JPH0833004B2 - Soil improvement method for cohesive soil - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to improvement of cohesive ground, particularly alluvial cohesive soil layer in a pressure-consolidated or unconsolidated state,
More specifically, the present invention relates to a method for consolidating the alluvial cohesive soil layer to improve the soil quality by using the loading embankment method and the groundwater level lowering method in the ground together.

(Prior Art) In recent years, development has been actively carried out in waterfront areas, so-called waterfront areas. The alluvial cohesive soil ground in such areas is often composed mainly of a soft cohesive soil layer that has not been consolidated or is in a normally consolidated state, and is used as the foundation ground of a structure or residential land. In order to do so, it is often necessary to improve the ground.

Conventionally, various construction methods have been proposed and implemented as measures to improve the soil quality of such soft cohesive soil, but as a construction method that is relatively economical and reliably obtains the improvement effect,
A preconsolidation method using loading embankments is known.

In this construction method, a compaction load corresponding to the structure load is preliminarily applied to the soft cohesive ground by the embankment to give sufficient proof stress as the structure foundation.

On the other hand, the sixth to lower the groundwater level of the viscous land in Release n as shown in Figure in addition to the, by reducing the pore pressure of the viscous soil layer n _1, the compaction phenomenon viscous soil layer n ₁ There is a method to improve the soil quality by generating it (see the Geotechnical Society "Soft ground countermeasure method").

(Problems to be solved by the invention) However, in the soil improvement method using the above-mentioned embankment, (1) If the layer thickness of the cohesive soil layer is large, the time required for consolidation is significantly prolonged, so the sand drain Auxiliary construction method such as construction method is required.

(2) A stepwise embankment method is often required, which requires a great deal of labor and cost.

(3) Since the embankment material needs to be transported, the environment of the transportation route deteriorates.

(4) In many cases, extra embankment is required, so it is necessary to remove the embankment by the thickness of the extra embankment.

(5) Dust may generate environmental problems around the construction site.

There was a problem.

Further, in the soil improvement method by lowering the groundwater level, (1) if the cohesive soil layer is thick, it takes a long time to obtain the improvement effect.

(2) Since the drop in groundwater level affects a wide range, consolidation settlement also occurs in the surrounding ground, and as a countermeasure for that, it is necessary to construct an impermeable wall around the construction site.

(3) With the soil improvement method by lowering the groundwater level, as shown in Fig. 7 (a), a large improvement effect cannot be obtained in the surface layer of the clayey ground.

There were various problems.

The present invention has been made in view of the above problems, and an object thereof is to combine ground embankment and groundwater level lowering in order to complement the above-mentioned drawbacks of the prior art, and thus ground conditions And to provide an economical and rational soil improvement method for cohesive soil that is suitable for the design conditions.

(Means for Solving the Problems) The main configuration of the soil improvement method of the cohesive soil of the present invention for achieving the above objects is to penetrate the cohesive soil layer in the improved soil and reach the lower permeable layer, In addition to forming a grid-like drainage ditch that is continuous in a plane, the groundwater level is lowered by forced drainage from the drainage ditch, and loading embankment is performed on the ground surface of the improvement target area surrounded by the drainage ditch to increase the viscosity. A soil improvement method for consolidating and improving a soil layer, characterized in that a water-impervious material is installed over the entire length of the outermost drainage groove in the drainage groove, and the cohesive soil layer is penetrated into the improved ground. Reaching the lower permeable layer and forming a planarly continuous grid-shaped drainage channel, lowering the groundwater level by forced drainage from the drainage channel, and the ground surface of the improvement range surrounded by the drainage channel Loading embankment on the ground to make it viscous A soil improvement method for consolidating and improving a layer, wherein the drainage groove is formed from a lattice-shaped thin groove and circular holes formed at intersections and intermediate portions of the thin groove. Characterized in that the water-blocking material is installed over the entire length of the outermost drainage groove in the outermost drainage groove, and the outermost drainage groove in the grid-like drainage groove is characterized by being formed by excavation deeper than other drainage grooves, Also, by making the above-mentioned loading embankment a loading embankment of a thickness commensurate with the consolidation settlement amount calculated in advance based on the consolidation theory, the ground height after improvement should be the same as the preset ground height. Is a feature characterized by.

