JP6512014B2 - Evaluation method of stability of excavated wall - Google Patents

Description

  The present invention relates to a method for evaluating the stability of an excavated wall close to an existing underground structure.

  2. Description of the Related Art Conventionally, when constructing an underground structure with open cut in construction such as underground continuous wall or cast-in-place pile, any one of methods to evaluate the stability of the excavated wall surface in excavated ditch or pile hole etc. The shear strength reduction finite element method (SSR-FEM) is adopted, which can accurately calculate the safety factor of slope under various conditions such as slope shape, soil heterogeneity, existence of pore water pressure.

  As described in Non-Patent Document 1, shear strength reduction finite element method is to find the safety factor of the entire system consistent with the concept of the safety factor based on the limit equilibrium method without assuming the slip surface. Is a method that can In addition, even when loads from existing structures and construction machines act on the ground surface in the vicinity of the excavated wall surface, it is a method that can evaluate the stability of the excavated wall surface while including these upper loading loads as conditions. .

Calculation method of overall safety factor of slope by elastic-plasticity FEM, Proceedings of the Society of Geotechnical Engineering Vol. 29, No. 2, pp. 190-195, 1989

  However, for example, when there is an existing foundation structure in proximity to the underground continuous wall to be constructed, if the stability of the excavated wall during the construction is to be evaluated by the above shear strength reducing finite element method, excavating Before the sliding surface that is continuous from the wall surface is formed, the fracture mechanism is formed in advance between the existing foundation structure and the ground, and the calculation diverges, and the stability of the excavated wall surface with accuracy. It was difficult to assess the

  On the other hand, if the ground is excavated in the vicinity of the existing foundation structure, there is a high possibility that the surrounding ground and the foundation structure will be affected. However, these effects affect the deformation of the surrounding ground and the supporting function of the foundation structure. It was not possible to evaluate the effects etc. by the shear strength reduction finite element method.

  The present invention has been made in view of such problems, and its main object is to use the shear strength reduction finite element method in the case where the excavated wall exists in proximity to the existing underground structure. The purpose is to provide a method for evaluating the stability of the excavated wall which can carry out both the stability assessment of the above and the impact assessment on the surrounding ground and the existing underground structure by the ground excavation.

  In order to achieve the above object, the stability evaluation method of the excavated wall surface according to the present invention sets an analysis model based on the finite element method in an analysis target area including the existing underground structure and the excavated wall adjacent to this. It is a stability evaluation method of excavated wall surface which evaluates stability of a wall surface by a shear strength reduction finite element method, which is an element forming the ground in contact with an element forming the existing underground structure in the analysis model, It substitutes as a physical property value adjustment element, gives the adjustment Young's modulus adjusted to a value smaller than the Young's modulus of the existing underground structure to the physical property value adjustment element, and expresses the boundary between the existing underground structure and the ground. It is characterized by

  Further, in the stability evaluation method of the excavated wall surface according to the present invention, the adjustment Young's modulus includes the Young's modulus of the existing underground structure and the Young's modulus applied to an element forming the ground adjacent to the physical property value adjustment element. Is adjusted to a value located between.

  According to the present invention, when the excavated wall surface is located adjacent to the existing underground structure, the boundary between the ground and the existing underground structure is modeled by the physical property value adjustment element to which the adjusted Young's modulus is applied. By performing analysis using an analysis model, the failure mechanism resulting from the large difference in rigidity between the ground and the existing underground structure may be formed prior to the failure mechanism that occurs in other areas of the analysis model Instead, it is possible to form a sliding surface close to the actual behavior for the ground near the excavated wall.

  As a result, it becomes possible to evaluate the stability of the excavated wall surface with high accuracy while taking into consideration the influence of the existing underground structure on the excavated wall surface, and underground excavation is carried out in the surrounding ground or existing underground construction It will also be possible to assess the impact on things.

It is a figure which shows the object area of the underground continuous wall construction plan. It is a figure which shows the analysis model analyzed by shear strength reduction finite element method. It is a figure which shows a mode that the boundary of a foundation pile and a ground was modeled by the physical-property value adjustment element. It is a figure which shows the result analyzed by shear strength reduction finite element method. It is an image-analysis figure at the time of making a sliding surface generate | occur | produce in the vicinity of the excavated wall surface adjacent to a foundation pile in a centrifugal model test.

