JP4055838B2 - Improved foundation ground structure of existing structure - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a foundation ground structure of an existing structure that is installed on soft ground such as sand that may be liquefied during an earthquake.
[0002]
[Prior art]
Various measures for liquefaction during earthquakes have been studied for existing structures such as the foundation ground of existing cylindrical storage tanks such as oil tanks. And as a foundation ground improvement construction method for liquefaction measures, there are a groundwater level lowering construction method, a drain type construction method, a construction method using a steel ring, and the like.
[0003]
The groundwater level lowering method is intended to prevent liquefaction from occurring by lowering the groundwater level by surrounding the existing cylindrical storage tank with a sheet pile or the like and pumping up the internal groundwater by a pump.
However, there is a problem that the foundation ground structure improved by the groundwater level lowering method tends to cause uneven settlement due to the groundwater level drop. In addition, it is necessary to keep pumps operating for earthquakes that do not know when they occur, and there is a problem that maintenance costs are high.
[0004]
In addition, the drain method is unequal by inserting many pipes into the surrounding ground of the existing cylindrical storage tank or creating a crushed stone column and discharging the pressure water generated in the lower ground of the existing cylindrical storage tank during an earthquake. It is intended to prevent settlement.
However, in the drain type construction method, since the construction machine is relatively large, it is not possible to place the construction machine when the pipes are densely packed around the tank like an oil tank, and the existing pipes attached to the cylindrical storage tank. Need to be removed. Therefore, there is a problem that it cannot be applied to the dense conditions of the existing cylindrical storage tank.
[0005]
Moreover, the construction method using a steel ring is a method in which a steel sheet pile is driven so as to surround an existing cylindrical storage tank to prevent sliding to the side of the ground and to suppress liquefaction.
However, as in the case of the drain method, this method requires the removal of the existing pipes attached to the cylindrical storage tank, and in the case of a large tank, the joint strength of the steel sheet pile is insufficient, and the countermeasure effect has been verified. There are various problems such as absence.
[0006]
All the conventional methods mentioned in the above examples are to prevent liquefaction using the attached equipment in the existing cylindrical storage tank. As a common problem, installation space for the attached equipment is required, and the cylindrical storage tank is densely packed. There is a big problem that it can not be implemented in places.
[0007]
Therefore, what has been proposed is a ground improvement method by injection solidification, and FIG. 5 shows a side cross section of the ground improved by this method. In the figure, 1 is an existing cylindrical storage tank, 2 is sandy ground, 3 is groundwater level, and 4 is improved ground.
In this construction method, a plurality of pipes are inserted into a liquefaction tank (sandy ground) 2 below the existing cylindrical storage tank 1, and cement-based solidified material is injected through the pipes. Then, the injected solidified material is solidified by replacing the water in the gap between the soil particles to form the improved ground 4 over the entire foundation ground below the existing tank 1 to stabilize the ground.
[0008]
However, the foundation ground structure by the ground improvement method by the injection solidification mentioned above has the following problems.
In the ground improvement method by injection solidification, generally the improved ground diameter calculated by the following equation (1) is required.
Improved ground diameter = tank diameter + 2 x (2/3) x liquefaction layer depth (1)
Therefore, considering the depth of a general liquefaction tank, the improved ground diameter is at least the tank diameter +10 m or more from equation (1), the volume of the improved ground is large, the improvement takes a long time and the cost is high. Become.
[0009]
In view of this, a basic ground structure based on a partial ground improvement method disclosed in, for example, Japanese Patent Laid-Open No. 8-128054 has been proposed to solve such a problem.
As shown in FIG. 6, the basic ground structure by this partial ground improvement construction method is a non-liquefied layer 9 only in a portion immediately below the base portion 5 constructed on the bottom outer peripheral portion of the existing cylindrical storage tank 1 (existing structure in the publication). The cylindrical solidified columnar body 7 is constructed from the solidified material until the solid cylindrical material 7 is reached, and the existing cylindrical storage tank 1 is supported by the solidified columnar body 7.
And in the foundation ground structure by said partial ground improvement construction method, what is necessary is just to solidify only the lower part of the grounding part of the existing cylindrical storage tank 1, and to improve the ground. According to the report, it is not necessary to improve the ground in the entire area below, and material costs and construction time can be reduced.
