPa). Because feldspar and mica exist joints, the granite have three groups of primary joints, and because the expansion coefficient of quartz and feldspar is difference nearly doubled, the granite surface easily cracks in the process of expansion and contraction, so the granite, especially coarse grain structure granite, weathered easily (Wu et al., 24). Granite is influenced by the long-term geological tectonic movement and climate changes in the special geographical environment. Because of the different physical and chemical properties of granite mineral composition, granite has been weathered in different degrees of weathering, and decomposed granitic soils are generally classified as weak weathering, weathering, strong weathering and residual (WCHEG, 27). Weathered granite covers more than 9% from the land area of China (Cui et al., 27). Ease of availability means that these soils are usually used as construction materials such as 685

2 back fill or highway subgrade materials as a result of the scarcity of better materials (Li et al., 22; Guo et al., 29; Yang et al., 29; Xiao et al., 21; Niu, 213). Some researchers have conducted a series of studies and have preliminarily understood the engineering characteristics of weathered granite (Lee et al., 1995; Ng et al., 24; Ham et al., 21), but there has been a lack of comprehensive understanding about the basic road performance of weathered granite soil in previous studies. In this study, besides specific gravity test, X-ray diffraction test, sieving test, liquid and plastic limit test, and CBR test, a sires of compaction tests were carried out on claymixed weathered granite soil in order to clarify the influence of clay contents on their compaction behavior. PREPARATION OF SAMPLES The weathered granite soil used in this research was sampled from a site near Yunzhong Mount, a Branch of Luliang Mount, on the eastern outskirts of Xinzhou, Shanxi, China. The weathered granite soil was used in Xinbao (Xinzhou to Baode) highway subgrade. There were three color kinds of weathered granites in the field, including red brown, yellow brown and gray, shown in Fig. 1. The in situ dry unit weight and moisture content were kn/m 3 and %, respectively. 2cm Red brown Yellow brown Gray BASIC ROAD PERFORMANCE TEST Figure 1. Photograph of Rock Samples. Specific gravity test The bottle method (T , Test methods of soils for Highway Engineering) was used to acquire the specific gravity of weathered granite samples in this research. Through specific gravity test, it was found that the specific gravity of red brown, yellow brown and gray granite samples are 2.62 g/cm 3, g/cm 3 and 2.69 g/cm 3, respectively. X-ray diffraction test The X-ray diffraction test is a classic analytical method on the mineral composition of soils and rocks. A X-ray diffraction equipment PSD-34 was used to analyze the primary mineral composition of weathered granite soil sample in this paper. According to the X- ray diffraction curve, the Joint Committee on Powder Diffraction Standards (JCPDS) Methods was adopted in this research. Fig. 2 shows the rock samples composition in this work. As can be seen from Fig. 2, the color of the samples becomes gradually shallow with the increase of the content of quartz, 686

3 and the incremental content of potassium feldspar and plagioclase feldspar is the main cause that the rocks becomes light. Geng (24) has claimed that the amphibole is blue green and the biotite is yellow in Yunzhong Mount, and that the main body of granite is coarse grain (porphyritic granite), while fine grained granite on the edge. The three kinds of samples used in this research had quartz (mean 4.3%) and feldspar (mean 54.3%) as the dominant minerals present, with smaller amounts of mica (mean 1.3%), amphibole (mean 2%), hematite (.5%), vermiculite (1%) and unknown (1%). Kinds of sample Yellown borwn Red borwn Gray quart potassium feldspar plagioclase feldspar mica amphibole haematite vermiculite unknown Whight percent: % Figure 2. Mineral Composition of Weathered Granite Samples. Sieving test and liquid and plastic limit test Fig. 3 shows the measured particle size distributions of the three samples using sieve analysis. Table 1 shows the uniformity coefficient (C u ) and curvature coefficient (C c ) of the samples tested in the study. According to the test methods of soils for Highway Engineering (JTG E4-27), when the uniformity coefficient of coarse grained soil is greater than or equal to 5 and the curvature coefficient is equal to 1-3, it is a good gradation. It is can be found from Fig. 3 and Table 1 that the red brown is finest and the gray coarsest, and the gradation of red brown is best and the gray worst. 1 Percentage passing: % Rad brown sample Yellow brown sample Hoar sample Particle size: mm Figure 3. Particle-Size Distribution of Samples. 687