Hereafter, the stability evaluation method of the excavation wall surface of this invention is demonstrated using FIGS. 1-5.
In the present embodiment, as shown in FIG. 1, a case where a foundation pile 3 supporting an existing structure 2 exists in a target area planned to be constructed in the underground continuous wall 1 will be described as an example. In addition, the new underground structure to be built is not necessarily limited to the underground continuous wall 1, for example, if it is a structure with excavating in construction such as cast-in-place piles, any new underground A structure may be adopted. In addition, the existing underground structure is not necessarily limited to the foundation pile 3, and any existing underground structure may be adopted, such as measures for other foundation structures, earth retaining walls, etc. .

  The method of evaluating the stability of the excavated wall surface is to construct a new underground structure in the case of constructing a new underground structure involving excavation in construction of the underground continuous wall 1 etc. in the vicinity of the existing underground structure such as the foundation pile 3 etc. Evaluation of the stability of the excavated wall 1a constructed at the construction stage of the construction and the influence on the existing underground structures such as deformation of the surrounding ground and deterioration of the support function of the foundation pile 3 due to ground excavating shear strength reduction finite element method (SSR-FEM) is a method of evaluation. And in adopting this shear strength reduction finite element method, a three-dimensional elastic-plastic ground analysis program GA3D is adopted in this embodiment.

  GA3D is a well-known program widely known in the field of ground FEM analysis, so details are omitted, but it is based on non-relevant flow measurement using Mohr-Coulomb formula as yield standard and Drucker-Prager formula as plastic potential. It is a complete plasticity model, and uses a mixed method in which the modified Newton-Raphson method is combined with the incremental method as the solution method of the nonlinear equation.

The input physical property values include at least formation conditions, soil conditions corresponding to each layer (Young's modulus Es, Poisson's ratio ,, adhesion ratio c, internal friction angle φ, dilatancy angle ψ, unit volume weight γ, etc.), groundwater level, stabilizer Water level, presence or absence of guide wall 1b (in the case of thickness, depth from ground surface, material constant (Young's modulus, adhesive force)), structure of the structure where load acts on the ground surface near excavated wall surface 1a Presence (in the case of presence, coordinates of distributed load, load per unit area), presence or absence of line load acting on ground surface near excavated wall surface 1a (in case of presence position and magnitude of concentrated load), It is. Incidentally, the Young's modulus Es of the ground, for example in sandy soil 2000N (kN / m ²⁾ approximately, is about 4000N (kN / m ²⁾ in cohesive soil. Here, N refers to the N value by a standard penetration test).

  In the case where the new underground structure to be constructed is the underground continuous wall 1 as a range in which the analytical model 4 is set, when the length is L, the X axis direction is the underground continuous wall The length L of 1 or more is twice or more, the Y-axis direction is equal to or less than the length L of the underground continuous wall 1, and the Z-axis direction is equal to or more than the digging depth of the underground continuous wall 1. In addition, the setting method of the analysis model 4 is not necessarily limited to this, and as long as it is a range which can form the slide surface which generate | occur | produces to a X direction, you may set it to any.

  Further, in the mesh division method of the analysis model 4, the division width and the number of divisions may be appropriately adjusted according to the purpose of analysis. In the present embodiment, as shown in FIG. 2, the interval is gradually extended from the excavated wall surface 1a in the X-axis direction and divided, the Y-axis direction is equally divided, and in the Z-axis direction, the overhang is projected on the excavated wall surface 1a. The height range that may occur and the height range near the groundwater level 5 are finely divided, and the other height ranges are equally divided, etc., so that the range in which the behavior is desired to be grasped is finely divided.

By the way, in this embodiment, as stated above, the foundation pile 3 which supports the existing structure 2 exists in the vicinity of the planned construction area of the underground continuous wall 1. For this reason, the distance in the X direction from the excavated wall surface 1a in the analysis model 4 is determined from measured values such as the position, diameter, depth, number and arrangement interval of the foundation pile 3, and modeling is performed using solid elements. Physical property values (Young's modulus E, Poisson's ratio 等, etc.) are input. The Young's modulus E of the foundation pile 3 is, for example, about 2.1 × 10 ⁸ (kN / m ² ) in steel pipe construction and about 2.1 × 10 ⁷ (kN / m ² ) in concrete construction.

