Basic properties and weathering process of strong weathered granite

Transcription

1 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 Yao iv) and Jian Han iv) i) Departent of Civil Engineering, Shanxi University, 9, Wucheng Road, Taiyuan, China. ii) College of Architecture and civil engineering, Taiyuan University of Technology, 79, Yingzexi Street, Taiyuan, China. iii) Key Lab of Highway Construction & Maintenance Technology in Loess Region, 36, Xutanxi Street, Taiyuan, China. iv) School of Transportation Science and Engineering, Beihang University, 37, Xueyuan Road, Beijing, China. ABSTRACT Strong weathered granite is abundant in China and it is used extensively as a fill aterial. However, systeatic research on this aterial is relatively less as copared to sedientary type of soils like sand and clay. In this study, a series of laboratory tests were perfored, including X-ray diffraction test, sieving test, liquid and plastic liit test, copaction test, and California bearing ratio (CBR) test, to study the basic characteristic of strong weathered granite. Specially, in order to evaluate the effects of clay content on the echanical properties of the copacted weathered granite, the extra copaction test and CBR test were ade by using saples with the five different clay contents of 4, 8, 1, and 1% by weight. Results of the laboratory tests indicated that the clay content of 8% could be a good indicator to deterine the copaction characteristics of weathered granitic with different clay contents. Thus, clay content of about 8% would be advantageous to the copaction rate of subgrade and the stability of roads. However, The California bearing ratio of strong weathered granite with clay content of about 4% is largest, 95%. A preliinary relationship between weathering process paraeters (, r) and burying granite depth (h) was established to iprove the understanding of weathering process and degree of this aterial. It is found in this research that as the depth h of weathered granite increases, the geological year s paraeter becoes saller in a power function, but the geoetric progression constants r becoes larger in a power function. Keywords: strong weathered granite, laboratory test, basic characteristic, weathering process 1 INTRODUCTION Granite is a kind of igneous rocks which is fored by Maga in inside the earth,and the ain coponents of igneous rocks are quartz, feldspar, ica and hornblende (Lu & Sang, ). Granite shows a full crystal grain structure, hard texture and unifor property, and high copressive strength (1-MPa). Because feldspar and ica exist joints, the granite have three groups of priary 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., 4). Granite is influenced by the long-ter geological tectonic oveent and cliate changes in the special geographical environent. Because of the different physical and cheical properties of granite ineral coposition, granite has been weathered in different degrees of weathering, and decoposed granitic soils are generally classified as weak weathering, weathering, strong weathering and residual (WCHEG, 7). Weathered granite cover ore than 9% fro the soil area of China (Cui et al., 7). Ease of availability eans that these soils are frequently used as construction aterials such as back fill or highway subgrade aterials as a result of the scarcity of better aterials (Li, ; Guo, 9; Yang, 9; Xiao, 1; Niu, 13). Soe researchers have conducted a series of studies and have preliinarily understood the engineering characteristics of weathered granite (Lee, 1995; Ng, 4; Ha, 1). As entioned above, weathered granite is produced by physical and cheical weathering. However, it is not so easy for engineers to deterine the degree of weathering of these soils since so any factors are involved. In this study, besides specific gravity test, X-ray diffraction test, sieving test, liquid and plastic liit test, and CBR test, a sires of copaction tests were carried out on clay-ixed strong weathered granite in order to clarify the influence of clay contents on their copaction behavior. The index of power function in this research can be used as evaluate the weathering degree of weathered granite. 8

