Foundation settlement calculation method considering construction leveling

Abstract

A kind of foundation settlement calculation method considering construction leveling, comprising the steps of: determining the unloading amount, depth range and layer thickness of foundation pit excavation area;According to Boussinesq solution, the additional stress of the center of the jth layer soil caused by foundation pit unloading is calculated, and the self-weight stress distribution of foundation is calculated, the additional stress is obtained by MATLAB programming calculation;The total rebound amount is obtained by layer summation method;The soil particle volume of layer soil thickness is obtained based on the total rebound amount;The lower limit of soil plasticity index limit is taken as the moisture content of the rebound part of soil body, and the soil particle volume of the rebound part of soil body is obtained;The volume of the first layer of residual soil particles after construction leveling is obtained;The first layer settlement is calculated, and the total settlement after construction leveling is obtained by adding the settlement of other layers.The present application considers the construction leveling factor in foundation pit engineering to calculate the foundation settlement, deduces the pore ratio of pit bottom after construction leveling in theory, and saves a lot of time by programming calculation of additional stress, and avoids the error of manual table lookup value.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of soft soil foundation settlement monitoring, and particularly relates to a foundation settlement calculation method considering construction leveling. BACKGROUND

[0002] With the acceleration of urban construction, the utilization rate of land resources is getting higher and higher, and the demand for underground shopping malls and parking lots is increasing, so the foundation pit engineering develops to a larger and deeper field. The foundation settlement is one of the important factors affecting the safety of foundation engineering, and the settlement exceeding the allowable value will affect the normal use of buildings. At present, although there are many theoretical methods for calculating foundation settlement, such as the layer summation method, residual stress method and complex function method, these methods do not consider the factor that the rebounding soil is dug out due to construction leveling. However, the foundation pit excavation is a process of unloading in the soil, which will inevitably break the original stress field and cause the rebound deformation of the foundation pit, especially the rebound amount of the deep foundation pit is relatively large. The existing theoretical calculation method of settlement does not consider the situation that the soil rebounding part is dug out in order to level during the excavation process, and the compensation foundation or over-compensation foundation, which theoretically will not appear settlement, but the actual measurement proves that there is still settlement. At present, researchers have analyzed the mechanism of compensation foundation from the aspect that the soil properties at the bottom of the foundation pit change greatly after the construction leveling, but no calculation method is given. The traditional layer summation method calculates the stress by checking the table to obtain the additional stress coefficient through the size of the foundation pit, which is time-consuming and has the error of manual table checking. SUMMARY

[0003] The technical problem to be solved by the present application is to provide a foundation settlement calculation method considering construction leveling, which considers the construction leveling factor in the real foundation pit engineering to calculate the foundation settlement, and theoretically deduces the pore ratio at the bottom of the foundation pit after construction leveling, so as to calculate the settlement after construction leveling according to the layer summation method, greatly saving the table checking time, and avoiding the error of manual table checking value.

[0004] The technical scheme adopted by the present application to solve the above technical problem is:

[0005] A foundation settlement calculation method considering construction leveling, comprising the following steps:

[0006] S1, determining the unloading amount of the foundation pit excavation area BxL , and respectively represent the unit effective gravity and thickness of the soil layer above the bottom of the corresponding excavation layer th layer;

[0007] S2, determining the layer thickness in the settlement calculation depth range;

[0008] S3. Determine the depth range for settlement calculation, and take the ratio of additional stress to self-weight stress as 0.1;

[0009] S4. Calculate the first step caused by foundation pit unloading based on Boussinesq solution. Additional stress at the center of the soil layer The distribution of foundation self-weight stress was calculated, and the additional stress was obtained through MATLAB programming.

[0010] S5. Calculate the rebound amount using the layered summation method. The formula for the rebound amount of each soil layer is as follows: ; It is the rebound amount of the j-th layer (the center or sidewall of the foundation pit); It is the first Layer thickness; It is the spring modulus of the j-th layer; It is the first The resilience coefficient of the layer; It is based on the instantaneous excavation. The porosity is obtained from the layer's self-weight stress curve using the ep curve. It is the first time that long-term excavation has been carried out. The void ratio is obtained from the ep curve of the layer's self-weight stress; since the rebound shape of the foundation pit is convex, the rebound amount at the center of the foundation pit is also calculated. and the rebound amount of the foundation pit sidewall Then calculate the average value to represent the average rebound of the j-th layer. , The total rebound amount is obtained by summing the average rebound amounts of each layer. (Represents the amount of rebound when the foundation pit is leveled).

