OCR of Shanghai soft clay and its geological causes
GAO Yan-bin1, CHEN Zhong-qing2
1. Department of Geotechnical Engineering, Tongji University, Shanghai 200092, China; 2. School of Civil Engineering, Shaoxing University, Shaoxing 312000, China
Abstract：Through the field vane strengths SuFV of 19 projects involving two sedimentary facies of coastal plain and lacustrine plain in Shanghai and combined with the worldwide research findings, the overconsolidation ratio (OCR) and its geological causes of three Shanghai soft clay layers are analyzed. The OCR is calculated according to strength ratio SuFV/ ( is the effective overburden pressure), and then its variation with depth is given. The results show that the Shanghai soft clay is overconsolidated in general. The OCR of shallow layers is larger and more discrete (from 1.3 to 5.0). The OCR decreases rapidly with the increase of depth and almost maintains a stable value (1.6 in averages) after 13 m deep. The OCR of Shanghai soft clay is significantly underestimated according to the laboratory compression tests in the past. Based on the model given by Hanzawa and Tanaka, the strength and geological causes of overconsolidation of Shanghai soft clay are analyzed. It is conformed that for the shallow layers of the clay the main cause is the cement effect varying in a wide range, and for deep layers the main cause is the secondary compression effect being in a relatively stable range. The contribution of normal compression, cement effect and secondary compression effect to the total strength of the soft clay is also given.
[1] CHANG M F. Interpretation of overconsolidation ratio from in situ tests in Recent clay deposits in Singapore and Malaysia[J]. Can Geotech J, 1991, 28: 210-225. [2] 魏道垛, 胡中雄. 上海浅层地基土的前期固结压力及有关压缩性参数的试验研究[J]. 岩土工程学报, 1980, 2(4): 13-22. (WEI Dao-duo, HU Zhong-xiong. Experimental study of preconsolidation pressure and compressibility of Shanghai subsoil[J]. Chinese Journal of Geotechnical Engineering, 1980, 2(4): 13-22. (in Chinese)) [3] 张诚厚, 王伯衍, 汪兆京. 上海黄浦江岸边淤泥质黏土的固结状态及强度特性[J]. 水利水运科学研究, 1981, 3(1): 12-33. (ZHANG C H, WANG B H, WAN Z J. The consolidated condition and shear strength behavior of soft clay at the bank of Huangpu River, Shanghai [J]. Hydro-science and Engineering, 1981, 3(1): 12-33. (in Chinese)) [4] 张诚厚. 上海黏土的准超压密特性[J]. 岩土工程学报, 1982, 4(3): 62-69. (ZHANG Cheng-hou. The apparent over consolidated behavior of Shanghai clay[J]. Chinese Journal of Geotechnical Engineering, 1982, 4(3): 62-69. (in Chinese)) [5] 冯铭璋. 软土压缩曲线特征[J]. 岩土工程学报, 1992, 14(5): 95-100. (FENG Ming-zhang. Compression curve of soft cohesive soil[J]. Chinese Journal of Geotechnical Engineering, 1992, 14 (5): 95-100. (in Chinese)) [6] JAMIOLKOWSKI M, LADD C C, GERMAINE J T, et al. New developments in field and laboratory testing of soils[C]// Proceedings of the 11th International Conference on Soil Mechanics and Foundation Engineering. San Francisco, 1985: 57-153. [7] CHANDLER R J. The in-situ measurement of the undrained shear strength of clays using the field vane[C]// Field and Laboratory Studies, ASTM STP 1014, American Society for Testing and Materials. Philadelphia, 1988: 13-44. [8] SKEMPTON A W. Vane tests in the alluvial plain of the River Forth near Grangemouth[J]. Géotechnique, 1948, 1: 111-124. [9] MAYNE P W, MITCHELL J K. Profiling of over- consolidation ratio in clays by field vane[J]. Canadian Geotechnical Journal, 1988, 25(4): 150-157. [10] SCHMERTMANN J H. The mechanical aging of soils[J]. J Geotech Engineering Div, ASCE, 1991, 117(9): 1288-1329. [11] BJERRUM L. Problems of soils mechanics and construction on soft clays and structurally unstable soils collapsible, expensive and others[J]. Proceedings of the 8th International Conference on Soil Mechanics and Foundation Engineering, 1973, 17(2): 83-119. [12] 胡中雄. 土力学与环境土工学[M]. 上海: 同济大学出版社, 1997. (HU Zhong-xiong. Soil mechanics and environmental geotechnique[M]. Shanghai: Tongji University Press, 1997. (in Chinese)) [13] HANZAWA H, TANAKA H. Normalized undrained strength of clay in the normally consolidated state and in the field[J]. Soils and Foundations, 1992, 32(1): 132-148.