4 Table 1. C u and C c of Weathered Granite Samples. Styles of samples d 1 (mm) d 3 (mm) d 6 (mm) C u C c Red brown Yellow brown Gray A Liquid Plastic Combine Tester was used in this research to obtain the liquid limit and plastic limit index of red brown weathered granite soil. The results of liquid and plastic limit test showed that the liquid limit, the plastic limit and the plasticity index of red brown weathered granite are 32.8%, 23.5% and 9.2, respectively. Compaction test On the basis of Test methods of soils for Highway Engineering (JTG E4-27), the heavy compaction test method was used in the three color kinds of typical samples. Through this test, it was found that the maximum dry density (MDD) of pure red brown sample, yellow brown sample and gray sample are 2.13 g/cm 3, 2.25 g/cm 3 and 2.18 g/cm 3, respectively. The maximum dry densities of finer red brown sample and the coarser gray sample are smaller than that of yellow brown sample, this is because that the compaction character of weather gratin is synthetically effected on the size distribution of soils, the hardness degree rock and the degree of weathering. The weathering granite stratum was covered by Pleistocene (Q 3 ) and Pleistocene (Q 4 ) Loess Holocene in the surrounding of Xinbao highway. In the process of filling subgrade, specially shoveling and transport weathering material, it is inevitable that the clay in situ will be mixed into the weathering filling material. In order to understand the characteristic of the weathering granite mixed with different clay content, the heavy compaction tests were made by using samples with the five different clay contents of 4%, 8%, 1% and 12% by weight. These clay samples were obtained from Xinzhou (Shanxi, China), which liquid limit and plasticity index are 33.2% and 15.5%, respectively. The gray weathered granite locates at depths of more than 3 m in the earth, but red brown and yellow brown weathered granite is at depths of less than 3 m. The filling materials used for subgrade constriction are mainly obtained at depths of less than 3 m, so the heavy compaction test been aimed at red brown and yellow brown samples. Fig. 4 shows the compaction curves for red brown and yellow brown samples with different clay content. It can be seen from Fig. 4 that the peak MDD value occurs most significantly in the range of about 7.5%-1% fine aggregate content. The results of test clearly show also that as clay content increases, the MDD tends to be considerably reduced after it reaches the peak MDD. The MDD of red brown and yellow brown weathered granite is separately 2.32 g/cm 3 and 2.38 g/cm 3. Furthermore, the clay content relative to peak MDD of red brown is 1% larger than that of yellow brown. In addition, the higher the clay content, the larger the optimum moisture content, this relationship is approximately linear. These results also indicate that because the gradation of red brown samples is finer than yellow brown, the optimum moisture content (OMC) of red brown samples is.1% smaller than yellow brown. It is noted that tendency to OMC increase slowed down slightly in the samples with more than 8% of clay content. 688

5 Maximum dry density: g/cm MDD of red brown sample MDD of yellow brown sample OMC of red brown sample OMC of yellow brown sample clay content: % Figure 4. Compaction Index for Weathered Granite Samples with Different Clay Content. California bearing ratio (CBR) test Because the red brown weathered granite soil is most widespread in the field, the CBR test was carried out for the red sample. Fig. 5 represents the relationship of clay content and CBR obtained from different compaction times test, which times are separately 3, 5 and 98. As seen in Fig. 5, the CBR come to a peak value when the compaction times are 5 or 98. Because the compaction power under 3 times is not enough to make the rock in a dense state, the CBR of sample of 3 compact times don t arise a peak value. But, with increasing clay content, the sample of compaction 3 times will eventually arise a peak CBR. The experimental results shows, with the increase of compaction times, the peak value of CBR is on the increase. But, as the compaction times increase, the clay content at the point of peak CBR decreases. It is noted that the clay content at the point of peak CBR in 98 compact times is approximately 4%, which is 4% less than the clay content at the point of peak maximum dry density. The cause of the above results is mainly that when clay content exceeds a certain value (such as 4%), the interlocking structure of compacted weathered granite was opened by the clay in samples, the internal friction angle (φ) was decreased, and the penetration resistance subsequently declined. Finally, the clay content at the point of peak CBR is larger than that at the point of peak maximum dry density Optimum moisture content: % CBR: % ` Compaction times: 3 Compaction times: 5 Compaction times: caly content: % Figure 5. Change of CBR for the Soils with Different Clay Content. 689

