JP2020180454A - Ground improvement foundation structure - Google Patents

Abstract

To provide a ground improvement foundation structure that can efficiently reduce the vibrating of the superstructure due to the surface ground amplification of seismic motion.SOLUTION: The present invention is a ground improvement foundation structure constructed by an improved body in which the ground and a cement-based solidifying material are mixed and agitated. In short, by making the total cross-sectional area of a plurality of large-diameter improved bodies 2 10% -80%, which is the ratio to the total area when the lower ground of a building 3 is set to an improvement range 31, adjustments have been made to deviate a predominant eigenperiod of the improvement range from an eigenperiod of the building.SELECTED DRAWING: Figure 1

Description

The present invention relates to a ground improvement foundation structure constructed by an improved body obtained by mixing and stirring the ground and a solidifying material.

When building a building such as a house on the ground, in addition to investigating whether the ground has enough bearing capacity to support the weight of the building, it also investigates whether the ground is damaged by the earthquake. It is desirable to do. For example, in Patent Document 1, the acceleration response spectrum of the seismic foundation and the amplification factor of the surface layer ground are obtained, and the seismic design of the building is performed using the natural period (excellent natural period) obtained based on the result. The method is disclosed.

Further, in Patent Document 2, regarding the ground improvement foundation structure provided with a wall-shaped improved body so as to surround the lower ground of the superstructure for the purpose of reducing liquefaction damage, the portion bearing the vertical load and the liquefaction It is proposed that the structure will be rational and economical by clarifying the parts to be arranged to increase the rigidity.

Here, when it is determined that there is concern about surface ground amplification of seismic motion, it is conceivable to simply increase the rigidity of the ground in order to reduce the amplification. Due to site restrictions and economic efficiency, ground improvement of small-scale buildings is mostly due to small-diameter steel pipe piles or improved bodies in which the ground and cement-based solidifying material are mixed and agitated.

Japanese Unexamined Patent Publication No. 2011-80905 Japanese Unexamined Patent Publication No. 2015-200173

However, if the rigidity of the supporting ground changes due to the construction of the improved body, it may approximate the vibration characteristics of the superstructure, so it is necessary to adjust so that resonance does not occur in the building.

Therefore, an object of the present invention is to provide a ground improvement foundation structure capable of efficiently reducing the shaking of the superstructure due to the surface ground amplification of the seismic motion.

In order to achieve the above object, the ground improvement foundation structure of the present invention is a ground improvement foundation structure constructed by an improved body obtained by mixing and stirring the ground and a cement-based solidifying material, and a plurality of the above-mentioned improvements are arranged. By setting the total cross-sectional area of the body to 10% -80%, which is the ratio of the total area of the lower ground of the superstructure to the improvement range, the predominant natural period of the improvement range and the upper part It is characterized in that it is adjusted so as to deviate from the natural period of the structure.

Here, the improved body may have a columnar shape and a pile diameter of 0.7 m-2.0 m. Further, the pile length of the improved body can be 1.0 m-8.0 m.

The ground improvement foundation structure of the present invention constructed in this way is a ground improvement foundation structure constructed by an improved body obtained by mixing and stirring the ground and a solidifying material, and has an excellent natural period of the supporting ground including the improved body. Adjustments are made to deviate from the natural period of the superstructure. Then, the adjustment is to set the improvement rate, which is the ratio of the total cross-sectional area of the improved body to the total area of the improved range, to 10% -80%.

By setting such an improvement rate, it is possible to avoid the occurrence of a state in which the vibration characteristics of the superstructure such as a building resemble the supporting ground and cause resonance, and the superstructure is amplified by the surface ground of the seismic motion. It is possible to efficiently construct a ground improvement foundation structure that can reduce shaking.

It is a perspective view explaining the schematic structure of the ground improvement foundation structure of this embodiment. It is a perspective view explaining the schematic structure of another ground improvement foundation structure of this embodiment. It is a flowchart explaining the process flow of the ground investigation method. It is explanatory drawing which showed typically the model of the ground which has not improved the ground. It is explanatory drawing which showed typically the model of the ground which improved the surface layer. It is explanatory drawing which showed typically the model of the ground which improved the columnar. This is an example of a chart for determining the necessity of earthquake shaking reduction measures in the ground where the ground has not been improved. This is an example of a chart for determining the necessity of earthquake shaking reduction measures in the ground with surface improvement. This is an example of a chart for determining the necessity of earthquake shaking reduction measures in the ground with columnar improvement.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The ground improvement foundation structure of the present embodiment is constructed on the ground as the foundation of a building that becomes an upper structure such as a house or a small-scale apartment building. Here, the ground on which the ground improvement foundation structure that supports the superstructure from below is provided is referred to as "supporting ground", and the area projected on the ground surface is referred to as "improvement range".

