JPH03281818A - Soil improvement method for cohesive soil ground - Google Patents

Abstract

PURPOSE:To accelerate the consolidation of a cohesive soil layer to improve soil of the ground by piercing a cohesive soil ground with a cohesive soil layer to form drain ditches reaching a lower water permeable layer, draining water forcedly from the drain ditches and, at the same time, making a loaded filling on the ground. CONSTITUTION:A cohesive soil layer (n1) is pierced into a cohesive soil ground (n) to reach a lower water permeable layer (n2), and plane and latticed drain ditches 1 and 1 are formed. Water is forcedly drained from the drain ditches 1 and 1 to lower the ground-water level W and, at the same time, a loaded filling M is made on the ground n to accelerate the consolidated phenomenon of the cohesive soil layer (n1). In addition, the drain ditches 1 are constituted of latticed thin type ditches 1a, circular holes 1b, 1b and 1c, the drain ditches 1 are filled with water permeable materials. The ultimate outside drain ditches 1A and 1A are excavated deeper than another drain ditches 1, and impermeable materials 7 and 7 are provided to the full lengths of them. The loaded filling M has the thickness equivalent to a settlement quantity due to consolidation of the ground 1, and the height of the ground after the improvement is the same height as that set in advance.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to the improvement of cohesive soil layers, particularly alluvial cohesive soil layers in a normally consolidated state or an unconsolidated state. This relates to a construction method that aims to improve soil quality by consolidating alluvial clay layers by using a construction method and a method of lowering the groundwater level in the ground.

(Prior Art) In recent years, development has been actively carried out in coastal areas, so-called waterfront areas.

The alluvial cohesive soil in such areas is often unconsolidated, or even in a properly consolidated state, is mainly composed of a soft cohesive soil layer, making it difficult to use it as the foundation ground for structures or as housing land. In many cases, some form of ground improvement is required.

In the past, various construction methods have been proposed and implemented as measures to improve the soil quality of such soft and cohesive soil, but as a method that is relatively economical and can reliably produce improvement effects, pre-planning methods such as loading embankments have been proposed. Consolidation method is known.

This construction method attempts to apply a consolidation load corresponding to the structural load to the soft, cohesive ground in advance using loaded embankments, thereby providing sufficient bearing capacity for the structure's foundation.

On the other hand, in addition to the above, by lowering the groundwater level in the cohesive soil layer n as shown in Fig. 6 and reducing the pore water pressure of the cohesive soil layer n, a consolidation phenomenon is caused in the cohesive soil layer n1. There is a construction method that improves the soil quality by compacting it (see "Soft ground countermeasures method" by Japan Society of Geotechnical Engineers).

(Problems to be Solved by the Invention) However, in the soil improvement method using loaded embankment as described above, (1) If the thickness of the cohesive soil layer is large, the time required for consolidation becomes significantly long, so sand drain Requires auxiliary construction methods such as construction methods.

(2) Staged embankment construction methods are often required, which requires a great deal of labor and cost.

(3) Since it is necessary to transport embankment materials, the environment of the transport route deteriorates.

(4) Since extra fill is often required, it is necessary to remove the embankment for the thickness of the extra fill.

(5) It may cause environmental problems around the construction site where dust etc. will be generated.

There was a problem.

In addition, in the soil improvement method by lowering the groundwater level, (1) If the clay layer is thick, it takes a long time to obtain the improvement effect.

(2) Since the decline in the groundwater level affects a wide area, consolidation subsidence also occurs in the surrounding ground, and as a countermeasure for this, it is necessary to construct water walls around the construction site.

(3) As shown in Fig. 7(a), the soil improvement method by lowering the groundwater level has various problems in that it cannot achieve a large improvement effect on the surface layer of the clayey ground.

The present invention was made in view of the above-mentioned problems, and its purpose is to improve the ground conditions by combining loading embankment and groundwater level lowering, in order to compensate for the above-mentioned drawbacks of the conventional technology. The purpose of the present invention is to provide an economical and rational soil improvement method for clay soil that is suitable for design conditions.

