JP2012031662A - Ground displacement absorbing base isolation structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ground displacement absorbing base isolation structure in which prolonged stability of an underground base isolation wall is secured and stress acting on an underground structure such as an open-cut tunnel at the time of an earthquake is reduced.SOLUTION: The present invention is an underground base isolation wall 4 which has a continuous wall structure made of a clay-based material with a water swelling property and is placed between an underground structure 2 and the surrounding ground. The underground base isolation wall 4 has a width D of 0.2 to 2.5 m and is made of the clay-based material which is a mixture of bentonite and water having a bentonite effective dry density of 300 to 1200 kg/min a region filled with the mixture.

Description

  The present invention relates to a ground displacement absorbing base isolation structure for reducing stress on underground structures such as open tunnels during an earthquake.

  Conventionally, as an underground seismic isolation wall for the purpose of reducing stress during earthquakes to underground structures such as open tunnels, it has been common to install continuously in the extending direction in contact with the underground structure If it is this installation position, it can be constructed simultaneously with the construction of the underground structure. For example, Patent Document 1 proposes a ground displacement absorbing method in which polymer improved soil is cast as an underground seismic isolation wall for the purpose of reducing stress on underground structures during an earthquake.

  On the other hand, Non-Patent Document 1, for example, discloses an underground seismic isolation wall for an underground structure such as an open-cut tunnel that is not continuously installed in the extending direction of the underground structure. In this nonpatent literature 1, the vertical cylinder-shaped polymer improvement soil is intermittently arrange | positioned in the ground along the underground structure.

JP 2006-112182 A

Tsuyoshi Murono, Masaru Tateyama, Fumi Kiryu, Shosuke Kobayashi, “Reinforcement of existing excavated tunnels with polymer seismic isolation walls”, Foundation work, 2007.3, P69-71

However, the conventional underground seismic isolation wall has the following problems.
That is, in the construction method for placing the polymer improved soil as disclosed in Patent Document 1, the polymer seismic isolation material is greatly compressed and deformed due to the influence of normal earth pressure and the like. Therefore, the wall thickness of the predetermined underground seismic isolation wall set at the time of construction cannot maintain the initial wall thickness for a long time, and there is a concern that the seismic isolation wall may be crushed in some cases. Such a change in the thickness of the base isolation wall is caused by a decrease in the displacement absorption performance of the polymer base isolation wall, and the effect of absorbing the ground displacement by the underground base isolation wall is not fully exhibited. There was a problem that the reduction effect was reduced.

As a countermeasure, Non-Patent Document 1 is considered in which the underground seismic isolation wall is intermittently arranged so as to avoid the continuous arrangement along underground structures such as open tunnels. However, in the seismic isolation walls that are not arranged continuously, the surrounding ground and the structure are connected by soil, so that the seismic isolation effect of sufficiently absorbing the original displacement is significantly reduced. there were.
Moreover, since the polymer material is dissolved in water in the ground where the groundwater level exists, there is a problem that the construction is not easy. Furthermore, although a predetermined reinforcing agent must be added to the polymer seismic isolation material, there is a problem that an expensive reinforcing agent is required for environmental considerations and costs are increased.

  In this way, when an underground seismic isolation wall is provided along an underground structure such as an open-cut tunnel, it can sufficiently resist the earth pressure acting on the underground isolation wall and ensure long-term stability. On the other hand, in order to exert the seismic isolation effect, it is desirable that the material has low rigidity, and there is a need for an underground seismic isolation wall that provides a good balance between the two. There was room for.

  The present invention has been made in view of the above-described problems. In addition to ensuring the long-term stability of the underground seismic isolation wall, it is possible to reduce stress on underground structures such as an open tunnel during an earthquake. The purpose is to provide a ground displacement absorbing base isolation structure.

  In order to achieve the above object, in the ground displacement absorbing structure according to the present invention, a continuous wall-shaped underground seismic isolation wall made of a clay-based material having water absorption and swelling is installed between the surrounding ground and the structure. It is characterized by doing.

In the present invention, the underground seismic isolation wall is composed of a clay-based material having water-absorbing swellability, and expands in response to soil pressure due to the water-absorbing swellability of the material, so It can resist earth pressure, keep the wall width of underground seismic isolation walls constant, ensure long-term stability, and excel in stability of material density change.
And since clay system material can set rigidity small compared with the surrounding ground, a seismic isolation effect can be exhibited by relieving the deformation of the ground at the time of an earthquake.