Ground improvement For the ground on which the foundation structure is provided, a ground survey is conducted at one or more points in or around the improvement area. Ground surveys include surface wave exploration tests and penetration tests.

The surface wave exploration test, which is one method of ground investigation, is a method of detecting Rayleigh waves artificially generated by a shaker applied to the surface of the ground by multiple sensors (detection) installed at a position away from the shaker. It is a survey method to check the hardness of the ground by measuring with a vessel). In short, it is a method to estimate the degree of hardness such as whether the ground to be surveyed is hard or soft from the magnitude of the propagation speed by utilizing the fact that the propagation speed increases as the substance becomes hard. ..

Specifically, two sensors installed at different distances from the oscillator detect the time when the Rayleigh wave applied to the ground from the oscillator is detected. Here, since the two sensors are installed at different positions, there is a time lag in the detection time.

Therefore, the propagation velocity of the surface wave (S wave velocity) is calculated from the distance between the two sensors and the time difference of the detection time. In order to accurately determine this time difference, it is necessary to use a spectrum analyzer to completely remove noise from the detection signal.

On the other hand, the penetration test includes a dynamic penetration test, such as a standard penetration test, in which boring is performed to the target layer and the hardness of the soil layer is evaluated by the penetration amount when the weight is dropped on the layer. is there. In addition, as in the Swedish sounding test, there is a static penetration test that indirectly evaluates the support performance of the layer from the presence or absence of subsidence when a weight is loaded on the rod and the degree of resistance when rotating and penetrating. ..

Here, if a vibration exciter is attached to the penetration device of the boring or Swedish sounding test to dynamically hit the soil layer to be investigated, it is the same as performing a dynamic loading test. Therefore, the rigidity of the layer at the time of an earthquake is being evaluated. Therefore, it is possible to think that an S-wave velocity correlating with this can be obtained.

From the S-wave velocity obtained from the results of these ground surveys, it is possible to determine whether or not the ground shaking is greatly amplified during an earthquake. Here, "amplification of ground motion (earthquake motion)" means that seismic waves are greatly amplified locally depending on ground conditions such as cut embankment.

By identifying in advance the ground where such ground motion is amplified, it becomes possible to reduce or prevent earthquake damage. In short, "sway reduction measures" refer to measures to reduce building damage caused by large ground shaking during a large earthquake.

Amplification determination as to whether or not seismic motion is amplified is made based on, for example, the surface ground amplification factor. The surface ground amplification factor is a numerical value of the magnitude of shaking of the surface ground near the ground surface during an earthquake, and indicates the weakness of the ground against an earthquake.

This surface layer ground amplification factor can be calculated from, for example, the S-wave velocity of the surveyed ground obtained by a surface wave exploration test or a penetration test. Then, by analyzing this, the surface layer ground amplification factor for each cycle can be obtained.

It is said that the larger the value of the surface layer ground amplification factor, the weaker the ground and the greater the shaking. In addition, when the natural period of the structure to be built is in the period zone where the surface layer ground amplification factor of the ground is high, the seismic motion that matches the natural period of the structure becomes larger, causing more deformation than expected and causing damage. May affect.

From these facts, based on the knowledge of the characteristics of the building and the results of past experiments, for example, the surface ground amplification factor α = 2.0 times was used as the reference value in a period of 0.5 sec, and the surface ground amplification factor above the reference value was calculated. In that case, it may be judged that the shaking reduction measures are necessary, and if it is less than the standard value, the shaking reduction measures are unnecessary.

Here, the value of the S-wave velocity for each depth varies from ground to ground. When determining the surface ground amplification factor in a house, the soil layer (hard ground) with sufficient hardness is used as the "engineering foundation". For example, the ground with _S -wave velocity V _S = 400 m / s corresponds to the engineering foundation.

