| 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. |
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