JP6338096B2 - Ground improvement method and improved ground - Google Patents

Description

  The present invention relates to a ground improvement method and ground for preventing liquefaction of the ground.

  Conventionally, as a countermeasure for liquefaction of the ground when an earthquake occurs, a ground improvement method for constructing a grid-like improved body formed in a grid has been widely used. This ground improvement method suppresses shear deformation of the ground and prevents liquefaction of the ground by a highly rigid grid-like improvement body constructed on the ground. (For example, refer to Patent Document 1).

Japanese Patent No. 5190655

  By the way, the lattice-shaped improvement body is comprised from the improvement wall arrange | positioned at the grid | lattice form, and it is necessary to make the space | interval of adjacent improvement walls into several m-10 several m by the conditions of the ground. And if the building area of the building (structure) built on the upper side is large, many improvement walls are required, so a lot of labor is required for the construction of the lattice-shaped improvement body, and costs and construction periods are required. There is a problem.

  Then, an object of this invention is to provide the ground improvement construction method and improved ground which can suppress a construction period, labor, and cost.

In order to achieve the above object, the ground improvement method according to the present invention is to improve the grid by arranging a plurality of improvement walls in a grid shape in which the lower end reaches the low-permeability lower ground on the ground to prevent liquefaction. A grid-like improvement body construction process for constructing a body, a well is installed on the ground where the grid-like improvement body is built, and groundwater is pumped, and the groundwater level of the ground where the grid-like improvement body is built is determined as a natural water level. A groundwater level that is lowered from the groundwater level to a predetermined groundwater level, and the groundwater level that has been lowered by the pumping step is restored to the natural water level, so that the ground in which the grid-like improvement body is constructed becomes an unsaturated ground in which bubbles are mixed possess a desaturation process, the improved rate of ground in the lattice-shaped improved body construction step, the excess pore water pressure ratio occurring ground when the shear strain amount and the desaturation of the ground upon which desaturate Unsaturated ground evaluated from the relationship Characterized in that it designed based on liquefaction strength.
In the ground improvement method according to the present invention, the excess pore water pressure ratio Δu / σ _v ′ generated in the ground when the desaturation is performed may be calculated from the following mathematical formula.

Further, the improved ground according to the present invention is constructed of a grid-like improved body in which a plurality of improved walls that are constructed in the ground and reach the lower-layer ground having a low-permeability bottom portion are arranged in a grid shape, and the grid-like improved body is constructed. The ground in which the grid-like improvement body is constructed is lowered from the natural water level to a predetermined groundwater level when the groundwater is pumped up by the well. and, when the pumping of groundwater by the well is stopped, reduced water table is restored to the natural water level, Ri Do unsaturated ground bubbles are mixed, improvement ratio of the ground which the lattice-shaped improved body is constructed Is designed based on the liquefaction strength of unsaturated ground evaluated from the relationship between the amount of shear strain of the ground when desaturated and the excess pore water pressure ratio generated in the ground when desaturated. Features.

In the present invention, since the ground on which the grid-like improvement body is constructed can be desaturated to become an unsaturated ground, the liquefaction strength of the ground can be increased as compared with the saturated ground.
And the unsaturated ground can reduce the volume of the grid improvement body compared to the case where the grid improvement body is constructed on the saturated ground due to the high liquefaction strength of the ground compared to the saturated ground, Since the interval between adjacent improvement walls can be increased and the thickness of the improvement walls can be reduced, the construction period, labor and cost for ground improvement can be suppressed.

In the ground improvement method according to the present invention, the improvement wall includes an outer improvement wall arranged at an outer edge portion of the lattice-like improvement body, and an inner improvement wall arranged in a lattice shape inside the outer improvement wall. The inner improved wall is formed with a water passage portion through which one wall surface side communicates with the other wall surface side and through which the ground water can pass. In the pumping process, a plurality of inner improved walls are formed by the inner improved wall. It is preferable to arrange the wells at one or more places in the area divided into two and to pump water in the wells.
The ground on which the grid-like improvement body is constructed is divided into a plurality of areas by the inner improvement walls arranged in a grid, but if a well is installed in at least one of the divided areas, The groundwater in the area where is not installed can also be pumped by passing through the water passing part of the inner improvement wall.
Thereby, since it is not necessary to install a well for every area | region divided by the inner side improvement wall in the ground where the lattice-shaped improvement body was constructed | assembled, the construction period, labor, and cost concerning installation of a well can be suppressed.

