JP6691751B2 - Liquefaction seismic isolation structure - Google Patents

Description

  The present invention relates to a liquefaction seismic isolation structure.

  Conventionally, as a basic structure of a structure when constructing the structure on soft ground, in order to prevent excessive subsidence due to insufficient bearing capacity of the ground, for example, as shown in Patent Document 1 below, concrete, steel pipe, etc. It is known that the foundation pile of (1) has a structure in which the load of the structure is supported by a supporting ground composed of hard ground below the soft ground.

JP, 10-46619, A

However, when building a building on soft ground, as mentioned above, construction is performed to increase the bearing capacity by driving foundation piles up to a solid support ground to form a foundation structure that supports the building. .. When the foundation pile is driven until reaching the support ground, if the support ground is deep, there is a problem that the pile length of the foundation pile becomes long, the placing becomes large, and the member cost also increases.
Furthermore, a method of reducing the load of the structure to reduce the ground contact pressure against the ground is also used, but in order to reduce the load, there is a limit to the reduction of the ground contact pressure due to the restriction of the structural members, It was not possible to secure sufficient stability, and there was room for improvement in that respect.

  The present invention has been made in view of the above-mentioned problems, and by low-cost and simple construction, it is possible to exert a well-balanced effect of seismic isolation due to liquefaction and the effect of suppressing subsidence and differential subsidence. The purpose is to provide a liquefaction seismic isolation structure that can ensure structural stability.

In order to achieve the above object, the liquefaction seismic isolation structure according to the present invention is a liquefaction seismic isolation structure used as a seismic isolation base structure for a structure, and the structure is provided above the liquefaction layer, In the liquefied layer in the lower region of the structure, a ground improvement body is formed over the entire vertical direction, and the ground improvement body is made by infiltrating a special silica-based agent into sand, and the liquefaction is used. It is provided with the height of the entire layer thickness from the upper surface to the lower surface of the layer, and exhibits a behavior that combines the repeated characteristics of the liquefaction layer and the repeated softening characteristics of the ground improvement body during an earthquake. I am trying.

In the present invention, when the liquefied layer is liquefied, the rigidity is reduced and the strain is increased to be a liquid-like behavior, so that the seismic isolation function of the structure can be exhibited. On the other hand, the soil improvement body provided in the liquefaction layer should have the function of seismic isolation because the shear rigidity decreases and softens during repeated shearing due to the occurrence of earthquakes above the set level. You can That is, in the liquefaction seismic isolation structure of the present invention, the seismic isolation effect can be exhibited by the liquefaction of the liquefaction layer and the softening of the ground improvement body.
Moreover, after the liquefaction, the liquefaction layer sinks, but the soil improvement body formed in the liquefaction layer undergoes almost no volume change after repeated shearing, and vertical deformation of the soil improvement body is suppressed. Since the ground improvement body supports the structure from below even when the liquefaction layer is liquefied, it is possible to suppress the subsidence and the non-uniform settling of the structure.
Thus, in the present invention, when the liquefied layer is liquefied, it is possible to exert an excellent seismic isolation effect while suppressing subsidence and differential subsidence by low-cost and simple construction.

  Further, the liquefaction seismic isolation structure according to the present invention, on the support ground, the structure is provided via the liquefaction layer, the side wall is provided around the entire periphery of the structure at intervals. It is preferable that the liquefaction layer in which saturated sand set in a saturated state is cast is formed in a region surrounded by the support ground and the side wall.

In this case, the liquefaction layer is provided in the region surrounded by the support ground and the side wall, and buoyancy acts on the structure, so that the ground pressure of the structure becomes small and liquefaction easily occurs. Then, after the liquefaction, the saturated sand sinks, but since the structure is supported by the ground improvement body formed below the structure, it is possible to suppress the settlement or the differential settlement of the structure.
Further, even after the liquefaction layer is liquefied, the saturated sand in the liquefaction layer does not flow out of the region surrounded by the sidewall and the support ground and move. Therefore, no gap is formed below the structure, and structural stability can be continued.