6 Large-scale triaxial test According to T (MOTPRC, 27), the coarse grained soil samples were not saturated, unconsolidated and undrained during triaxial compression test. However, as subgrade filling, in view of the actual situation, the granite weathered soil should be tested after saturation and consolidation, so T cannot meet the actual situation. With reference to the method of coarse grained soil triaxial shear test in the Code for coarsegrained soil tests for hydropower and water conservancy engineering, the large-scale triaxial shear tests on the red brown granite weathered soil were conducted after consolidation and drain (CD) in this research (CEC, 26). In this study, using surface vibration method, the soil specimens were prepared at the optimum water rate of 6.1% and the compactness of 93%. The specimen s diameter is 3 mm and the specimens were sheared after four different effective confining pressures of 1, 2, 3 and 4 kpa. Fig. 6 shows this typical stress-strain relationship of the CD tests on four representative red brown pure weathered soil samples after four confining pressures, where, σ 1 is the axial stress, σ 3 is confining pressure, and ε 1 is the axial strain. Fig. 7 shows this typical ε v -ε 1 relationship of the CD tests, where, ε v in this figure is the volumetric strain. It can be found in Fig. 6 the specimens transit gradually from strain softening to strain hardening with the increasing of confining pressure sample, and the peak deviator stress increases as the confining pressure increases. It can be observed from Fig. 7 that the samples transit generally from shear dilation to shear shrinkage and the volumetric strain of the transition point of specimen increases gradually with the increasing of confining pressure. All in all, as the confining pressure increases, the volumetric strain at failure changes towards the volume contraction (σ 1-σ 3) : 1 2 kpa 1 5 1kPa 3kPa 2kPa 4kPa ε 1 : % Figure 6. Stress-Strain Relationship. 69

7 kPa 3kPa 2kPa 4kPa ε v: % Figure 7. Curves of v - 1. ε 1 : % According to the results of large-scale triaxial tests, the Mohr s circles and the strength failure envelope can be depicted in Fig.8 on the basis of the Mohr-Coulomb strength criterion. The linear Mohr s envelope of the weathered granite soil in this work can be expressed as τ f =σ tan (1) where, τ f is peak shear stress and is normal stress. τ : 1 2 kpa strength envelope (linear function) 1kPa 2kPa y =.8775x kPa 4kPa σ: 1 2 kpa Figure 8. Mohr Strength Envelope of Red Brown Samples. It is can be found from equation (1) that the cohesive strength of the sample is 55 kpa, but, because the weathered granite soil belongs to coarse grained soil, the cohesive strength of samples should be close to zero. It has been proved that the particles of coarse grained soil might be broken and its peak strength is nonlinear. As can be seen from Fig. 9, the ratio (τ f /σ) is not constant but incremental as σ increase, and the Mohr envelope is bending-downward. The degree of bend reduces as the confining pressures increase, which reflects that the particle breakage is controlled by the confining pressures observably. 691

8 τ : 1 2 kpa strength envelope (power function) 1kPa 2kPa 3kPa 4kPa σ: 1 2 kpa Figure 9. Actual Strength Envelope of Red Brown Samples. Referring to the power function of De Mello (1977), the power function of weathered granite soil in this study can be expressed as follows:.83 f (3) As shown in Fig. 9, the fitting-envelope has a good correlation, which is a further explanation that the weathered granite soil has the property of strength nonlinearity because of particle breakage. CONCLUSIONS In this study, a series of physical and mechanical tests was performed. The main conclusions obtain from this study are summarized below. (1) The major cause that the color of the weathered granite soil becomes light is the increase in the content of quartz. Also, the increase in the content of potassium feldspar and plagioclase make the color of weathered granite become dark. (2) The pure weathered granite soil shows similar compaction characteristics of sands. The characteristics of compression of weathered granite soil vary significantly at the clay content of about 8%. This indicates that the clay content of 8% can be a good indicator to determine the compaction characteristics of weathered granitic with different clay contents. Thus, the clay content of about 8% would be advantageous to the compaction rate of subgrade and the stability of roads. (3) The California bearing ratio of typical weathered granite is 15-5%, and it can meet the requirements of subgrade bearing capacity, and the weathered granite soil in this study can be used as the filling of highway subgrade. The clay content is about 4% CBR value of the largest, up to 95%. The California bearing ratio of weathered granite soil with clay content of about 4% is largest, 95%. (4) As we can see from the large-scale triaxial test, it is the particle breakage that leads to the obvious characteristic of strength nonlinearity of the granite weathered soil in this research. ACKNOWLEDGMENTS The joint financial support provided by the Fundamental Research Funds for the Central Universities of Chang an University ( ), the Key Lab of Highway Construction & Maintenance Technology in Loess Region of Shanxi Transportation Research Institute (KLTLR-Y12-9) and Student Research Training Program of Shanxi University ( ) is greatly appreciated. 692

