JPH0649991B2 - Floating prevention method for underground structures in sand ground that tends to liquefy - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for preventing floating of an underground structure in a sand ground that is easily liquefied, and in particular, the water pressure of excess pore water in the ground that sandwiches the underground structure. The present invention relates to a construction method capable of preventing the floating of underground structures by balancing the above.

《Prior art》 Underground structures such as underground tunnels with a hollow interior have a small apparent specific gravity, so if such an underground structure is built in the sand ground, excess voids will be generated in the ground during an earthquake. Due to the rise of water pressure, the sand ground is liquefied, and the damage of floating these underground structures has often occurred in the past earthquakes.

As countermeasures against such a problem, there are improvement of the ground to be liquefied and construction method for lowering groundwater level.

However, in the ground improvement method, if the target ground is deep or it covers a wide area, a huge amount of public expense is required. In addition, the groundwater level lowering method must continue draining for a long period of time, and there are restrictions on site conditions and maintenance problems.
In fact, it is not often adopted.

Therefore, recently, the gravel drain pile method has been adopted. As shown in FIG. 3, this method involves liquefying the pile-like drainage channel 2 from the ground surface in which the ground E on both sides of the underground structure 1 is filled with a gravel made of a member having good water permeability such as gravel and gravel. It is driven almost vertically to the target ground, and when liquefaction occurs, excess pore water is absorbed to the surface side through this drainage channel 2.

<< Problems to be Solved by the Invention >> However, since the pile drainage channel 2 is provided only on both sides of the underground structure 1 in this construction method, when the width S of the underground structure 1 is wide, Excessive pore water generated immediately below the structure 1 cannot be absorbed only by the drainage channel 2, and there is a drawback that water pressure for floating is generated.

The present invention has been made in view of such conventional problems, and is a construction method capable of preventing the floating of the underground structure is easy and economical, and a method for preventing the floating of the underground structure in the sand ground that is easily liquefied. The purpose is to provide.

<< Means for Solving Problems >> In order to achieve the above object, the present invention forms a lower gravel layer and an upper gravel layer on at least the lower surface and the upper surface of an underground structure in a sand ground that is easily liquefied. By connecting the gravel layers through the underground structure, the water pressure of excess pore water in the sand ground on the upper and lower surfaces of the underground structure is maintained in an equilibrium state.

<Operation> According to the above floating prevention method, when seismic force acts on the sand ground and it liquefies and the pore water pressure rises, excess pore water is collected in the lower gravel layer, and It tries to generate a large levitation pressure, but due to the water pressure difference with the upper gravel layer, it moves into the upper gravel layer and keeps the upper and lower water pressures in equilibrium, so that the underground structure remains stationary. You can

<Example> Hereinafter, a preferred example of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows an embodiment of a method for protecting underground structures on sand ground which is easily liquefied according to the present invention.

The underground structure 10 shown in the figure is a concrete structure such as an underground runner having a hollow rectangular cross section,
It is located at a predetermined depth in the sand ground E that is easily liquefied.

In the construction of the floating prevention method, the lower gravel layer 16 having a predetermined thickness is formed below the bottom slab 14 of the structure 10 before the underground structure 10 is constructed.

As the lower gravel layer 16, gravel or gravel having a predetermined size and good water permeability is used, and the lower gravel layer 16 has a size corresponding to the area of the bottom slab 14 to be constructed and is spread over a predetermined thickness dimension.

The lower gravel layer 16 can also serve as paving masonry that is usually carried out.

When the gravel layer 16 is formed in this manner, the bottom slab 14 is constructed on the upper surface thereof, and the side wall 18 and the top plate 20 are constructed on the bottom slab 14, whereby the underground structure 10 is completed.

After the construction of the underground structure 10, the upper gravel layer 22 is formed on the upper surface of the top plate 20.

As the upper gravel layer 22, gravel or gravel having a predetermined size is used like the lower gravel layer 16,
This has a size corresponding to the area of the top 20,
It is spread to the same thickness as the lower gravel layer 16.