  Further, in the liquefaction seismic isolation structure according to the present invention, it is preferable that the ground improvement body is provided in a part of a lower region of the structure.

  In the present invention, by providing the ground improvement body partially in the lower region of the structure, the behavior that combines the repeated characteristics due to the liquefaction layer and the repeated softening characteristics due to the ground improvement body is remarkably exhibited, and excellent seismic isolation is provided. It can be effective.

  According to the liquefaction seismic isolation structure of the present invention, it is possible to exert a well-balanced seismic isolation effect due to liquefaction and the effect of suppressing subsidence or differential subsidence by low cost and simple construction, and further structural stability It is possible to secure the sex.

It is a longitudinal section showing the liquefaction seismic isolation structure according to the first embodiment of the present invention. FIG. 2 is a cross-sectional view taken along the line AA shown in FIG. 1, and is a horizontal cross-sectional view showing an arrangement state of a plurality of columnar improving portions. It is an example showing the effect of the liquefaction seismic isolation structure, and is a diagram showing the relationship between the stress and the strain of the saturated sand. It is an example which shows the effect of a liquefaction seismic isolation structure, and is a figure which shows the relationship of the stress of a ground improvement body by chemical injection and strain. It is a longitudinal section showing a liquefaction seismic isolation structure according to a second embodiment. FIG. 6 is a cross-sectional view taken along line BB shown in FIG. 5, and is a horizontal cross-sectional view showing an arrangement state of a plurality of columnar improving portions. FIG. 9 is a horizontal cross-sectional view showing an arrangement state of a ground improvement body according to a first modification, and is a view corresponding to FIG. 2. It is a horizontal cross-sectional view which shows the arrangement state of the ground improvement body by a 2nd modification, Comprising: It is a figure corresponding to FIG. FIG. 9 is a horizontal cross-sectional view showing an arrangement state of a ground improvement body according to a third modified example, and is a view corresponding to FIG. 2.

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

(First embodiment)
As shown in FIGS. 1 and 2, the liquefaction seismic isolation structure 1 according to the first embodiment is used as a seismic isolation foundation structure for a structure 10, and the foundation portion of the structure 10 is directly constructed in the ground. It applies when you do. The structure 10 is an office, a residence, or the like, and is not limited to the size, shape, application, etc. of the structure on the foundation.

The ground G of the present embodiment is composed of four layers of a first non-liquefied layer G1, a saturated sand layer G2 (liquefied layer), a second non-liquefied layer G3, and a support layer G4 from the upper layer toward the lower layer. Is intended for. The base portion of the structure 10 is arranged and supported in the first non-liquefied layer G1.
Here, as the saturated sand layer G2, for example, silica sand or the like is assumed to be in a saturated state in which the degree of saturation is 95% or more.

  In the area below the structure 10 (two-dot chain line shown in FIG. 2) in the saturated sand layer G2, the ground improvement body 2 in which the ground is improved by chemical solution injection is formed over the entire layer thickness range of the saturated sand layer G2. .. The ground improvement body 2 is formed in a prismatic shape or a cylindrical shape as indicated by reference numeral 2A, and a plurality of the columnar improvement portions 2A are provided at intervals in the vertical and horizontal directions in the plan view shown in FIG. The configuration of the horizontal cross-section of these columnar improvement portions 2A, such as the size and spacing, can be set as appropriate. Further, the dimension (height dimension) in the depth direction of the columnar improved portion 2A is preferably the entire layer thickness from the upper layer surface G2a to the lower layer surface G2b of the saturated sand layer G2 as described above.

