上浮极限状态下饱和土的浮力模型试验和理论分析
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摘要
饱和黏土中的长期浮力是否需要折减是一个具有争议的问题。为研究饱和黏土中的长期浮力是否小于相同水头高度下的静水压力,通过地基上浮失稳的模型试验,实测了上浮极限状态下(即有效应力约等于0时)饱和砂土和黏土中的浮力折减系数。实验装置由模型槽、土样、凡士林、浮筒、配重及压重设施组成,通过浮筒上浮瞬间的受力平衡得到失稳时的实际上浮力。实验测得饱和砂土中的浮力折减系数为1,饱和高岭土和饱和蒙脱石中的浮力折减系数分别为0. 973±0. 024和0. 959±0. 016。试验结果表明上浮极限状态下,饱和土中的浮力与纯水中的浮力差别很小。即便在高塑性黏土中,模型基础失稳时受到的上浮力与纯水中的浮力相比,仅折减了不到5%。基于饱和土有效应力原理的分析表明,上浮极限状态下的浮力折减系数与土样固结前的Skempton B值互为倒数。大量实测数据并未发现饱和黏土的Skempton B值明显大于1,故饱和黏土的浮力折减系数也不可能显著小于1。本文模型试验和理论分析表明,在地基抗浮承载力验算时,饱和黏土中的长期浮力不宜进行显著折减。
Whether the long-term buoyancy in saturated clay is less than that in pure water is still a controversial issue. The theoretical analysis of this issue is lacked and the existing experimental researches lead to inconsistent opinions. A model test of up-lifting is conducted in both the cohesionless and cohesive saturated soils,and the reduction coefficient of buoyancy is obtained during the ultimate limit state of up-lifting( ULSU,i. e.,the effective stress approaches zero). The experiment is based on geometry measurements,thus the inaccuracy of force measurement is avoid. The results indicate that the reduction coefficient of buoyancy is close to unity for different saturated soils. The uplift force in the saturated soil is approximately the same as that in pure water,and no significant reduction is observed. For the saturated soils,theoretical analyses based on the effective stress principle show that the reduction coefficient of buoyancy during the ULSU is the reciprocal of the Skempton's B value before consolidation. However,the B-value significantly greater than unity is not observed for the varied saturated clays. Therefore,during the ULSU in the saturated soils,significant reduction in buoyancy is not supported by the experimental and theoretical results.
Whether the long-term buoyancy in saturated clay is less than that in pure water is still a controversial issue. The theoretical analysis of this issue is lacked and the existing experimental researches lead to inconsistent opinions. A model test of up-lifting is conducted in both the cohesionless and cohesive saturated soils,and the reduction coefficient of buoyancy is obtained during the ultimate limit state of up-lifting( ULSU,i. e.,the effective stress approaches zero). The experiment is based on geometry measurements,thus the inaccuracy of force measurement is avoid. The results indicate that the reduction coefficient of buoyancy is close to unity for different saturated soils. The uplift force in the saturated soil is approximately the same as that in pure water,and no significant reduction is observed. For the saturated soils,theoretical analyses based on the effective stress principle show that the reduction coefficient of buoyancy during the ULSU is the reciprocal of the Skempton's B value before consolidation. However,the B-value significantly greater than unity is not observed for the varied saturated clays. Therefore,during the ULSU in the saturated soils,significant reduction in buoyancy is not supported by the experimental and theoretical results.
引文
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[19] LADE P V,DE BOER R. The concept of effective stress for soil,concrete and rock[J]. Géotechnique,1997,47(1):61-78.
[20] SINGH P N,WALLENDER W W. Effective Stress from Force Balance on Submerged Granular Particles[J]. International Journal of Geomechanics,2007,7(3):186-193.
[21] CHARLEY R C,STEVENS E,SHETH N. Suggested test method for determination of degree of saturation of soil samples by B value measurement[J]. Geotech Test J,1979,2(3):158-162.
[22] BRAJA M D. Advanced soil mechanics[M]. 3rd ed.New York:Taylor&Francis,2009.
[2] DEWEY R R,REICH R W,SAOUMA V E. Uplift modeling for fracture mechanics analysis of concrete dams[J]. J Struct Eng,1994,120(10):3025-3044.
[3] U S Department of the Interior. Design of gravity dams[R]. Washington, D C:Bureau of Reclamation,1976.
[4] FERC. Engineering guidelines for the evaluation of hydropower projects[R]. Washington,D C:Office of Hydropower Licensing,1991.
