JPH03212514A - Foundation structure of underground construction - Google Patents

Abstract

PURPOSE:To hold down floating of an underground construction to a very little amount in case that liquefaction is caused in a sand layer by constructing a cut-off wall up to an impermeable layer, forming a drain layer inside the cut-off wall, building up an underground construction on the drain layer, and backfilling sediment. CONSTITUTION:A cut-off wall 2 is constructed in such a manner that the lower end thereof is extended to an impermeable layer C in such a position as to surround an underground construction 1 and a sand layer S just under the construction, and after digging to a depth for forming an inside drain layer D, the dug portion is backfillied with ballast or the like to form a drain layer D. Subsequently, after the underground construction 1 is built up on the drain layer, further sediment is backfilled. In the case where earthquake force or the like is applied to the sand layer S to cause liquefaction in the sand layer S in the cut-off wall 2 just under the underground construction 1, the ground in the periphery is checked from its wraparound into the cut-off wall 2, and pore water due to a very little floating is discharged to the drain layer D. Thus, the excess water pressure in the sand layer can be quickly dissipated, and a drainage work having a reliable drain effect can be executed.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" This invention is applicable to foundation structures of truverts, common ditches, buried pipes, and other underground structures that are completely buried in a sand layer. This is a related story.

“Conventional technology” In recent years, the development of reclaimed land and the development of waterfront
As development in this area becomes more active, liquefaction of sandy ground during earthquakes becomes a problem, and countermeasures against this liquefaction include ground compaction methods such as the Sand Compact Noyon Pile method and vibration compaction method. Construction methods that prevent ground liquefaction were being implemented, such as ■Vertical drain methods such as gravel drain method and pipe drain method, and ■Deep mixing method.

However, when using the ground compaction method (■), there is a problem in that construction work in urban areas is now difficult due to noise and vibration. The deep mixing method is relatively expensive, and its effectiveness in preventing ground liquefaction was questionable.

"Problem to be Solved by the Invention" By the way, the specific gravity of sandy ground is about 1.9, so in the case of a structure built on sandy ground, the apparent specific gravity of the underground part of the structure is In other words, if the ratio of the total weight of the structure divided by the volume of the underground part of the structure is 1.9 or less, there is a risk that the structure will float if the surrounding ground liquefies during an earthquake. . If the specific gravity of the structure is greater than 1.0, lifting due to hydrostatic pressure will not occur even if the groundwater level in the sandy ground reaches close to the ground surface.

In a normal structure, the apparent specific gravity is 1.
9 or less is often the case with underground structures where the entire structure is buried in the ground. Therefore, especially when constructing underground structures on soft ground such as sandy ground,
From a cost perspective, it is more desirable from a cost perspective to create a foundation structure that takes into account the prevention of uplift caused by ground liquefaction during earthquakes, etc., than to support the foundation on deep bedrock to prevent it from sinking due to its own weight.

For example, as an underground structure having an apparent specific gravity of 1.9 or less, there is a culvert l buried in a sand layer S as shown in FIG. The floating of an underground structure such as the culvert L in the sand layer S is caused by the liquefaction of the sand layer S, which causes the earth and sand that has turned into mud to flow directly under the culvert L, as shown in Figure 4. That culvert is crowded! This occurs by pushing up from the bottom (see the arrow in Figure 4).

This invention has been made in view of the above circumstances, so that even if ground liquefaction during an earthquake or the like reaches the ground directly under the underground structure, the surrounding ground will not flow directly under the underground structure. It is an object of the present invention to provide a foundation structure for an underground structure that can suppress the uplift of the underground structure to an extremely small amount below an allowable limit by preventing this.

"Means for Solving the Problems" The foundation structure of an underground structure of the present invention is a foundation structure of an underground structure entirely buried in a sand layer, and the bottom of the underground structure has gravel etc. At the same time, a water cutoff wall is provided at a position surrounding the underground structure and the sand layer immediately below it, and the lower end of the water cutoff wall is connected to the impermeable layer below the sand layer. Or it has reached a layer that does not liquefy.