Moreover, in the ground displacement absorption seismic isolation structure according to the present invention, the wall width of the underground seismic isolation wall is preferably 0.2 to 2.5 m.
In the present invention, if the wall width of the underground seismic isolation wall is in the range of 0.2 to 2.5 m, the shear force reduction rate (the shear force generated in the underground structure when the underground isolation wall is provided) The value divided by the shear force generated in the underground structure when there is no underground seismic isolation wall) is smaller than 1, which has the effect of reducing the shear force generated in the underground structure. This is because the range of countermeasures (planar area where underground seismic isolation walls are located) is smaller than the conventional polymer method when vertical cylindrical polymer improved soil is placed in the ground. A reduction effect can be obtained and cost reduction can be achieved.

Further, in the ground displacement absorbing base isolation structure according to the present invention, the shear wave velocity ratio of the clay-based material of the surrounding ground and the underground base isolation wall is preferably 0.6 or less.
In this case, the shear rigidity is remarkably smaller than that of the surrounding ground, and it is not necessary to use a soft member.

Moreover, in the ground displacement absorption seismic isolation structure according to the present invention, the clay-based material is preferably a mixture of bentonite and water.
Moreover, in the ground displacement absorption seismic isolation structure according to the present invention, the clay-based material may be a mixture of bentonite, aggregate, and water.
In the present invention, since the water absorption and expansion characteristics of bentonite are fully utilized, the construction is easy even in a ground environment where the groundwater level is high. And even if the groundwater level is low, if the ground is not dry, bentonite will exhibit its water absorption swellability and keep the water retained at the time of construction, so the rigidity does not change due to drying, and the groundwater level Even if it is low, the function is not lost. In addition, bentonite has the property of absorbing and expanding, and even if cracking or some damage occurs, it can self-repair under conditions where groundwater penetrates, protecting the features as a continuum. However, there is an advantage that the wall width of the underground seismic isolation wall does not decrease due to earth pressure from the surrounding ground.

Moreover, in the ground displacement absorption seismic isolation structure according to the present invention, the region filled with a material composed of a mixture of bentonite and water in the viscous earth material is 300 to 1200 kg / m ³ in terms of an effective dry density of bentonite. More preferred.
Moreover, in the ground displacement absorption seismic isolation structure according to the present invention, in a material composed of a mixture of bentonite, aggregate, and water in a viscous earth-based material, the region filled with bentonite and water has a bentonite effective dry density of 300 to More preferably, it is 1200 kg / m ³ .
In the present invention, since the water absorption swelling pressure is 0.03 to 0.3 MPa, it is assumed that the soil underwater mass is about 1 g / cm ^3, and the lateral earth pressure is 1 times the earth covering pressure. It can withstand earth pressure up to 30m. In addition, a more effective underground seismic isolation wall can be provided by appropriately adjusting the bentonite effective dry density of the material according to the depth at which the underground isolation wall is installed.

  According to the ground displacement absorbing base isolation structure of the present invention, the clay-based material constituting the underground seismic isolation wall has water-absorbing swell, can resist normal earth pressure received from the surrounding ground, and the underground seismic isolation wall Since the wall width can be kept constant, long-term stability can be ensured. In addition, it has the effect of reducing stress on underground structures such as open tunnels during an earthquake.

It is the schematic perspective view which showed typically an example of the underground seismic isolation wall of the ground displacement absorption seismic isolation structure by embodiment of this invention. It is a figure which shows the relationship between a bentonite effective dry density and swelling pressure. It is a figure which shows the triaxial test result of the mixture by a bentonite mixing | blending. It is a figure which shows the bentonite repetitive nonlinear characteristic (stress-strain relationship) calculated | required with the dynamic triaxial compression test apparatus. It is a figure which shows the analysis model of the underground seismic isolation wall by an Example. It is a figure which shows the analysis result by an Example, Comprising: It is a figure which shows the relationship between the wall width of an underground seismic isolation wall, and a shear-force reduction rate. It is a figure which shows the analysis result by an Example, Comprising: It is a figure which shows the relationship between the shear wave velocity ratio of a ground, and a shear force reduction rate. It is a figure which shows the analysis result by an Example, Comprising: It is a figure which shows the relationship between the distance from an underground structure, and a shear-force reduction rate.

  Hereinafter, a ground displacement absorption seismic isolation structure according to an embodiment of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, the ground displacement absorption seismic isolation structure 1 according to the present embodiment is for reducing stress during an earthquake that acts on an underground structure 2 such as an open-cut tunnel. The underground structure 2 is a reinforced concrete structure such as a box culvert, and is constructed to extend in a predetermined direction (in the direction of the arrow X shown in FIG. 1) while being embedded in the upper ground 3A. The upper layer ground 3A in which the underground structure 2 is embedded is a soft ground having an elastic wave velocity Vs of, for example, 100 to 200 m / s, and exists on the lower layer ground 3B having an elastic wave velocity Vs of, for example, 300 to 500 m / s. ing.