  According to the present invention, it is possible to reduce the volume of the lattice-shaped improvement body, as compared with the case of constructing the lattice-shaped improvement body on the saturated ground, increase the interval between adjacent improvement walls, or improve the thickness of the improvement wall. Therefore, it is possible to reduce the construction period, labor and cost for ground improvement.

It is a top view explaining an example of the improvement ground by the embodiment of the present invention. It is the sectional view on the AA line of FIG. It is a perspective view which shows an example of a water flow part. It is a graph which shows the relationship between the unit drainage amount (water flow rate) of a ground, and a saturation degree. It is a graph which shows the relationship between the saturation degree of a ground, and the increase rate of liquefaction strength. It is a graph which shows the relationship between the initial shear strain and excess pore water pressure ratio in the design of the lattice improvement body of a saturated ground. It is a graph which shows the relationship between the initial shear strain and excess pore water pressure ratio in the design of the lattice improvement body of unsaturated ground. The top view which shows the shape of the lattice-shaped improvement body constructed | assembled in the saturated ground, (b) is a top view which shows the shape of the lattice-shaped improvement body constructed | assembled in the unsaturated ground. (A) is a top view which shows the shape of a lattice-shaped improvement body, (b) is a top view which shows the other shape of a lattice-shaped improvement body.

Hereinafter, a ground improvement method and an improved ground according to an embodiment of the present invention will be described with reference to FIGS.
As shown in FIGS. 1 and 2, the improved ground 1 improved by the ground improvement method according to the present embodiment includes a grid-like improved body 2 constructed on the ground 11 to prevent liquefaction, and a grid-like improved body. And a deep well (well) 3 capable of pumping ground water of the ground 11 on which 2 is constructed.
In the ground 11 on which the improved ground 1 is constructed, an upper ground 12 (see FIG. 2) composed of a water-permeable sandy layer or the like is arranged on the ground surface 11a (see FIG. 2), and the lower side of the upper ground 12 Is provided with a low-permeability lower layer ground 13 (see FIG. 2).
And the structure 14 (refer FIG. 2), such as a building, is constructed in the upper part of this improved ground 1. FIG.

The grid-like improvement body 2 includes a plurality of outer improvement walls (improvement walls) 4 arranged to form a square tube on the ground 11, and a plurality of inner improvement walls arranged in a grid pattern inside the outer improvement wall 4. (Improved wall) 5. For this reason, the improved ground 1 is divided into a plurality of regions by the inner improved wall 5.
The outer improved wall 4 and the inner improved wall 5 have lower end portions 4a and 5a (see FIG. 2) reaching the lower layer ground 13, respectively, and upper end portions 4b and 5b (see FIG. 2) reach the vicinity of the ground surface 11a. ing.

Further, the plurality of inner improved walls 5 are respectively formed with water passage portions 51 that allow one wall surface side and the other wall surface side to communicate with each other and allow ground water of the ground to pass therethrough. As shown in FIG. 3, in the present embodiment, the water flow portion 51 is configured by a notch formed at the lower end portion 5 a of the inner improved wall 5. In addition, the number of the water flow parts 51 formed in the inner improved wall 5 and the place to be formed may be set as appropriate.
Moreover, although the water flow part 51 like the inner improvement wall 5 is not formed in the some outer improvement wall 4, water is completely sealed between the lower end part 4a of the outer improvement wall 4 and the lower layer ground 13. Although it is not carried out, the one wall surface side and the other wall surface side of the outer improved wall 4 are small in volume compared to the inner improved wall 5, but water is passed.

Returning to FIG. 2, the deep well 3 is installed so as to reach a slightly upper side of the lower layer ground 13, and is configured to be able to pump the ground water of the upper layer ground 12.
The deep well 3 is installed in a part of the improved ground 1 divided into a plurality of areas by the inner improved wall 5 (hereinafter referred to as a deep well installation ground 15).
And when ground 11 groundwater is pumped in the deep well 3, groundwater other than the deepwell installation ground 15 in the improved ground 1 (hereinafter referred to as the deepwell non-installation ground 16) also flows through the inner improved wall 5. It passes through the part 51, moves to the deep well installation ground 15, and is pumped by the deep well 3.
Thus, the groundwater of the ground 11 of the ground 11 is pumped by the deep well 3 so that the groundwater of the entire ground 11 inside the outer improved wall 4 is pumped.

