超深超大基坑回弹变形计算方法的试验研究
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摘要
随着我国城市化进程和城市地下空间开发利用,超深超大基坑工程越来越多。随着基坑开挖深度加大,基坑开挖中的回弹变形在沉降中所占的比例亦越高,如何准确预估基坑开挖回弹变形成为亟待解决的问题。目前回弹变形研究主要以工程实测结合室内土工试验成果分析预测基坑开挖回弹变形量与再压缩变形量。本文的研究以室内压缩回弹试验为基础,通过大量试验数据分析了土体卸荷比—回弹模量、卸荷比—回弹比率、卸荷比—回弹率之间的关系及不同土性土样回弹变形的差异；通过室内基坑开挖模型试验,探究新的回弹变形观测方法,对回弹变形在基底处及基底以下土体中的分布规律进行了分析研究,得出回弹变形基本规律；通过原位载荷试验,研究了土体在原位状态下卸荷回弹变形基本规律；在室内模型试验测试方法基础上加以改进后在现场实测中进行应用,一方面验证其在实际工程中的适用性,另一方面对实际工程条件下的回弹变形规律进行研究；结合压缩回弹试验、基坑开挖模型试验、现场实测试验成果,对回弹变形计算方法进行研究,探索新的计算方法。通过大量试验与数值分析,得出一些具有参考价值和实用意义的结论：
1.通过室内压缩回弹试验,得到土体回弹变形基本规律,土体的回弹变形与固结压力、卸荷比、土性密切相关。
(1)土样卸荷回弹过程中,当卸荷比R<0.4时,产生的回弹变形不到总回弹变形量的10%；当卸荷比增大至0.8时,已完成的回弹变形约占总回弹变形量的40%；而当卸荷比介于0.8-1.0时,发生的回弹量约占总回弹变形量的60%,这一阶段土体卸荷量较大,产生的回弹变形在总回弹变形中所占比例最高,处于此卸荷比范围内的土体是回弹变形发展最为强烈的区域,这一阶段也是回弹变形发展最快的阶段。
(2)土样再压缩过程中,当再加荷量为卸荷量的20%时,土样再压缩变形量已接近回弹变形量的40%-60%；之后土样再压缩变形量增长速率降低,当再加荷量为卸荷量40%时,土样再压缩变形量为回弹变形量的70%左右；当再加荷量为卸荷量的60%时,土样产生的再压缩变形量接近回弹变形量的90%。在土样再压缩过程中,再加荷初始阶段再压缩变形增长速率较大,之后增长速率随着加荷量的增加反而逐渐降低。当加荷量为卸荷量的80%时,再压缩变形量与回弹变量大致相等,当再加荷量与卸荷量相等时,再压缩变形量约为回弹变形量的1.2倍。
(3)在相同固结压力下,不同土性土样的回弹率存在明显差异。在相同固结压力下,淤泥及淤泥质土的最终回弹率最大,粘土和粉质粘土次之,砂土的最终回弹率最小。土体回弹变形具有一定滞后性,其滞后性与固结压力、卸荷比、土性密切相关。在相同固结压力下,随着时间的发展,淤泥及淤泥质土比粘性土、砂土表现出更为明显的回弹滞后性。土性也是影响土体回弹变形的主要因素之一。
(4)土体回弹变形是随着卸荷量逐渐增加土体回弹模量逐渐减小的过程。
2.刚性变形深标在模型试验回弹变形观测中取得良好效果,基坑工程空间效应对回弹变形的影响显著。
(1)基底以下同一深度处土体回弹变形沿基坑宽度的分布形状类似倒扣的“锅底形”,基坑中心点处最大,越靠近基坑边缘其回弹变形量越小。
(2)在模型试验中,在基底以下约0.57倍开挖深度处土体的回弹变形约占总回弹变形量的35%,在基底以下约一倍开挖深度处土体的回弹变形约为基底处回弹变形量的20%,可见在一倍开挖深度范围内回弹变形沿深度的衰减最快,这也是回弹变形发展最为剧烈的区域。在基底中心点下1.86倍开挖深度处土体的回弹变形约为基底处回弹变形的10%,该深度以下的土体回弹变形已不明显。
(3)当填土开始阶段,即再加荷荷载较小时,土体已产生明显的再压缩变形,当加荷量为开挖卸荷量的20%时,各点所产生的沉降已超过回弹变形量的40%,当再加荷至卸荷量的80%左右时,回弹变形被完全压缩。
3.通过工程实测,证明刚性变形深标对测试基底下一定深度处土体回弹变形具有实用性。
(1)实测结果表明回弹变形影响深度不仅与基坑开挖深度、基坑形状与尺寸有关,还与基底下一定深度范围内土层土性密切相关。在确定回弹变形影响深度时,基底以下土层工程地质状况也是关键因素之一。
(2)在基底处采用回弹标志进行回弹变形的观测准确性较高,对于基底下一定深度处土体回弹变形的观测,采用刚性变形深标进行观测较为有利,两种方法结合使用对于研究基底处及其以下一定深度处的回弹变形较为合理。
4.结合试验成果,充分利用土工试验中回弹模量与卸荷比之间的密切关系,针对现有回弹变形计算方法中确定回弹模量的方法加以改进,得到了预估基底开挖回弹变形的简化方法与数值计算方法,简化方法中没有考虑基坑开挖过程中开挖面以下土体剪应力的变化,更适于预估基坑中心点处回弹变形；而数值计算方法是通过调整建模计算过程,将土工试验成果应用于回弹变形计算,最终求得良好的回弹变形分布规律,不仅基坑中心点处,对于回弹变形在基坑内的分布规律也取得良好计算结果。
数值方法计算回弹变形要点为：
(1)根据工程地质条件,取样后针对土样埋深做相应固结压力下的压缩回弹试验,得到其卸荷比—回弹模量关系曲线,进而得到其相应表达式；
(2)根据工程实际状况进行数值计算,得其相应开挖步骤下的卸荷应力场,借助竖向应力的卸荷比这一参数,确定开挖面下各点土体卸荷状态,代入相应的卸荷比—回弹模量表达式取得其相应开挖工况下的回弹模量;
(3)重新采用与第二步相同工程条件下的数值计算过程,在每步开挖前将由土工试验得到的回弹模量赋予相应的单元体,之后进行开挖计算,求得此步开挖工况下土体回弹变形增量；
(4)各开挖工况下回弹变形增量累加即得基坑开挖最终回弹变形量。
结合以上主要结论,本文创新点如下：