[0011] S6. Calculate the total rebound amount based on step S5. According to the formula Find the volume of soil particles for each layer thickness. denoted as , where is the thickness of the first layer of soil at the bottom of the pit; e is the void ratio obtained by checking the ep curve based on the self-weight stress after the pit has been exposed for a long time since excavation. It is the pore water volume after rebound (before construction leveling); B represents the volume of soil particles after rebound (before leveling during construction), and L represents the width of the foundation pit and the length of the foundation pit.

[0012] S7. Take the lower limit of the soil plasticity index as the moisture content of the rebound portion of the soil. Calculate the volume of soil particles in the rebound portion of the soil. , For soil particle density, The density of water;

[0013] S8. Based on the calculation results of steps S6 and S7, calculate the volume of the remaining soil particles after the first layer has been leveled during construction. Then according to the formula Calculate the void ratio after leveling during construction. To determine the pore water volume after leveling during construction, the soil is assumed to be saturated. ;

[0014] S9. Calculate the settlement of the first layer, and add it to the settlement of the other layers to obtain the total settlement after leveling. The formula for calculating the settlement is: ,in, It is the first The compressive modulus of the layer; It is the first The compressibility coefficient of the layer; It is based on the first The porosity is obtained from the layer's self-weight stress curve using the ep curve. It is the first The porosity is obtained by referring to the ep curve from the sum of the layer's self-weight stress and the additional stress. It is the first under structural load Additional stress in the layer; n is the number of layers.

[0015] According to the above plan, it also includes step S10, considering the settlement at a certain moment during construction leveling: ,in, It is settlement caused by long-term excavation. It is settlement caused by instantaneous excavation. It is the one-dimensional consolidation degree of Terzaghi. ; , , It is the permeability coefficient. It is the compressibility modulus. Here, t is the specific gravity of water, t is time, and H is the depth at which the calculation is performed.

[0016] According to the above scheme, in step S2, the layer thickness is no more than 0.4 times the width of the foundation pit or no more than 4m.

[0017] According to the above scheme, in step S4, additional stress The equation is as follows:

[0018]

[0019] In the formula: The area of ​​the foundation pit excavation. p is the integral variable; when calculating the rebound, p is the excavation load of the foundation pit, and when calculating the settlement, p is the structural load. These are the coordinates of the calculated point in a rectangular coordinate system.

[0020] Compared with the prior art, the present application has the beneficial effects that:

[0021] 1、The present application does not consider two situations of construction leveling and does not consider construction leveling, and the settlement is calculated according to the layering summation method, and the two situations are divided into instantaneous excavation and long-time excavation, and the actual excavation situation is between the instantaneous excavation and the long-time excavation, the settlement of the center of the actual foundation pit is calculated by introducing the one-dimensional consolidation degree of Terzaghi; the instantaneous excavation and the long-time excavation are calculated respectively, the instantaneous excavation means that the foundation pit is immediately excavated and the foundation and the upper structure are constructed, because the time is instantaneous, the stress unloading of the foundation pit is not released in time, and the structure load is directly added, the foundation pit will not rebound, and therefore there is no construction leveling situation; the long-time excavation is that the foundation pit keeps the excavation state for a long time, and the stress of the soil body at the bottom of the foundation pit is released, so that the soil body has enough time to rebound, the present application starts from the fact that the rebounding soil body is excavated in order to level the construction process, thereby greatly changing the shape of the soil body at the bottom of the pit, on the basis of the simple layering summation method, the characteristics of the soil body at the bottom of the pit caused by the construction leveling are considered, the characteristics of the change of the void ratio are analyzed from the microcosmic aspect, the void ratio of the first layer after the construction leveling is derived, and the foundation settlement is calculated according to the void ratio, the traditional e-p curve is used to calculate the foundation settlement, the parameters are few, and the method is suitable for various soils with the existing e-p curve, and the foundation settlement can be predicted and calculated;

[0022] 2、Compared with the traditional stress calculation through the size of the foundation pit, the present application theoretically derives the void ratio of the pit bottom after the construction leveling, so as to calculate the settlement after the construction leveling according to the layering summation method, the two situations are only different in the starting point of the self-weight stress calculation, other steps are the same, the one-dimensional consolidation degree of Terzaghi is introduced to consider the actual foundation pit situation, and the additional stress value of the soil body below the pit bottom caused by the foundation pit excavation and the structure load is quickly and accurately calculated through the MATLAB programming, the table searching time is greatly saved, and the error caused by the manual table searching value is also avoided;