  Moreover, when performing the ground improvement construction method like this embodiment, the structure 14 constructed in the upper part of the lattice-shaped improvement body 2 becomes an underground seismic isolation structure in many cases, and in the case of construction of this structure 14 For example, a deep well is used for excavation work in order to perform excavation work of about 3 m. For this reason, the deep well 3 of the improved ground 1 according to the present embodiment uses the deep well 3 used in the excavation work.

Next, the ground improvement method according to the present embodiment will be described.
(Lattice improvement body construction process)
First, the grid-like improved body 2 is constructed so that the outer improved wall 4 and the inner improved wall 5 are arranged in a grid pattern on the ground 11. At this time, a water flow portion 51 is formed at the lower end portion 5 a of the inner improved wall 5.
The outer improved wall 4 and the inner improved wall 5 are constructed by a known method, for example, by mixing and stirring cement or cement-based solidified material and in situ soil to form a wall shape.

(Pumping process)
Subsequently, the deep well 3 is installed in an area that becomes the deep well installation ground 15 in the improved ground 1. In this embodiment, the deep well used for the excavation work of the ground 11 is used for the deep well 3.
And the groundwater of the improved ground 1 is pumped up by the deep well 3, and the groundwater level of the improved ground 1 is lowered from the natural water level 17 (refer FIG. 2). Then, when the groundwater level drops to a predetermined groundwater level 18 (see FIG. 2), the pumping is stopped.

(Ground unsaturation process)
Subsequently, the lowered predetermined groundwater level 18 is recovered to the natural water level 17. In order to restore the lowered predetermined groundwater level 18 to the natural water level 17, the groundwater is passed from the ground 11 outside the outer improved wall 4 to the improved ground 1 inside the outer improved wall 4, or the ground surface. The water is poured from 11a and the ground 11 is submerged.
Thereby, air (bubbles) is mixed in the ground 11 and the ground 11 becomes unsaturated, and the liquefaction strength of the ground 11 can be increased.

  In addition, since the saturation degree of the ground 11 changes with time, the ground water of the ground 11 is pumped regularly or irregularly such as once every six months to several years so that the ground 11 is maintained in an unsaturated state. .

Here, as can be seen from FIG. 4, by performing the pumping process and the ground desaturation process, the degree of saturation of the improved ground 1 can be about 70 to 90%, and the improved ground 1 is an unsaturated ground. be able to.
Further, as can be seen from FIG. 5, the unsaturated ground having a saturation degree of 70% can have a liquefaction strength approximately three times that of the saturated ground having a saturation degree of 100%, and the saturation degree is 90%. In unsaturated ground, the liquefaction strength can be about twice that of saturated sand with 100% saturation.
In this way, the liquefaction strength of the improved ground 1 can be increased by performing the pumping process and the ground desaturation process and making the improved ground 1 an unsaturated ground.

Then, the design method of the improved ground 1 by this embodiment is demonstrated.
The design method of the improved ground 1 is performed based on, for example, the “evaluation method of the deformation amount of the partially improved ground” described in Japanese Patent No. 5190655.
In this “evaluation method of deformation amount of partially improved ground”, the relationship between the initial shear strain of the improved ground and the excess pore water pressure ratio generated during liquefaction is derived from the following formulas (1) to (4).

  In the present embodiment, since the ground is unsaturated, the formula (2) is changed to the following formula (2) ′. Other than this, the design is performed in the same manner as the “evaluation method of the deformation amount of the partially improved ground” described in Japanese Patent No. 5190655.

Subsequently, a design example of the grid-like improved body 2 of the improved ground will be described.
First, for example, shear wave velocity V _S = 140 m / s, wet density ρ = 1.8 g / cm ³ , initial shear stiffness G ₀ = 35 MPa, reference shear strain γ _ref = 2.9 e−4, maximum attenuation h _max = 0.12, constant beta = 1.46, vertical effective stress _σ'v = 54kPa, equivalent cyclic shear stress ratio τ _d / _σ'v = 0.41, liquefaction resistance ratio _{R 20 = τ lq / σ'v} = An improved ground for constructing a grid-like improved body 2 having a shear wave velocity V _s = 500 m / s is created for a saturated ground having an experimental constant k = −0.25 and an experimental constant α _rf = 0.7. To do.
At this time, if the improvement rate R of the ground is 20%, the liquefaction safety factor of the improved ground is 1.5. In addition, the liquefaction safety factor of the ground when not improved is 0.37.
FIG. 6 shows the relationship between the initial shear strain and the excess pore water pressure ratio in the design of the grid improvement body of the saturated ground.

