JPH02112527A - Technique of preventing floating of underground construction in liquescent sand bed - Google Patents

Abstract

PURPOSE:To balance the water pressure of excessive interstitial water in a sandy subsoil of up and down surfaces of a construction by forming a lower gravel layer and upper gravel layer on the lower surface and upper surface of an underground construction and, at the same time, connecting between gravel layers each other across the underground construction. CONSTITUTION:Before constructing an underground construction 10, a lower gravel layer 16 with well permeable gravel or stone having the same area as that of a bottom slab 14 and a specific thickness is formed downward of the bottom slab 14. After that, the bottom slab 14 is constructed thereon, a side wall 18 and top slab 20 are constructed on the bottom slab 14, and an upper gravel layer 22 is formed on the top slab 20 as well as the lower gravel layer 16. A plurality of water pipes 24a-24c connecting the upper and lower gravel layers vertically are piped, and the lower end is opened to the lower gravel layer 16 and the upper end is opened to the upper gravel layer 22. According to the constitution, excessive interstitial water caused by an earthquake is collected to the lower gravel layer 16, however, the difference of water pressure is up and down is always kept its balance by water pipes, and the stand-still condition of the underground construction 10 can be maintained.

Description

Detailed Description of the Invention (Industrial Application Field) This invention relates to a method for preventing floating of underground structures in sandy ground that is prone to liquefaction, and in particular, to prevent water pressure of excess pore water in the ground between the underground structures. This invention relates to a construction method that can prevent underground structures from surfacing by balancing the

(Conventional technology) Underground structures with hollow interiors, such as underground tunnels, have a small apparent specific gravity. Therefore, when such underground structures are constructed in sandy ground, excessive voids occur in the ground during an earthquake. Past earthquakes have often caused damage in which sandy ground liquefies due to increased water pressure, causing these underground structures to float up.

Countermeasures against such problems include improving the ground subject to liquefaction and methods of lowering the groundwater level.

However, ground improvement methods require a huge amount of public expense if the ground is deep or covers a wide area. In addition, the groundwater level lowering method requires continuous drainage over a long period of time, and there are constraints on location and maintenance issues, so it is not often adopted in practice.

Therefore, the gravel drain pile method has recently been adopted. As shown in Figure 3, this construction method involves creating a pile-shaped drainage channel 2 filled with gravel made of highly permeable materials such as gravel and gravel in the ground E on both sides of an underground structure 1, which is liquefied from the ground surface. It is driven almost vertically to the target ground, and if liquefaction occurs, excess pore water is absorbed to the ground surface through this drainage channel 2.

(Problem to be solved by the invention) However, since this construction method provides pile drainage channels 2 only on both sides of the underground structure 1, when the width S of the underground structure 1 is wide, Excess pore water generated directly under the structure 1 cannot be absorbed by the drainage channel 2 alone, and there is a drawback that water pressure for floating is generated.

This invention was made in view of these conventional problems, and provides an easy and economical construction method for preventing surfacing of underground structures on sandy ground that is prone to liquefaction. The purpose is to provide

(Means for Solving the 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 upper surface of an underground structure in sandy ground that is easily liquefied. By sandwiching the underground structure and communicating between the gravel layers, the water pressure of excess pore water in the sandy ground on the upper and lower surfaces of the underground structure is maintained in an equilibrium state.

(Function) According to the above floating prevention method, when seismic force acts on the sandy ground and it liquefies and the pore water pressure increases, the excess pore water is collected in the lower gravel layer and is damaged against underground structures. However, due to the water pressure difference with the upper gravel layer, it moves into the upper gravel layer, keeping the upper and lower water pressures in equilibrium, and as a result, the underground structure remains stationary. be able to.

(Embodiments) Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows an embodiment of the method for protecting underground structures in sandy ground that is prone to liquefaction according to the present invention.

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

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

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

Note that this lower gravel layer 16 can also be used for paving stone work carried out during normal construction.

Once the gravel layer 16 is formed in this way, the bottom slab 14 is constructed on the upper surface thereof, and the side walls 18 and the top slab 20 are constructed on the bottom slab 14, thereby completing the underground structure 10.

After constructing the underground structure 10, an upper gravel layer 22 is formed on the upper surface of the ceiling plate 20.

As with the lower gravel layer 16, gravel or gravel having a predetermined size is used for this upper gravel layer 22,
This is a size corresponding to the area of the heaven plate 20,
The upper gravel layer 22 can be formed to have the same thickness as the lower gravel layer 16. For example, the upper surface of the top plate 20 is exposed using a cut-and-cover method, and the upper surface is then filled in. good.

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.
One or more water pipes 24a to 24c are arranged.

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

Further, the water pipe 22b is installed along the inside of the right side wall 18, and opens its upper and lower ends into the respective gravel layers 16 and 22.

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

Now, in the floating prevention method configured as described above, under normal conditions, 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 seismic force acts on sandy ground E and pore water pressure increases,
Excess pore water collects in the lower gravel layer 16, which has good water permeability, and tries to generate buoyancy due to water pressure.However, since water pressure due to excess pore water has not yet been generated in the upper gravel layer 22, the water flow pipe 24a ~ 24c, the water is quickly drained to the upper gravel layer 22 side, and the water pressure is balanced between the top and bottom,
This suppresses the increase in buoyancy.

FIG. 2 shows a second embodiment of the invention.

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

In this embodiment, when seismic force acts on the sandy ground E and liquefaction occurs, and excess pore water is collected on the lower side of the gravel layer 32, the collected water evenly distributes its water pressure. The periphery of the underground structure 30 is moved to balance the vertical and horizontal water pressures. Therefore, in this embodiment as well, the generation of buoyant force is suppressed, and the floating of the underground structure can be prevented.

Note that the upper gravel layer 22 may be provided so as to reach the ground surface. Moreover, the underground structure 10.30 does not necessarily have to be located entirely in the ground that is easily liquefied; for example, there may be a ground layer that is easily liquefied below the lower gravel layer 16.

(Effects of the Invention) As explained in each of the embodiments above, according to the method for preventing floating of underground structures in sandy ground that is prone to liquefaction according to the present invention, seismic force acts on the sandy ground, causing it to liquefy. When pore water pressure increases, excess pore water collects in the lower gravel layer and tries to create a large uplift pressure on the underground structure, but due to the water pressure difference with the upper gravel layer, the excess pore water collects in the lower gravel layer. It moves to keep the water pressure above and below balanced, allowing the underground structure to remain stationary.

Furthermore, this floating prevention construction method is easier to construct than any of the conventional construction methods, and is economically advantageous.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing the first embodiment of the method of the present invention;
The figure is a sectional view showing a second embodiment of the present invention, and FIG. 3 is a sectional view showing a conventional gravel drain vibe construction method. Figure 3 10.30・・・・・・・・・Underground structure 16・
・・・・・・・・・・・・・・・・・・Lower gravel layer 22・・・・・・・・・・・・・・・・・・・・・
Upper gravel layer 24a to 24c... Water pipe

Claims (1)

[Claims] (1) By forming a lower gravel layer and an upper gravel layer on at least the lower and upper surfaces of an underground structure in sandy ground that is prone to liquefaction, and by sandwiching the underground structure and communicating between the gravel layers, A method for preventing surfacing of underground structures in sandy ground that is prone to liquefaction, characterized by maintaining the water pressure of excess pore water in sandy ground in a balanced state on the upper and lower surfaces of the underground structure.