1.提出回弹比率的概念,依据卸荷比—回弹比率与卸荷比—回弹模量变化关系,将土体卸荷回弹变形分为三个阶段：当卸荷比R<0.4时,卸荷量较小,此时土体回弹变形少于总回弹变形量的10%；随着土体卸荷量增大,当卸荷比增大至0.8时,土体回弹变形约占总回弹变形量的40%;当卸荷比介于0.8-1.0时为第三阶段,土体产生的回弹变形约占总回弹变形量的60%。
2.粉质粘土、粘土、砂土、淤泥及淤泥质土等在相同固结压力下,各种土的最终回弹率,淤泥及淤泥质土最大,粘土和粉质粘土次之,砂土最小。
3.土体回弹变形具有一定滞后性,且与土体土性有关。在相同固结压力下,淤泥及淤泥质土比粘性土、砂土表现出更为明显的回弹变形滞后性。
4.模型试验得出在基底以下约一倍开挖深度处土体的回弹变形约为基底处回弹变形量的20%,可见在一倍开挖深度范围内回弹变形沿深度的衰减最快；在基底中心点下1.86倍开挖深度处土体的回弹变形约为基底处回弹变形量的10%,该深度以下的土体回弹变形已不明显。
5.提出回弹变形工程实测的方法。基底处土体回弹变形采用埋设回弹标志,开挖后测量其回弹量；基底以下某一深度处土体的回弹变形观测采用设置刚性变形深标的方法更为合理。两种方法结合使用对于研究基底处及其以下一定深度处的回弹变形较为合理。
6.提出结合室内土工试验成果的回弹变形数值计算方法。
With the development of urbanization and underground space utilization, the scale of foundation pit become deeper and larger, in such case, rebound deformation of foundation pit excavation take a large proportion in the settlement, so sound rebound deformation becomes a urgent problem need to be solved. Recently, the researching methods on rebound deformation are mainly depended on field measurement, combined with the results from soil tests to forecast the rebound and recompression deformation. In this paper, compression and rebound test is performed, the relation between i) unloading ratio with resilience modulus; ii) unloading ratio with rebound proportion; iii) unloading ratio with rebound ratio; iv) the difference of rebound deformation for different soils are analyzed and discussed. Laboratory foundation pit excavation model experiment is carried out, new measuring method is introduced to analyze the distribution law of rebound deformation on the foundation base or below it, the basic rules of rebound deformation are received; Through in situ bearing test, the rebound deformation law of soil under in situ stress is analyzed. Based on measurement used in the laboratory model test, an improving measuring method is applied in field measurement to verify its applicability in practice, at the same time, the law of rebound deformation in engineering is investigated; Combination the achievements from compression and rebound test, foundation pit excavation modeling test and field measurement test, a new calculating method of rebound deformation is derived. Through the tests and numerical analysis, some primary conclusions can be drawn:
1. Through the compression and rebound test, the basic law of rebound deformation is derived; the rebound deformation of soil is closely related to the consolidation pressure; unloading ratio and the characteristic of soil.
