JPH02136419A - Ground improvement of artificial island - Google Patents

Abstract

PURPOSE:To increase grade of subsoil by draining ground water from a drain member to tamp the subsoil, providing a cut-off and earthquake resistance layer, applying preload from water to consolidate the ground, and maintaining a ground-water level at the lower end of a reclaimed layer. CONSTITUTION:Two steps of up and down horizontal drains 6 and 7 are placed to the upper and of a soft layer 3 and a middle layer, and a vertical drain member 8 is driven into the soft layer 3 at a specific interval. After that, a cut-off wall 9 attaining to the soft layer 3 is provided to the inside of a revetment 5 to lower a ground-water level, and a reclaimed layer 4 is tamped with a weight. Then, a ship's bottom shaped cut-off and earth-quake resistance layer 13 of a non-water permeable elastic substance is formed in the reclaimed layer 4 in an area 12 divided by providing a levee 10, central levee 11 and the like. Water 14 as a preload is filled thereinto to draw up the ground water by the drain members 6 and 7, and the ground-water level is reduced to a level of the drain member 7 to maintain the level until the ground is sufficiently solidified.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for improving the ground of an artificial island constructed on the coast of Tokyo Bay.

[Problem to be solved by the invention]

The ground conditions of the artificial island built on the Tokyo Bay coast are such that the soft layer is thick, the supporting layer is deep at 60 to 70 m, and the supporting layer has a depth of 15 to 20 m.
There is an uneven ground of m. The reclaimed layer is generally made of fine sand, silt, etc. and is very loose, so it is unsuitable as the foundation ground for large-scale buildings that require great strength and rigidity.

This invention was made in view of the above circumstances. The purpose is to propose a ground improvement method for artificial islands that achieves high-grade ground improvement that makes it possible to construct large-scale buildings.

[Means to solve the problem]

This invention is a ground improvement method for an artificial island in which a reclaimed layer mainly made of fine sand, silt, etc. is provided on a soft layer, and a drain material is installed across the lower layer of the reclaimed layer and the soft layer, A water wall was built around the island to lower the groundwater level, the ground was consolidated and compacted, and levees were built around and within the island. A water-stop and seismic isolation layer made of permeable elastic material is installed, the area is filled with water and preloaded, groundwater is pumped up through drain material, the ground is consolidated, the water is removed, and the groundwater level is lowered using drain material. is maintained at the level of the lower end of the reclaimed layer.

In this construction method, ■The strength of the soil is greatly affected by the amount of water contained in the soil. ■Directly applied to the skeleton of soil particles - large force even at degrees (
When the material is compacted under the influence of effective stress (effective stress), it reliably exhibits supporting capacity up to that force and deformation is small. ■Soil that has been adjusted to the appropriate moisture level is easily compacted, and compacted soil exhibits great strength as a foundation that supports buildings. ■Artificial islands built on the coast of Tokyo Bay are made of soil with a high water content, and considering the soft nature of deep underground, we have achieved high-grade ground improvement using methods that primarily involve water removal. This is what I did.

[Examples] Hereinafter, the embodiments shown in the drawings will be explained. Figure 1 shows an artificial island 1 that is the target of ground improvement, and a supporting layer 2. Landfill N4 is constructed on top of the soft layer 3. Reclamation layer 4 is fine sand,
The soft layer 3 is made of silt, etc. and is about 20 m thick. The soft layer 3 is made of alluvial clay and is divided into two layers: an upper soft layer 3-1 and a lower soft layer 3-2. It is 40 to 50 m thick, and its upper surface is the ocean floor. ing. The support layer 2 has a depth of 60-70 m and a depth of 15-20 m.
There is an uneven ground of m. A cofferdam seawall 5 whose tip reaches the support layer 2 is constructed around the island.

In order to improve the ground of this artificial island, as shown in Figure 2, it is necessary to install horizontal drains in two stages, the upper and lower layers of the lower layer of the reclaimed layer 4, that is, a layer with a depth of 20 m, and the intermediate layer of the soft layer 3, that is, a layer that is approximately 35 m deep. The materials 6 and 7 are arranged in a grid pattern at a pitch of 50 to 100 m.

Further, vertical drain materials 8 are placed at a predetermined interval from the bottom of the reclaimed layer 4 to a depth of approximately 50 m within the soft layer 3.

The drain material is selected from paper drains, pack drains, sand drains, etc.

Before and after the installation of the drain material, a cut-off wall 9 is installed inside the seawall 5 around the island, the lower end of which reaches a depth of approximately 40 m within the soft layer 3.
is installed to lower the groundwater level, and when the water content of the reclaimed layer 4 reaches an appropriate level, the reclaimed layer 4 is compacted by tamping with a weight (dynamic consolidation method), and the alluvial clay layer (soft layer 3) is stimulated. to promote compaction.

Next, around the island is an embankment 10 with a height of 10 m and a width of 20 m, a spine-shaped central embankment 11 with a length of 10 m and a width of 40 m that bisects the island.
Furthermore, a rib-shaped embankment connecting embankment 10 and central embankment 11 (
(not shown) to divide the entire island into a plurality of area works 2 surrounded by embankments. In this example, the embankment was constructed using embankment using geotextile, but the construction method is not necessarily limited to this.