(Means for Achieving the Object) The main structure of the soil improvement method for cohesive soil of the present invention to achieve the above-mentioned objects is to penetrate the cohesive soil layer to be improved within the ground within the area to be improved. A lattice-shaped drainage ditch that reaches the lower permeable layer and is continuous in a plane is formed, and drainage from the ditch lowers the groundwater level and reduces the pore water pressure of the cohesive soil layer, and The method is characterized in that a loading embankment is carried out to cause a consolidation phenomenon in a clayey soil layer, and the lattice-shaped drainage ditch includes a thin ditch excavated in a lattice-like manner, and an intersection and a middle part of the thin ditch. The drain groove is constructed by filling a water-permeable material into the drain groove, and the outermost drain groove in the lattice-shaped drain groove is separated from the other drain grooves. In addition to deep excavation, a water-blocking material is installed along the entire length of the outside of the drainage ditch. By doing so, the improved ground height is made to be the same as the preset ground height.

(Function) According to the above-mentioned configuration, it is possible to obtain a consolidation promotion effect equal to or greater than that obtained when consolidation methods such as sand drains are used together, and also to reduce the embankment height and groundwater level drop according to the design conditions. Since combinations of the above can be selected as appropriate, it is possible to provide an economical and rational soil improvement method. Furthermore, the necessary and sufficient improvement effect can be obtained even in the ground surface layer.

(Example) Hereinafter, an example of the present invention will be described in detail based on the drawings.

FIGS. 1 and 2 are a plan view and a cross-sectional view of the soil improvement method for clay soil according to the present invention.

In the present invention, first of all, drain grooves 1 in the form of a lattice as shown in FIG.

As shown in Fig. 3, the thin groove excavator A has a pair of left and right side casings 3 equipped with a pilot bit 3a at the lower end.
A pair of left and right disc cutters 5 are rotatably supported by the side gauging 3, and a reverse bit 4a is provided at the lower end of the cutter 5, and a reverse pipe 4b is provided at the lower end. It consists of a built-in center casing 4, the pilot bit 3a, a disc cutter 5, and an underwater motor 6 that rotationally drives the reverse bit 4a.

Further, the center casing 4 is a side casing 3.
The left and right pilot bits 3a and the disc cutter 5 rotate in opposite directions, thereby canceling out their rotational torques.

Using the thin trench excavator A having the above configuration, a thin trench 1a and circular holes 1b, lc as shown in FIG. Dig all the way.

As shown in FIG. 5, when the thin groove excavator A is rotated 90 degrees around the circular hole 1c of the thin groove 1a, the thin grooves 1a intersect with each other around the circular hole 1c. is formed by excavation.

By repeating the above-mentioned excavation in sequence, a drainage ditch 1 consisting of a thin groove 1a excavated in a lattice pattern and circular holes 1b and 1c excavated at the intersections and midpoints of the thin groove 1a is formed. Formed by drilling.

At this time, the distance a between the drain grooves 1 can be arbitrarily determined by appropriately selecting a combination of the diameter φ of the disc cutter 5 of the thin trench excavator A and the center distance 1 of the pilot bit 3a and reverse bit 4a. Can be set.

Therefore, the spacing a of the drainage ditches 1 can be appropriately selected according to the soil constants (especially the value of the consolidation coefficient Cv) of the cohesive soil base n and the set construction period.

In addition to excavating the drainage ditch 1, it is also advisable to excavate an appropriate number of condensate valves m around the drainage ditch 1.

Next, as shown in FIG. 2, the outermost drainage ditch IA of the drainage ditches 1 thus excavated and formed is excavated and formed deeper than the other drainage ditches 1, and the water supply material 7 is installed on the outside thereof. do.

As the water-shielding material 7, a combination of steel and a solidifying material, a thin film-like water-shielding material, etc. can be used.