Then, using this engineering foundation as the base, the S-wave velocity, density, attenuation constant, and soil quality of the shallower ground are set. Here, in examining the surface layer ground amplification factor, if the target is soft ground, for example, documents such as "Guidelines for Basic Design of Small Buildings" (Architectural Institute of Japan, 2008) can be referred to. .. For example, assuming a viscous land, the density is 1.7t / m ³ , and the damping constant is assumed to be about 0.03, which is not excessive, and it is possible to conduct a study in advance according to the S-wave velocity shallower than the engineering base, which is a variable.

In the following, the processing flow of the ground survey method will be described with reference to FIG. In conducting a ground survey, the existing ground survey results accumulated up to that point and the existing ground data obtained from literature and the like are used.

In step S1, a ground survey is conducted on the survey target ground on which the building 3 is actually constructed. For the ground survey, for example, a surface wave exploration test is conducted. By this surface wave exploration test, the S-wave velocity for each depth of the surveyed ground is measured.

Therefore, the depth at which the S-wave velocity is 400 m / s or more is specified as the depth to the engineering base (step S2). Subsequently, in step S3, the S-wave velocities up to the engineering foundation are averaged and calculated as the average S-wave velocities of the surface layer ground. If the average S-wave velocity is not used, this step can be omitted.

Then, in step S4, the surface ground amplification factor is estimated from the relationship between the depth to the specified engineering base of the surveyed ground and the average S-wave velocity, etc., using the existing ground data as described above.

This estimated surface ground amplification factor serves as a reference value for determining the necessity of shaking reduction measures. For example, if α = 2.0 times or more in a cycle of 0.5 sec, it is determined that shaking reduction measures are necessary (step S5).

When it is determined from the results of the ground survey that measures to reduce shaking are necessary, it is necessary to provide a ground improvement foundation structure in which the natural period of the superstructure and the predominant natural period of the supporting ground are separated. Basically, if the horizontal rigidity is increased, the transition will occur on the short cycle side.

Therefore, as shown in FIG. 1, the lower ground of the building 3 as the superstructure is set as the improvement range 31, and the ground improvement foundation structure 1 is provided in the improvement range 31, so that the surface ground amplification of the seismic motion is reduced. To.

The ground improvement foundation structure 1 of the present embodiment is constructed by an improved body obtained by mixing and stirring the ground and a cement-based solidifying material. Such an improved body can be constructed by, for example, a deep mixing treatment method. In these construction methods, a slurry-like cement-based solidifying material such as cement milk is mixed into the ground in the original position where it was cut, and mixed and stirred to construct an improved body (soil cement).

The improved body shown in FIG. 1 is a columnar large-diameter improved body 2 having a relatively large diameter. For example, if the pile diameter of an improved body generally constructed in a detached house is 0.5m-0.6m, it will be constructed with a pile diameter of about 0.7m-2.0m.

On the other hand, the pile length can be shortened if necessary. That is, the construction cost can be reduced by shortening the portion that does not contribute to the reduction of the surface ground amplification of the earthquake motion. For example, if the pile length of an improved version of a general detached house is 2.0m-8.0m, the pile length will be about 1.0m-8.0m.

In order to shorten the pile length, it is necessary to increase the improvement rate and reduce the surface ground amplification of seismic motion. Here, the "improvement rate" is the ratio of the total cross-sectional area of the plurality of improved bodies to the total area (flat area) of the improved range 31, and is set to 10% -80%. For example, when there is no overlap between the improved bodies, the improvement rate = (cross-sectional area of the improved bodies × number) / (flat area of the entire improvement range 31) × 100. On the other hand, when the improved bodies have wraps, the total cross-sectional area of the improved bodies is calculated so that the cross-sectional areas of the overlapping portions are not added twice.

The large-diameter improved body 2 constituting the ground improvement foundation structure 1 shown in FIG. 1 has a pile diameter of 1.0 m, which is twice the pile diameter of 0.5 m of the improved body of a general detached house, and has a cross-sectional area of 4. Double. The pile length of the large diameter improved body 2 is 1.0 m, which is shorter than usual.

Then, in the improvement range 31, a plurality of large-diameter improved bodies 2, ... Are constructed at intervals in the horizontal direction, and the improvement rate is about 23%. The large diameter improved body 2 is mainly arranged in a place where a vertical load is likely to be concentrated, such as below a pillar of a building 3.