"Function" In the foundation structure of the underground structure of this invention, when seismic force etc. acts on the surrounding ground and liquefaction of the sand layer occurs, the sand layer will liquefy even in the ground inside the water stop wall directly under the underground structure. Although liquefaction will occur, since the ground from the surrounding area is prevented from moving inside the water-stopping wall, the lifting of the underground structure will be suppressed to a small amount. If the underground structure rises slightly, the drainage layer at the bottom of the underground structure immediately releases excess water pressure to /l! 1, the uplift of the underground structure will not increase to a harmful extent. On the contrary, since the slight uplift has the effect of reducing the excess water pressure in the sand layer, it is an effective preventive measure against uplift that is large enough to cause the collapse of the underground structure.

"Embodiments" Hereinafter, embodiments of the present invention will be described with reference to the drawings.

1 and 2 are diagrams showing an embodiment of the basic structure of an underground structure of the present invention, in which FIG. 1 is a longitudinal cross-sectional view, and FIG. 2 is a view taken along the line AA in FIG. 1. FIG. In these figures, reference numeral 1 is a concrete culvert shown as an example of an underground structure, 2 is a water stop wall, and 3 is IJ steel that reinforces the water stop wall.

The culvert 1 is buried in a sand layer S extending on an impermeable layer C, as shown in FIG. 1, for example. For example, in this sand layer S, the groundwater level reaches close to the ground surface,
Ground that does not lift due to hydrostatic pressure, but is at risk of lifting due to liquefaction (apparent specific gravity - approx. 1.
9).

The sand layer S near the culvert 1 has this culvert 1.
A water cutoff wall 2 is constructed to surround the sand layer S directly below the sand layer S, and the lower end of this water cutoff wall is connected to the sand layer S.
It has reached the impermeable layer C below the liquefaction layer or the layer that does not liquefy at a depth of 15 to 20 cm or more. The members forming this positive water wall 2 and their structure are specifications, and may be appropriately selected from well-known and commonly used means. Further, the shape of the ice stopping wall 2 in a plan view is arbitrary, and may be appropriately determined depending on the planar shape of the culvert 1, such as circular or rectangular.

A drainage layer made of gravel or the like is formed on the top of the sand layer S surrounded by this water-stop wall 2, and this drainage layer covers the bottom surface of the culvert 1.

In addition, the above drainage layer! In order to improve the water permeability of this drainage layer, a perforated pipe (not shown) with a tip protruding above the ground surface may be inserted as appropriate.

The method for constructing the foundation structure of an underground structure as described above is arbitrary, and no special construction method is required.
An example is shown below.

First, when constructing culvert I in sand layer S, a retaining wall is constructed around the position of sand layer S where culvert 1 is to be constructed, and the inside of the retaining wall is cut and raised. By excavating downward while supporting the culvert 1, a cut-off wall 2 is constructed at a position surrounding the culvert 1 and the sand layer S immediately below it, and the lower end of the cut-off wall 2 is covered with an impermeable layer as shown in FIG. Bring it up to C. The construction method of the water stop wall 2 is arbitrary, and well-known and commonly used construction methods such as the PIF construction method, the PIPW construction method, the SMWT method, the underground continuous wall construction method, the soil cement wall construction method, and the cast-in-place resisting wall construction method are suitable. It can be used for

After constructing the water stop wall 2 in this way and digging to the depth to form the drainage layer inside the water stop wall 2,
This excavated portion is backfilled with gravel or the like to form a drainage layer, then a culvert 1 is constructed on top of this drainage layer, and then earth and sand is backfilled on top of this culvert 1.

Therefore, in the foundation structure of such underground structures, the culvert! The sand layer S immediately below is isolated from the surrounding sand layer S by the water stop wall 2, and the lower end of the water stop wall 2 reaches the impermeable layer C or a layer that does not liquefy at a depth of 15 to 2011 or more. Seismic force acts on the surrounding ground (sand layer S), causing liquefaction of the sand layer S, and as a result, culvert! Even if liquefaction occurs in the sand layer S inside the water stop wall 2 directly below, the ground will be prevented from moving around from the surrounding area to the inside of the water cutoff wall 2, so the floating torsion of the culvert l will be prevented. The amount will be suppressed to a small amount.