As a method for forming the upper gravel layer 22, for example, an upper surface of the top plate 20 is exposed by a cutting method or the like, a covering operation is performed, and the upper portion thereof is backfilled.

Then, during or after the construction of the underground structure 10, the lower gravel layer 16 and the upper gravel layer 22 are vertically connected 1
One or a plurality of water flow pipes 24a to 24c are arranged.

In this embodiment, three hollow pipe-shaped water passages 24a to
24c is installed, and the water pipe 24a on the left side in the drawing penetrates the center of the inside of the side wall 18 and has the lower end at the lower gravel layer 16
In addition, the upper end is opened to the upper gravel layer 22.

The water pipe 22b is arranged along the inside of the right side wall 18 and has upper and lower ends opened to the gravel layers 16 and 22, respectively.

Further, the water distribution pipe 22c is arranged along the outer surface of the right side wall 18, and is opened with its upper and lower ends bent to the top plate 20 and the bottom plate 14 side.

Now, in the floating prevention method constructed as described above, in the normal state, the underground structure 10 is held in the ground E while being sandwiched between the lower gravel layer 16 and the upper gravel layer 22.

When the seismic force acts on the sand ground E and the pore water pressure rises,
Excessive pore water is collected in the lower gravel layer 16 having good water permeability and tries to generate buoyancy due to water pressure. However, since water pressure due to excess pore water is not yet generated in the upper gravel layer 22, the water pipe 24a. ~ 24c is quickly drained to the upper gravel layer 22 side to balance the water pressure up and down,
It suppresses the increase in buoyancy.

FIG. 2 shows a second embodiment of the present invention.

In the figure, an underground structure 30 constructed in a sand ground E is an underground tunnel made of a cylindrical concrete structure, and its entire outer peripheral portion is covered with a gravel layer 32 having a predetermined thickness.

In this embodiment, when seismic force acts on the sand ground E to cause liquefaction and excess pore water is collected on the lower side of the gravel layer 32, the collected water has a uniform water pressure distribution. In order to do so, the rim of the underground structure 30 is moved to balance the water pressure in the vertical and horizontal directions. Therefore, also in this embodiment, the generation of buoyancy is suppressed, and the floating of the underground structure can be prevented.

The upper gravel layer 22 may be provided so as to reach the surface of the earth. Further, the underground structures 10 and 30 do not necessarily have to be located in the ground where the whole is easily liquefied, and for example, there may be a ground layer that is easily liquefied below the lower gravel layer 16.

<< Effects of the Invention >> As described in each of the above embodiments, according to the method for preventing the floating of the underground structure in the sand ground that is easily liquefied according to the present invention, the seismic force acts on the sand ground to liquefy it. When the pore water pressure rises, excess pore water is collected in the lower gravel layer and tries to generate a large levitation pressure on the underground structure.However, due to the difference in water pressure between the upper gravel layer and the upper gravel layer, It moves and keeps the water pressure above and below in equilibrium, so that the underground structure can remain stationary.

Further, according to this floating prevention construction method, the construction is simpler than any of the conventional construction methods, and it is economically advantageous.

[Brief description of drawings]

FIG. 1 is a sectional view showing a first embodiment of the method of the present invention.
FIG. 3 is a sectional view showing a second embodiment of the present invention, and FIG. 3 is a sectional view showing a conventional gravel drain pipe construction method. 10,30 ... Underground structure 16 ... Lower gravel layer 22 ... Upper gravel layer 24a-24c ... Water pipe 32 ... Gravel layer E ... Sand ground

Claims (1)

[Claims] 1. A lower gravel layer and an upper gravel layer are formed on at least a lower surface and an upper surface of an underground structure in a sand ground which is easily liquefied, and the gravel layers are connected to each other with the underground structure sandwiched therebetween. A method for preventing floating of an underground structure in a sand ground that is easily liquefied, characterized in that the water pressures of excess pore water in the sand ground on the top and bottom surfaces of the underground structure are kept in a balanced state.