When using the thin film-like water-shielding material, the roll-shaped thin film-like water-shielding material is inserted into the circular hole 1b in the outermost drainage ditch IA, IC
into the circular hole 1b, and drain groove IA starting from the IC.
It is best to attach it to the outside wall.

After installing the water supply material 7 in the outermost drain IA in this way, each thin groove 1a and the circular hole 1 in the drain 1 are
b. A submersible pump P in which the IC is filled with a water-permeable material 2 mainly made of natural materials or artificial materials with a high hydraulic permeability coefficient such as sand, gravel, crushed stone, etc., and a discharge pipe is connected to the circular hole 1b and the IC.
are installed at appropriate intervals, and the submersible pump P forcibly drains groundwater to lower the groundwater level W (natural water level) within the construction site. In addition, W° in the figure indicates the groundwater level after the drop.

Incidentally, the groundwater forcibly drained at this time is normally discharged into the sewerage system, but it is preferable that it be returned to the condensate valve m around the construction site described above.

In this way, by installing the water shielding material 7 on the outside of the outermost drainage ditch IA in the drainage ditch 1 excavated and formed in the cohesive soil bed n, the drop in the groundwater level W (natural water level) will affect the surrounding ground. This can prevent consolidation settlement of the surrounding ground.

In addition, by forcibly draining the groundwater in the cohesive land plate n through the circular hole 1b and IC to lower the groundwater level W (natural water level), the consolidation of the cohesive land plate n is promoted (see Fig. 2). ).

Furthermore, since the lattice-shaped excavated holes exhibit a drain function, the consolidation is further promoted.

However, the strength of the cohesive soil after improvement due to a decrease in the groundwater level W (natural water level) cannot be significantly improved in the surface layer of the soil ((a) in Figure 7). In the present invention, a loading embankment M of an appropriate thickness is applied to the upper surface of the clay ground n in the area to be improved.

The embankment height H at this time can be determined economically and rationally from the required improved ground strength and the preset improved ground height.

In other words, there are many combinations of groundwater level decline Δh and embankment height H to obtain the required ground strength after improvement, but the amount of consolidation settlement within the construction period should be estimated in advance based on consolidation theory. Therefore, the height H of the embankment may be determined so that the height of the ground after subsidence of the two-cohesive ground n is the same as the design ground height.

The consolidation settlement amount S at this time can be determined by the following formula.

5=U-S.

Here, U・Average consolidation density during the construction period So: Total amount of settlement of the cohesive soil (This is the amount of settlement when U = 100%, and can be determined by the following formula, for example.) 0-e So; Σ H8 1+e.

So=Σmv−Δp−H.

Here, Ho: Layer pressure e of each consolidation layer, eo: Gap ratio mv after and before settlement of each consolidation layer
, Cc: Volume compression coefficient and compression index ΔP of each consolidated layer
, Po: Increment of consolidation pressure and consolidation yield stress of each consolidation layer. Figure 8 also shows the amount of settlement of the clayey soil in an example of the soil improvement method for the clayey soil that uses a combination of loading embankment and lowering of the groundwater level. This is an example of actual measurements.

In this example, after the loading embankment M is carried out first, the groundwater level W
The embankment height H is finally adjusted to a predetermined ground height by lowering the groundwater level W (natural water level), but in the present invention, the groundwater level W (natural water level) is lowered first. It is also possible to perform the loading embankment M later, and it is also optional to perform the lowering of the groundwater level W (natural water level) and the loading embankment M at the same time.

As mentioned above, the present invention provides a rational soil improvement method that utilizes the compaction phenomenon caused by the loading embankment M in a clayey soil platform and groundwater drop, and the principles of the improvement method and the ground strength after improvement are as follows. A schematic diagram is shown in FIG.

FIG. 7(a) is a conceptual diagram of the soil improvement method for clayey soil platform n according to the present invention, which shows that by rationally combining groundwater level lowering and loaded embankment, the water level decrease Δh and the embankment height H
This causes a consolidation phenomenon due to an increase in the effective stress in the cohesive soil bed corresponding to the above, and obtains the improved soil strength as shown in FIG. 7(b).