On the other hand, when the liquefaction strength of the unsaturated ground with the saturation degree S _r = 90% is R _un (90) = 2R ₂₀ = 2 × 0.15 = 0.3 , the ground improvement rate R = 5% Then, the liquefaction safety factor of the improved ground is 1.4. In addition, the liquefaction safety factor of the ground when not improved is 0.73.
FIG. 7 shows the relationship between the initial shear strain and the excess pore water pressure ratio in the design of the improved lattice structure of unsaturated ground.

From the above, when the ground improvement is performed with the improvement rate R being 20% in the saturated ground, and when the ground improvement is performed with the improvement rate being 5% in the unsaturated ground with the saturation degree S _r = 90% Then, it turns out that the liquefaction safety factor of the improved ground becomes substantially equal.
For this reason, in the ground improvement for constructing the grid-like improvement body 2, when the site area where the ground improvement is performed and the thickness of the outer improvement wall 4 and the inner improvement wall 5 of the grid-like improvement body 2 are equal, the saturation degree S _r = In the 90% unsaturated ground, the distance between the adjacent outer improved wall 4 and the inner improved wall 5 and the adjacent inner improved wall 5 can be increased as compared with the saturated ground.

For example, when performing ground improvement to construct a grid-like improvement body 2 in which the thickness of the outer improvement wall 4 and the inner improvement wall 5 is 1 m on a substantially square site of 80 m × 80 m, as shown in FIG. In the case of constructing a lattice-like improvement body 2B in which the interval between the adjacent outer improvement wall 4B and the inner improvement wall 5B or the two adjacent inner improvement walls 5B is 10 m, as shown in FIG. In the case of constructing the grid-like improved body 2 in which the interval between the adjacent outer improved wall 4 and the inner improved wall 5 and the adjacent inner improved wall 5 is 40 m in the unsaturated ground of S _r = 90%, the improved ground 1 1B, the liquefaction strength is almost the same.

Next, the above-described ground improvement method and the function and effect of the improved ground will be described.
In the ground improvement method and the improved ground according to the above-described embodiment, the improved ground 1 in which the lattice-like improved body 2 is constructed can be desaturated to become an unsaturated ground, and therefore the improved ground 1 compared to the saturated ground. The liquefaction strength can be increased.
And in unsaturated ground, the liquefaction strength of the improved ground 1 is higher than that of the saturated ground, so that the volume of the grid-like improved body 2 is reduced compared to the case where the grid-like improved body is constructed on the saturated ground. Since the distance between the adjacent outer improved walls 4 and the inner improved walls 5 can be increased, the construction period, labor, and cost for ground improvement can be suppressed.

In addition, since the inner improvement wall 5 is formed with a water flow portion 51, the deep well 3 is provided on at least a part of the deep well installation ground 15 in a region divided by the inner improvement wall 5 of the improved ground 1. If the deep well 3 is pumped up, the groundwater in the deep well non-installed ground 16 can also be pumped through the water passage 51 of the inner improved wall 5.
Thereby, since it is not necessary to install the deep well 3 for every area | region divided by the inner improvement wall 5 in the improved ground 1, the construction period, labor, and cost concerning installation of the deep well 3 can be suppressed. .

As mentioned above, although the ground improvement construction method by this invention and embodiment of the improved ground were demonstrated, this invention is not limited to said embodiment, In the range which does not deviate from the meaning, it can change suitably.
For example, in the above embodiment, the deep well 3 used in the ground improvement method uses the deep well used for the excavation work of the ground 11, but the deep well used for the excavation work of the ground 11 is removed, Another deep well may be installed for the ground improvement method. Further, the number of deep wells 3 to be installed may be set as appropriate. When a plurality of deep wells 3 are installed and the deep well non-installed ground 16 is eliminated, a water flow portion 51 is formed on the inner improved wall 5. It does not have to be.
Further, in the above-described embodiment, the water flow portion 51 is formed by a notch portion formed in the lower end portion 5 a of the inner improved wall 5, but is formed in the vicinity of the lower end portion 5 a of the inner improved wall body 5. The inner improved wall 5 may be configured with a hole that allows one wall surface side to communicate with the other wall surface side. Moreover, the number of the notch parts and hole parts of the water flow part 51 may be set as appropriate.