[0010]
[Problems to be solved by the invention]
However, in the foundation ground structure by the partial ground improvement method, since the inside of the solidified columnar body 7 constructed in a cylindrical shape is left an unimproved portion that may be liquefied, the portion sinks when liquefaction occurs. Or non-uniform subsidence occurs between improved parts with small subsidence. For this reason, the bottom plate of the existing cylindrical storage tank 1 is subject to subsidence, causing a severe deformation, and is likely to have a harmful effect on the existing cylindrical storage tank 1.
[0011]
Further, when liquefaction occurs in the unimproved portion immediately below the existing cylindrical storage tank 1, hoop tension is generated in the solidified columnar body 7 arranged in a cylindrical shape due to excessive pore water pressure. And generally, since the tensile strength of the solidified columnar body 7 improved by the solidified material is extremely small, a tensile failure occurs with respect to this hoop tension. When the solidified columnar body 7 is broken, local slip occurs in the broken portion, and the existing cylindrical storage tank 1 is likely to be tilted.
[0012]
As described above, the partial ground improvement method disclosed in Japanese Patent Application Laid-Open No. 8-128054 has advantages in terms of cost and construction time, but stabilizes the existing cylindrical storage tank during an earthquake, which is the original purpose. I was worried about the point of support.
[0013]
The present invention has been made to solve the above-mentioned problems, and can provide an improved foundation structure for an existing structure that can reduce cost and work time and can stably support an existing structure even during an earthquake. The purpose is to get.
[0014]
[Means for Solving the Problems]
The improved foundation ground structure of the existing structure according to the present invention has a cylindrical improved ground having a liquefaction strength higher than that of the surrounding ground below the existing structure installed on a soft sandy soil that is easily liquefied. In the foundation ground structure constructed so that the outer peripheral edge of the lower surface of the existing structure is disposed in the thickness portion of the cylinder of the improved ground when viewed in plan, the improved ground is non-alkaline A sodium silicate solution is formed by injecting into the sandy soft ground, and the improved ground is constructed below the groundwater level with the top position of the improved ground being the same position as the groundwater level. A space is provided between the surface of the sandy soft ground.
[0015]
The improved ground may be cylindrical .
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Before describing specific embodiments, the concept of the present invention will be described.
The conventional ground improvement by injection solidification is to solidify the lower ground of an existing structure with a solidifying material to form a solid ground, thereby supporting the existing structure.
However, in such a method, as described in the example of the existing cylindrical storage tank of the conventional example, it is necessary to improve the ground of the entire lower ground in order to stably support the existing structure. This is because the method disclosed in Japanese Patent Laid-Open No. 8-128054, which has been partially improved to reduce costs and time, may not achieve even the original purpose of stably supporting an existing cylindrical storage tank. Is also understood.
[0018]
Therefore, the inventors improved the lower part of the existing structure to a solid ground, prevented the subsidence of the improved ground, changed the conventional idea of supporting the existing structure on this improved ground, Based on the knowledge that the essential element of stable support is not to prevent subsidence but to prevent unequal subsidence, rather than to prevent subsidence, in other words to prevent unequal subsidence. It is intended to achieve stable support of existing structures by allowing for even subsidence but preventing uneven subsidence.
Hereinafter, a specific embodiment will be described by taking an existing cylindrical storage tank as an example of an existing structure.
[0019]
Embodiment.
FIG. 1 is an explanatory diagram of an embodiment of the present invention. The same reference numerals are given to the same parts as those in FIGS. 5 and 6 showing the conventional example.
In the figure, 1 is an existing cylindrical storage tank installed on the sandy ground 2, and 11 is a cylindrical improved ground formed below the existing cylindrical storage tank.
[0020]
The improved ground 11 is obtained by injecting a non-alkali silicate solution into the sandy ground 2 and improving the ground. The improved ground 11 has a liquefaction strength (for example, the ratio of the shear stress and the initial effective confining pressure at a repetition rate of N = 20 in the 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 11 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.
[0021]
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. The super multi-point injection method is a method in which a large number of nozzles of several tens to several hundreds are arranged in the ground, and injection can be performed simultaneously by performing low discharge injection from each nozzle. According to this construction method, the uniform improved ground 11 can be reliably formed, and it is economically superior.
[0022]
Next, the arrangement relationship of the improved ground 11 will be described.