[0023] 3、According to the example result, the factor of the construction leveling should be considered in the calculation of the foundation settlement, especially in the foundation pit excavation near the subway tunnel, the deformation of the subway tunnel is controlled in mm, and in this case, the construction leveling of the foundation pit should be considered. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 It is a foundation pit construction leveling schematic diagram;

[0025] Figure 2 It is a foundation pit excavation model schematic diagram of the embodiment of the present application;

[0026] Figure 3 It is a foundation pit profile and a calculation depth layering diagram of the embodiment of the present application;

[0027] Figure 4 This is a diagram showing the distribution of self-weight stress and additional stress in the instantaneous excavation stress calculation of an embodiment of the present invention;

[0028] Figure 5 This is a diagram showing the distribution of self-weight stress and additional stress during long-term excavation stress calculation in an embodiment of the present invention.

[0029] Figure 6 This is a graph showing the relationship between loading p and porosity e in an indoor test according to an embodiment of the present invention.

[0030] Figure 7 This is a graph showing the relationship between the degree of consolidation and time in an embodiment of the present invention. Detailed Implementation

[0031] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0032] For ease of calculation, the following assumptions are made: After the foundation pit has fully rebounded, the rebounded portion of the excavation is considered as another excavation, i.e., leveling. Only the first rebound is considered; subsequent rebounds are negligible. This excavation deteriorates the soil quality at the bottom of the pit, increasing the void ratio. Since leveling only occurs in the first layer of the layered soil, the greatest change in soil quality occurs in this layer. Therefore, only the change in void ratio in the first layer is calculated to obtain the average void ratio change for that layer. The void ratio changes in other layers are minor and can be considered constant. The changes in self-weight stress and additional stress caused by leveling are small and negligible. In actual projects, leveling often leads to over-excavation of the foundation pit, such as... Figure 1 As shown.

[0033] The steps are illustrated through numerical examples, such as... Figure 2 As shown, a two-story underground building is to be constructed on a single, deep, soft soil foundation formed by natural sedimentation. The proposed excavation pit dimensions are L=14m, B=10m, and h=5m; the foundation soil is silty clay with a natural unit weight of [missing information]. saturated bulk density The groundwater level is located at the surface, and the structural load... The foundation pit adopts a certain type of support structure (the influence of the support structure is not considered in the calculation). Surface soil samples were taken for indoor compression tests, and the data obtained are as follows: Figure 6 See Table 1.

[0034] Table 1. Hysteresis data of ep curves at different unloading points.

[0035]

[0036] The following results were obtained based on the calculation steps:

[0037] (1) Example: Excavation depth of 5m in the foundation pit, the foundation pit profile and calculation depth are as follows: Figure 3As shown, the basic assumption is that the soil is saturated, so the effective unit weight is ; the unloading amount of the foundation pit excavation area of 10m x 14m ;

[0038] (2) Basic assumption: stratify the foundation within the range of the main compression layer, and each layer of soil is homogeneously distributed; the example is homogeneous soil, and the stratification thickness is generally not greater than 0.4 times the foundation width B or not greater than 4m; according to the aforementioned excavation area, the stratification thickness is 2.5m;

[0039] (3) Calculate the calculation depth from the perspectives of instantaneous excavation and long-time excavation, obtain the self-weight stress and additional stress of the center of each layer of soil thickness according to the Boussinesq solution, and the stress calculation of instantaneous excavation and long-time excavation are shown in Figure 4 and Figure 5 respectively; the calculation results of instantaneous excavation and long-time excavation are shown in Table 2 and Table 3 respectively; the calculation depth of instantaneous excavation is 18.75m (calculated from the pit bottom), and there are 8 layers in total; the calculation depth of long-time excavation is 21.25m (calculated from the pit bottom), and there are 9 layers in total.

[0040] Table 2 Stress calculation table for instantaneous excavation

[0041]

[0042] Table 3 Stress calculation table for long-time excavation

[0043]

[0044] From the data in Table 2 and Table 3, it can be concluded that when the foundation pit is unloaded instantaneously, the stress in the soil cannot be released in time and is directly offset by the structural load, while when the foundation pit is excavated for a long time, the stress in the soil is fully released, causing the soil to rebound, and then the structural load is added, which increases the depth of influence of the structural load. This shows that in real engineering, the structural bottom plate should be poured as soon as possible after the completion of foundation pit excavation to reduce settlement, but it is impossible to achieve instantaneous excavation in real foundation pit engineering. Therefore, the unloading of the soil inside the foundation pit causes the soil to rebound, and in order to level, the rebounding part of the soil is excavated.

