JP3649140B2 - Liquefaction countermeasure structure and its construction method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a liquefaction countermeasure structure for sandy ground where liquefaction is possible, and a construction method thereof, in which underground underground structures such as buried pipes and shield tunnels are constructed.
[0002]
[Prior art]
When the surrounding sandy ground liquefies, it is considered that there are the following two major effects on the underground linear structure.
(1) In the completely liquefied state, the sandy ground behaves in the same manner as a fluid having an apparent unit weight of 1.8 to 2.0 Mg / m ³ . For this reason, the buoyancy resulting from the difference with the apparent unit weight of an underground linear structure arises, and an underground linear structure will float.
(2) If the surrounding ground does not flow completely and the surrounding ground flows laterally, lateral flow pressure due to relative displacement with the rigid underground linear structure is generated, and the underground linear structure Things will shift to the side.
[0003]
As a countermeasure against the influence of the above (1), for example, there is a liquefaction countermeasure method for a submarine pipeline disclosed in Japanese Patent Laid-Open No. 61-274191. The construction method is to provide a buried section that is not partially liquefied, and to prevent levitation of the submarine pipeline while allowing liquefaction of other surrounding ground.
In addition, as other examples, countermeasure methods such as fixing the submarine pipeline to the submarine ground not liquefied with an earth anchor, or adjusting the buoyancy by increasing the concrete coating thickness of the submarine pipeline (for example, “Koji Sekiguchi and Hiroshi Oishi: A study on liquefaction countermeasure method for submarine pipeline, Proceedings of Japan Society of Civil Engineers, No. 380, I-7, pp. 467-473, 19877.4”).
[0004]
Further, as a measure against the influence of the above (2), for example, there is a device for preventing permanent displacement of a buried object due to ground liquefaction disclosed in Japanese Patent Laid-Open No. 1-102129. In this apparatus, a method of reducing the lateral flow pressure by forming the shape of the buried object in a streamline shape has been proposed. Specifically, a symmetric streamline wing-like protective work is attached to the pipe, and the whole is pile-supported.
As another example, a construction method in which a steel sheet pile or a concrete wall is continuously placed on the mountain side of the pipeline, thereby suppressing the lateral flow itself.
[0005]
[Problems to be solved by the invention]
The influence of buoyancy described above is dominant on the horizontal ground seen in the plains of river basins and landfills.
Therefore, in this case, it is conceivable to apply the method of introducing a section that is not partially liquefied, the method of using an earth anchor, the method of weighting concrete coating, etc., introduced as measures against the influence of the above (1).
However, each of the above-mentioned construction methods has been conceived in the case where a new underground structure is installed, and has the following problems when applied to an existing structure.
[0006]
(1) Problem of construction method of providing a section that does not liquefy partially It is assumed that the section is not liquefied in order to provide a section that does not liquefy, and when applied to existing linear structures, It is necessary to remove the sandy ground around the medium-line structure and backfill with a material that does not liquefy. Therefore, not only a large cost and a long construction period are required, but there is a risk that excavation work may damage existing structures.
(2) Problems of construction method using earth anchor The ground anchor prevents the underground structure from floating, but in order to connect the anchor and the saddle installed on the underground structure, Excavation of up to about half of the medium-line structure is required. Therefore, as in the case of (1) above, a large amount of cost and a long construction period are required, and there is a risk that excavation work will damage existing structures.
(3) Problems of construction method due to weight increase of concrete coating, etc. Construction methods can be considered in which materials for weight increase (such as old rails and concrete) are installed inside underground structures. This is difficult, and the effective cross section is also reduced, which may impair the original function.
[0007]
The present invention has been made in order to solve the above-described problems, and without compromising the function of the existing underground line structure, at a low cost, in a short construction period, and in the existing underground line structure. The object is to obtain a liquefaction countermeasure structure and a method for constructing it without risk of damage.
[0008]
[Means for Solving the Problems]
The liquefaction countermeasure structure according to the present invention is a liquefaction countermeasure structure for sandy ground with liquefaction potential, in which an underground linear structure is constructed, along the underground linear structure. The liquefaction permissible section allowing liquefaction and the non-liquefaction section improved so as not to liquefy are alternately arranged, and the non-liquefaction section is in contact with the supporting ground and around the underground linear structure. It consists of an improved ground formed by an osmotic injection method.
[0009]
The improved ground is formed by injecting a non-alkali silicate solution into the sandy ground.
[0010]
Further, the construction method of the liquefaction countermeasure structure according to the present invention is a construction method of a liquefaction countermeasure structure for sandy ground where liquefaction is possible, wherein the underground linear structure is constructed, An improved ground surrounding the periphery of the underground linear structure is provided by arranging a chemical injection nozzle at the peripheral edge of the intermediate linear structure and injecting the chemical from the nozzle into the sandy ground to contact the supporting ground. A predetermined interval is formed along the underground linear structure.