If the culvert I rises slightly, the water is drained into the pore water of the sand layer S directly under the culvert I or into the two drainage layers, and this drainage layer immediately relieves excess water pressure. Since it is dissipated, no harmful upheavals that could cause the culvert to collapse, etc. occur. Rather, a slight uplift has the effect of reducing the excess water pressure in the sand layer S, so it is an effective preventive measure against uplift that is large enough to cause the culvert 1 to collapse.

Therefore, in this example, even if liquefaction occurs in the sand layer S directly under Culper 1-1, the excess water pressure in the sand layer S can be quickly dissipated, compared to the conventional gravel lane construction method described above. It is possible to construct a drainage layer that has a reliable drainage effect. Furthermore, unlike the conventional groundwater lowering method described above, permanent facilities such as drainage pumps are not required, and equipment costs, maintenance costs, etc. are almost unnecessary. Furthermore, in order to construct the underground part of the culvert I, the water stop wall 2 is required in any case for excavating the underground part, so this can be preferably used to prevent liquefaction of the sand layer S. . In addition, the water-stopping wall 2 only has to function to block the temporary increase in water pressure in the sand layer S that is surrounded by it during an earthquake, and there is no need to expect a complete water-stopping effect, so its construction can be simplified. It is possible because it can be

Note that the details of the basic structure of the underground structure of the present invention are not limited to the embodiments described above, and various modifications are possible.

- As an example, the above embodiment is based on Calvert! Although this method was applied to foundation structures, it can also be suitably applied to public ditches, buried pipes, and other underground structures.

"Effects of the Invention" As explained above, according to the foundation structure of the underground structure of the present invention, the foundation structure of the underground structure is entirely buried in a sand layer. A drainage layer made of gravel or the like is provided at the bottom, and a water stop wall is provided at a position surrounding the underground structure and the sand layer directly below it, and the lower end of the water stop wall is located below the sand layer. Since it has been made into an impermeable layer or a layer that does not liquefy, it can produce the following excellent effects.

In other words, if seismic force or the like acts on the surrounding ground and causes liquefaction of the sand layer, liquefaction of the sand layer will also occur in the ground inside the water cutoff wall directly below the underground structure, but the water cutoff wall Since the ground is prevented from moving inward from the surrounding area, the uplift of the underground structure is suppressed to a small amount. If the above-mentioned underground structure rises slightly, the excess water pressure is immediately dissipated by the drainage layer at the bottom of the underground structure, so that the lifting of the underground structure becomes harmful. It never gets bigger. Rather, a slight uplift has the effect of reducing the excess water pressure in the sand layer, so it is an effective preventive measure against uplift large enough to cause collapse of the underground structure.

In addition, even if liquefaction occurs in the sand layer directly beneath the underground structure, the existence of a drainage layer that spreads over a wide area will quickly dissipate excess water pressure in the sand layer. Can be constructed. Furthermore, permanent facilities such as drainage pumps are not required, and equipment costs, maintenance costs, etc. are almost unnecessary.

Furthermore, in order to construct the underground part of an underground structure,
Since a water-stop wall is required in any case for underground excavation, this is suitable because it can be used to prevent liquefaction of the sand layer. In addition, the cut-off wall only has to function to block the temporary rise in water pressure in the sand layer surrounding it during an earthquake, and does not need to be expected to have a complete water supply effect, which simplifies its construction. It is also possible.

[Brief explanation of drawings]

1 and 2 are diagrams showing one embodiment of the present invention, in which FIG. 1 is a longitudinal sectional view showing the basic structure of an underground structure, and FIG. 2 is a line taken along the line A-A in FIG. 1. FIG. Fig. 3 is a longitudinal cross-sectional view of a sand layer in which a culvert is buried, and Fig. 4 is a longitudinal cross-sectional view showing the lifting of the culvert in the sand layer due to ground liquefaction during an earthquake. l...Underground structure (culvert), 2...
...Water-stop wall, 3...Hli. S: Sand layer, D: Drainage layer, C: Impermeable layer.

Claims (1)

[Claims] The basic structure of an underground structure entirely buried in a sand layer, in which a drainage layer made of gravel or the like is provided at the bottom of the underground structure, and the underground structure and the sand layer immediately below it. An underground structure characterized in that a water-stop wall is provided at a position surrounding the water-stop wall, and the lower end of the water-stop wall extends to an impermeable layer or a non-liquefaction layer below the sand layer. Basic structure.