(Operation) According to the above-mentioned configuration, by using the groundwater level lowering and the loading embankment together, it is possible to obtain the consolidation promotion effect equal to or more than the precompacting method using the sand drain together, and the embankment height according to the design conditions. Since it is possible to select the optimum combination of the amount of water and the amount of groundwater level reduction, it is possible to provide an economical and rational soil improvement method.

In this case, the grid-shaped drainage channels can serve as a drain function to promote consolidation, and the outermost drainage channels are equipped with water-blocking materials, so the water level of the surrounding ground outside the target range is lowered or subsided. The adverse effects of can be reduced.

In addition, since the drainage groove is formed from a lattice-shaped thin groove and circular holes formed at the intersections and the middle portions of the thin grooves, the circular hole can be used as a pumping well, so that adverse effects on the surrounding ground can be prevented. There is no need to drill and form a separate pumping well.

Further, by forming the outermost drainage groove deeper than the other drainage grooves, it is possible to further significantly reduce the influence on the surrounding ground and environment.

In addition, the embankment height and groundwater level drop amount are set after the settlement.
It is possible to select the optimum combination of the embankment height and the groundwater level drop amount according to the ground condition and the design condition by setting the same level in advance so that the economical and rational soil improvement can be performed.

(Example) Hereinafter, one example of the present invention will be described in detail with reference to the drawings.

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

According to the present invention, first, a drainage groove 1 in a grid shape as shown in FIG.

As shown in FIG. 3, the thin groove excavator A has a pair of left and right side casings 3 each having a pilot bit 3a at the lower end thereof.
And a pair of left and right disc cutters 5 rotatably supported by the side casing 3 and a reverse bit 4a provided at the lower end of the side casing 3 and provided with a reverse pipe 4b for discharging sand and sand. It is composed of a built-in center casing 4, the pilot bit 3a, a disc cutter 5, and a submersible motor 6 for rotationally driving the reverse bit 4a.

Further, the center casing 4 has substantially the same diameter as the side casing 3, and the pair of left and right pilot bits 3a and the disc cutter 5 rotate in opposite directions to cancel the rotational torque. ing.

Using the thin groove excavator A having the above structure, the thin groove 1a and the circular holes 1b and 1c as shown in FIG.
Excavate to the lower permeable layer n ₂ through the cohesive soil layer n ₁ .

Then, as shown in FIG. 5, when the thin groove excavator A is rotated 90 degrees about the circular hole 1c at the center of the thin groove 1a, the thin groove 1a intersected around the circular hole 1c is formed. Excavated and formed.

In this way, by repeating the above excavation sequentially,
A drainage groove 1 including a thin groove 1a excavated in a lattice shape, and circular holes 1b and 1c excavated at an intersection and an intermediate portion of the thin groove 1a.
Is excavated and formed.

At this time, the distance a between the drain grooves 1 is determined by the diameter φ of the disc cutter 5 of the thin groove excavator A and the pilot bits 3a, 3a.
It can be set arbitrarily by appropriately selecting a combination with the center interval l of.

Therefore, it is possible to appropriately select the soil constant of the cohesive land n (particularly the value of the consolidation coefficient Cv) and the interval a of the drainage grooves 1 according to the set construction period.

Incidentally, it is advisable to excavate an appropriate number of condensate wells m around the drainage groove 1 along with the excavation of the drainage groove 1.

Next, as shown in FIG. 2, the outermost drainage groove 1A in the drainage groove 1 thus formed by excavation is formed deeper than the other drainage grooves 1 and the water-blocking material 7 is provided on the outer side thereof. Install.

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

When using the thin film waterproof material, the roll-shaped thin waterproof material is inserted into the circular holes 1b and 1c in the outermost drain groove 1A, and the circular holes 1b and 1c are used as starting points for the drain groove 1A. It is good to stick it to the outer wall.