(1) In the process of unloading:i) as the unloading ratio is less than 0.4, the rebound deformation are less than 10% of the total rebound deformation; ii) as the unloading ratio increase to 0.8, the rebound deformation is about 40% of the total ones; iii) as the unloading ratio is between 0.8 and 1.0, the rebound deformation is about 60% of the total ones; In this phase, the unloading capacity is much higher and the rebound deformation take a large proportion in the total deformation. The evolution of soil rebound deformation is much more severe within the unloading ratio between 0.8 and 1.0, in which the fastest development of rebound deformation can be found as well.
(2) In the process of recompression:i) as the reloading is 20% of unloading capacity, the recompression deformation is about 40% or 60% of rebound ones; and then the increase rate of recompression deformation reduced; ii) as the reloading is 40% of unloading capacity, the recompression deformation is about 70% of rebound ones; iii) as the reloading capacity is 60% of unloading capacity, the recompression deformation is 90% of rebound ones. In the beginning of reloading, the rate of recompression deformation increased, and then the increase rate of recompression formation reduced as the reloading capacity increase; iv) as the reloading capacity reached 80% of unloading capacity, the recompression is roughly equal to the rebound deformation; v) as the reloading capacity is equal with the unloading capacity, the recompression is 1.2 times to the rebound deformation.
(3) Under the same consolidation pressure, the rebound ratio of different soils is obviously different. The mucky soil is of maximum final rebound ratio, the clay and silty clay is lower, and the minimum rebound ratio can be found in sandy soil. The rebound deformation has the character of hysteresis, which is closely related to the consolidation pressure, unloading ratio and soil properties. Under the same consolidation pressure, the hysteresis of mucky soil is more obvious than clay and sandy soil with the time. The property of soil is the main factor influenced the rebound deformation as well.
(4) The rebound deformation of soil is the process of that the modulus of resilience become smaller as the unloading increased.
2. Deep deformation gauge is applied in laboratory model test to measure rebound deformation, the reasonable results are shown. The space effect of foundation pit has obvious influence on rebound deformation.
