Model tests and theoretical analyses of buoyancy in saturated soils during the ultimate limit state of up-lifting
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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。Abstract: 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.
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Key words:
- saturated soil /
- uplift force /
- reduction coefficient /
- model test /
- Skempton’s B value /
- pore pressure
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