[0011]
The improved ground is formed by injecting a non-alkali silicate solution into the sandy ground.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
1 is an axial sectional view showing an embodiment of the present invention, FIG. 2 is a sectional view taken along line AA in FIG. 1, and FIG. 3 is a sectional view taken along line BB in FIG.
The present embodiment is a liquefaction countermeasure structure for sandy ground 2 having a possibility of liquefaction in which an underground linear structure 1 having a circular cross section is constructed, and along the underground linear structure 1 Thus, the liquefaction permissible section L1 allowing liquefaction and the non-liquefaction section L2 improved so as not to liquefy are alternately arranged. And the non-liquefaction area L2 consists of the improved ground 4 formed so that the circumference | surroundings of the underground linear structure 1 may be enclosed while contacting the upper surface of the support ground 3 which is not liquefied.
[0013]
The reason why the lower end of the improved ground 4 is brought into contact with the upper surface of the supporting ground is to prevent the liquefied peripheral ground from entering the lower portion of the improved ground 4.
The upper position of the improved ground 4 is determined by the ultimate pulling resistance expected from the improved ground 4, but the ground above the groundwater level 5 is not liquefied, so the improved ground is generally at the groundwater level. Up to 5.
[0014]
By the way, the improved ground 4 is obtained by injecting a non-alkali silicate solution into the sandy ground 2 and improving the ground. The improved ground 4 has a liquefaction strength (for example, the ratio of shear stress and initial effective confining pressure at a repetition rate of N = 20 in a hollow torsion test, and both amplitude strains of 3%). It is set to be larger than the liquefaction strength of the ground 2.
Specifically, since the liquefaction strength of the sandy ground 2 is generally about 0.2 to 0.4, the liquefaction strength of the improved ground 4 is that of the sandy ground 2 that is the surrounding ground. It is set to be 0.3 to 2.0 on the assumption that it is higher than the control strength.
[0015]
Here, an injection method for injecting the non-alkali silicate solution into the sandy ground 2 will be described.
Even if a low-viscosity permanent chemical solution that easily permeates is injected into a soil layer with a permeability of around 10 ^-4 cm / sec in a soft soil layer that causes liquefaction, the chemical solution is pulsated in the conventional method. It may not be able to penetrate on average.
As a countermeasure in such a case, it is conceivable to increase the number of injection locations and to reduce the injection amount per hour from one injection hole to 1/10 or less of the conventional method.
And as a construction method that can realize this, for example, there is a super multi-point injection method that has been attracting attention recently. As shown in FIG. 4, the super multi-point injection method is provided with a plurality of bore holes 7 (seven in this example) in the sandy ground 2 to be a non-liquefaction section, and a plurality of nozzles in the bore holes 7. An injection pipe having 8 (31 in this example) is inserted, and a chemical solution is simultaneously injected from a plurality of nozzles 8.
[0016]
FIG. 5 is an explanatory view of the improved ground 4 formed by chemical injection. As shown in FIG. 5, the chemical liquid injected from each nozzle 8 improves the ground around the nozzle so that it is difficult to liquefy, and they are integrated to form the improved ground 4.
Thus, according to the super multi-point injection method, the periphery of the underground linear structure 1 including between the underground linear structure 1 and the supporting ground 3 can be easily improved.
[0017]
Next, the arrangement relationship between the liquefaction allowable section L1 and the non-liquefaction section L2 will be described. As the premise, the action at the time of liquefaction due to earthquake will be explained. When the liquefaction allowable section L1 is liquefied due to the occurrence of an earthquake, buoyancy acts on the underground linear structure 1 as described in the section of the conventional example. At this time, in the improved ground 4 in the non-liquefaction section L2, the ground below the underground linear structure 1 is not liquefied, so that buoyancy does not act on this portion. Further, since the ground above the underground linear structure 1 is not liquefied, the ground becomes heavy and acts to resist the floating of the liquefaction allowable section L1 due to the shear resistance of the improved ground 4. .
Therefore, as shown in FIG. 6, the underground linear structure 1 is displaced upward in the liquefaction permissible section L1, and acts to suppress this in the non-liquefaction section L2. The floating of the linear structure 1 can be suppressed.
[0018]
In this way, the non-liquefaction section L2 acts so as to restrain the floating of the liquid permissible section L1, but in order for this action to have an effect, the restraining force for this must be greater than the buoyancy. .
Accordingly, the section length of the non-liquefaction section L2 is determined by the pulling resistance force of the underground linear structure (so that it can resist the buoyancy acting on the liquid allowable section L1), The section length is determined according to the allowable flying height of the underground linear structure.
Therefore, a specific example for satisfying this requirement will be described below using an analysis model.