The improved ground 11 is constructed so that the outer peripheral edge of the lower surface of the existing cylindrical storage tank 1 is disposed in the thickness portion of the cylinder of the improved ground 11 when viewed in plan.
The reason for this arrangement is as follows.
In the event of an earthquake, the lower ground at the lower peripheral edge of the existing cylindrical storage tank 1 is likely to slip locally from the inside to the outside at the boundary between the part where the pressure is applied to the structure and the outer periphery. . And when a local slip occurs, the part will sink locally and an unequal settlement will generate | occur | produce. Therefore, in order to prevent such local slip and prevent uneven settlement due to this, the lower ground of the lower peripheral edge of the lower surface of the existing cylindrical storage tank 1 is improved so as to increase the liquefaction strength.
[0023]
The improved ground 11 is located below the groundwater level 3 below the existing cylindrical storage tank 1.
The reason for this arrangement is as follows. In other words, liquefaction occurs in the ground below the groundwater level. And since this patent does not support the existing cylindrical storage tank 1 by the ground which solidified the ground, it is not necessary to form the improved ground above the groundwater level 3 and thereby support the existing cylindrical storage tank 1. It is. For this reason, the range to improve may be small and time and cost can be reduced more. But you may improve to the upper part from a groundwater level.
[0024]
Next, the operation of the present embodiment configured as described above will be described.
If the ground has not been improved, if an earthquake occurs, the lower ground of the outer peripheral edge of the lower surface of the existing cylindrical storage tank 1 will cause local slip to the outside, and this part will sink locally, resulting in uneven settlement. To do.
However, in the case of the present embodiment, since the improved ground 11 having a higher liquefaction strength than the surrounding sandy ground 2 is formed, local slip occurs on the lower ground of the lower peripheral edge of the existing cylindrical storage tank 1. It does not occur and therefore unequal settlement does not occur.
[0025]
In addition, liquefaction may occur in the unimproved portion inside the cylindrical improved ground 11, and excessive pore water pressure may be generated. The improved ground 11 permeates and injects a non-alkali silicate solution into the sandy ground 2. The ground is improved and is not solidified by a cement-based solidifying material as in the conventional example, and is excellent in deformation performance. Therefore, the hoop tension does not occur as in the conventional example, and the improved ground 11 has a hoop tensile fracture. Will not occur.
Therefore, unlike the conventional example, unequal settlement due to the destruction of the improved ground 11 does not occur.
[0026]
Thus, in this Embodiment, since a local slip does not generate | occur | produce in the lower ground of the lower surface outer periphery part of the existing cylindrical storage tank 1, and there is no hoop tension fracture of the improved ground 11, the lower ground of the existing cylindrical storage tank 1 Therefore, the existing cylindrical storage tank 1 can be stably supported.
[0027]
【Example】
Next, specific examples will be described.
FIG. 2 is an analysis model diagram of the lower ground of the existing cylindrical storage tank, and portions corresponding to the respective portions shown in FIG. 1 are denoted by the same reference numerals.
Reference numeral 1 denotes a virtual cylindrical storage tank having a dimension of 16 m in diameter, a liquid depth of 13.7 m, and a capacity of 3000 kl. And in the ground where a liquefaction layer exists to GL-10m, as shown in FIG. 2, the example at the time of improving the area | region of thickness 4m into a cylindrical shape is introduced.
[0028]
Figure 3 shows the numerical analysis method based on effective stress (Program name: LIQCA: “Geotechnical Society: Symposium on liquefaction mechanism / prediction method and design method, committee report / paper collection, 1999/5”) The Hachinohe wave in the 1968 Tokachi-oki earthquake was GL. It is a figure which shows the deformation | transformation state of the existing cylindrical storage tank and foundation ground system at the time of inputting into a -10m point. (Deformation is shown in 3 times).
As shown in FIG. 3, although the existing cylindrical storage tank 1 sinks slightly, there is no large unequal settling that poses a problem for the tank foundation.
[0029]
FIG. 4 shows the relationship between the amount of subsidence of the foundation ground and the horizontal displacement of the ground of the existing cylindrical storage tank 1 in the case of no countermeasure (FIG. 4 (a)) and the case of the present embodiment (FIG. 4 (b)). Are shown in comparison.