On the other hand, FIG. 2 shows another ground improvement foundation structure 1A. The improved body constructed at intervals of the improved range 31A is a columnar long improved body 2A that is close to the pile diameter and pile length of the general improved body. For example, a long improved body 2A having a pile diameter of 0.7 m and a pile length of 7.0 m is constructed.

The improvement rate of the plurality of long improved bodies 2A, ... Arranged at intervals in the horizontal direction in the improvement range 31A is about 10%. The bearing capacity of the long improved body 2A is the sum of the tip bearing capacity, which is the ground resistance of the lower end surface, and the peripheral friction resistance due to adhesion to the ground.

The building 3 built on the supporting ground whose predominant natural period is adjusted by adjusting the improvement rate of the improvement ranges 31 and 31A in this way is, for example, a unit house in which members are manufactured in a factory. For example, by bridging beams between columns arranged at intervals, a skeleton structure serving as a structural member is formed.

The columns and beams constituting this framework structure can be formed of steel materials such as H-shaped steel, channel steel (C-shaped steel), and square steel pipes. Further, the frame structure may be a steel structure in which columns and beams are joined by bolts, or a rigid frame structure in which columns and beams are rigidly joined by welding.

Vertical loads such as the weight of the building in which the skeleton structure is formed by such columns and beams are intensively transmitted from the column base to the foundation. The arrangement of the large diameter improved body 2 and the long diameter improved body 2A shown in FIGS. 1 and 2 is at and around the position where the concentrated load acts from the pillar or the like.

Since the natural period of the building 3 constructed by manufacturing the unitized members in a factory or the like is known in advance, the predominant natural period of the supporting ground and the natural period of the building 3 should be separated from each other. Can be adjusted.

Basically, amplification can be reduced by increasing the rigidity of the ground, but it is designed so that the vibration characteristics of the supporting ground are close to the vibration characteristics of the building 3 and resonance is likely to occur. Is done.

When targeting small-scale houses, the target layer is shallower and the overall layer thickness is thinner than the soil layer targeted for medium to large-scale buildings. Furthermore, even if the target soil layer is large or small, the rigidity is low as a whole, and it can be said that the degree of influence is small even if a simple study is conducted. Therefore, the average S-wave velocity is calculated in advance for each layer thickness.

FIG. 4 is an explanatory diagram schematically showing a model of the ground to which the average S-wave velocity is applied. This ground model models the original ground (unreinforced ground) that has not been improved. On the other hand, for example, if a shallow layer improvement is performed to reinforce the ground by stirring with the soil of the local ground using a cement-based solidifying material, or a deep layer improvement is performed by continuously constructing columnar improved bodies to reinforce the ground, an earthquake will occur. The way the ground sways at the time changes.

FIG. 5 is an explanatory diagram schematically showing a model of the ground where the surface layer improvement M1 is performed as the ground improvement. When improving the ground, if the conventional soft ground countermeasure method is used and the horizontal rigidity of the improved ground can be predicted, it is possible to consider the layer to be improved. That is, it is possible to separately calculate the average S-wave velocity only for the improved portion, prevent the range where there is a clear difference in rigidity from being averaged, and perform modeling close to the actual ground.

On the other hand, FIG. 6 is an explanatory view schematically showing a model of the ground in which the columnar improvement M2 is performed as the ground improvement. Then, for these ground models, the depth to the engineering foundation is changed, and the average S-wave velocity of the surface ground is changed to calculate the surface ground amplification factor based on the two relationships.

FIG. 7 shows an example of a matrixed chart for examining the surface ground amplification factor of the original ground. Here, for example, when the reference value is set to 1.7 times, the surface ground amplification factor of the corresponding portion exceeds the reference value from the average S wave velocity and the depth to the engineering foundation (the mass shown darkly in FIG. 7). It is judged that measures to reduce shaking are necessary for the area of the eyes). On the other hand, if the surface layer ground amplification factor of the corresponding portion is less than the reference value, it can be determined that the shaking reduction measures are unnecessary.

For example, in depth 6m to engineering bedrock, when the average S wave velocity until said depth of ground is V _S = 50 m / s, according to FIG. 7, if left local board, surface ground Since the amplification factor is less than the standard value, ground reinforcement is not necessary to reduce the shaking of the earthquake.