In this way, after installing the water blocking material 7 in the outermost drainage groove 1A, each thin groove 1a and circular hole 1b in the drainage groove 1,
1c is filled with a water-permeable material 2 mainly composed of a natural material or an artificial material having a large water permeability such as sand, gravel, and crushed stone, and a submersible pump P in which a discharge pipe is connected to the circular holes 1b and 1c is provided at appropriate intervals. The groundwater is forcedly drained by the submersible pump P to lower the groundwater level W (natural water level) in the construction site. W'in the figure indicates the groundwater level after the decrease.

The groundwater that is forcibly discharged at this time is normally discharged to the sewer, but it is desirable to return it to the condensate well m around the construction site.

In this way, by installing the water-blocking material 7 outside the outermost drainage groove 1A in the drainage groove 1 formed by excavation in the cohesive land n, a decrease in the groundwater level W (natural water level) affects the surrounding ground. Can be prevented to prevent consolidation settlement of the surrounding ground.

Further, the groundwater in the cohesive landbed n is forcibly discharged from the circular holes 1b and 1c to lower the groundwater level W (natural water level), thereby promoting the consolidation of the cohesive landbed n (second
Figure).

Further, since the grid-shaped drill holes 1 have a drain function, the consolidation is further promoted.

However, the strength of the cohesive ground n after improvement due to the decrease of the groundwater level W (natural water level) cannot obtain such a great improvement effect in the surface layer portion of the ground n (see (a) of FIG. 7). ), In the present invention, a loading embankment M having an appropriate thickness is performed on the upper surface of the cohesive ground n in the improvement target range.

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

That is, there are many combinations of the groundwater level decrease amount Δh and the embankment height H to obtain the required improved ground strength, but the consolidation settlement amount within the construction period is calculated in advance based on the consolidation theory. Therefore, the embankment height H may be determined so that the ground height after the subsidence of the cohesive ground n is the same as the design ground height.

The compaction settlement amount S at this time can be obtained by the following formula.

S = U ・ So where U: average pressure density during the construction period So: total settlement of cohesive soil (subsidence when U = 100%, which can be calculated by the following formula, for example) Where H _O : bed pressure of each consolidation layer e, e _O : void ratio mv after and before subsidence of each consolidation layer, Cc: volumetric compression coefficient and compression index ΔP of each consolidation layer, Po: each consolidation layer Increment of consolidation pressure and consolidation yield stress Fig. 8 shows an example of actually measuring the subsidence amount of the cohesive land in the example of the soil improvement method of the cohesive land using both loading embankment and lowering groundwater level.

In this embodiment, after carrying out the loading embankment M first, the groundwater level W (natural water level) is lowered, and the embankment height H is finally adjusted so as to reach a predetermined ground height. In the invention, it is possible to perform the loading embankment M after lowering the groundwater level W (natural water level) first, or to simultaneously lower the groundwater level W (natural water level) and the loading embankment M. It is optional.

As described above, the present invention provides a rational soil improvement method using the loading embankment M on the cohesive land n and the consolidation phenomenon due to the decrease in groundwater. The principle of the improvement method and the model of the ground strength after improvement The figure is shown in FIG.

FIG. 7 (a) is a conceptual diagram of the soil improvement method for the cohesive land n according to the present invention. By reasonably combining groundwater level lowering and loading embankment, the water level lowering amount Δh and the embankment height H can be adjusted. A corresponding consolidation phenomenon is caused by an increase in effective stress in the cohesive soil n, and the improved soil strength as shown in FIG. 7 (b) is obtained.

(Effects of the Invention) The present invention has the following effects by being configured as described above.

In the improved ground, it penetrates the cohesive soil layer to reach the lower permeable layer, and forms a planar continuous grid-like drainage channel, which lowers the groundwater level by forced drainage from the drainage channel and Equal to or more than the pre-compaction method using sand drain etc. by loading embankment on the ground surface of the improvement area surrounded by the groove and installing the water blocking material over the entire length of the outermost drain in the drain. In addition to the effect of promoting consolidation, it is possible to select an optimum combination of embankment height and groundwater level reduction amount according to design conditions, so that an economical and rational soil improvement method can be provided.