[0019]
An example of application of the present invention to a steel water pipe having an outer diameter of 2000 m, a plate thickness of 20 mm, and an embedding depth of 3000 mm is shown. As an analysis model, a beam model on an elastic bearing shown in FIGS. 7 (a) and 7 (b) was considered. In the liquefaction zone, the buoyancy caused by the difference between the unit volume weight γs = 20.0 kN / m ³ of the liquefied sand and the apparent unit volume weight γu = 10.0 kN / m ³ of the water pipe, etc. It was loaded upward as a distributed load.
In addition, the spring characteristics of the improved area were assumed to be an elasto-plastic type, and the ground reaction force coefficient Kh = 10000 kN / m ³ and the unit pull-out resistance q = 30 kN / m ² were assumed.
When the allowable floating displacement of the pipe is limited to 10 cm and the allowable axial bending stress of the pipe is limited to 140000 kN / m ² , it is calculated that the liquefaction zone length is 50.0 m or less and the improvement zone is 27.0 m or more. Is done.
The results are shown in FIG. 8 and FIG. 9 which is a sectional view taken along the line AA in FIG. 8 and FIG. 10 which is a sectional view taken along the line BB in FIG.
[0020]
As described above, according to the present embodiment, when the surrounding ground is liquefied during an earthquake, the floating of underground structures can be reduced. The adverse effects on can be avoided.
Moreover, since construction can be performed while the underground structure is in service, there is no burden on the user.
Furthermore, since the non-liquefaction sections are discretely arranged, measures can be taken at a low cost and in a short construction period.
Furthermore, liquefaction countermeasures by this construction method can be an effective investment if a construction method (such as a pipe-in-pipe construction method) in which a new pipeline is laid inside even if the underground linear structure itself is renewed in the future. There is no need to take new measures for liquefaction. That is, it is also effective from the viewpoint of life cycle cost considering future updates.
[0021]
In the above-described embodiment, the super multi-point injection method has been described as an example of the infiltration injection method for forming the improved ground, but any method that can form a predetermined improved ground using a durable chemical solution. For example, the injection method is not limited at all.
[0022]
【The invention's effect】
As described above, in the present invention, sections that allow liquefaction and non-liquefaction sections improved so as not to be liquefied are alternately arranged along the underground linear structure, and the non-liquefaction sections are arranged. Has a structure consisting of an improved ground formed by an infiltration method so as to contact the supporting ground and surround the underground structure, so that if the surrounding ground liquefies during an earthquake, Since the floating of the structure can be reduced and the non-liquefaction sections are discretely arranged, construction can be performed at low cost and in a short construction period.
[Brief description of the drawings]
FIG. 1 is an axial cross-sectional view showing an embodiment of the present invention.
FIG. 2 is a cross-sectional view taken along the line AA in FIG.
FIG. 3 is a cross-sectional view taken along the line BB in FIG.
FIG. 4 is a layout diagram of injection nozzles according to an embodiment of the present invention.
FIG. 5 is a conceptual diagram of the formation status of improved ground in one embodiment of the present invention.
FIG. 6 is an explanatory diagram of the operation of the embodiment of the present invention.
FIG. 7 is a model diagram of stress-displacement analysis in one embodiment of the present invention.
FIG. 8 is an axial cross-sectional view of a specific example of an embodiment of the present invention.
9 is a cross-sectional view taken along the line AA in FIG.
10 is a cross-sectional view taken along the line BB in FIG.
[Explanation of symbols]
1 underground structure 2 sandy ground 3 support ground 4 improved ground 5 groundwater level

Claims (4)

Liquefaction countermeasure structure for sandy ground with liquefaction potential, where underground linear structures are constructed,
Along with the underground linear structure, liquefaction permissible sections allowing liquefaction and non-liquefaction sections improved so as not to liquefy are alternately arranged, and the non-liquefaction sections are in contact with the supporting ground. A liquefaction countermeasure structure characterized by comprising an improved ground formed by an infiltration injection method so as to surround the periphery of the underground linear structure. The liquefaction countermeasure structure according to claim 1, wherein the improved ground is formed by pouring a non-alkali silicate solution into the sandy ground. A method for constructing a liquefaction countermeasure structure for sandy ground where liquefaction is possible, in which underground linear structures are constructed,
The chemical solution injection nozzle is arranged at the peripheral edge of the underground linear structure, and the chemical solution is injected from the nozzle into the sandy ground so as to come into contact with the support ground and surround the periphery of the underground linear structure. A construction method of a liquefaction countermeasure structure, characterized in that the ground is formed at a predetermined interval along the underground linear structure. The method for constructing a liquefaction countermeasure structure according to claim 3, wherein the improved ground is formed by injecting a non-alkali silicate solution into the sandy ground.

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