非饱和土试样优选及循环荷载作用下的变形特性研究

发布时间：2018-03-29 16:47

本文选题：非饱和土　切入点：SWCC　出处：《大连理工大学》2013年博士论文

【摘要】：无论是受不断出现的实际工程问题的驱使还是遵循从简单到复杂的科学发展规律,当前土力学研究的焦点已转向更为广泛也更具代表性的非饱和土领域。目前,基于重塑试样的室内试验是研究非饱和土的主要手段之一,而对于试验设备昂贵、试验耗时严重的非饱和土研究,试样是否均一已成为非饱和土试验结果能否有效应用与分析、研究的关键性问题。传统观念认为相同制样条件下的重塑试样都是均一的,但这些试样在非饱和条件下却可能因孔隙结构的不同而产生差异,而这种差异在实际研究中并未考虑。本文从理论分析出发,通过压力板仪测量多个试样的土水特征曲线(SWCC),确定相同基质吸力下试样的平衡饱和度存在差异,即试样在非饱和状态下并不均一；根据直剪试验确定试样平衡饱和度差异将对试样的力学性质造成影响；通过调整初始制样条件,改善试样均一性,分析土水特征曲线的变化以及试样尺寸的影响,并对试验过程中影响土水特征曲线的各种因素进行分析总结；综合考虑脱湿试验和无侧限抗压强度试验的结果,分析饱和度-时间关系曲线与非饱和土试样力学性质之间的关系,根据研究成果提出非饱和土试样均一性的判别准则；根据非饱和土试样均一性的判别准则对试样进行优选,采用二次改进的“土工静力-动力液压三轴-扭转多功能剪切仪”对优选试样进行循环加载试验,分析循环荷载作用下非饱和土试样的累积变形、动孔隙水压力、体变等与循环应力、基质吸力、固结压力的关系。基于上述研究,得到如下结论： (1)通过双孔隙模型,从理论角度分析得出同一基质吸力下试样可能具有明显不同的平衡饱和度；对四个三轴试样的SWCC进行对比分析,证实平衡饱和度差异的确存在。拟合结果表明由于引入修正系数C(ψ),因此FredlundXing模型更为合理。基于压力板仪与直剪仪的组合试验证实平衡饱和度差异对试样的力学性质将产生影响,因此必须重视平衡饱和度差异的影响。 (2)通过改变试样的初始条件,可以在一定程度上减弱试样差异性对SWCC的影响,即试样均一性提高,但是这种试样差异性仍难以消除。同时,试验结果表明：颗粒级配不良也将引起孔隙分布不均,导致在一定基质吸力范围内SWCC的差异明显；蒸发效应在压力板仪试验中持续存在,并在平衡后期成为失水的重要因素；高径比较大的试样对侧面防护有明显的依赖性,在没有侧面防护时,蒸发将成为失水的主要因素,因此笔者认为在非饱和土研究中采用完整的亚克力制作的重塑筒作为侧面防护值得推广。 (3)根据脱湿试验与无侧限抗压强度试验的结果,对饱和度-平衡时间关系曲线和非饱和土试样力学性质之间的关系进行对比分析,结果表明：当试样的饱和度-平衡时间关系曲线一致时,它们力学性能以及孔隙结构几乎一致,可认为这些试样是均一的；在相同基质吸力下,平衡饱和度较低的试样具有更稳定的力学性质。基于上述结论提出非饱和土试样均一性的判别准则,并指出应选用平衡饱和度较低的试样。通过总结制样方法以及土的基本特性,针对制样不均、干密度难以控制、试样在包裹橡胶膜等过程中易受扰动等问题,设计了解决上述问题的三轴制样器。该三轴制样器将对开模定位、密封合为一体,提高试样尺寸精度与密封效果；通过两级控制模式保证试样的干密度；并通过对开模保护试样,消除了额外扰动对试样的影响。 (4)对临界循环应力作用下的累积变形进行研究,结果表明：随着基质吸力的增大,试样的累积应变逐渐减小,并趋于稳定；在基质吸力一定时,试样累积应变随固结压力的增大而增大。进一步分析表明在指定固结压力下,非饱和土试样的初始应变与累积应变的比值为一定值,而初始应变与基质吸力呈线性关系,因此可根据饱和试样的初始应变预测一定基质吸力范围内的非饱和土试样在临界循环应力作用下的累积应变。 (5)对100kPa基质吸力和100kPa固结压力下的累积变形进行研究,结果表明：在特定基质吸力范围内,拉伸破坏将成为非饱和土试样动力失稳的主要因素,并且土体薄弱处成为拉伸破坏发生的主要区域；拉伸破坏与循环应力密切相关,同时拉伸破坏也明显受到基质吸力的制约,因此试样累积变形与基质吸力未呈现单调变化,而是受到循环应力与基质吸力综合作用,即拉伸破坏不存在基质吸力的临界值,而是随基质吸力增大出现拐点,这里将基质吸力的拐点定义为关键基质吸力。在本次试验中的关键基质吸力为100kPa。 (6)从循环应力、基质吸力、固结压力三方面对循环荷载作用下的孔隙水压力与体变进行对比分析,研究表明：非饱和土的孔隙水压力受到基质吸力的明显影响；在低基质吸力下,非饱和土试样性质近似于饱和土,依然会产生超孔隙水压力,并表现出随循环应力增大而增大的趋势,但在高基质吸力下其变化明显受限,随循环应力增大规律性并不明显。非饱和土试样的体变受循环应力影响明显,特别在rc=0.9时由试样由体缩突变为体胀,并且这种变化不受基质吸力和固结压力的影响,这一现象进一步表明拉应力对非饱和土的影响显著。
[Abstract]:Whether it is driven by the emergence of practical engineering problems or follow the scientific development rule from simple to complex, the current focus of the study of soil mechanics has shifted to broader and more representative of the unsaturated soil. At present, the indoor test sample based on remodeling is one of the main means of unsaturated soil, and for the test equipment is expensive, time-consuming test of unsaturated soil research, if the specimen has become uniform unsaturated soil test results can be effective application and analysis, a key problem in the research. The traditional idea that the remolded samples under the condition of the same sample preparation is homogeneous, but these samples in non saturated conditions may produce difference because the pore structure is different, and the difference in the actual research is not taken into account. Based on the theoretical analysis of the soil water characteristic curve measurement of multiple samples by the pressure plate instrument (SWCC),. The balance of the saturation of sample under the same matric suction are different, the sample is not uniform in the unsaturated state; according to the direct shear test to determine the sample equilibrium saturation difference will affect the mechanical properties of the sample; by adjusting the initial conditions of sample preparation, improve sample homogeneity, change analysis of soil water characteristic curve and the influence of the specimen size and various factors which affect the soil water characteristic curve during the test were analyzed and summarized; considering the wetting test and unconfined compressive strength test results, analysis of the relationship between saturation time curve and unsaturated soil mechanical properties, according to the research results of unsaturated soil sample homogeneity criterion; according to the non saturated soil sample homogeneity criterion to optimize the sample, using two improved soil static and dynamic universal triaxial and torsional power three Cyclic shear tests were carried out on the selected samples. The cumulative deformation, the dynamic pore water pressure and the volumetric strain of unsaturated soil samples under cyclic loading were related to the cyclic stress, the matrix suction and the consolidation pressure. Based on the above research, the following conclusions were reached.
(1) through the double porosity model from the angle of theory analysis, it is concluded that the same suction specimens under different saturation may have obvious balance; on the four axis three specimens of SWCC were analyzed, confirmed the equilibrium saturation differences do exist. The fitting results show that due to the introduction of the correction coefficient C (PSI), so the FredlundXing model is more reasonable combination test. The pressure plate instrument and direct shear apparatus that mechanical properties of equilibrium saturation differences on the specimen will have an impact on, so we must attach importance to the influence of equilibrium saturation difference.