[0045] (4) Calculate the additional stress of the soil below the pit bottom using the Boussinesq solution, and according to the self-weight stress calculated by instantaneous excavation and long-time excavation, look up Table 1 to obtain the void ratio by linear interpolation method ; according to the self-weight stress calculated by instantaneous excavation and long-time excavation + structural load , look up Table 1 to obtain the void ratio by linear interpolation method ; then according to the formula of calculation step S9, the foundation settlement can be calculated without considering the construction leveling factor, and the calculation results of instantaneous excavation settlement and long-time excavation settlement without considering the construction leveling factor are shown in Table 4 and Table 5 respectively;

[0046] Table 4: Instantaneous excavation settlement calculation table without considering construction leveling factors

[0047]

[0048] Table 5: Long-time excavation settlement calculation table without considering construction leveling factors

[0049]

[0050] (5) Calculate the rebound amount according to the formula in step S5. Since the shape of the rebound amount is irregular, the rebound amounts of the foundation pit center and the foundation pit side wall are calculated separately, and an average value is calculated to represent the foundation pit shape rebound amount. The rebound amount is obtained according to the following table is 4.365 cm, and the calculation process of the rebound amount of the foundation pit center and the foundation pit side wall is shown in Table 6 and Table 7, respectively;

[0051] Table 6: Foundation pit center rebound amount calculation table

[0052]

[0053] Table 7: Foundation pit side wall rebound amount calculation table

[0054]

[0055] Note: is the void ratio obtained from Table 1 according to the instantaneous excavation self-weight; is the void ratio obtained from Table 1 according to the long-time excavation self-weight.

[0056] (6) Calculate the soil particle volume of the first layer of soil according to the formula , e is the void ratio obtained from Table 1 according to the first layer self-weight stress, and according to , obtain from Table 1; is the pore water volume after rebound (before construction leveling); is the soil particle volume after rebound (before construction leveling); ;

[0057] (7) Calculate the soil particle volume of the rebound part of the soil body ; the soil particle volume of the rebound part is the space filled with the rebound height, without voids, but in reality it is impossible, it must exist voids, the specific calculation is according to the different soil plasticity index limit, because the bottom of the pit is saturated soil body, so the rebound part of the soil particle must contain combined water, the plasticity limit range of silty clay is greater than 17 and less than 28, take the lower limit of the plasticity index limit as the water content of the uplift soil particle, that is ; the dry density is ; by , is the volume of the rebounding part of the soil containing pore water, , the volume of the rebounding part of the soil is to be removed, and the true volume of the rebounding soil particles is obtained ;

[0058] (8) The volume of the soil particles remaining after the first layer of construction is leveled is obtained: ;

[0059] The void ratio after the construction is leveled is obtained ;

[0060] (9) The settlement of the first layer is obtained: ; and the total settlement after the construction is leveled is obtained by adding the settlement of the other layers: ;

[0061] (10) The time effect of the actual excavation of the foundation pit is considered, and the settlement at a certain time after the construction is leveled is obtained: ; the permeability coefficient of the silty clay ; it is assumed that the foundation only undergoes vertical settlement, i.e., the soil is in a lateral stress state; and only the consolidation settlement is considered, without considering the instantaneous settlement and the secondary consolidation settlement; the consolidation degree curve is obtained according to different times t as shown in Figure 7 ; and it can be seen from Figure 7 that: with the increase of time, the consolidation degree continuously increases until 100%, and after two days, the consolidation settlement of the foundation exceeds half of the final consolidation settlement.

[0062] Table 8 is the calculated settlement at times of 1 to 6 days. Because the permeability coefficient of the silty clay is large, the total settlement has reached 56.04 cm after 6 days.