  Further, in the present embodiment, a ground shallower than the underground water level 5 is replaced as the upper load, and the distributed load is applied to the entire surface 5 a equal to the underground water level 5 of the analysis model 4. However, in the present embodiment, analysis is performed by the above-described modeling. However, the method of modeling is not limited to this method. Furthermore, the load of the existing structure 2 is applied to each of the foundation piles 3 as a concentrated load. The load of the existing structure 2 may be assumed to act as a distributed load in the range surrounded by the four foundation piles 3.

  With respect to the analysis model 4 set in this way, the water level of the stabilizer 6 in the underground continuous wall 1 is lowered stepwise and the water level is lowered at each stage. Overall safety factor F by shear strength reduction method (SSR) Calculation of and estimation of slip surface. In addition, as a result of analysis, it is natural to understand the output generally obtained by the finite element method, such as nodal displacement, element strain, element stress, etc., grasp the member force of the foundation pile 3 and strain generated in the member Is also possible.

  A concrete analysis procedure virtually reduces the shear strength parameter c 'and' 'of the ground to cr' and rr 'with a reduction factor fr, and increases the reduction factor fr gradually to virtually shear strength To form a continuous sliding surface from the excavated wall surface 1 a to the surface 5 a equal to the groundwater level 5. Then, the reduction factor fr immediately before the slip surface is formed is taken as the overall safety factor F. Generally, the reduction coefficient fr is increased by 0.01.

  By the way, in the above analysis model 4, before the slip surface which continues continuously from the excavated wall surface 1a to the surface 5a equal to the groundwater level 5 is formed due to the large difference in rigidity between the foundation pile 3 and the ground. A phenomenon is likely to occur in which the yield zone is continuous and the fracture mechanism is formed prior to the boundary between the foundation pile 3 and the ground.

  In the case where elements with greatly different physical property values such as the ground and the underground structure are adjacent to each other, the finite element method generally has a thin layer between the two as a modeling method to express these boundaries. Various methods have been proposed such as a method of providing an element, a method of connecting the two with a spring, and a method of providing an interface element. On the other hand, the point which corresponds by providing the physical-property value adjustment element 7 with respect to the interaction problem of these grounds and the foundation pile 3 in this embodiment is the big characteristics.

  That is, as shown by the cross-sectional view of the analysis model 4 in FIG. 3, among the elements 4b forming the ground, the element 4b forming the ground in contact with the element 4a forming the foundation pile 3 is replaced with the physical property value adjusting element 7, By inputting the adjustment Young's modulus to the value adjustment element 7, the phenomenon that the boundary between the foundation pile 3 and the ground breaks in advance is suppressed. Here, the adjustment Young's modulus input to the physical property value adjustment element 7 is intended to reduce the difference in rigidity between the foundation pile 3 and the ground, so the Young's modulus E of the foundation pile 3 and the physical property value adjustment element 7 It is adjusted to be a value located between the Young's modulus Es applied to the element 4b forming the adjacent ground.

  And preferably, the failure mechanism due to the difference in rigidity between the element 4a forming the foundation pile 3 and the element 4b forming the ground is not formed prior to the failure mechanisms occurring in other areas of the analysis model 4 And it is good to adjust an optimal numerical value so that the sliding surface which continues in a row continuously from the excavation wall surface 1a to the surface 5a equal to the groundwater level 5 may be formed precisely.

  In addition, when a plurality of formations exist in the ground of analysis object, since Young's modulus Es of a ground differs for every formation, it is given to element 4b which makes a ground adjacent to each of a plurality of physical property value adjustment elements 7 It is assumed that there are different Young's modulus Es. In this case, the physical property value adjustment element 7 is a value smaller than the Young's modulus E of the foundation pile 3 and larger than the Young's modulus Es of the ground adjacent to the physical property value adjustment element 7 and adjusted to an optimal value. Calculate as the adjusted Young's modulus, and give this. However, in the case where the calculated adjusted Young's modulus is similar for each of the plurality of physical property value adjusting elements 7, all the physical property values are represented by using one adjusted Young's modulus selected in the calculated adjusted Young's modulus as a representative value. It may be input to the adjustment element 7.

