JP3876655B2 - Deformation prediction method and program for excavation bottom ground - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique for predicting / evaluating deformation of a bottom ground associated with earth retaining excavation work.
[0002]
[Prior art]
For example, in construction where the construction area is limited, such as urban civil engineering work, a retaining wall such as a steel sheet pile or a side sheet pile is erected from the meaning of effectively using the limited construction area. A lot of construction work involving earth retaining excavation, which is supported by supporting works such as digging, is carried out. However, this soil excavation has been pointed out to generate phenomena that should be noted depending on the properties of the construction ground and groundwater. This phenomenon is a deformation phenomenon of the bottom ground such as bulging, boiling, and heaving. Of these, for example, regarding the blisters, the water pressure of the aquifer (such as sand / gravel layer) contained in the ground at the bottom of the excavation includes an impermeable layer that exists above the confined aquifer. This is a phenomenon that pushes the ground upward, and prediction and evaluation methods for this phenomenon have been proposed. FIG. 1 shows the concept of board bulging in the conventional method.
[0003]
In the conventional prediction / evaluation method, the water pressure U of the confined aquifer (permeable layer) derived from the confined water head hw and the ground (impermeable layer and I layer) above the confined aquifer are described. By comparing the magnitude with the weight W (overburden pressure), it is predicted that if the water pressure wins, the occurrence of overburden is a concern, and if the overburden pressure wins, there is less fear of overburden. I was going to estimate the occurrence of blistering on the entire excavated ground.
[0004]
[Problems to be solved by the invention]
However, according to the conventional method, the contact resistance between the soil retaining wall side surface and the ground, which is estimated to be more effective resistance to water pressure than the soil covering pressure, is considered while considering the construction process and elapsed time. Never considered. In addition, since the analysis of the comparison between the water pressure and the drag force such as soil covering pressure was conducted as a vertical one-dimensional problem, it is possible to predict and evaluate the deformation of the entire bottom of the excavation bottom, but spatial and temporal No changes were predicted or evaluated. Therefore, the deformation of the bottom ground predicted and evaluated by the conventional method tends to be an overestimation, and even if the countermeasure work for the predicted deformation is designed here, the area of the bottom ground and the excavation stage Every time there was a risk of excessive or insufficient countermeasures.
[0005]
Therefore, the present invention has been made paying attention to such a conventional problem, and the excavation bottom that can easily and reliably predict the deformation of the excavation bottom ground for each region or each construction stage with excellent accuracy. The present invention provides a ground deformation prediction evaluation method.
[0006]
[Means for Solving the Problems]
The present invention has been made to achieve the above object, and the first invention is a method for predicting the deformation of the bottom ground due to earth excavation work, wherein the bottom ground is below the bottom ground. The water pressure in the confined aquifer that causes deformation, the earth covering pressure on the confined aquifer, the contact resistance between the retaining wall or the pile and the ground, and the viscous soil deeper than the bottom of the retaining wall or the bottom of the pile A drag value and a water pressure value for spatial discretization and time discretization in the bottom ground by applying an FEM analysis method based on a multi-dimensional consolidation theory to the drag force against the water pressure including any of the adhesive forces of the layers And the drag / water pressure ratio is calculated for each construction process and each construction area.
[0007]
In analyzing the contact resistance generated in the contact portion between the retaining wall or the pile and the ground in the first invention, the second invention is the same nonlinear model as the ground around the contact portion in an appropriate region including the contact portion. And a material constant is applied, and when the stress state of the contact portion exceeds the ground strength, an analysis process for eliminating the contact resistance at the contact portion is performed. The present invention is suitable for predicting deformation of a viscous ground, and it is preferable to apply an elastoplastic constitutive equation proposed by Sekiguchi and Ota as a nonlinear model.
[0008]
In analyzing the contact resistance generated in the contact portion between the earth retaining wall or the pile and the ground in the first or second invention, the third invention applies a nonlinear model to an appropriate region including the contact portion, It is characterized by using measured data regarding contact resistance as a material constant. The present invention is suitable for prediction of deformation of gravel ground, and it is preferable to apply a Drucker-Prager model as a nonlinear model.
[0009]
4th invention shall make the deformation | transformation prediction program for functioning on a computer the deformation | transformation prediction method in any one of 1st-3rd.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of a method for predicting the deformation of a bottom of excavation ground according to the present invention will be described below with reference to the accompanying drawings. FIG. 2A is a plan view showing a bottom of excavation and an analysis region in the present embodiment, and FIG. 2B is an explanatory diagram showing a geological situation and a construction section in which earth excavation is performed. FIG. 3A is an explanatory diagram showing the situation of finite element modeling at the construction site, and FIG. 3B is an explanatory diagram showing the planar arrangement of cast-in-place piles and the application status of the finite element mesh.