(1) In the same depth, the distribution shape of rebound deformation along the width of foundation base is like the bottom of the pot upturned, the closer to the edge of the foundation pit, the smaller rebound deformation was found, the maximum rebound deformation is found in center points.
(2) In the model test, the rebound deformation, in the one time of excavation depth, is about 20% of that in foundation base, it indicates the rebound deformation decrease fastest in this field. The rebound deformation, in the depth about 1.86 time of excavation depth, is about 10% of that in foundation base; however, the rebound deformation is not obvious below this level of depth.
(3) In the beginning of filling, although the reloading is lower, recompression deformation is obvious; i) as the reloading capacity is about 20% of unloading capacity in excavation, the settlement of each points is more than 40% of rebound deformation; ii) as the reloading capacity is about 80% of unloading capacity in excavation, the rebound deformation has been compressed completely, some settlement occurred.
3. Through field test, the deep deformation gauge has the advantages in the measurement of rebound deformation for deep soils under the foundation base.
(1) The results certify that the rebound deformation is closely related to the excavation depth; the shape and scale of foundation pit; the properties of soil in a certain depth below foundation base. The geology status of soil under the foundation base is also the important factor when definition the influence depth of rebound deformation.
(2) In the foundation base, measuring the rebound deformation by pre-buried rebound equipment before excavation is accurate; however, measuring the deformation of the soil in certain depth under foundation base, rigid deep deformation gauge is more favorable, so the combination of the methods is reasonable to measuring the rebound deformation of soil in any depth.
4. Combination with results of tests, full use of the close relation between unloading ratio and modulus of resilience, the method used to define modulus of resilience in existing calculating method is improved; the calculating approach for prediction rebound deformation in foundation excavation is derived. In this simple calculating method, the shear stress of soil under the excavation base during the excavation is ignored, so this method is more suitable for calculating the rebound deformation in centre points of foundation pit. However, in the numerical calculating method, the results of soil test are applied in the calculating method by adjusting the process of modeling and calculating, reasonable distribution of rebound deformation was found, which is not only for the points in the center of foundation pit, but also for other points within the pit.
The key points of numerical calculation for rebound deformation are:
(1) According to geological conditions, compression and rebound test under different consolidation pressure are performed, the curve of unloading ratio versus modulus of resilience can be drawn, and the corresponding formula is derived.
(2) According to engineering practice, numerical computation is carried out, the stress field of unloading in different excavation procedure is described, unloading state of each points under the excavation surface is described by using the parameter of vertical stress unloading ratio, and then substituted into the formula of unloading ratio-modulus of resilience, the modulus of resilience in the corresponding excavation status can be determined.
(3) The procedure of numerical computation is the same with second step, the modulus of resilience derived from the soil test is given to the corresponding unit before each step excavation, then computation performed, the increment of rebound deformation in each excavation step is obtained.
(4) Cumulation the rebound deformation of each excavation step, the final rebound deformation is obtained.
According to the above conclusions, the innovation of this paper as follows:
1. The concept of rebound proportion proposed, according to the relation of unloading ratio-rebound proportion and unloading ratio-modulus of resilience, the development of rebound deformation can be divided into three phases:i) as the unloading ratio is less than 0.4, the rebound deformation is about 10% of the total rebound deformation; ii) as the unloading ratio increased to 0.8, the rebound deformation is about 40% of the total rebound deformation; iii) as the unloading ratio is between 0.8 and 1.0, the rebound deformation is about 60% of the total rebound deformation.
2. Under the same consolidation pressure, the mucky soil is of maximum final rebound ratio, the clay and silty clay is lower, and the minimum rebound ratio can be found in sandy soil.
3. The rebound deformation has the character of hysteresis, it is closely related to the soil properties. Under the same consolidation pressure, the hysteresis of mucky soil is much more obvious than clay and sandy soil.
