JP7089891B2 - Freezing expansion analysis method of the ground - Google Patents

Description

The present invention relates to a method for analyzing the expansion (stress state) of the ground when the freezing method is applied.

Conventionally, as a protective work for stabilizing the ground and forming a waterproof layer, a freezing method in which a bearing wall or a waterproof wall is formed by a frozen soil wall has been used (see, for example, Patent Document 1).

On the other hand, in the freezing method, the earth pressure on the outer surface of the frozen soil wall may increase due to the displacement due to freezing expansion. This increase in stress is called freeze expansion pressure, and the additional stress on structures such as shafts is called frozen earth pressure. As an idiomatic formula for predicting these, there is a cylindrical theoretical formula of Takashi (1972), and there is also a prediction method using FEM analysis.

Japanese Unexamined Patent Publication No. 2005-264717

However, Takashi's cylindrical theoretical formula approximates the frozen soil wall to a cylinder and predicts the freezing expansion displacement and expansion pressure of the outer surface of the frozen soil wall, and it is difficult to apply it to frozen soil other than the cylindrical shape. Furthermore, it is not possible to properly evaluate the effects of initial earth pressure and changes in ground stress due to excavation.

On the other hand, in FEM analysis, sequential analysis is possible for complicated frozen soil shapes, but when sandy soil with a small freezing expansion coefficient and cohesive soil with a large freezing expansion rate are present, excessive stress is concentrated at the boundary of the geology. Therefore, it is not used for calculating the freezing expansion pressure or designing frozen soil.

In view of the above circumstances, it is an object of the present invention to provide a method for analyzing the freezing and expanding of the ground, which makes it possible to accurately obtain the expansion (stress state) of the ground when the freezing method is applied.

In order to achieve the above object, the present invention provides the following means.

The method for analyzing freezing expansion of the ground of the present invention determines the stress state of the ground when the freezing method is applied to the ground having a sandy soil layer having a relatively small freezing expansion ratio and a cohesive soil layer having a relatively large freezing expansion ratio. This is a method for analyzing freezing and expansion of the ground for obtaining, and a contact surface element having a lower deformation coefficient and shear rigidity than the sandy soil layer and the cohesive soil layer is provided at the boundary between the sandy soil layer and the cohesive soil layer. The contact surface element is extended not only to the frozen soil range but also to the ground side, and the analysis is performed in order for each construction step of the initial stress analysis step, the freeze expansion step, and the excavation step. The feature is that the maximum principal stress, the minimum principal stress, and the safety factor are obtained in each step .

In the freeze-expansion analysis method of the ground of the present invention, it is possible to accurately predict the freeze-expansion displacement / expansion pressure even for a complicated frozen soil shape.

In addition, the effects of initial earth pressure and changes in ground stress due to excavation can be appropriately evaluated.

Furthermore, the stability of frozen soil and the ground can be evaluated, and it can be suitably used for frozen soil design.

It is a figure used for the explanation of the freeze-expansion analysis method of the ground which concerns on one Embodiment of this invention. It is a figure which shows the analysis model of both the case without a contact surface element and with a contact surface element (the present invention). It is the analysis result at the time of freezing expansion of both cases with no contact surface element and with contact surface element (the present invention). It is an analysis result at the time of excavation of both cases without a contact surface element and with a contact surface element (the present invention).

Hereinafter, a method for analyzing freezing and expanding of ground according to an embodiment of the present invention will be described with reference to FIGS. 1 to 4. Here, the present embodiment relates to a method for accurately obtaining the expansion (stress state) of the ground (frozen soil) when the freezing method is applied.

Specifically, in the freeze-expansion analysis method of the ground of the present embodiment, as shown in FIG. 1, sandy soil (sandy soil layer) having a relatively small freeze-expansion rate and cohesive soil (viscosity) having a relatively large freeze-expansion rate. When there is a soil layer), a thin element (“contact surface element”) with a reduced horizontal deformation coefficient and shear rigidity (for example, 1/100) is provided at the geological boundary and modeled and FEM. Analysis (freeze expansion analysis) was performed. In addition, the analysis range will be extended not only to the frozen soil range but also to the ground side.

In the freeze-expansion analysis method of the ground of the present embodiment, the sandy soil and the cohesive soil behave as one in the vertical direction due to the presence of the contact surface element, but in the horizontal direction, a deviation occurs and almost all tensile stress is generated. This will not be the case, and it will be possible to make predictions that are close to the actual behavior.

In other words, in the freeze-expansion analysis method of the ground of the present embodiment, by providing a contact surface element at the geological boundary and modeling, "familiarity at the stratum boundary" at the time of actual frozen soil formation and "frozen soil growth at the time" "Slip" can be reflected, that is, the behavior becomes close to the actual one, and it becomes possible to accurately predict the freeze expansion displacement / expansion pressure by FEM analysis.

[Example]
Here, an example in which a three-dimensional FEM sequential analysis (excavation analysis + freeze-expansion analysis) is performed in an underground shaft for underground widening work using the ground freeze-expansion analysis method of the present embodiment will be described.