A water stop/seismic isolation layer 13 made of an impermeable elastic body and shaped like the bottom of a ship is formed in the reclaimed layer 4 having the area 12 surrounded by the embankment as a unit. This seismic isolation layer 13 is made of rubber asphalt mixture 2
It is formed by injecting a synthetic resin matrix, a resin-forming composition, etc. into the reclaimed layer, and an isocyanate-based prepolymer composition having an isocyanate-reactive group (for example, "Hicel-OH" by Toho Chemical Industry Co., Ltd.) is added to the jet column. When it is injected into a predetermined depth position in the reclaimed layer 4 using a construction method, it is reacted with water to form a urethane resin, and an impermeable and elastic layer consisting of a mixture of fine sand, etc. in the reclaimed layer 4 and urethane resin is formed. This is even more suitable.

The area 12 configured in this manner is filled with water 14 serving as a preload load, and groundwater is pumped up from the upper and lower horizontal drain members 6 and 7, and the groundwater level is raised to the level of the lower horizontal drain member 7, i.e. The level will be lowered to a depth of 35m and level 3 will be maintained until the ground is sufficiently compacted.

After the compaction of the ground is completed, the water 14 is removed and the groundwater level is controlled using horizontal drain materials 6°7 installed in two stages.
It is maintained at the lower end level of the landfill layer 4, that is, at a depth of about 20 m. In this example, 25 to 30 tons of water is deposited on the ground due to the movement of water.
This means that a preload of approximately /rrr was applied.

To construct a building on an artificial island that has been improved by ground improvement in this way, the ground must be excavated to a depth appropriate to the building load, as shown in Figure 3. In addition, a detailed ground investigation will be carried out, and a high-rise building 15 will be constructed in a place where the supporting layer 2 is shallow, as shown in FIG.

Directly below the high-rise building 15, ground improvement will be carried out using the deep mixing method up to the water-stop seismic isolation layer 13.

[For production]

This invention consists of the above configuration. This construction method pumps up and drains groundwater through drain materials, compacts the ground, installs a water-stop seismic isolation layer, applies a preload load with water, and further consolidates the ground to maintain the groundwater level at the lower end of the reclaimed layer. It is. Therefore, the ground made of compacted soil can exhibit great strength to support buildings. In addition, there is a water-stop seismic isolation layer made of elastic material within the reclaimed layer, and the groundwater level is low, so shaking during an earthquake is small and there is no risk of the ground liquefying.

Since the water-stop seismic isolation layer is shaped like a ship's bottom, the seismic force is absorbed and attenuated by the diagonal rising portion of the seismic isolation layer, and the seismic force acting on the cofferdam is alleviated. Since the preload is filled with water, the load can be removed by removing the water after the preload is completed, which is advantageous because there is no need for embankment work or disposal of leftover soil.

In addition to forming a space inside to fill with preload water, the top of the levee can be used as a road, and functions to prevent underground objects from floating up, absorb stress from the seismic isolation layer, and prevent the collapse of cofferdams.

Further, in this embodiment, horizontal drain materials were installed in two stages, upper and lower, but by using two stages, the groundwater level can be lowered rapidly, and groundwater can be pumped up from both during preloading. When the planned consolidation state is reached, the lower drain material is used as a charge pipe to condense water and restore the pore water pressure of the soft layer (alluvial clay layer), and the upper drain material is used as a constant drainage pipe to lower the groundwater level. Enables you to reliably and easily maintain the desired water level.

〔Effect of the invention〕

This invention is as described above, and this ground improvement method for an artificial island improves the ground of an artificial island, which consists of a reclaimed layer of fine sand, silt, etc. on a soft layer, to a high-grade ground on which high-rise buildings, etc. can be constructed. be able to.

[Brief explanation of the drawing]

The drawings are longitudinal cross-sectional views showing the ground improvement method for an artificial island in the order of steps. Figure 1 is the state before improvement, Figure 2 is the state filled with water and preloaded, and Figure 3 is after the improvement. Figure 4 shows the state where the ground has been excavated, and the state where the building has been erected. l...artificial island, 2...support layer, 3...soft layer, 4
... Reclamation layer, 5... Closure revetment, 6... Upper horizontal drain material, 7... Lower horizontal drain material, 8... Vertical drain material, 9... Water cutoff wall, 10...・Dike, 11
... Central embankment, 12 ... Area, 13 ... Water stop and seismic isolation layer, 14 ... Water, 15 ... High-rise building.

Claims (1)

[Claims] (1) A ground improvement method for an artificial island in which a reclaimed layer mainly made of fine sand, silt, etc. is provided on a soft layer, and drain material is installed across the lower layer of the reclaimed layer and the soft layer. A water wall was constructed around the island to lower the groundwater level, the ground was consolidated and compacted, and an embankment was constructed around and within the island, and an impermeable ship bottom-shaped water wall was constructed within the reclaimed layer in the area surrounded by the embankment. A water stop and seismic isolation layer made of elastic elastic material is installed, the area is filled with water and preloaded, groundwater is pumped up through drain material, the ground is compacted, the water is removed, and the groundwater level is lowered using drain material. A ground improvement method for artificial islands, which is characterized by maintaining the soil at the level of the lower end of the reclaimed layer.