4. In the model test, the rebound deformation, in the depth about one time of excavation depth, is about 20% of the rebound deformation in foundation base, it indicates that the decrease of rebound deformation is fastest in the depth about one time of excavation depth under foundation base; the rebound deformation, in the depth about 1.86 time of excavation depth under foundation base, is about 10% of the rebound deformation in foundation base; the rebound deformation is not obvious below this level of depth.
5. The method of measuring rebound deformation proposed, in the foundation base, measuring the rebound deformation after excavation by pre-buried rebound equipment before excavation is accurate; in certain depth under foundation base, the deep deformation gauge used to measure the rebound deformation is seriously reasonable; so the combination of the methods is reasonable to measuring the rebound deformation of soil in any depth.
6. Combination the achievements of compression and rebound test, numerical computation method for calculating rebound deformation introduced.
1.通过室内压缩回弹试验,得到土体回弹变形基本规律,土体的回弹变形与固结压力、卸荷比、土性密切相关。
(1)土样卸荷回弹过程中,当卸荷比R<0.4时,产生的回弹变形不到总回弹变形量的10%；当卸荷比增大至0.8时,已完成的回弹变形约占总回弹变形量的40%；而当卸荷比介于0.8-1.0时,发生的回弹量约占总回弹变形量的60%,这一阶段土体卸荷量较大,产生的回弹变形在总回弹变形中所占比例最高,处于此卸荷比范围内的土体是回弹变形发展最为强烈的区域,这一阶段也是回弹变形发展最快的阶段。
(2)土样再压缩过程中,当再加荷量为卸荷量的20%时,土样再压缩变形量已接近回弹变形量的40%-60%；之后土样再压缩变形量增长速率降低,当再加荷量为卸荷量40%时,土样再压缩变形量为回弹变形量的70%左右；当再加荷量为卸荷量的60%时,土样产生的再压缩变形量接近回弹变形量的90%。在土样再压缩过程中,再加荷初始阶段再压缩变形增长速率较大,之后增长速率随着加荷量的增加反而逐渐降低。当加荷量为卸荷量的80%时,再压缩变形量与回弹变量大致相等,当再加荷量与卸荷量相等时,再压缩变形量约为回弹变形量的1.2倍。
(3)在相同固结压力下,不同土性土样的回弹率存在明显差异。在相同固结压力下,淤泥及淤泥质土的最终回弹率最大,粘土和粉质粘土次之,砂土的最终回弹率最小。土体回弹变形具有一定滞后性,其滞后性与固结压力、卸荷比、土性密切相关。在相同固结压力下,随着时间的发展,淤泥及淤泥质土比粘性土、砂土表现出更为明显的回弹滞后性。土性也是影响土体回弹变形的主要因素之一。
(4)土体回弹变形是随着卸荷量逐渐增加土体回弹模量逐渐减小的过程。
2.刚性变形深标在模型试验回弹变形观测中取得良好效果,基坑工程空间效应对回弹变形的影响显著。
(1)基底以下同一深度处土体回弹变形沿基坑宽度的分布形状类似倒扣的“锅底形”,基坑中心点处最大,越靠近基坑边缘其回弹变形量越小。
(2)在模型试验中,在基底以下约0.57倍开挖深度处土体的回弹变形约占总回弹变形量的35%,在基底以下约一倍开挖深度处土体的回弹变形约为基底处回弹变形量的20%,可见在一倍开挖深度范围内回弹变形沿深度的衰减最快,这也是回弹变形发展最为剧烈的区域。在基底中心点下1.86倍开挖深度处土体的回弹变形约为基底处回弹变形的10%,该深度以下的土体回弹变形已不明显。
(3)当填土开始阶段,即再加荷荷载较小时,土体已产生明显的再压缩变形,当加荷量为开挖卸荷量的20%时,各点所产生的沉降已超过回弹变形量的40%,当再加荷至卸荷量的80%左右时,回弹变形被完全压缩。
3.通过工程实测,证明刚性变形深标对测试基底下一定深度处土体回弹变形具有实用性。
(1)实测结果表明回弹变形影响深度不仅与基坑开挖深度、基坑形状与尺寸有关,还与基底下一定深度范围内土层土性密切相关。在确定回弹变形影响深度时,基底以下土层工程地质状况也是关键因素之一。
(2)在基底处采用回弹标志进行回弹变形的观测准确性较高,对于基底下一定深度处土体回弹变形的观测,采用刚性变形深标进行观测较为有利,两种方法结合使用对于研究基底处及其以下一定深度处的回弹变形较为合理。
4.结合试验成果,充分利用土工试验中回弹模量与卸荷比之间的密切关系,针对现有回弹变形计算方法中确定回弹模量的方法加以改进,得到了预估基底开挖回弹变形的简化方法与数值计算方法,简化方法中没有考虑基坑开挖过程中开挖面以下土体剪应力的变化,更适于预估基坑中心点处回弹变形；而数值计算方法是通过调整建模计算过程,将土工试验成果应用于回弹变形计算,最终求得良好的回弹变形分布规律,不仅基坑中心点处,对于回弹变形在基坑内的分布规律也取得良好计算结果。
数值方法计算回弹变形要点为：
(1)根据工程地质条件,取样后针对土样埋深做相应固结压力下的压缩回弹试验,得到其卸荷比—回弹模量关系曲线,进而得到其相应表达式；