  In the present embodiment, the lower surface 10a of the structure 10 and the lower surface 10a are located in the first non-liquefied layer G1 and are vertically separated from the ground improvement body 2 (2A). There is no limitation. That is, the lower surface 10a of the structure 10 may reach the saturated sand layer G2 and may be provided in contact with the upper surface of the ground improvement body 2 (2A). The second non-liquefied layer G3 may be omitted.

Next, the operation of the liquefaction seismic isolation structure having the above-described configuration will be described with reference to FIGS. 1 and 2.
In the present embodiment, when the saturated sand layer G2 is liquefied, as shown in FIG. 3, the rigidity decreases and the behavior becomes large while drawing an inverted S shape. Then, in the middle of this behavior, the behavior becomes like a liquid in which the strain increases in a state where the shearing force is almost zero, so that the seismic isolation function of the structure 10 can be exhibited. FIG. 3 shows data with a constraint pressure of 100 kPa in relation to the stress and strain of saturated sand with a relative density of 63%. The horizontal axis represents shear strain γ (%) and the vertical axis represents shear stress (kPa). There is.

On the other hand, the ground improvement body 2 provided in the saturated sand layer G2 does not liquefy, but as shown in FIG. 4, the shear rigidity decreases and softens during repeated shearing due to the occurrence of an earthquake of a set level or higher. By drawing a non-linear loop, the structure 10 can have a seismic isolation function. FIG. 4 shows the relationship between the stress and strain of the ground improvement body by the chemical solution injection, where the horizontal axis represents the axial strain γ (%) and the vertical axis represents the shear stress (kPa). As the ground improvement body in this case, a ground improvement body in which 4% of a special silica-based agent is infiltrated in Toyoura sand having a relative density _Dr of 65% is used, and the binding pressure is 100 kPa.
As described above, in the liquefaction seismic isolation structure 1 of the present embodiment, in the case of a medium- or large-scale earthquake in which a seismic motion of a predetermined level or more is input, the saturated sand layer G2 is liquefied and the ground improvement body 2 is With softening, seismic isolation effect can be exhibited.

  Moreover, after the liquefaction, the saturated sand layer G2 sinks, but the soil improvement body 2 formed in the saturated sand layer G2 undergoes almost no volume change after repeated shearing, and vertical deformation of the soil improvement body 2 is suppressed. Therefore, even if the saturated sand layer G2 is liquefied by the ground improvement body 2, the structure 10 is supported from below, so that the subsidence and the heterogeneous subsidence of the structure 10 can be suppressed.

  Further, in the present embodiment, the columnar improvement portion 2A is partially provided in the lower region of the structure 10, so that the behavior that combines the repeated characteristics of the saturated sand layer G2 and the repeated softening characteristics of the ground improvement body 2 (that is, , The hysteresis characteristics shown in FIGS. 3 and 4 are remarkably exhibited, and an excellent seismic isolation effect can be exhibited.

  Thus, according to the liquefaction seismic isolation structure of the present embodiment, by low cost and simple construction, seismic isolation effect due to liquefaction and the effect of suppressing subsidence or differential subsidence can be exerted in a balanced manner, Furthermore, structural stability can be secured.

(Second embodiment)
As shown in FIG. 5 and FIG. 6, the liquefaction seismic isolation structure 1A according to the second embodiment has a space around the support layer 6 (support ground) and the structure 10 supported on the support layer 6. A side wall 3 which is opened over the entire circumference and a saturated sand layer 4 (liquefied layer) in which saturated sand is cast in a region surrounded by the support layer 6 and the side wall 3 (hereinafter referred to as a pit portion 5) are provided. I have it.
Here, on the upper layer of the support layer 6, the soft ground G is provided.

The solid ground forming the support layer 6 is a solid ground such as hard clay, rock, dense sand, chemically improved ground, or the like.
The structure 10 is arranged such that a part of the saturated sand layer 4 is interposed between the structure 10 and the upper surface 6 a of the support layer 6.