2 PREPARATION OF SAMPLES The strong weathered granite used in this research was sapled fro a site near Yunzhong Mount, a Branch of Luliang Mount, on the eastern outskirts of Xinzhou, Shanxi, China. The strong weathered granite 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 oisture content were kn/ 3 and % respectively. and feldspar (ean 54.3%) as the doinant inerals present, with saller aounts of ica (ean 1.3%), aphibole (ean %), haeatite (.5%), vericulite (1%) and unknown (1%). Kinds of saple Yellown borwn Red borwn Gray quart potassiu feldspar plagioclase feldspar ica aphibole haeatite vericulite unknown Whight percent: % Red brown Yellow brown Gray Fig. 1. Photograph of rock saples. 18c 3 BASIC CHARACTERISTICS OF WEATHERED GRANITIE 3.1 Specific gravity test The bottle ethod (T , Test ethods of soils for Highway Engineering) was used to acquire the specific gravity of weathered granite saples in this research. Through specific gravity test, it was found that the specific gravity of red brown, yellow brown and gray granite saples are.6g/c 3,.636g/c 3 and.69g/c 3 respectively. 3. X-ray diffraction test The X-ray diffraction test is a classic analytical ethod on the ineral coposition of soils and rocks. A X-ray diffraction equipent PSD-34 was used to analyze the priary ineral coposition of strong weathered granite saple in this paper. According to the X-ray diffraction curve, the Joint Coittee on Powder Diffraction Standards (JCPDS) Methods was adopted in this research. Fig. shows the the rock saples coposition in this work. As can be seen fro Fig. 1, the color of the saples becoes gradually shallow with the increase of the content of quartz, and the increental content of potassiu feldspar and plagioclase feldspar is the ain cause that the rocks becoes light. Geng (4) has claied that the aphibole is blue green and the biotite is yellow in Yunzhong Mount, and that the ain body of granite is coarse grain (porphyritic granite), while fine grained granite on the edge. The three kinds of saples used in this research had quart (ean 4.3%) Fig. Mineral coposition of weathered granite saples 3.3 Sieving test and liquid and plastic liit test Fig. 3 shows the differences of easured particle size distribution through sieving tests on the previous three different saples. Table 1 shows the unifority coefficient (C u ) and curvature coefficient (C c ) of the saples tested in the study. According to the test ethods of soils for Highway Engineering (JTG E4-7), when the unifority 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 fro 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. Percentage passing: % Rad brown saple Yellow brown saple Hoar saple Particle size: Fig. 3. Copaction curves for the soils with different clay content Table 1. C u and C c of weathered granite saples Styles of saples d 1 () d 3 () d 6 () C u C c Red brown Yellow brown Gray A Liquid Plastic Cobine Tester was used in this research to obtain the liquid liit and plastic liit index of red brown strong weathered granite. The results of liquid and plastic liit test showed that the liquid liit, the plastic liit and the plasticity index of red brown weathered granite are 3.8%, 3.5% and 9. 9

3 respectively. 3.4 Copaction test On the basis of Test ethods of soils for Highway Engineering (JTG E4-7), the heavy copaction test ethod was used in the three color kinds of typical saples. Through this test, it was found that the axiu dry density (MDD) of pure red brown saple, yellow brown saple and gray saple are.13g/c 3,.5 g/c 3 and.18 g/c 3 respectively. The axiu dry densities of finer red brown saple and the coarser gray saple are saller than that of yellow brown saple, this is because that the copaction character of weather gratin is synthetically effected on the size distribution of soils, the hardness degree rock and the the degree of weathering. The weathering granite stratu 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 shovelling and transport weathering aterial, it is inevitable that the clay in situ will be ixed into the weathering filling aterial. In order to understand the characteristic of the weathering granite ixed with different clay content, the heavy copaction tests were ade by using saples with the five different clay contents of 4, 8, 1, and 1% by weight. These clay saples were obtained fro Xinzhou (Shanxi, China), which liquid liit and plasticity index are 33.% and 15.5% respectively. Gray weathered granite locate at depths of ore than 3 in the earth, but red brown and yellow brown weathered granite is at depths of less than 3. The filling aterials used for subgrade constriction are ainly obtained at depths of less than 3, so the heavy copaction test been aied at red brown and yellow brown saples. Fig. 4 shows the copaction curves for red brown and yellow brown saples with different clay content. It can be seen fro Fig. 4 that the peak MDD value occurs ost 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.3g/c 3 and.38g/c 3. Furtherore, 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 optiu oisture content, this relationship is approxiately linear. These results also indicate that because the gradation of red brown saples is finer than yellow brown, the optiu oisture content (OMC) of red brown saples is.1% saller than yellow brown. It is noted that tendency to OMC increase slowed down slightly in the saples with ore than 8% of clay content. Maxiu dry density: g/c MDD of red brown saple MDD of yellow brown saple OMC of red brown saple OMC of yellow brown saple clay content: % Fig. 4. Copaction curves for weathered granite saples with different clay content 3.5 California bearing ratio (CBR) test Because the red brown strong weathered granite is ost widespread in the field, the CBR test was carried out for the red saple in the field. Fig. 5 represents the relationship of clay content and CBR obtained fro different copaction ties test, which ties are separately 3, 5 and 98. As seen in Fig. 5, the CBR coe to a peak value when the copaction ties are 5 or 98. Because the copaction power under 3 ties is not enough to ake the rock in a dense state, the CBR of saple of 3 copact ties don t arise a peak value. But, with increasing clay content, the saple of copaction 3 ties will eventually arise a peak CBR. The experiental results shows, with the increase of copaction ties, the peak value of CBR is on the increase. But, as the copaction ties 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 copact ties is approxiately 4%, which is 4% less than the clay content at the point of peak axiu dry density. The cause of the above results is ainly that when clay content exceeds a certain value (such as 4%), the interlocking structure of copacted weathered granite was opened by the clay in saples, 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 axiu dry density. CBR: % Copaction ties: 3 Copaction ties: 5 Copaction ties: clay content: % Fig. 5. Copaction curves for the soils with different clay content Optiu oisture content: % 3

