压电陶瓷弯曲元剪切波速测试及饱和海洋软土动力特性研究
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摘要
地震和波浪等动荷载作用下海洋软土的不排水特性是海洋结构设计和海床稳定分析时必须考虑的问题,为了保证海洋结构的安全与稳定,减少海洋工程事故的发生,研究饱和原状海洋土的小应变剪切模量和循环荷载作用下海洋土的不排水特性有重要意义。压电陶瓷弯曲元是一种多功能传感器,在国外已被应用于室内测试土的剪切波速和研究土的工程性质,但合理的安装方法和高精度波速测试方法尚未完全解决,需要进一步研究。
本文在总结了目前关于弯曲元剪切波速测试方法和循环荷载作用下海洋土特性的有关研究成果之基础上,通过动力试验,进行了如下研究工作:
(1)在HX-100型多功能三轴仪上开发了压电陶瓷弯曲元剪切波速测试装置,通过弯曲元试验和Drnevich Long-Tor型共振柱试验,研究了准确测试不同种类和刚度土样剪切波速的方法以及剪切波速弥散性问题。研究结果表明,对不同种类和刚度的土样,通过选用合理的激发波形和频率就可以消除接收波形的近场效应和过冲现象,准确确定土样剪切波波速和小应变剪切模量;在所使用的激发频率范围内,剪切波速不具弥散性。
(2)通过弯曲元试验和共振柱试验,研究了原状海洋土的小应变剪切模量G_(max)。研究结果表明,平均有效固结应力σ_0、土的初始孔隙比e和塑性指数I_P是影响海洋土G_(max)的主要因素;海洋土的G_(max)与σ_0成正比,与e或I_p成反比;对于所研究的低塑性海洋土,可根据土的初始孔隙比e、塑性指数I_p、密度ρ和埋深d等基本物性指标,采用G_(max)=(1437-234e-26Ip)σ_0可以快速方便地估算海洋土的G_(max)。
(3)通过循环三轴试验和弯曲元试验,研究了海洋土动剪切模量和阻尼特性。研究表明,杭州湾海洋土动剪切模量随剪应变衰减曲线低于其它海洋土的衰减曲线,在整个应变范围内,双曲线模型和Ramberg-Osgood模型均可较好地拟合杭州湾海洋土的动剪切模量随剪应变的衰减,对于给定的剪应变水平,G/G_(max)
The property of soft marine clay subjected to undrained cyclic loading such as earthquake and storm waves is an important consideration in the design of offshore installations and analysis of the stability of the seafloor. To ensure the safety of offshore facilities and stability of seafloor, it is meaningful to investigate the small-strain shear modulus and the undrained properties of soft marine clay under cyclic loading. Piezoceramic bender elements are versatile transducer and are widely installed in soil instruments to measure shear velocity and the small-strain shear modulus of soil. But reasonable installation and the method for precisely determining shear wave velocity of soil have not been completely solved yet and have to be further investigated.On the base of summarizing the important advances in the field of the methods of shear wave velocity measurement from bender elements and the properties of soft marine clay subjected to undrained cyclic loading, some subjects as follow were investigated through tests in this dissertation:(1) The piezoceramic bender element system for shear wave velocity was developed in the HX-100 static/dynamic triaxial apparatus. The method for precisely determining shear wave velocity of soil from bender element tests was studied, and the dispersion of shear wave velocity was investigated, and the value of shear wave velocity from bender element tests was compared with that of resonant column tests in Drnevich Long-Tor apparatus. It is concluded that the near-field effects and overshooting during the bender element tests can be eliminated by using suitable waveform and rational frequency of input signals, and the value of shear wave velocity and the very small-strain shear modulus can be precisely determined. It is also concluded that the value of shear wave velocity from the bender element tests does not vary with frequencies of input signals.