For reference, also for the ground model of FIG. 5 in which the surface improvement M1 which is a shallow ground improvement is performed, the depth to the engineering foundation is changed and the average S-wave velocity of the surface ground is changed. The surface ground amplification factor based on the two relationships is calculated respectively.

FIG. 8 shows an example of a matrixed chart for examining the surface layer ground amplification factor when the surface layer improvement M1 having a layer thickness of about 2 m is applied. By using this matrix-based determination database, it is possible to determine whether or not it is possible to reduce the shaking of an earthquake by improving the shallow ground when the shaking of the earthquake becomes large with the original ground as it is.

In short, when it is necessary to improve the ground in order to secure the bearing capacity because of the soft ground, it is possible to predict in advance whether the surface improvement M1 may increase the shaking of the earthquake. become able to. Then, when it is predicted that the shaking will increase, it is possible to consider switching to other construction methods such as deep ground improvement and pile-shaped ground reinforcement with low horizontal rigidity.

Similarly, FIG. 9 shows an example of a matrixed chart for examining the surface layer ground amplification factor when continuous columnar improved M2 for increasing the horizontal rigidity up to a depth of about 4 m is applied. By using this matrixed determination database, it becomes possible to determine whether or not it is possible to reduce the shaking of an earthquake by the columnar improvement M2.

Furthermore, when it is necessary to apply the columnar improvement M2 for soft ground, it becomes possible to predict in advance the possibility that the shaking of the earthquake will increase. Then, if it is expected that the shaking will increase, it is possible to consider switching to another construction method such as pile-shaped ground reinforcement with low horizontal rigidity.

In short, the horizontal rigidity of the supporting ground can be adjusted by replacing the continuous columnar improved M2 shown in FIG. 6 with a pile-shaped thick-diameter improved body 2 or a long-diameter improved body 2A. Therefore, the ground improvement of the present embodiment as described above is applied to the ground showing a large amplification in the range of the depth to the engineering foundation of 2.5 m-20 m and the average S wave velocity of 110 m / s-290 m / s. The foundation structure 1, 1A is constructed.

Next, the operation of the ground improvement foundation structures 1, 1A of the present embodiment will be described.
The ground improvement foundation structure 1, 1A of the present embodiment configured as described above is composed of an improved body (2, 2A) in which the ground and the cement-based solidifying material are mixed and agitated.

Then, adjustments are made so as to dissociate the predominant natural period of the supporting ground including the improved body (2, 2A) from the natural period of the building 3. This adjustment is performed so that the improvement rate is set to 10% -80%, for example, by increasing the diameter of the improved body (2, 2A). It can also be adjusted by the number of improved bodies and the pile length.

In short, by avoiding the occurrence of a state in which the vibration characteristics of the superstructure such as the building 3 and the vibration characteristics of the supporting ground are close to each other and resonance occurs, the shaking of the building 3 due to the surface ground amplification of the seismic motion is reduced. Construct a ground improvement foundation structure 1, 1A that is possible.

On the other hand, in soft ground that does not contribute to the reduction of surface ground amplification of seismic motion and the bearing capacity cannot be expected to increase due to the peripheral frictional resistance with the ground, the pile length can be shortened to reduce the construction cost. ..

Although the embodiment of the present invention has been described in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and design changes to the extent that the gist of the present invention is not deviated are described in the present invention. Included in the invention.
For example, in the above-described embodiment, the large-diameter improved body 2 and the long-diameter improved body 2A have been described as an example, but the present invention is not limited to this, and an intermediate improved body thereof can also be constructed.

1,1A: Ground improvement foundation structure 2: Large diameter improved body (improved body)
2A: Long improved body (improved body)
3: Building (superstructure)
31, 31A: Improvement range

Claims (3)

It is a ground improvement foundation structure constructed by an improved body in which the ground and a cement-based solidifying material are mixed and agitated.
The improvement range is set by setting the total cross-sectional area of the improved bodies in which a plurality of the improved bodies are arranged to be 10% -80%, which is a ratio to the total area when the lower ground of the superstructure is set as the improvement range. A ground improvement foundation structure characterized in that the predominant natural period of the above is adjusted so as to deviate from the natural period of the superstructure. The ground improvement foundation structure according to claim 1, wherein the improved body is columnar and has a pile diameter of 0.7 m to 2.0 m. The ground improvement foundation structure according to claim 2, wherein the pile length of the improved body is 1.0 m to 8.0 m.