4, the horizontal axis is the vertical displacement ρ (m) of the foundation ground (point S in FIG. 2) of the existing cylindrical storage tank 1, and the vertical axis is the horizontal displacement of the ground (point P in FIG. 2) below the lower peripheral edge of the existing cylindrical storage tank. Respectively.
[0030]
As can be seen from FIG. 4, when no countermeasure is taken (FIG. 4 (a)), the amount of horizontal displacement in the ground is larger than the amount of settlement of the existing cylindrical storage tank 1, suggesting the possibility of local destruction of the ground. It has become the result.
On the other hand, in the case of the present embodiment (FIG. 4B), the horizontal displacement amount in the ground is suppressed to about 1/2 of that in the case where no countermeasure is taken, and the ground is stably deformed during an earthquake. It is suggested that the load of the existing cylindrical storage tank 1 is supported.
[0031]
Note that the above analysis model is assumed to be in a solid state, and there is a limit in handling the analysis of a completely liquefied state. It is thought that displacement occurs.
Therefore, it can be considered that there is actually a difference more than the above model analysis between the non-measures and this embodiment, and it was proved that this embodiment is effective against uneven settlement. It is accepted.
[0032]
In the present embodiment, a water glass-based material is used as an injection material for ground improvement, so that the liquefaction strength of the improved ground can be easily adjusted and work efficiency is good. However, in the present invention, it is not limited to the water glass type, and even if it is a cement type, the liquefaction strength can be adjusted by controlling the injection amount, etc., so this can be used. It is.
[0033]
In the above description, the lower ground of the existing cylindrical storage tank is taken as an example, but the present invention is not limited to this, and other shapes installed on the soft ground that is liable to liquefy. It is also effective as an improved foundation ground structure for existing structures.
[0034]
【The invention's effect】
Since the present invention is configured as described above, the following effects can be obtained.
[0035]
When the cylindrical improved ground having a higher liquefaction strength than the surrounding ground is viewed in plan below the existing structure, the outer peripheral edge of the bottom surface of the existing structure is the thickness portion of the cylinder of the improved ground. In the foundation ground structure constructed so as to be disposed in, the improved ground is formed by injecting a non-alkali silicate solution into the sandy soft ground, and the position of the upper end of the improved ground is the groundwater level. By constructing the improved ground below the groundwater level at the same position and providing a gap between the upper end of the improved ground and the surface of the sandy soft ground, the outer periphery of the lower surface of the existing structure Since local slip does not occur in the lower ground, and there is no hoop tensile failure of the improved ground, uneven settlement does not occur in the lower ground of the existing structure, and the existing structure can be supported stably.
[0036]
Further, the improved ground is formed by infiltrating and injecting a non-alkali silicate solution into the soft ground, and it is easy to adjust the liquefaction strength of the improved ground, and the work efficiency is good.
[0037]
Furthermore, since the improved ground is constructed below the groundwater level, the improved volume is small and costs and time can be further reduced.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing an embodiment of the present invention.
FIG. 2 is an analysis model diagram of one embodiment of the present invention.
FIG. 3 is a diagram showing a deformed state after vibration by a numerical analysis method based on effective stress.
FIG. 4 is a graph showing the relationship between the amount of settlement of the foundation ground and the amount of horizontal displacement of the ground of the existing cylindrical storage tank in the present embodiment.
FIG. 5 is a cross-sectional view showing an example of a basic ground structure by a ground improvement method by conventional injection solidification.
FIG. 6 is a sectional view showing an example of a basic ground structure by a conventional partial ground solidification improvement method.
[Explanation of symbols]
1 Existing cylindrical storage tank 2 Sandy ground 3 Groundwater level 11 Improved ground

Claims (2)

When the cylindrical improved ground having a higher liquefaction strength than the surrounding ground is seen below the existing structure installed on the soft sandy soil that is liable to liquefy, the existing structure In the foundation ground structure constructed so that the outer peripheral edge of the lower surface is disposed in the thickness portion of the cylinder of the improved ground, the improved ground is injected with a non-alkali silicate solution into the sandy soft ground. And forming the improved ground below the groundwater level with the top position of the improved ground being the same position as the groundwater level, and providing a gap between the top of the improved ground and the surface of the soft sandy ground Improved foundation ground structure of existing structures characterized by the fact that   The improved foundation ground structure for an existing structure according to claim 1, wherein the improved ground is cylindrical.

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