(2)根据工程实际状况进行数值计算,得其相应开挖步骤下的卸荷应力场,借助竖向应力的卸荷比这一参数,确定开挖面下各点土体卸荷状态,代入相应的卸荷比—回弹模量表达式取得其相应开挖工况下的回弹模量;
(3)重新采用与第二步相同工程条件下的数值计算过程,在每步开挖前将由土工试验得到的回弹模量赋予相应的单元体,之后进行开挖计算,求得此步开挖工况下土体回弹变形增量；
(4)各开挖工况下回弹变形增量累加即得基坑开挖最终回弹变形量。
结合以上主要结论,本文创新点如下：
1.提出回弹比率的概念,依据卸荷比—回弹比率与卸荷比—回弹模量变化关系,将土体卸荷回弹变形分为三个阶段：当卸荷比R<0.4时,卸荷量较小,此时土体回弹变形少于总回弹变形量的10%；随着土体卸荷量增大,当卸荷比增大至0.8时,土体回弹变形约占总回弹变形量的40%;当卸荷比介于0.8-1.0时为第三阶段,土体产生的回弹变形约占总回弹变形量的60%。
2.粉质粘土、粘土、砂土、淤泥及淤泥质土等在相同固结压力下,各种土的最终回弹率,淤泥及淤泥质土最大,粘土和粉质粘土次之,砂土最小。
3.土体回弹变形具有一定滞后性,且与土体土性有关。在相同固结压力下,淤泥及淤泥质土比粘性土、砂土表现出更为明显的回弹变形滞后性。
4.模型试验得出在基底以下约一倍开挖深度处土体的回弹变形约为基底处回弹变形量的20%,可见在一倍开挖深度范围内回弹变形沿深度的衰减最快；在基底中心点下1.86倍开挖深度处土体的回弹变形约为基底处回弹变形量的10%,该深度以下的土体回弹变形已不明显。
5.提出回弹变形工程实测的方法。基底处土体回弹变形采用埋设回弹标志,开挖后测量其回弹量；基底以下某一深度处土体的回弹变形观测采用设置刚性变形深标的方法更为合理。两种方法结合使用对于研究基底处及其以下一定深度处的回弹变形较为合理。
6.提出结合室内土工试验成果的回弹变形数值计算方法。
With the development of urbanization and underground space utilization, the scale of foundation pit become deeper and larger, in such case, rebound deformation of foundation pit excavation take a large proportion in the settlement, so sound rebound deformation becomes a urgent problem need to be solved. Recently, the researching methods on rebound deformation are mainly depended on field measurement, combined with the results from soil tests to forecast the rebound and recompression deformation. In this paper, compression and rebound test is performed, the relation between i) unloading ratio with resilience modulus; ii) unloading ratio with rebound proportion; iii) unloading ratio with rebound ratio; iv) the difference of rebound deformation for different soils are analyzed and discussed. Laboratory foundation pit excavation model experiment is carried out, new measuring method is introduced to analyze the distribution law of rebound deformation on the foundation base or below it, the basic rules of rebound deformation are received; Through in situ bearing test, the rebound deformation law of soil under in situ stress is analyzed. Based on measurement used in the laboratory model test, an improving measuring method is applied in field measurement to verify its applicability in practice, at the same time, the law of rebound deformation in engineering is investigated; Combination the achievements from compression and rebound test, foundation pit excavation modeling test and field measurement test, a new calculating method of rebound deformation is derived. Through the tests and numerical analysis, some primary conclusions can be drawn:
1. Through the compression and rebound test, the basic law of rebound deformation is derived; the rebound deformation of soil is closely related to the consolidation pressure; unloading ratio and the characteristic of soil.
