WANG Zhe-chao1, 2, QIAO Li-ping3, LI Shu-cai1, LIN Chun-jin1
1 . Research Center of Geotechnical and Structural Engineering, Shandong University, Jinan 250061, China ; 2. Department of Civil Engineering, University of Calgary, Calgary, Alberta T2N1N4, Canada; 3. Department of Engineering Mechanics, Shandong University, Jinan 250061, China
Abstract: Soil creep is such a process that the deformation of soil develops with time under a state of constant stress. Following the tradition of continuum mechanics, the time-dependent creep rate is always employed in the investigations of the soil creep. However, in practices, soils undergo complex loading-unloading cycles and are allowed to creep on the stages at different stress and strain levels. In this complex situation, the initial states (or configurations) of the creep stages are different and thus the traditional continuum mechanics is incapable in describing the soil creep. Different from the traditional continuum mechanics, the internal-variable theory is based on the concept of internal variable, which describes the internal structure of materials. In this study, an internal-variable creep model is proposed, in which the creep rate of soils is dependent on not only applied stress, but also irreversible strain, which is adopted as internal variable for soil deformation. A series of laboratory tests have been performed to verify the proposed creep model. The parametric relation between the proposed internal-variable and the traditional power law creep models is derived. A method to accelerate creep tests is proposed based on the proposed creep model. It is found that the proposed internal-variable creep model is capable in describing the soil creep under complex conditions.
王者超,乔丽苹,李术才,林春金,. 土的内变量蠕变模型研究[J]. 岩土工程学报, 2011, 33(10): 1569-1575.
WANG Zhe-chao, QIAO Li-ping, LI Shu-cai, LIN Chun-jin . An internal-variable creep model for soils. Chinese J. Geot. Eng., 2011, 33(10): 1569-1575.
4 讨 论 4.1 土的内变量蠕变模型与传统蠕变模型参数关系 由于塑性应变与时间无关,所以: ,于是式( 3 )可写为 。 (7) 图 11 冰渍土分级加载蠕变试验数据和模型预测对比 Fig. 11 Comparison between test data and model prediction for stepwise creep tests 图 12 冰渍土多阶段蠕变试验数据和模型预测对比 Fig. 12 Comparison between test data and model prediction for multiple stage creep tests 式( 7 )对时间积分有 , (8) 式中, 为蠕变初始状态下土的最大不可恢复应变。式( 8 )对时间求导可得传统幂函数时间硬化蠕变模型表达式: 。 (9) 4.2 加快蠕变试验方法 土蠕变性质研究的一个重要目的是预测土的长期力学性质。在常规蠕变试验中,一般采取简单加载方式获得土的蠕变性质。在这些试验中,土样被加载到指定应力水平,然后试验持续进行。基于目前的技术水平,这种方式开展的蠕变试验一般不超过几年,而这与多数与土有关建筑物的设计寿命是不能相比的。因此提出一个可以加快蠕变试验的方法无疑是有重要意义的 [8, 15] 。 根据本文的讨论,土的蠕变性质取决于施加外力的大小和土的不可恢复应变。而土的不可恢复应变可通过蠕变过程或塑性变形过程产生。相比蠕变过程,塑性变形过程可在较短时间内获得较大的不可恢复变形。以图 8 中冰渍土的蠕变试验为例,如果采用常规蠕变试验,需要很长时间才能获得偏应力水平为 360 kPa 下后面两阶段( 360-2 和 360-3 )的蠕变数据。而本文试验中,通过加卸载循环,试样的不可恢复应变在较短时间内得以增加,从而得到了土的长期蠕变行为。根据式( 9 )可以得到阶段 360-2 和 360-3 的等效蠕变时间,即在常规蠕变试验中所对应时间。经过计算,在常规试验中到达阶段 360-2 和 360-3 起点所需时间分别为 28176 和 63875 h 。图 13 给出了这两个阶段在常规蠕变试验曲线上的位置。从图中不难看出,采用加快蠕变试验可以在较短时间内获得土的长期蠕变行为。 图 13 冰渍土各蠕变阶段在常规蠕变曲线上位置 Fig. 13 Positions of creep stages on creep curve of till obtained in .. conventional triaxial creep tests 5 结论与展望 ( 1 )提出采用不可恢复应变作为土变形的内变量。 ( 2 )开展了复杂条件下土的蠕变试验,具体讨论了不可恢复应变和应力对土蠕变性质的影响。 ( 3 )提出了土的内变量蠕变模型,并利用试验数据进行了验证。 ( 4 )讨论了土的内变量蠕变模型与传统时间硬化幂函数蠕变模型参数间的关系,提出了一种加快蠕变试验的方法。 然而,土的蠕变性质与土的种类、土所处应力应变状态和加载速率等因素均有密切关系。目前为止,本文模型只是在三轴正常固结黏土蠕变试验中得到了验证。因此,该模型是否适用于其他情况则有待于进一步验证。 [1] SINGH A, MITCHELL J K. 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