(2) The small-strain shear moduli of soft marine clay were investigated by bender element tests and resonant column tests. The test results show that the mean effective stress ct0 , initial (or natural) void ratio e, and plasticity index IP of marine soil are main factors affecting G max of marine soils. For the marine clay in low plasticity index of this study, its Gmax can be estimated through the formula Gmax =(\437-234e-26Ip)cr0 from this study by initial void ratio e, plasticity index IP, mass density p and depth d of marine soil.(3) The dynamic modulus and the damping ratio for the soft marine clay were studied by using bender element and cyclic triaxial devices. The test results demonstrate that the modulus reduction curves versus cyclic shear strain of marine clay at Hangzhou Bay is below that of other marine clays, and Hardin-Drnevich model and Ramberg-Osgood model can fit them well, and its G/Gmax increases with the increase of plasticity index IP for the given shear strain. The test results also show that the damping curves versus cyclic shear strain of marine clay at Hangzhou Bay have a broad distribution, and Hardin-Drnevich model can fit them.(4) The small-strain shear moduli Gmax of saturated soils during undrained cyclic loading were investigated through cyclic triaxial tests and bender element tests. It is concluded that Gmax of saturated sand, and undisturbed silt and soft marine clayduring undrained cyclic loading have different properties. The Gmax of saturated sand during undrained cyclic loading is approximately equal to that of sand confined by same effective stress at static state. For the saturated silt and marine clay, the Gmax during undrained cyclic loading is similar to that of sand when its strain is less than its threshold strain, but the Gmax is less than that of saturated silt and marine clayconfined by same effective stress at static state when its strain is bigger than its threshold strain. So it is valid in dynamic effective-stress analysis of soil structure and construction site to use the Gmax of saturated sand determined by Hardin Equation during undrained cyclic loading; but it is unsafe to use the Gmax of saturated silt andmarine clay determined by Hardin Equation during undrained cyclic loading.(5) Based on a series of cyclic triaxial tests, the effect of cyclic frequency such as earthquake and ocean wave on the undrained behaviors of undisturbed marine clay was investigated. The test results indicate that, for a given dynamic stress ratio, the
accumulated pore water pressure and dynamic strain increase with the number of cycles. There exists a threshold value for both the accumulated pore water pressure and dynamic strain, below which the effect of cyclic frequency is very small, but above which the accumulated pore water pressure and dynamic strain increase intensely with the decrease of cyclic frequency for a given number of cycles. The dynamic strength increases with the increase of cyclic frequency, whereas the effect of cyclic frequency on it gradually diminishes to zero when the number of cycles is large enough, and the dynamic strengths at different frequencies tend to the same limiting minimum dynamic strength. The test results also manifest that the reasons for the frequency effect on the undrained soil behaviors are both the creep effect induced by loading rates and the decrease of sample effective confining pressure caused by the accumulated pore water pressure. Analysis of an ocean project indicates that wave frequency has effect not only on the magnitude of dynamic strength of seabed soil but on that of shear stress induced by wave also, and has significant influence on the stability and the liquefaction depth of seabed soils.Finally, some conclusions were drawn and advices for further study were given.
本文在总结了目前关于弯曲元剪切波速测试方法和循环荷载作用下海洋土特性的有关研究成果之基础上,通过动力试验,进行了如下研究工作:
(1)在HX-100型多功能三轴仪上开发了压电陶瓷弯曲元剪切波速测试装置,通过弯曲元试验和Drnevich Long-Tor型共振柱试验,研究了准确测试不同种类和刚度土样剪切波速的方法以及剪切波速弥散性问题。研究结果表明,对不同种类和刚度的土样,通过选用合理的激发波形和频率就可以消除接收波形的近场效应和过冲现象,准确确定土样剪切波波速和小应变剪切模量;在所使用的激发频率范围内,剪切波速不具弥散性。
(2)通过弯曲元试验和共振柱试验,研究了原状海洋土的小应变剪切模量G_(max)。研究结果表明,平均有效固结应力σ_0、土的初始孔隙比e和塑性指数I_P是影响海洋土G_(max)的主要因素;海洋土的G_(max)与σ_0成正比,与e或I_p成反比;对于所研究的低塑性海洋土,可根据土的初始孔隙比e、塑性指数I_p、密度ρ和埋深d等基本物性指标,采用G_(max)=(1437-234e-26Ip)σ_0可以快速方便地估算海洋土的G_(max)。
(3)通过循环三轴试验和弯曲元试验,研究了海洋土动剪切模量和阻尼特性。研究表明,杭州湾海洋土动剪切模量随剪应变衰减曲线低于其它海洋土的衰减曲线,在整个应变范围内,双曲线模型和Ramberg-Osgood模型均可较好地拟合杭州湾海洋土的动剪切模量随剪应变的衰减,对于给定的剪应变水平,G/G_(max)