[5]《岩土工程手册》编写委员会.岩土工程手册[M].北京:中国建筑工业出版社:1994.[Editorial Committee of Handbook of Geotechnical Engineering.Handbook of Geotechnical Engineering[M]. Beijing:China Architecture&Building Press, 1994.(in Chinese)]
[6]崔岩,崔京浩,吴世红,等.浅埋地下结构外水压折减系数试验研究[J].岩石力学与工程学报,2000,19(1):82-84.[CUI Y,CUI J H,WU S H,et al. Testing study of reducement factor for the pore water pressure of underground structure[J]. Chinese Journal of Rock Mechanics and Engineering,2000,19(1):82-84.(in Chinese)]
[7]张乾,宋林辉,梅国雄.黏土地基中的基础浮力模型试验[J].工程勘察,2011(9):37-41.[ZHANG Q, SONG L H, MEI G X. Model experiment on foundation buoyancy in clay[J].Geotechnical Investigation&Surveying,2011(9):37-41.(in Chinese)]
[8]梅国雄,宋林辉,宰金珉.地下水浮力折减试验研究[J].岩土工程学报,2009,31(9):1476-1480.[MEI G X, SONG L H, ZAI J M.Experimental study on reduction of groundwater buoyancy[J]. Chinese Journal of Geotechnical Engineering,2009, 31(9):1476-1480.(in Chinese)]
[9]向科,周顺华,詹超.浅埋地下结构浮力模型试验研究[J].同济大学学报(自然科学版),2010,38(3):346-357.[XIANG K,ZHOU S H,ZHAN C.Model test study of buoyancy on shallow underground structure[J]. Journal of Tongji University(Natural Science),2010,38(3):346-357.(in Chinese)]
[10]宋林辉,刘益,梅国雄,等.黏土地基中的水浮力试验研究[J].水文地质工程地质,2008,35(6):80-84.[SONG L S,LIU Y,MEI G X,et al.Experimental study on buoyancy effect on deep foundation in clay[J]. Hydrogeology&Engineering Geology,2008,35(6):80-84.(in Chinese)]
[11]崔岩,崔京浩,吴世红.地下结构浮力模型试验研究[J].特种结构,1999(1):32-39.[CUI Y,CUI J H, WU S H. Model test of buoyancy of underground structure[J]. Special Structures,1999(1):32-39.(in Chinese)]
[12]倪伟杰,朱斌,陈仁朋,等.回填软土中管道上浮力测试及计算方法[J].岩土工程学报,2014,36(3):569-573.[NI W J,ZHU B,CHEN R P,et al. Tests and methods for buoyancy of pipelines in backfill soft clay[J]. Chinese Journal of Geotechnical Engineering, 2014, 36(3):569-573.(in Chinese)]
[13]中华人民共和国建设部. GB 50021-2001岩土工程勘察规范[S].北京:中国建筑工业出版社,2009.[Ministry of Construction of the People’s Republic of China. GB 50021-2001 Code for investigation of geotechnical engineering[S].Beijing:China Architecture&Building Press,2009.(in Chinese)]
[14]高涛.黏性土中地下水浮力探讨[C]//吉林省土木建筑学会2014年学术年会.长春:吉林省土木建筑学会,2014:76-77.[GAO T. Discussion on up lift force of underground water in cohesive soil[C]//2014 Conference of Jilin Society of Civil Engineering. Changchun:Jilin Society of Civil Engineering,2014:76-77.(in Chinese)]
[15]龚晓南.土力学[M].北京:中国建筑工业出版社,2002.[GONG X N. Soil mechanics[M].Beijing:China Architecture and Building Press,2002.(in Chinese)]
[16]王洪新.水土压力统一计算理论的证明及水土共同作用下的压力计算[J].岩石力学与工程学报,2012, 31(2):392-398.[WANG H X.Verification of unified calculation theory of water and earth pressures and calculation of pressure under interaction of water and earth[J]. Chinese Journal of Rock Mechanics and Engineering,2012,31(2):392-398.(in Chinese)]
[17]李大鹏,崔传安,唐德高,等.基于充分浮力理论的有效应力原理公式推导[J].水文地质工程地质,2012,39(1):24-30.[LI D P,CUI C A,TANG D G, et al. Derivation of the formula of effective stress principle based on theory of adequate rising force[J]. Hydrogeology&Engineering Geology,2012,39(1):24-30.(in Chinese)]
[18] WONG I H. Methods of resisting hydrostatic uplift in substructures[J]. Tunn Undergr Sp Technol,2001,16(3):77-86.
[19] LADE P V,DE BOER R. The concept of effective stress for soil,concrete and rock[J]. Géotechnique,1997,47(1):61-78.
[20] SINGH P N,WALLENDER W W. Effective Stress from Force Balance on Submerged Granular Particles[J]. International Journal of Geomechanics,2007,7(3):186-193.
[21] CHARLEY R C,STEVENS E,SHETH N. Suggested test method for determination of degree of saturation of soil samples by B value measurement[J]. Geotech Test J,1979,2(3):158-162.
[22] BRAJA M D. Advanced soil mechanics[M]. 3rd ed.New York:Taylor&Francis,2009.
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