4 4 WEATHERING PROCESS OF GRANITIE 4.1 Weathering odel It is difficult to describe quantificationally the weathering process of granite by a certain atheatical odel. Fukuoto (199), in Ritsueikan University, Japan, through soe assuptions and siplifications, akes it possible to describe the process of granite weathering by atheatical ethods. Based on a large nuber of field investigation and a series of sieving tests, Fukuoto (199) has proposed a grad equation for decoposed grante soil through geoetric fractal theories and atheatical statistics ethod, and has used it to evaluate the particle breakage of weathering rock. Afterwards, any researchers have achieved fruitful achieveents by Fukuoto s odel, so this research also uses this odel to express the weathering process of granite. Before establishing weathering atheatical odel, Fukuoto (199) has ade the following assuptions: (1) In a state of weathering, the retained percentage of soil show statistically geoetric progressional distribution, it is constant r ( r 1). () The decoposed granite soil, which passed geological tie of y years, was sieved by one series of sieves with sieve ratio of λ. (3) The percent retained of nth sieve fro axiu grain size D ax toward saller size, is expresses as (1-r) r n-1. Eq. (1) and Eq. () are called a grading equation of decoposed granite soil in Fukuoto s paper (199). n 1 1 i i p C r r (1) n 1 1 i1 Dax log D n n () log in which, p n is the percent passing which pass esh opening D n. n in Eq. (1) is related with D n as in Eq. (). The calculation steps of Fukuoto s equation are follows as: (1) The position of n on the horizontal axis in Fig,6 ust be deterined. Applying D ax, the axiu grain size by actual easureent, and , the sieve ratio assuing Tyler Series, n log Dax Dn is obtained fro Eq. (3). Using this relation and cobining with grain size D n on the horizontal axis, graduation of n scale in figure is possible. Depending on the n scale, read the value of percent passing p n on vertical axis corresponding to integer n (dotted line in figure). Then, obtain the percent retained q n. Fig. 6 Fitting of grading equation (after Fukuoto, 199) () Obtain ean μ and variance σ by the following equation n (3) n (4) Since Eq. (5) and Eq. (6) stand, and r are obtained by Eq. (7) and Eq. (8). r (5) 1 r r (6) (1 r) r 1 (7) (8) Using Eq. (1) and Eq. () by the substitution of and r, the theoretical curve which is indicated by thin line in Fig. 6 is obtained. 4. Field sapling and testing In order to evaluate the degree of weathering of granite, Fukuoto s weathering odel was used in this research. Based on the thorough investigation on the distribution of weathered granite in the field (as shown in Fig. 7 and Fig. 8), a series of sieving testes were ade on a large nuber of granite saples obtained fro two typical sections, and geological year's paraeter and geoetric progression constants r of different depth granite saples were calculated by Fukuoto s grading equation. 4.3 Matheatical description of weathering process Fig. 9, Fig. 1, Fig. 11 and Fig. 1 illustrate the change of paraeters and r for different depth saples in section A and B. Results of the statistical analysis showed that there is a power function relationship between depth of saples and paraeters (or r), and the correlation of relationship is relatively good. The power function relationships in section A are showed as follows: h (1).153 r1. 43h (11) 31

5 where h is the depth of granite saples, which is easured fro the surface of ount; 1 and r 1 is geological year's paraeter and geoetric progression constants in section A, respectively..6.4 y =.48x.153 R =.8983 r Depth: Fig. 1. Change of paraeters r for different depth saples in section A Fig. 7. Photograph of section A (Yellow brown) 8 6 y = 3.79x R = Fig. 8. Photograph of section B (Red brown) The power function relationships in section B are showed as follows: h (1).168 r. 69h (13) where 1 and r 1 is geological year's paraeter and geoetric progression constants in section B, respectively Depth: Fig. 11. Change of paraeters for different depth saples in section B r.6.4 y =.691x.1679 R = y = 3.338x R = Depth: Fig. 9. Change of paraeters for different depth saples in section A Depth: Fig. 1. Change of paraeters r for different depth saples in section B 5 CONCLUSIONS In this study, specific gravity test, X-ray diffraction test, sieving test, liquid and plastic liit test, 3