[0063] Because the internal stress of the soil is not released in time due to the instantaneous excavation, the soil does not rebound, and there is no construction leveling. It can be seen from Table 9 that in the case of long-term excavation, the final settlement considering the construction leveling and not considering the construction leveling differs by 2.91 cm, which is relatively small in the foundation settlement, but in the case of excavation of a foundation pit adjacent to an existing tunnel, the deformation of the tunnel is controlled in mm, so the situation where the rebounding part of the soil is removed due to construction leveling during the excavation of the foundation pit needs to be considered. Moreover, the soil in the example is silty clay, which has a large rebound modulus, resulting in a small rebound amount. If other soil with a small rebound modulus is used, the rebound amount will be large, and the construction leveling amount will also increase, resulting in a larger void ratio of the first layer and a larger settlement during compression. In this case, the construction leveling factor needs to be considered in order to take engineering measures to reduce the foundation settlement.

[0064] Table 8 Relationship between the settlement of the foundation pit considering the construction leveling factor and time

[0065]

[0066] Table 9 Summary of Results

[0067]

[0068] The application is not restricted to the details of the foregoing description and the accompanying drawings, and various modifications and alterations can be made without departing from the scope of the application as defined in the claims.

Claims

1. A method for calculating ground settlement taking into account construction leveling, characterized by, The method comprises the following steps: S1, determine the unloading amount of the foundation pit excavation area BxL , and respectively represent the unit effective gravity and thickness of the soil layer above the bottom of the excavation layer layer S2, determining the layer thickness in the settlement calculation depth range; S3, determining the settlement calculation depth range, taking the ratio of the additional stress and the self-weight stress as 0.1; S4, according to Boussinesq solution to calculate the foundation unloading caused by the first Additional stress at the center of the soil layer And calculate the stress distribution of the foundation self-weight, through MATLAB programming calculation, additional stress; S5. Calculate the rebound amount using the layered summation method. The formula for the rebound amount of each soil layer is as follows: , It is the springback of the j-th layer. It is the first The thickness of the layer, It is the resilient modulus of the j-th layer. It is the first The resilience coefficient of the layer, It is based on the instantaneous excavation. The porosity is obtained from the layer's self-weight stress curve using the ep curve. It is the first time that long-term excavation has been carried out. The void ratio is obtained from the ep curve of the layer's self-weight stress; since the rebound shape of the foundation pit is convex, the rebound amount at the center of the foundation pit is also calculated. and the rebound amount of the foundation pit sidewall Then calculate the average value to represent the average rebound of the j-th layer. , The total rebound amount is obtained by summing the average rebound amounts of each layer. ; S6、total rebound amount calculated based on step S5 , the volume of soil particles in the layer is calculated according to the formula is the thickness of the first layer at the bottom of the pit; e is the void ratio obtained by looking up the e-p curve according to the stress after the pit is excavated for a long time; is the volume of pore water after rebounding; is the volume of soil particles after rebounding, B is the width of the pit, and L is the length of the pit;​ S7, taking the plasticity index limit lower limit of the soil body as the moisture content of the resilient part of the soil body , the soil particle volume of the resilient part of the soil body is obtained , is the soil particle density, is the water density; S8, according to the calculation results of step S6 and step S7, the volume of the residual soil particles after the first layer is leveled by construction is calculated ; and then according to the formula , the void ratio after the construction leveling is calculated , the void water volume after the construction leveling, the soil body is assumed to be saturated soil ; S9. Calculate the settlement of the first layer, and add it to the settlement of the other layers to obtain the total settlement after leveling. The formula for calculating the settlement is: ,in, It is the first The compressive modulus of the layer; It is the first The compressibility coefficient of the layer; It is based on the first The porosity is obtained from the layer's self-weight stress curve using the ep curve. It is the first The porosity is obtained by referring to the ep curve from the sum of the layer's self-weight stress and the additional stress. It is the first under structural load Additional stress in the layer; n is the number of layers.

2. The method for calculating ground settlement considering construction leveling according to claim 1, wherein, The method further comprises the following steps: S10, consider the settlement of construction leveling at some time: where, is the settlement of long-time excavation, is the settlement of instantaneous excavation, is the one-dimensional consolidation degree of Terzaghi, ; , , is the permeability coefficient, is the compression modulus, is the unit weight of water, t is time, and H is the calculation depth.

3. The method for calculating ground settlement considering construction leveling according to claim 1, characterized in that, In the step S2, the layer thickness is not greater than 0.4 times the width of the foundation pit or not greater than 4 m.

4. The method for calculating ground settlement considering construction leveling according to claim 1, wherein, In the step S4, additional stress The equation is as follows: ; In the formula: is the area of the foundation pit excavation, is the integral variable; p is the foundation pit excavation load when calculating the rebound, and p is the structural load when calculating the settlement; is the coordinate of the calculation point in the rectangular coordinate system.

Images