Moreover, in said embodiment, although the space | interval of the adjacent outer improvement wall 4, the inner improvement wall 5, and the adjacent inner improvement wall 5 is enlarged compared with the case where a grid | lattice improvement body is constructed | assembled in a saturated ground. One of the outer improvement wall 4 and the inner improvement wall 5 is set so that the interval between the adjacent outer improvement wall 4 and the inner improvement wall 5 and the adjacent inner improvement wall 5 is the same as that when the lattice-like improvement body is constructed on the saturated ground. Or you may make both thickness thin compared with the case where a lattice-shaped improvement body is constructed | assembled in a saturated ground.
Moreover, in said embodiment, as shown in FIG.1 and FIG.9 (a), the grid | lattice-like improvement body 2 is comprised from the outer side improvement wall 4 and the several inner improvement wall 5 arrange | positioned at grid | lattice form. However, it may be composed only of the outer improved wall 4 formed in a rectangular shape in plan view. Moreover, the lattice-shaped improvement body 2 is good also as a structure by which only one inner improvement wall 5 is provided in the inside of the outer improvement wall 4 formed in the rectangular shape in planar view. The outer improved wall 4 may be formed in a shape other than a rectangular shape.

1 Improved ground 2 Lattice improved body 3 Deep well
4 outer improved wall (improved wall)
5 inner improved wall (improved wall)
11 Ground 13 Lower ground 17 Natural water level 18 Predetermined groundwater level 51 Water flow section

Claims (4)

In the ground to prevent liquefaction, a grid-like improvement body construction step of constructing a grid-like improvement body by arranging a plurality of improvement walls in a grid shape with the lower end reaching the low-permeability lower ground,
A well is installed on the ground where the lattice-like improvement body is constructed to pump groundwater, and the groundwater level of the ground where the lattice-like improvement body is constructed is lowered from a natural water level to a predetermined groundwater level, and
The groundwater level was reduced by該揚water step by restoring the natural water level, the ground in which the lattice-shaped improved body is constructed possess the soil desaturation step of the unsaturated soil bubbles are mixed, and
The improvement rate of the ground in the grid-like improvement body construction process is that of the unsaturated ground evaluated from the relationship between the shear strain amount of the ground when desaturated and the excess pore water pressure ratio generated in the ground when desaturated. A ground improvement method that is designed based on liquefaction strength . Excess pore water pressure ratio Δu / σ generated in the ground when desaturated _v 'Is calculated from the following mathematical formula, the ground improvement method according to claim 1.

The improvement wall is composed of an outer improvement wall arranged at the outer edge of the lattice-like improvement body, and an inner improvement wall arranged in a lattice shape inside the outer improvement wall,
The inner improved wall is formed with a water passage portion through which one wall surface side and the other wall surface side communicate with each other and through which the groundwater can pass.
3. The ground improvement method according to claim 1 , wherein in the pumping step, the well is disposed at one or more locations in a region partitioned by the inner improvement wall, and pumping is performed in the well. A lattice-shaped improvement body in which a plurality of improvement walls that are constructed on the ground and reach the lower ground where the lower end portion is hardly permeable are arranged in a lattice shape,
A well capable of pumping ground ground water in which the grid-like improvement body is constructed,
The ground where the lattice-shaped improvement body is constructed is reduced when the groundwater is pumped by the well, the groundwater level is lowered from the natural water level to a predetermined groundwater level, and the pumping of the groundwater by the well is stopped. the underground water level is restored to the natural water level, Ri Do the unsaturated soil in which bubbles are mixed,
The improvement rate of the ground on which the grid-like improvement body was constructed was determined based on the relationship between the amount of shear strain of the ground when desaturated and the excess pore water pressure ratio generated in the ground when desaturated. Improved ground, which is designed based on the liquefaction strength of

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