(1) In the process of unloading:i) as the unloading ratio is less than 0.4, the rebound deformation are less than 10% of the total rebound deformation; ii) as the unloading ratio increase to 0.8, the rebound deformation is about 40% of the total ones; iii) as the unloading ratio is between 0.8 and 1.0, the rebound deformation is about 60% of the total ones; In this phase, the unloading capacity is much higher and the rebound deformation take a large proportion in the total deformation. The evolution of soil rebound deformation is much more severe within the unloading ratio between 0.8 and 1.0, in which the fastest development of rebound deformation can be found as well.
(2) In the process of recompression:i) as the reloading is 20% of unloading capacity, the recompression deformation is about 40% or 60% of rebound ones; and then the increase rate of recompression deformation reduced; ii) as the reloading is 40% of unloading capacity, the recompression deformation is about 70% of rebound ones; iii) as the reloading capacity is 60% of unloading capacity, the recompression deformation is 90% of rebound ones. In the beginning of reloading, the rate of recompression deformation increased, and then the increase rate of recompression formation reduced as the reloading capacity increase; iv) as the reloading capacity reached 80% of unloading capacity, the recompression is roughly equal to the rebound deformation; v) as the reloading capacity is equal with the unloading capacity, the recompression is 1.2 times to the rebound deformation.
(3) Under the same consolidation pressure, the rebound ratio of different soils is obviously different. The mucky soil is of maximum final rebound ratio, the clay and silty clay is lower, and the minimum rebound ratio can be found in sandy soil. The rebound deformation has the character of hysteresis, which is closely related to the consolidation pressure, unloading ratio and soil properties. Under the same consolidation pressure, the hysteresis of mucky soil is more obvious than clay and sandy soil with the time. The property of soil is the main factor influenced the rebound deformation as well.
(4) The rebound deformation of soil is the process of that the modulus of resilience become smaller as the unloading increased.
2. Deep deformation gauge is applied in laboratory model test to measure rebound deformation, the reasonable results are shown. The space effect of foundation pit has obvious influence on rebound deformation.
(1) In the same depth, the distribution shape of rebound deformation along the width of foundation base is like the bottom of the pot upturned, the closer to the edge of the foundation pit, the smaller rebound deformation was found, the maximum rebound deformation is found in center points.
(2) In the model test, the rebound deformation, in the one time of excavation depth, is about 20% of that in foundation base, it indicates the rebound deformation decrease fastest in this field. The rebound deformation, in the depth about 1.86 time of excavation depth, is about 10% of that in foundation base; however, the rebound deformation is not obvious below this level of depth.
(3) In the beginning of filling, although the reloading is lower, recompression deformation is obvious; i) as the reloading capacity is about 20% of unloading capacity in excavation, the settlement of each points is more than 40% of rebound deformation; ii) as the reloading capacity is about 80% of unloading capacity in excavation, the rebound deformation has been compressed completely, some settlement occurred.