(2) the change of the initial conditions, can affect the sample difference of SWCC decreases to a certain extent, the homogeneity of the sample increased, but the difference of the sample is still difficult to eliminate. At the same time, experimental results show that the particle size distribution is bad will cause uneven distribution of pore, lead to differences in SWCC in a certain range of matric suction the obvious; evaporation effect persists in pressure plate test, and become an important factor in the water balance later; samples of high diameter is significantly dependent on side shields, with no side door, evaporation will become the main factor of water loss, so I think in the study of unsaturated remodeling tube the soil with complete production of acrylic as a side door is worthy of promotion.
(3) according to the desorption test and unconfined compressive strength test results of saturation equilibrium time curve and unsaturated soil samples the relationship between mechanical properties were analyzed, the results show that when the sample saturation equilibrium time curve is consistent, their mechanical properties and pore structure can be almost the same. Think of these specimens is uniform; under the same suction, the sample equilibrium saturation with lower mechanical properties more stable. Based on the above conclusion to put forward the criterion of unsaturated soil sample homogeneity, and points out that we should choose sample equilibrium saturation is low. By summarizing the characteristics of sample preparation method and system for soil. Not all, dry density is difficult to control, the disturbed sample problems such as the package rubber membrane process, three axis sample making device to solve the above problems. The design of three axis sample making device of mold Positioning, sealing and integration as a whole, improve the size accuracy and sealing effect of the sample, ensure the dry density of the sample through the two level control mode, and eliminate the influence of additional disturbance on the specimen by opening the mold to protect the sample.
(4) the critical cyclic stress accumulation under the action of stress deformation was studied. The results show that with the increase of suction, cumulative strain of specimen decreases, and tends to be stable; in some specimens of matric suction, cumulative strain with the consolidation pressure increases. Further analysis showed that in the specified consolidation pressure, the ratio of the initial strain and strain accumulation of unsaturated soil samples to a certain value, and the initial strain and the matrix suction is linear, so according to the initial strain of saturated prediction of the matric suction range of unsaturated soil samples in the critical cyclic stress strain under the action of force.
(5) the cumulative 100kPa matric suction and 100kPa consolidation deformation under pressure is studied, the results showed that: in particular the matric suction range, the tensile failure of unsaturated soil specimen power will become the major factor for instability, and weak soil become the main area of tensile failure occurs; tensile failure and cyclic stress is closely related. At the same time, the tensile failure also obviously constrained by the matrix suction, so the sample cumulative deformation and suction is not monotonic, but under cyclic stress and matric suction, the critical value of tensile failure does not exist in the matric suction, but with the increase of matric suction inflection point, here will be defined as the key point of suction suction the key of matric suction in this experiment is 100kPa.
(6) from the cyclic stress, matrix suction and consolidation pressure of three aspects of comparative analysis, the cyclic loading of pore water pressure and volume change study shows that the effect of pore water pressure of unsaturated soil by suction; in low suction, unsaturated soil specimen properties similar to the saturated soil still, will produce excess pore water pressure, and with the cyclic stress increasing, but at high matric suction changes under cyclic stress was limited, with increasing regularity is not obvious. The volume change of unsaturated soil specimen under cyclic stress significantly, especially in the rc=0.9 by the sample by volume contraction for expansion and mutation, this change is not affected by the influence of matric suction and consolidation pressure, this phenomenon further showed that the influence of tensile stress on unsaturated soil significantly.

【学位授予单位】：大连理工大学
【学位级别】：博士
【学位授予年份】：2013
【分类号】：TU41

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