The property of soft marine clay subjected to undrained cyclic loading such as earthquake and storm waves is an important consideration in the design of offshore installations and analysis of the stability of the seafloor. To ensure the safety of offshore facilities and stability of seafloor, it is meaningful to investigate the small-strain shear modulus and the undrained properties of soft marine clay under cyclic loading. Piezoceramic bender elements are versatile transducer and are widely installed in soil instruments to measure shear velocity and the small-strain shear modulus of soil. But reasonable installation and the method for precisely determining shear wave velocity of soil have not been completely solved yet and have to be further investigated.On the base of summarizing the important advances in the field of the methods of shear wave velocity measurement from bender elements and the properties of soft marine clay subjected to undrained cyclic loading, some subjects as follow were investigated through tests in this dissertation:(1) The piezoceramic bender element system for shear wave velocity was developed in the HX-100 static/dynamic triaxial apparatus. The method for precisely determining shear wave velocity of soil from bender element tests was studied, and the dispersion of shear wave velocity was investigated, and the value of shear wave velocity from bender element tests was compared with that of resonant column tests in Drnevich Long-Tor apparatus. It is concluded that the near-field effects and overshooting during the bender element tests can be eliminated by using suitable waveform and rational frequency of input signals, and the value of shear wave velocity and the very small-strain shear modulus can be precisely determined. It is also concluded that the value of shear wave velocity from the bender element tests does not vary with frequencies of input signals.
(2) The small-strain shear moduli of soft marine clay were investigated by bender element tests and resonant column tests. The test results show that the mean effective stress ct0 , initial (or natural) void ratio e, and plasticity index IP of marine soil are main factors affecting G max of marine soils. For the marine clay in low plasticity index of this study, its Gmax can be estimated through the formula Gmax =(\437-234e-26Ip)cr0 from this study by initial void ratio e, plasticity index IP, mass density p and depth d of marine soil.(3) The dynamic modulus and the damping ratio for the soft marine clay were studied by using bender element and cyclic triaxial devices. The test results demonstrate that the modulus reduction curves versus cyclic shear strain of marine clay at Hangzhou Bay is below that of other marine clays, and Hardin-Drnevich model and Ramberg-Osgood model can fit them well, and its G/Gmax increases with the increase of plasticity index IP for the given shear strain. The test results also show that the damping curves versus cyclic shear strain of marine clay at Hangzhou Bay have a broad distribution, and Hardin-Drnevich model can fit them.(4) The small-strain shear moduli Gmax of saturated soils during undrained cyclic loading were investigated through cyclic triaxial tests and bender element tests. It is concluded that Gmax of saturated sand, and undisturbed silt and soft marine clayduring undrained cyclic loading have different properties. The Gmax of saturated sand during undrained cyclic loading is approximately equal to that of sand confined by same effective stress at static state. For the saturated silt and marine clay, the Gmax during undrained cyclic loading is similar to that of sand when its strain is less than its threshold strain, but the Gmax is less than that of saturated silt and marine clayconfined by same effective stress at static state when its strain is bigger than its threshold strain. So it is valid in dynamic effective-stress analysis of soil structure and construction site to use the Gmax of saturated sand determined by Hardin Equation during undrained cyclic loading; but it is unsafe to use the Gmax of saturated silt andmarine clay determined by Hardin Equation during undrained cyclic loading.(5) Based on a series of cyclic triaxial tests, the effect of cyclic frequency such as earthquake and ocean wave on the undrained behaviors of undisturbed marine clay was investigated. The test results indicate that, for a given dynamic stress ratio, the
accumulated pore water pressure and dynamic strain increase with the number of cycles. There exists a threshold value for both the accumulated pore water pressure and dynamic strain, below which the effect of cyclic frequency is very small, but above which the accumulated pore water pressure and dynamic strain increase intensely with the decrease of cyclic frequency for a given number of cycles. The dynamic strength increases with the increase of cyclic frequency, whereas the effect of cyclic frequency on it gradually diminishes to zero when the number of cycles is large enough, and the dynamic strengths at different frequencies tend to the same limiting minimum dynamic strength. The test results also manifest that the reasons for the frequency effect on the undrained soil behaviors are both the creep effect induced by loading rates and the decrease of sample effective confining pressure caused by the accumulated pore water pressure. Analysis of an ocean project indicates that wave frequency has effect not only on the magnitude of dynamic strength of seabed soil but on that of shear stress induced by wave also, and has significant influence on the stability and the liquefaction depth of seabed soils.Finally, some conclusions were drawn and advices for further study were given.