In this case, the grid-shaped drainage channels can serve as a drain function to promote consolidation, and the outermost drainage channels are equipped with water-blocking materials, so the water level of the surrounding ground outside the target range is lowered or subsided. The adverse effects of can be reduced.

By forming the drainage groove from a lattice-shaped thin groove and a circular hole formed at an intersection and a midway portion of the thin groove,
Since the circular hole can be used as a pumping well, adverse effects on the surrounding ground are reduced, and it is not necessary to separately excavate and form a pumping well.

By forming the outermost drainage trench deeper than other drainage trenches, it is possible to significantly reduce the impact on the surrounding ground and environment.

By presetting the embankment height and groundwater level drop to the same level as the setting GL after subsidence, the optimum combination of embankment height and groundwater level drop according to the ground conditions and design conditions can be selected, Economically and reasonably possible soil improvement.

[Brief description of drawings]

FIG. 1 is a plan view of the drainage groove of the present invention, FIG. 2 is a sectional view of the same,
FIG. 3 is a front view of a thin groove excavator, FIG. 4 is a plan view showing one element of a drainage hole formed by a thin excavator, and FIG. 5 is a plan view showing a method of excavating a drainage groove. Fig. 6 is a cross-sectional view of the conventional groundwater level lowering method, and Fig. 7 (a) is a schematic diagram showing the increased effective stress by the soil improvement method using the loading embankment of the present invention and the groundwater level lowering method in combination. (B)
Is a schematic diagram showing the ground strength after improvement, and FIG. 8 is a diagram measuring the subsidence amount of the cohesive ground to adjust the embankment height to match the design GL. Also, in the figure, 1: drainage ditch n: cohesive landbed n ₁ : cohesive soil layer n ₂ : lower permeable layer W: groundwater level (natural water level), M: loading embankment.

 -------------------------------------------------------------------------------------------------------------- ─── Continuation of front page (72) Inventor Masahiro Nakagawa 1-7-1, Kyobashi, Chuo-ku, Tokyo Toda Construction Co., Ltd. (72) Inventor Yuji Tategawa 1-1-7, Kyobashi, Chuo-ku, Tokyo Toda Construction Co., Ltd. (72) Inventor Masamizu Ochiai 1-7-1 Kyobashi, Chuo-ku, Tokyo Toda Construction Co., Ltd. (72) Inventor Takaaki Kubota 1-1-7 Kyobashi, Chuo-ku, Tokyo (56) References JP-A 1-226913 (JP, A) JP-A 61-49017 (JP, A) JP-B 52-23163 (JP, B1)

Claims (4)

[Claims] 1. A groundwater level is lowered in the improved ground by penetrating a cohesive soil layer to reach a lower permeable layer and forming a planarly continuous grid-shaped drainage channel, and forcibly draining from the drainage channel. In addition to the above, the soil improvement method is a method of consolidating and improving the cohesive soil layer by performing loading embankment on the ground surface in the area to be improved surrounded by the drainage trench, in which the outermost drainage trench of the drainage trench is covered over the entire length. A soil improvement method for cohesive soil characterized by the installation of water material. 2. A ground drainage level is formed in the improved ground by penetrating the cohesive soil layer to reach the lower permeable layer and forming a grid-like drainage groove that is continuous in a plane and by forced drainage from the drainage groove. Along with it, it is a soil improvement method of compacting and improving the cohesive soil layer by performing loading embankment on the ground surface of the improvement target area surrounded by the drainage groove, and the drainage groove is a lattice-shaped thin groove,
Formed from circular holes formed at the intersections and the middle of the thin grooves, a water-impervious material is installed over the entire length of the outermost drain in the grid-shaped drains. Soil improvement method. 3. The soil improvement method for a cohesive land according to claim 1, wherein the outermost drain of the grid-shaped drain is formed deeper than the other drains. 4. The loading embankment having a thickness commensurate with the consolidation settlement amount calculated in advance based on the consolidation theory so that the ground height after improvement becomes the same as the preset ground height. The soil improvement method for cohesive soil according to any one of claims 1, 2 and 3, wherein