(Effects of the Invention) The present invention has the following effects by having the above configuration.

■ Forming a grid-like drainage ditch that is continuous in a plane, lowering the groundwater level by draining water from the drainage ditch and reducing the pore water pressure in the layer of cohesive soil, and carrying out loading embankment on the ground. As a result, the height of the embankment can be set to the minimum necessary level, and the necessary and sufficient improvement effect can be obtained even in the ground surface layer.

■ A lattice-shaped drainage ditch is formed by thin grooves excavated in a lattice pattern and circular holes drilled at the intersections and midpoints of the thin grooves, and the drainage ditch is filled with a water-permeable material. As a result, a consolidation promotion effect equal to or greater than that obtained when the vertical drain method is adopted can be obtained, so that the consolidation time of the cohesive soil layer can be shortened, and the construction period can be shortened.

Furthermore, since the intervals between the drainage ditches can be set arbitrarily, it is possible to form a lattice-shaped drainage ditch that matches the soil constants and scheduled construction period.

■ The outermost drain in the lattice-shaped drain was excavated deeper than the other drains, and a water barrier was installed along the entire length of the drain, thereby creating a separate impermeable wall around the construction site. It is possible to reduce the groundwater level within the area without degrading the environment around the construction site, even without installing a can.

■ Since the embankment height and groundwater level reduction can be set in advance to be at the same level as the set GL after settlement, it is possible to select the optimal combination of embankment height and groundwater level reduction according to the ground conditions and design conditions. It is possible to provide an economical and rational soil improvement method.

■ Since the circular hole is used as a pumping well, there is no need to install a separate pumping well, which contributes to shortening the construction period and reducing construction costs.

[Brief explanation of the drawing]

FIG. 1 is a plan view of the drainage ditch of the present invention, FIG. 2 is a sectional view of the same,
FIG. 3 is a front view of the excavator for thin trenches, FIG. 4 is a plan view showing one element of a drain hole formed by the thin excavator, FIG. Figure 6 is a cross-sectional view of the conventional groundwater level lowering method, and Figure 7(a) is a schematic diagram showing the increased effective stress due to the soil improvement method using the loaded embankment of the present invention and the groundwater level lowering method in combination. (b) is a schematic diagram showing the soil strength after improvement, and FIG. 8 is a diagram showing the amount of settlement of the cohesive soil base measured in order to adjust the embankment height to match the design GL. In the figure, 1: Drainage ditch n: Cohesive soil layer nl: Cohesive soil layer n2: Lower permeable layer W: Groundwater level (natural water level) M: Loading embankment.

Claims (4)

[Claims] (1) A lattice-shaped drainage ditch is formed in the ground in the area to be improved, penetrating the clayey soil layer to be improved, reaching the lower permeable layer, and continuing on a plane, and draining water from the drainage ditch. A method for improving the soil quality of a cohesive soil bed, characterized by lowering the groundwater level to reduce the pore water pressure in the cohesive soil layer, and performing a loading embankment on the ground to cause a consolidation phenomenon in the cohesive soil layer. (2) The lattice-shaped drainage ditch consists of thin grooves excavated in a lattice pattern, and circular holes drilled at the intersections and midpoints of the thin grooves, and the drainage grooves are filled with a water-permeable material. 1. A method for improving the soil quality of a clay ground according to claim 1, characterized in that the method comprises: (3) Claims characterized in that the outermost drain in the lattice-shaped drain is excavated deeper than the other drains, and a water-blocking material is installed along the entire length of the outside of the drain. A soil improvement method for clayey soil as described in paragraph 2 or 3. (4) By making the above-mentioned loading embankment a loading embankment with a thickness commensurate with the amount of consolidation settlement calculated in advance based on consolidation theory, the ground height after improvement was made to be the same as the preset ground height. Claims 1-1 are characterized in that:
A soil improvement method for clayey soil as described in any of paragraph 3.