  Between the underground structure 2 and the upper ground 3A of the surrounding ground, a continuous wall-shaped underground seismic isolation wall 4 made of a clay-based material having water absorption and swelling properties is installed. The underground seismic isolation wall 4 has a predetermined wall width D (see FIG. 1), is arranged in contact with both the left and right sides of the underground structure 2, and is a wall continuous in the extending direction of the underground structure 2. State. The underground seismic isolation wall 4 is disposed in the upper ground 3 </ b> A, and is provided from the ground surface portion to substantially the same depth as the lower end of the underground structure 2. In addition, the underground seismic isolation wall 4 is not limited to being provided over the entire length of the underground structure 2 to be extended, and may be provided partially in the extending direction.

As a clay-based material constituting the underground seismic isolation wall 4, a mixture of bentonite and water (hereinafter referred to as "first mixture") or a mixture of bentonite, aggregate and water (hereinafter referred to as "second mixture"). Is used.
And the area | region filled with the material which consists of a 1st mixture is 300-1200 kg / m < ³ > in a bentonite effective dry density. In the material composed of the second mixture, the region filled with bentonite and water is the bentonite effective dry density (in the case of a material in which bentonite and aggregate are mixed, the density of the bentonite portion that fills the aggregate gap). It is 300 to 1200 kg / m ³ in terms of dry density. In addition, as the aggregate of the second mixture, a soil material such as sand or gravel, or an artificial material such as glass beads that hardly deteriorates for a long time can be employed.
In addition, in the case of the material which does not contain aggregate in the 1st mixture mentioned above, since it is only a bentonite density, it is a bentonite dry density, but here it uses uniformly as "bentonite effective dry density" below.

The wall width D of the underground seismic isolation wall 4 is preferably 0.2 to 2.5 m, and more preferably 0.25 to 1.0 m. The wall width D of the underground seismic isolation wall 4 is preferably 0.5 to 1.0 m because a general construction apparatus can be used.
Such a clay-based material has a characteristic that when a stress is repeatedly applied during an earthquake, it absorbs the earthquake energy by hysteresis damping and plastically deforms, and returns to its original state after the earthquake ends.

Next, the effect | action of the ground displacement absorption seismic isolation structure 1 mentioned above is demonstrated in detail.
As shown in FIG. 1, in the ground displacement absorbing base isolation structure 1, if a clay soil material selected as the underground seismic isolation wall 4 is used, a continuous wall shape can be formed. The reduction effect can be increased. And, by making the clay-based material less than 0.6 times the rigidity of the surrounding upper ground 3A, the effect of reducing the shear force of the underground structure 2 can be obtained, and the deformation of the ground during an earthquake can be mitigated And can exhibit seismic isolation effects.

  Further, when the clay-based material of the underground seismic isolation wall 4 is a first mixture of bentonite and water, or a second mixture of bentonite, aggregate, and water, by adjusting the bentonite effective dry density, Since the swelling pressure can be exerted, it is possible to secure a reaction force that resists the normal earth pressure received from the surrounding ground.

And in the case of a 1st mixture, since the area | region filled with bentonite and water is the range of 300-1200 kg / m < ³ > in the value of a bentonite effective dry density, if it is this density range, as shown in FIG. The water absorption swelling pressure is 0.03 to 0.3 MPa. Therefore, assuming that the underwater mass of the ground (upper ground 3A) is about 1 g / cm ³ , the soil pressure up to a depth of 30 m can be withstood if the lateral earth pressure is 1 times the earth covering pressure. According to the depth which the underground seismic isolation wall 4 installs, it can do more effectively by adjusting and adjusting the density of material suitably.
Here, FIG. 2 is described in Non-Patent Document 2 (“Study on Measurement Method of Swelling Pressure of Compacted Bentonite Sample”, 40th Geotechnical Research Conference, July 2005, page 2574). Note that “effective bentonite dry density” in Non-Patent Document 2 is the same as “bentonite effective dry density”.