6 copaction test, and CBR test were conducted to investigate the basic physical and echanical characteristic of strong weathered granite. The geological year's paraeter and geoetric progression constants r of different depth granite saples in two typical sections were calculated by Fukuoto s grading equation. The ain conclusions obtain fro this study are suarized below. (1) The ajor cause that the color of the strong weathered granite becoes light is the increase in the content of quartz. Also, the increase in the content of potassiu feldspar and plagioclase ake the color of weathered granite becoe dark. () The pure strong weathered granite show siilar copaction characteristics of sands. Characteristics of copression of the decoposed granitic vary significantly at the clay content of about 8%. This indicates that the clay content of 8% can be a good indicator to deterine the copaction characteristics of weathered granitic with different clay contents. Thus, clay content of about 8% would be advantageous to the copaction rate of subgrade and the stability of roads. (3) The California bearing ratio of typical weathered granite is 15-5%, and it can eet the requireents of subgrade bearing capacity, and the strong weathered granite 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 strong weathered granite with clay content of about 4% is largest, 95%. (4) As the depth h of weathered granite increases, the geological year s paraeter becoes saller in a power function, but the geoetric progression constants r becoes larger in a power function. The index of power function in this research can be used as evaluate the weathering degree of weathered granite. Building Materials, 1(), 197- (in Chinese). 5) Ha, T., Nakata, Y., Orense, R. P. and Hyodo, M. (1): Influence of gravel on the copression characteristics of decoposed granite Soil, Journal of Geotechnical and Geoenvironental Engineering, 136(11), ) Lee, I. K. and Coop, M. R. (1995): The intrinsic behaviour of a decoposed granite soil, Geotechnique, 45(1), ) Li, J. and Tang, Y. (): Experient study of odifying roadbed filler of seriously weathered granite by blending lie and technological process of construction, Journal of Hunan University (Natural Sciences Edition), 9(3), (in Chinese). 8) Lu, F. and Sang, L. (): Petrology, ISBN , Beijing: Geological Publishing House (in Chinese). 9) Ng, C. W., Fung, W. T., Cheuk, C. Y, and Zhang, L. (4): Influence of stress ratio and stress path on behavior of loose decoposed granite, Journal of Geotechnical and Geoenvironental Engineering, 13(1), ) Niu, X. (13): Study on echanical properties and subgrade filling technology of strongly weathered granite, Ph.D thesis, Chang an University, Xi an (in Chinese). 11) The Writing Coittee of Handbook of Engineering Geology (WCHEG). (7): Handbook of Engineering Geology, ISBN , Beijing: China Building Industry Press (in Chinese). 1) Wu, N., Zhao, C. and Hou, W. (4): Research on the cause of foration, distribution and engineering characteristics of the granite residual soil, Journal of Pingdingshan Institute of Technology, 13(4), 1-4 (in Chinese). 13) Xiao, H., Jiang, G. and Wang, J. (1): Experiental research on settleent of railway ebankent on deep layer copletely decoposed granite foundation based on centrifugal odel test, Journal of Highway and Transportation Research and Developent, 7(8), 4-44 (in Chinese). 14) Yang, W. (9): Research on application of weathered granite aterial road perforance, Ph.D thesis, Chang an University, Xi an (in Chinese). ACKNOWLEDGEMENTS The joint financial support provided by the Fundaental Research Funds for the Central Universities of Chang an University ( ) and the Key Lab of Highway Construction & Maintenance Technology in Loess Region of Shanxi Transportation Research Institute (KLTLR-Y1-9) is greatly appreciated. REFERENCES 1) Cui, Z., Yang, J. and Chen, Y. (7): The type and evolution of the granite landfors in China, ACTA Geographica SINICA, 6(7), (in Chinese). ) Fukuoto, T. (199): A grading equation for decoposed granite soil, Soils and Foundations, 199, 3(1), ) Geng, Y., Yang, C., Song, B. and Wan, Y. (4): Post-orogenic granites with an Age of 18 Ma in Luliang area, north China craton: constraints fro isotopic geochronology and geocheistry, Geological Journal of China Universities, 1(4), (in Chinese). 4) Guo, N., Wang, X. and Zhao, Y. (9): Mix design of granite stone astic asphalt (SMA) ixture, Journal of 33