3. Through field test, the deep deformation gauge has the advantages in the measurement of rebound deformation for deep soils under the foundation base.
(1) The results certify that the rebound deformation is closely related to the excavation depth; the shape and scale of foundation pit; the properties of soil in a certain depth below foundation base. The geology status of soil under the foundation base is also the important factor when definition the influence depth of rebound deformation.
(2) In the foundation base, measuring the rebound deformation by pre-buried rebound equipment before excavation is accurate; however, measuring the deformation of the soil in certain depth under foundation base, rigid deep deformation gauge is more favorable, so the combination of the methods is reasonable to measuring the rebound deformation of soil in any depth.
4. Combination with results of tests, full use of the close relation between unloading ratio and modulus of resilience, the method used to define modulus of resilience in existing calculating method is improved; the calculating approach for prediction rebound deformation in foundation excavation is derived. In this simple calculating method, the shear stress of soil under the excavation base during the excavation is ignored, so this method is more suitable for calculating the rebound deformation in centre points of foundation pit. However, in the numerical calculating method, the results of soil test are applied in the calculating method by adjusting the process of modeling and calculating, reasonable distribution of rebound deformation was found, which is not only for the points in the center of foundation pit, but also for other points within the pit.
The key points of numerical calculation for rebound deformation are:
(1) According to geological conditions, compression and rebound test under different consolidation pressure are performed, the curve of unloading ratio versus modulus of resilience can be drawn, and the corresponding formula is derived.
(2) According to engineering practice, numerical computation is carried out, the stress field of unloading in different excavation procedure is described, unloading state of each points under the excavation surface is described by using the parameter of vertical stress unloading ratio, and then substituted into the formula of unloading ratio-modulus of resilience, the modulus of resilience in the corresponding excavation status can be determined.
(3) The procedure of numerical computation is the same with second step, the modulus of resilience derived from the soil test is given to the corresponding unit before each step excavation, then computation performed, the increment of rebound deformation in each excavation step is obtained.
(4) Cumulation the rebound deformation of each excavation step, the final rebound deformation is obtained.
According to the above conclusions, the innovation of this paper as follows:
1. The concept of rebound proportion proposed, according to the relation of unloading ratio-rebound proportion and unloading ratio-modulus of resilience, the development of rebound deformation can be divided into three phases:i) as the unloading ratio is less than 0.4, the rebound deformation is about 10% of the total rebound deformation; ii) as the unloading ratio increased to 0.8, the rebound deformation is about 40% of the total rebound deformation; iii) as the unloading ratio is between 0.8 and 1.0, the rebound deformation is about 60% of the total rebound deformation.
2. Under the same consolidation pressure, the mucky soil is of maximum final rebound ratio, the clay and silty clay is lower, and the minimum rebound ratio can be found in sandy soil.
3. The rebound deformation has the character of hysteresis, it is closely related to the soil properties. Under the same consolidation pressure, the hysteresis of mucky soil is much more obvious than clay and sandy soil.
4. In the model test, the rebound deformation, in the depth about one time of excavation depth, is about 20% of the rebound deformation in foundation base, it indicates that the decrease of rebound deformation is fastest in the depth about one time of excavation depth under foundation base; the rebound deformation, in the depth about 1.86 time of excavation depth under foundation base, is about 10% of the rebound deformation in foundation base; the rebound deformation is not obvious below this level of depth.
5. The method of measuring rebound deformation proposed, in the foundation base, measuring the rebound deformation after excavation by pre-buried rebound equipment before excavation is accurate; in certain depth under foundation base, the deep deformation gauge used to measure the rebound deformation is seriously reasonable; so the combination of the methods is reasonable to measuring the rebound deformation of soil in any depth.
6. Combination the achievements of compression and rebound test, numerical computation method for calculating rebound deformation introduced.
引文
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