Similarly, the shear stiffness of bentonite also has different characteristics depending on the bentonite effective dry density. This is because, when the proportion of the aggregate volume in the material is 50% or less, the aggregate particles do not form a particle structure in which the aggregate particles contact each other and transmit stress to each other. This is because a bentonite gel (a mixture of bentonite and water) is interposed between them, so that the shear characteristics of the material are mainly determined by the characteristics of the bentonite gel.
Therefore, by adjusting the bentonite effective dry density, the shear stiffness of the contact material can be made smaller than the surrounding ground, absorbing the ground deformation by repeated deformation at the time of earthquake, reducing the adverse effect on the frame, The effect of absorbing external force can be expected.

Moreover, since it is characterized by fully utilizing the water absorption and expansion characteristics of bentonite, the construction is easy even in a ground environment where the groundwater level is high. That is, if the surrounding ground is not completely dry, the water retention capacity of bentonite is maintained, so that the wall width D and the material density can be maintained for a long time.
Further, since the clay-based material that is an inorganic natural mineral is used as the underground seismic isolation wall 4, there is no need to worry about the environmental impact on the surroundings.

As shown in FIG. 3, it can be seen that the material of the underground seismic isolation wall 4 is considerably less rigid than the results of Toyoura sand.
The non-linear characteristics shown in FIG. 4 indicate the stress-strain relationship obtained by the dynamic triaxial compression test apparatus for the material of the bentonite compound 3 (ρd = 0.7 Mg / m ³ ) shown in FIG. Since hysteresis is drawn (during repeated shearing), it is suitable as a hysteresis damping material (damper material) by energy absorption. This hysteresis damping effect can be increased by mixing sand into bentonite.
Therefore, the constraint pressure dependency is not as great as that of the ground material, and the bentonite effective dry density can be appropriately adjusted.

  Thus, in this ground displacement absorption seismic isolation structure 1, since the water absorption expansion characteristic of bentonite is fully utilized, construction is easy even in an environment where the groundwater level is high. Even if the groundwater level is low, if the ground is not dry, bentonite will exhibit its water absorption swellability and keep the water retained at the time of construction. Even if it is low, the function is not lost.

In addition, bentonite has the property of absorbing and expanding, and even if cracking or some damage occurs, the damage can be self-repaired under the condition that groundwater penetrates. That is, there is an advantage that the wall width of the underground seismic isolation wall 4 is not reduced by earth pressure from the surrounding ground.
Furthermore, since the clay-based material constituting the underground seismic isolation wall 4 is a natural inorganic mineral material, there is no change in quality, and the water retention state is hardly changed, so that the maintenance is unnecessary.

  Moreover, if the wall width D of the underground seismic isolation wall 4 is in the range of 0.2 to 2.5 m, the shear force reduction rate (the shear generated in the underground structure 2 when the underground seismic isolation wall 4 is provided) The value obtained by dividing the force by the shearing force generated in the underground structure when there is no underground seismic isolation wall 4 is smaller than 1, and the shearing force of the underground structure 2 is reduced. This is because the countermeasure area (planar area where the underground seismic isolation wall 4 is arranged) is smaller than the conventional polymer construction method when the vertical cylinder-shaped polymer improved soil is placed in the ground and installed. A great reduction effect can be obtained, and the cost can be reduced.

When the underground structure 2 is newly constructed, the underground seismic isolation wall 4 is constructed so as to be in contact therewith. In such a case, a construction space necessary around the underground structure 2 is constructed. Can be reduced.
Moreover, although it is assumed that the underground seismic isolation wall 4 is constructed later in the vicinity of the existing underground structure, it is effective even if the wall width D of the underground seismic isolation wall 4 is small. Therefore, it can also be installed as a seismic isolation measure for existing structures in a narrow place where there are adjacent underground structures or inside a narrow site.
Furthermore, even in the seismic reinforcement work for underground structures 2 that do not satisfy the earthquake resistance standards, the use of this ground displacement absorbing base isolation structure 1 makes it difficult for existing structures to be seismically strengthened. It can be used effectively for reinforcement.

As described above, in the ground displacement absorption seismic isolation structure according to the present embodiment, the clay-based material constituting the underground seismic isolation wall 4 has water absorption swelling property, and can resist normal earth pressure received from the surrounding ground, Since the wall width D of the underground seismic isolation wall 4 can be kept constant, long-term stability can be ensured.
In addition, there is an effect that it is possible to reduce the stress on the underground structure 2 such as an open tunnel during an earthquake.

[Example]
Next, examples carried out to support the effects of the ground displacement absorbing base isolation structure according to the above-described embodiment will be described below.