  In addition, even when there are multiple formations in the analysis target ground, if the ground conditions are relatively good (for example, when the N value of each layer is 30 or more), Young's modulus E of the foundation pile 3 For example, if the ground condition is relatively poor (for example, if the N value of each layer is less than 30), the Young's modulus E of the foundation pile 3 is, for example, 1/1000 of that of the foundation pile 3 The adjustment Young's modulus is determined to be smaller than this based on the Young's modulus E and larger than the Young's modulus Es of the ground adjacent to the physical property value adjustment element 7, and the adjustment Young's modulus covers the outer periphery of the foundation pile 3 It may be input to all of the physical property value adjustment elements 7.

  Furthermore, in the present embodiment, in the analysis model 4, the physical property value adjustment element 7 is formed in a thinner layer than the other ground elements 4b, and the width of the physical property value adjustment element 7 in the ground is made smaller. The invention is not limited to this, and it may be an element having the same thickness as that of the other ground element 4b.

  The analysis result using the analysis model 4 incorporating the above-mentioned physical property value adjustment element 7 is shown in FIG. FIG. 4 (a) shows that the water level of the stabilizer 6 is G.I. L. The case where the water level of the stabilizing solution 6 is lowered stepwise is shown in FIG. 4 (b) to FIG. 4 (d).

  Looking at this, the water level of the stabilizer 6 is G.I. L. In the case where the first slide surface 8 is continuously formed continuously from the excavated wall surface 1a to the surface 5a equal to the groundwater level 5, no yield zone is formed around the foundation pile 3 The influence of the presence of the foundation pile 3 on the stability of the ground wall surface 1a, the deformation of the surrounding ground due to ground excavation, and the influence of the support function of the foundation pile 3 are not recognized. However, as the water level of the stabilizer 6 decreases, a yield zone 9 is generated around the foundation pile 3 to affect the support function, and the water level of the stabilizer 6 is G.I. L. In FIG. 4 (d), which is 0.9 m lower than the above, it can be seen that the yield regions 9 generated in the vicinity of the foundation pile 3 are continuously connected toward the excavated wall surface 1a to form the second sliding surface 10.

  This is a phenomenon observed also in the centrifugal model experiment shown in FIG. In the centrifugal model test, as shown in the photograph in FIG. 5 (a), the stabilizer is lowered stepwise while the plurality of targets 11 are evenly arranged in the ground and the water level in the ground is kept constant. . Then, as shown in the image analysis diagram of FIG. 5B, the movement of the target 11 that fluctuates according to the change in the water level of the stabilizer is image-analyzed, and the slip plane is estimated from the result.

  In the image analysis result of FIG. 5 (b), a first slide surface 8 not formed by the foundation pile 3 and a second slide surface 10 extending from the vicinity of the foundation pile 3 toward the excavated wall surface 1 a are formed. These two slide surfaces correspond to the analysis result by the stability evaluation method of the excavated wall surface in the present embodiment. Therefore, in the present embodiment, the method of evaluating the stability of the excavated wall surface can evaluate the stability of the excavated wall surface 1a with high accuracy in consideration of the influence of the foundation pile 3 on the excavated wall surface 1a. At the same time, it is also possible to evaluate the influence of underground excavation on the surrounding ground and the support mechanism of the foundation pile 3.

  In addition, the stability evaluation method of the excavation wall surface of this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention.

Reference Signs List 1 underground continuous wall 1a excavated wall 2 existing structure 3 foundation pile 4 analysis model 4a pile element 4b ground element 5 groundwater level 5 surface equal to groundwater 6 stabilizer 7 property value adjustment element 8 first slip surface 9 Yield region 10 Second sliding surface 11 Target

Claims (2)

An excavated wall surface in which an analysis model based on the finite element method is set in an analysis target area including an existing underground structure and an excavated wall surface adjacent thereto, and the stability of the excavated wall surface is evaluated by a shear strength reduced finite element method Stability evaluation method of
Replace the element forming the ground in contact with the element forming the existing underground structure in the analysis model as a physical property value adjustment element,
An excavating characterized in that an adjusted Young's modulus adjusted to a value smaller than a Young's modulus of the existing underground structure is given to the physical property value adjusting element to express the boundary between the existing underground structure and the ground. Wall stability evaluation method. In the stability evaluation method of the excavated wall according to claim 1,
The adjusted Young's modulus is adjusted to a value located between the Young's modulus of the existing underground structure and the Young's modulus applied to an element forming the ground adjacent to the physical property value adjustment element. Evaluation method of stability of excavated wall.