[0011]
In the present embodiment, during the earth retaining excavation, a description will be given by focusing on the ground bulge among the deformation events that are likely to occur on the ground at the bottom of the excavation (hereinafter referred to as the bottom ground). Suppose that the actual site of concern at the time of excavation construction is subjected to 3D ground / groundwater coupled FEM analysis based on biot's multidimensional consolidation theory, and the occurrence of overburden is predicted. In addition, we analyze how the stress state of the bottom ground changes depending on the construction conditions.
[0012]
1. The construction conditions and the finite element modeling construction area, that is, the area where the excavation is performed is an approximately rectangular area of 80 m in length and 90 m in width as shown in FIG. (B) Refer to the figure, thickness t = 1.0 m, length L = 38 m). A ground improvement body with a thickness of 0.7m is appropriately constructed on the inner periphery of this RC continuous underground wall to increase the independence of the ground, and the outer periphery of the excavation area is adapted to the progress of excavation (this embodiment) In the case of the 7th excavation), the construction of deep well DW for groundwater pumping is assumed. In addition, a relief well RW for drainage is provided in the excavation area with the same length as that of the RC continuous underground wall to achieve dry excavation. In such an excavation region, excavation is performed over seven steps until flooring, and reverse slab placement is performed in combination with the above-described step excavation to form slabs from 1F to B5F. In addition, it is assumed that a cast-in-place pile (φ2.0 m, L = 19 m) is constructed from the flooring surface to GL-43 m according to the analysis conditions.
[0013]
On the other hand, as the ground condition of the excavation area, as shown in the geological columnar diagram in FIG. 2 (b), the uppermost alluvial upper sand layer is deposited up to about 6m below the surface (hereinafter referred to as GL-), and the alluvial layer below it. The (high plasticity) clay layer is thickly deposited up to around GL-6 m to 22 m. In the following, diluvial clay layers and diluvial or alluvial sand layers are alternately laminated. In the analysis, the above-mentioned alluvial clay layer and the upper clay layer and intermediate clay layer under the flooring level (GL-23.8m) where the occurrence of overburden is a concern are the bullets proposed by Sekiguchi and Ota. Using a plastic constitutive equation (reference: Sekiguchi, H. and Ohta, H .: Induced anisotropy and time dipendency in clay, 9th ICSMFE, Tokyo, Proc. Specialty session 9, pp.229-239, 1977) Set a nonlinear constant value based on the result.
[0014]
The initial deformation coefficient of each of the alluvial and diluvial sand layers and the diluvial clay layer is determined by PS logging, and is based on the Drucker-Pragger fracture condition (circle inscribed in the Mohr-Coulomb fracture condition) under triaxial stretching conditions. The deformation coefficient of the reached ground element is set to be reduced to 1/100. The Poisson's ratio is 0.35 based on the experimental results of Yokota et al. (Reference: Yokota, Konno, Kurita: The Poisson's ratio of soil, 15th Geotechnical Engineering Conference, pp.529-532, 1980). It is said. The permeability coefficient of each sand layer shall be determined based on the results of on-site permeability tests.
[0015]
The analysis region and the finite element mesh are shown in FIGS. 3 (a) and 3 (b). The analysis region here includes the center of the excavation region from the back of the retaining wall (see also FIG. 2A), and considers the symmetry of deformation due to the lattice placement of the cast-in-place piles described above. Use a shell element to simulate the earth retaining wall and the inverted slab provided at each excavation stage, and use a solid element for the cast-in-place pile at the bottom of the excavation and its upper column (back load receiving). I will give it. Here, a thin layer of ground element (appropriate region including the contact part) is provided between the retaining wall and the ground (contact part) and between the cast-in-place pile and the ground (contact part). The contact resistance was expressed. In this case, the contact resistance generated in the thin layer is analyzed so that the contact resistance in the thin layer disappears when the stress state of the thin layer exceeds the ground strength. In analyzing the contact resistance in this way, the Drucker-Prager model may be applied to an appropriate region including the thin layer, and measured data regarding the contact resistance may be used as a material constant.
[0016]
Under the above conditions, we will analyze the prediction of board bulge by sequentially tracking the repeated construction of stepped excavation and backlash slab as in the actual construction. At this time, in order to simulate dry excavation by the relief well RW, the total head value of the corresponding node is constrained so that the inner water level is kept at a position 1 m deeper than the excavation level in each subsequent excavation stage, and the final seventh excavation is performed. For deep well DW pumping, which is sometimes implemented as a countermeasure against blistering, the total head value at the nodes of the target intermediate sand layer is reduced by △ -6.0m and restrained by the same amount as in practice. Simulate.