In this example, in order to verify the effect of the underground seismic isolation wall 4, as shown in FIG. 5, the investigation and analysis by dynamic simulation is performed on the underground structure 2 made of box culvert, and the following conditions are changed. The effects were compared. Here, FIG. 5 shows a model used in this study analysis.
The wall width D of the underground seismic isolation wall 4 is 0 to 5 m, the shear rigidity difference between the upper ground 3A and the underground isolation wall 4 is 0.1 to 0.6 in terms of elastic wave velocity Vs, The distance between the structure 2 and the underground seismic isolation wall 4 was set to 0 to 2 m.

6 to 8 show the results obtained from the study analysis using the “Tokai / Tonankai Earthquake Predicted Earthquake” as the input seismic wave.
FIG. 6 shows the wall width and shear force reduction rate of the underground seismic isolation wall 4 made of a clay-based material having water absorption swelling property (the shear force generated in the underground structure when the underground isolation wall is provided). The value is divided by the shear force generated in the underground structure when there is no underground seismic isolation wall, and is used as an index of the seismic isolation effect.

As shown in FIG. 6, the shear force reduction rate is effective when the value is smaller than 1.0.
Therefore, if the wall width of the underground seismic isolation wall 4 is preferably in the range of 0.2 to 2.5 m, there is an effect of reducing the shearing force of the underground structure 2. In addition, the 0.2 m underground seismic isolation wall 4 has an effect of reducing shearing force by about 20%, and a conventional polymer in the case where a vertical cylindrical polymer improved soil is disposed in the ground. Greater reduction effect can be obtained with a countermeasure range smaller than the construction method.
Here, when the wall width of the underground seismic isolation wall 4 becomes too large, the shear force reduction rate becomes larger than 1, and the effect of reducing the shear force of the underground structure 2 is lost (inverse effect). This is presumably because the underground seismic isolation wall 4 and the frame of the underground structure 2 begin to swing independently, and the inertial force of the vibration of the underground structure 2 itself cannot be suppressed.

Next, FIG. 7 shows the result of trial calculation of the shear force reduction rate by dynamic simulation when the ratio of the shear wave velocity Vs of the clay-based material of the surrounding ground and the underground seismic isolation wall is different. The wall width was set to two cases of 0.5 m and 1.0 m.
In this analysis result, the stress reduction effect of the underground structure 2 is obtained at a shear wave velocity ratio of 0.6 or less. That is, it can be confirmed that it is not necessary to use a material that is significantly softer (small shear rigidity) than the surrounding ground, and this makes it difficult to design the material.

  FIG. 8 shows the analysis result regarding the installation position of the seismic isolation wall. The shearing force is approximately 40% under the condition that the seismic isolation wall is in contact with the underground structure 2, that is, when the separation distance from the underground structure 2 is zero. Although there is a reduction effect, it has been confirmed that a reduction effect of about 10% can be obtained even in a case 1 m away.

  From these analysis results, it has been demonstrated that it contributes to reducing the stress of underground structures such as open-cut tunnels and box culverts during earthquakes, and exhibits a stress reduction effect equivalent to or better than the conventional polymer method. Therefore, it can be said that the applicability to existing structures is greater.

  As mentioned above, although embodiment of the ground displacement absorption seismic isolation structure by this invention was described, this invention is not limited to said embodiment, In the range which does not deviate from the meaning, it can change suitably.

1 Ground displacement absorption seismic isolation structure 2 Underground structure 3A Upper ground 3B Lower ground 4 Underground seismic isolation wall D Wall width of underground seismic isolation wall

Claims (7)

  A ground displacement absorbing base isolation structure characterized in that a continuous wall-shaped underground base isolation wall made of clay-based material having water absorption and swelling properties is installed between the surrounding ground and the structure.   The ground displacement absorbing base isolation structure according to claim 1, wherein a wall width of the underground base isolation wall is 0.2 to 2.5 m.   The ground displacement absorbing base isolation structure according to claim 1, wherein a shear wave velocity ratio of the clay-based material of the surrounding ground and the underground base isolation wall is 0.6 or less.   The ground displacement absorbing seismic isolation structure according to any one of claims 1 to 3, wherein the clay-based material is a mixture of bentonite and water. 5. The ground displacement absorption resistance according to claim 4, wherein a region filled with a material composed of a mixture of bentonite and water in the clay-based material is bentonite effective dry density of 300 to 1200 kg / m ^3. Seismic structure.   The ground displacement absorption seismic isolation structure according to any one of claims 1 to 3, wherein the clay-based material is a mixture of bentonite, aggregate, and water. In the material composed of a mixture of bentonite, aggregate, and water in the clay-based material, a region filled with bentonite and water is 300 to 1200 kg / m ³ in terms of bentonite effective dry density. The ground displacement absorption seismic isolation structure according to 6.

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