引文
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12. Christian, J. T., Taylor, P. K., Yen, J. K. C., and Erali, D. R. Large diameter underwater pipeline for nuclear power plant designed against soil liquefaction. Proceedings of Offshore Technology Conference, Dallas, OTC 2094, pp.597-606.
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2. Ansal, A. M. and Erken, A. Undrained Behavior of Clay under Cyclic Shear Stresses. Journal of Geotechnical Engineering, ASCE, 1989, 115(7), pp.968-983.
3. Azzour, A., et al. Cyclic behaviour of clays in undrained simple shear. Journal of Geotechnical Engineering, ASCE, 1989, 115(5), pp.637-657.
4.白冰、肖宏彬编著,软土工程若干理论与应用,中国水利水电出版社,2002年。
5.白冰、周健,周期荷载作用下粘性土的变形及强度特性述评,岩土力学,1999,20 (3),pp.84-90。
6. Bea, R. G.. How seafloor slides affect offshore structures. Oil and Gas Journal, 1971, Vol.29, pp.88-92.
7. Bea, R. G.. Wave-induced slides in South Pass Block 70, Mississippi Delta. Journal of Geotechnical Engineering Division, ASCE, 1983, 109(4), pp.619-644.
8. Boulanger, Ross W., et al. Dynamic properties of Sherman island peat. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 1998, Vol.124, No.1, 12-20.
9. Brewer, J. H. The Response of Cyclic Stress in a normally Consolidated Saturated Clay, Thesis Presented to the Civil Engineering Dept., North Carolina State Univ., at Raleigh, North Carolina, in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy, 1972.
10. Brown, S. F., et al. Repeated load triaxial testing of a silty clay. Geotechnique, 1975, 25(1), pp.95-114.
11. Castro, G., et al. Shear strength of soils and cyclic loading. Journal of Geotechnical Engineering, ASCE, 1976, 102(9), pp.887-894.
12. Christian, J. T., Taylor, P. K., Yen, J. K. C., and Erali, D. R. Large diameter underwater pipeline for nuclear power plant designed against soil liquefaction. Proceedings of Offshore Technology Conference, Dallas, OTC 2094, pp.597-606.
13.顾小芸,海洋工程地质的回顾与展望,工程地质学报,2000,8 (1),pp.40～45。
14. Hardin, B. O., and Black, W. L. Vibration modulus of normally consolidated clay. Journal of soil mechanics and foundation division, ASCE, 1968, Vol. 94, No. GT2, 353-369.
15. Hicher, P. Y., et al. Rotation of principal directions in K_0-consolidated clay. Journal of Geotechnical Engineering, ASCE, 1987, 113(7), pp. 774-788.
16. Hyde, A. F. L., Yasuhara, K., and Hirao, K. Stability Criteria for Marine Clay under One-Way Cyclic Loading, Journal of Geotechnical Engineering, ASCE, 1993, 119(11), pp. 1771-1889.
17. Hyde, A. F. L., et al. A pore pressure and stability model for a silty clay under repeated loading. Geotechnique, 1985, 35(2), pp. 113-125.
18. Hyde, A. F. L., et al. Cyclic triaxial tests on remoulded clays. Journal of Geotechnical Engineering, ASCE, 1987, 113(6), pp.665-669.
19. Idriss, I. M., Dobry, R., and Sihgh, R. D. Nonlinear behavior of soft clay during cyclic loading. Journal of Geotechnical Engineering Division, 1978, ASCE, 104(12), pp. 1427-1447.
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