9 REFERENCES China Electricity Council (CEC). (26). DL/T "Code for Coarse-grained Soil Tests for Hydropower and Water Conservancy Engineering." ISBN , Beijing: China Electric Power Press (in Chinese). Cui, Z., Yang, J. and Chen, Y. (27). "The type and evolution of the granite landforms in China." ACTA Geographica SINICA, Vol. 62 (7): (in Chinese). DE MELLO V.F.B. (1977). "Reflections on decisions of practical significance to embankment dam construction." Géotechnique, Vol. 27 (3): Geng, Y., Yang, C., Song, B. et al. (24). "Post-orogenic granites with an Age of 18 Ma in Luliang area, north China craton: constraints from isotopic geochronology and geochemistry." Geological Journal of China Universities, Vol. 1 (4): (in Chinese). Guo, N., Wang, X. and Zhao, Y. (29). "Mix design of granite stone mastic asphalt (SMA) mixture." Journal of Building Materials, Vol. 12 (2): (in Chinese). Ham, T., Nakata, Y., Orense, R.P., et al. (21). "Influence of gravel on the compression characteristics of decomposed granite soil." Journal of Geotechnical and Geoenvironmental Engineering, Vol. 136 (11): Lee, I.K. and Coop, M.R. (1995). "The intrinsic behaviour of a decomposed granite soil." Geotechnique, Vol. 45 (1): Li, J. and Tang, Y. (22). "Experiment study of modifying roadbed filler of seriously weathered granite by blending lime and technological process of construction." Journal of Hunan University (Natural Sciences Edition), Vol. 29 (3): (in Chinese). Lu, F. and Sang, L. (22). "Petrology." ISBN , Beijing: Geological Publishing House (in Chinese). Ministry of Transport of the People s Republic of China (MOTPRC). (27). "JTG E4-27 Test Methods of Soil for Highway Engineering." ISBN , Beijing: People Communications Press (in Chinese). Ng, C.W., Fung, W.T., Cheuk, C.Y, et al. (24). "Influence of stress ratio and stress path on behavior of loose decomposed granite." Journal of Geotechnical and Geoenvironmental Engineering, Vol. 13 (1): Niu, X. (213). "Study on mechanical properties and subgrade filling technology of strongly weathered granite." Ph.D thesis, Chang an University, Xi an (in Chinese). The Writing Committee of Handbook of Engineering Geology (WCHEG). (27). "Handbook of Engineering Geology." ISBN , Beijing: China Building Industry Press (in Chinese). Wu, N., Zhao, C. and Hou, W. (24). "Research on the cause of formation, distribution and engineering characteristics of the granite residual soil." Journal of Pingdingshan Institute of Technology, Vol. 13 (4): 1-4 (in Chinese). Xiao, H., Jiang, G. and Wang, J. (21). "Experimental research on settlement of railway embankment on deep layer completely decomposed granite foundation based on centrifugal model test." Journal of Highway and Transportation Research and Development, Vol. 27 (8): 4-44 (in Chinese). Yang, W. (29). "Research on application of weathered granite material road performance." Ph.D thesis, Chang an University, Xi an (in Chinese). 693

Basic properties and weathering process of strong weathered granite

Japanese Geotechnical Society Special Publication The 6th Japan-China Geotechnical Syposiu Basic properties and weathering process of strong weathered granite Xirong Niu i, iv), Yanfang Sun ii, iii), Yangping