[0017]
2. Stress State of Excavation Bottom Ground FIG. 4 shows the general stress state of the bottom ground at the time of final excavation, and FIG. 5 shows the stress depth distribution near the center of the excavation part. FIG. 4 shows a contour map of the ratio (σv / pw) of the total vertical stress σv to the pore water pressure pw as an analysis result obtained by carrying out the deformation prediction method of the present invention under the above conditions. The effect of deep well DW and the effect of cast-in-place piles are shown in comparison with the case of no countermeasure work. When the σv at the upper end of the aquifer (eg sand / gravel layer) falls below pw (σv / pw <1), there is a risk of board bulge, but there is an unmeasured case (DW is also a place) In the case of no piles), the analysis result in which the region (black region) where σv / pw <1 extends near the center of the excavation region is clearly obtained as was feared in actual construction.
[0018]
On the other hand, in the case where the deep well DW is considered, the region of σv / pw <1 does not occur. Further, in the case where the cast-in-place pile is provided, the result of analysis that the value of σv / pw is greatly improved without using the deep well DW. In this way, it is possible to obtain an analysis result that spatially overlooks the entire excavation area at each construction stage. Therefore, it can be said that in which area the occurrence of the bulge is concerned, or the effect of the countermeasure work on that part is obvious. On the other hand, in FIG. 5, it is clear that the vertical total stress σv of the bottom ground remains largely due to the contact resistance exerted between the cast-in-place pile and the ground. If the prediction method for the board bulge according to the present invention is carried out, the effects of reducing the lifting pressure by the deep well DW and the residual soil covering pressure (total stress) by the cast-in-place piles can be simply, reliably and visually as described above. Therefore, it will lead to construction with full countermeasures against board blisters. If the deformation prediction method of the present invention is configured as a program that can function on a computer and functions on the computer, of course, the above-described deformation prediction is performed automatically, simply, and quickly. It will be.
[0019]
【The invention's effect】
As described above in detail, according to the method for predicting the deformation of the bottom of the excavated bottom according to the present invention, the contact resistance between the excavation area and the surrounding geometric conditions, and the side wall of the retaining wall and the ground is specifically considered. In addition to being able to execute deformation prediction, it is possible to analyze the comparison between the water pressure and the drag force such as soil covering pressure both spatially and temporally.
Therefore, not only the deformation prediction and evaluation of the entire excavation bottom ground can be performed, but also the deformation of the bottom ground can be predicted and evaluated locally at a predetermined construction stage.
In other words, it is possible to easily and reliably obtain the result of deformation prediction that leads to the effective implementation of various countermeasures such as the installation of pumping wells, etc. according to the deformation of each place without excess or deficiency.
In addition, the present invention has an advantage that it is possible to perform deformation prediction in consideration of the pressure reduction effect due to pumping to the pressured aquifer, which is a countermeasure for blistering.
Accordingly, it is possible to provide a method for predicting the deformation of the bottom of the excavation bottom that can easily and reliably perform the deformation prediction and evaluation of the bottom of the excavation bottom for each region or each construction stage with excellent accuracy.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an explanatory diagram showing a conventional way of thinking about a board bulge.
FIG. 2A is a plan view showing an excavation bottom and an analysis region in the present embodiment, and FIG. 2B is an explanatory diagram showing a geological situation and a construction cross section in which earth excavation is performed.
FIG. 3A is an explanatory diagram showing a state of finite element modeling at a construction site, and FIG. 3B is an explanatory diagram showing a planar arrangement of cast-in-place piles and an application state of a finite element mesh.
FIG. 4 is a contour map showing the ratio between the total vertical stress remaining on the excavation bottom ground and the pore water pressure for each countermeasure work.
FIG. 5 is an explanatory view showing the depth distribution of stress / pore water pressure near the center of the bottom ground.

Claims (4)

A method for predicting a deformation of a bottom ground due to a soil excavation work, wherein a water pressure in a confined aquifer under the bottom ground and causing the deformation of the bottom ground, and on the confined aquifer The resistance to the water pressure, including either the earth pressure of the earth, the contact resistance between the retaining wall or the pile and the ground, and the adhesive force of the viscous soil layer deeper than the bottom of the retaining wall or the bottom of the pile Applying the original FEM analysis method to analyze the drag value and water pressure value for spatial discretization and time discretization in the bottom ground, and to calculate the drag / water pressure ratio for each construction process and each construction region Characteristic deformation prediction method. In analyzing the contact resistance generated in the contact portion between the retaining wall or the pile and the ground, the same nonlinear model and material constant as the ground around the contact portion are applied to an appropriate region including the contact portion, and the stress state of the contact portion The deformation prediction method according to claim 1, wherein an analysis process for eliminating the contact resistance at the contact portion is performed when the ground strength exceeds the ground strength. In analyzing the contact resistance generated in the contact portion between the retaining wall or the pile and the ground, a nonlinear model is applied to an appropriate region including the contact portion, and measured data regarding the contact resistance is used as a material constant. The deformation prediction method according to claim 1. A deformation prediction program for causing the deformation prediction method according to claim 1 to function on a computer.

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