FIG. 2 is a diagram showing an analysis model (FIG. 2 (a): overall), and in this embodiment, there is no contact surface element (FIG. 2 (b)) and there is a contact surface element (FIG. 2 (c) :. Both cases of the present invention) were analyzed, and the superiority of the ground freeze-expansion analysis method of the present embodiment was confirmed by comparison.

In addition, the analysis was performed in order for each of the construction steps of the initial stress analysis step, the freeze expansion step, and the excavation step.

FIG. 3 shows the results of obtaining the maximum principal stress, the minimum principal stress, and the safety factor at the time of freeze expansion. FIGS. 3 (a), 3 (c), and 3 (e) show the maximum principal stress without a contact surface element. , Minimum principal stress and safety factor are shown, respectively, and FIGS. 3 (b), 3 (d) and 3 (f) show the maximum principal stress, minimum principal stress and safety factor with contact surface elements (in the present invention), respectively. Shows.

Comparing FIGS. 3 (a) and 3 (b) of the maximum principal stress during freezing expansion, when there is no contact surface element (FIG. 3 (a)), excessive tension is applied to the layer boundary (sandy soil side). While stress is generated, when there is a contact surface element (FIG. 3 (b): the present invention), the excessive tensile stress at the layer boundary (sandy soil side) is eliminated.

Comparing FIGS. 3 (c) and 3 (d) of the minimum principal stress during freeze expansion, when there is no contact surface element (FIG. 3 (c)), an excessive compressive stress is applied to the layer boundary (cohesive soil side). However, when there is a contact surface element (FIG. 3 (d): the present invention), the excessive compressive stress at the layer boundary (cohesive soil side) is eliminated.

Comparing FIGS. 3 (e) and 3 (f) of the safety factor at the time of freeze expansion, the safety factor near the stratum boundary is extremely reduced when there is no contact surface element (FIG. 3 (e)). On the other hand, when there is a contact surface element (FIG. 3 (f): the present invention), the extreme decrease in the safety factor is eliminated.

Next, FIG. 4 shows the results of obtaining the maximum principal stress, the minimum principal stress, and the safety factor at the time of excavation, and FIGS. 4 (a), 4 (c), and 4 (e) show the maximum without the contact surface element. The principal stress, the minimum principal stress, and the safety factor are shown, respectively. FIGS. 4 (b), 4 (d), and 4 (f) show the maximum principal stress, the minimum principal stress, and the safety factor with the contact surface element (the present invention). Are shown respectively.

Comparing FIGS. 4 (a) and 4 (b) of the maximum principal stress during excavation, as in the case of freeze expansion, when there is no contact surface element (FIG. 4 (a)), there is a layer boundary (sandy soil side). ), While the excessive tensile stress at the layer boundary (sandy soil side) is eliminated when there is a contact surface element (Fig. 4 (b): present invention). There is.

Comparing FIGS. 4 (c) and 4 (d) of the minimum principal stress during freeze expansion, as in the case of freeze expansion, when there is no contact surface element (FIG. 4 (c)), there is a layer boundary (cohesive soil side). ), While the excessive compressive stress at the layer boundary (cohesive soil side) is eliminated when there is a contact surface element (FIG. 4 (d): present invention). ..

Comparing FIGS. 4 (e) and 4 (f) of the safety factor during freeze expansion, the safety factor near the layer boundary is higher when there is no contact surface element (FIG. 4 (e)) as in the case of freeze expansion. While it is extremely reduced, when there is a contact surface element (FIG. 4 (f): the present invention), the extremely reduced safety factor is eliminated.

Therefore, according to the freeze-expansion analysis method of the ground of the present embodiment, it is possible to accurately predict the freeze-expansion displacement / expansion pressure even for a complicated frozen soil shape.

In addition, the effects of initial earth pressure and changes in ground stress due to excavation can be appropriately evaluated.

Furthermore, the stability of frozen soil and the ground can be evaluated, and it can be suitably used for frozen soil design.

Although one embodiment of the ground freeze-expansion analysis method according to the present invention has been described above, the present invention is not limited to the above-mentioned one embodiment and can be appropriately changed without departing from the spirit of the present invention.

Claims (1)

This is a freezing expansion analysis method for the ground to obtain the stress state of the ground when the freezing method is applied to the ground where a sandy soil layer with a relatively small freezing expansion rate and a cohesive soil layer with a relatively large freezing expansion rate exist. hand,
An analysis model was created by providing a contact surface element having a deformation coefficient and shear rigidity lower than that of the sandy soil layer and the cohesive soil layer at the boundary between the sandy soil layer and the cohesive soil layer.
The contact surface element extends not only to the frozen soil range but also to the ground side.
The analysis should be performed in order for each construction process of the initial stress analysis step, the freeze expansion step, and the excavation step.
A method for analyzing freezing expansion of the ground, characterized in that the maximum principal stress, the minimum principal stress, and the safety factor are obtained in each step .

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