The side wall 3 has a water blocking function such as a sheet pile, a soil cement wall, a reinforced concrete (RC) wall or the like piled up to reach the support layer 6 for the soft ground G, or a retaining wall. To be done. The type, configuration, and strength of the side wall 3 are appropriately set according to the load and shape of the structure 10, the ground conditions of the soft ground G, and the like. That is, it is preferable that the structure can support the difference in earth pressure between the ground G and the ground formed of the saturated sand layer 4. Further, the height of the side wall 3 (depth of the pit portion 5) may be arbitrarily set as long as the set amount of the saturated sand layer 4 can be placed in the pit portion 5, and can be arbitrarily set.
The side wall 3 has a function of receiving the earth pressure of the soft ground G and preventing groundwater from flowing into the pit portion 5. Specifically, the pit portion 5 may be provided with a waterproof function such as embedding a waterproof sheet (not shown) as needed.

For the saturated sand layer 4 formed by being cast in the pit portion 5, for example, sand having a uniform particle size such as silica sand can be applied, and the saturation degree is set to 95% or more, and the relative density is set. 20-50% of saturated sand (sand without positive dilatancy) is used.
The side wall 3 is stable because the saturated sand layer 4 always maintains a solid state. At any time, the structure can be expected to increase the supporting force due to the floating effect of the structure 10. Also, in the event of an earthquake, it easily liquefies into a liquid with a specific gravity of about 2.0. As a result, the buoyancy is larger than that of water, and the saturated sand layer 4 immediately below the structure is easily liquefied, and the seismic isolation effect can be enhanced.

  In the area below the structure 10 (two-dot chain line shown in FIG. 6) in the saturated sand layer 4, the ground improvement body 2 in which the ground is improved by chemical solution injection is formed over the entire layer thickness range of the saturated sand layer 4. .. The ground improvement body 2 is formed in a prismatic shape or a columnar shape as indicated by reference numeral 2A, and a plurality of the columnar improvement portions 2A are provided at intervals in the vertical and horizontal directions in a plan view as in the first embodiment described above. ing. The configuration of the horizontal cross-section of these columnar improvement portions 2A, such as the size and spacing, can be set as appropriate. The dimension (height dimension) in the depth direction of the columnar improved portion 2A is the entire layer thickness from the lower surface 10a of the structure 10 to the upper surface 6a of the support layer 6.

The height of the saturated sand layer 4 is preferably managed by an appropriate management means after the structure 10 is constructed, and well-known techniques such as control using a sensor can be adopted.
As a method of managing the saturated state of the saturated sand layer 4, a method such as monitoring the dryness of the surface of the sand layer can be adopted.

The height (upper surface position) of the saturated sand layer 4 is set to be equal to the groundwater level or higher than the upper surface 4a of the saturated sand layer 4.
When the upper surface 4a of the saturated sand layer 4 is below the groundwater level, the support layer 6 and the side wall 3 do not need to be waterproof (non-permeable). When the groundwater level is lower than the upper surface 4a of the saturated sand layer 4 and higher than the lower surface of the saturated sand layer 4 (upper surface 6a of the supporting layer 6), the supporting layer 6 does not need water stopping and the side wall 3 does not need to stop. Although it does not need to be water-based, it is preferable to have water-stopping property. Furthermore, when the lower surface of the saturated sand layer 4 (upper surface 6a of the support layer 6) is higher than the groundwater level, it is preferable that both the support layer 6 and the side wall 3 have water stopping properties.

  The distance between the structure 10 and the side wall 3 in the pit 5 in the horizontal direction is determined to be a sufficient size so that the structure 10 and the side wall 3 do not come into contact with each other when the structure 10 and the side wall 3 relatively move in the horizontal direction due to an earthquake. There is.

As described above, in the second embodiment, the saturated sand layer 4 is provided in the pit portion 5 surrounded by the support layer 6 and the side wall 3, and the buoyancy acts on the structure 10. Therefore, the ground pressure of the structure 10 is reduced. Becomes smaller and liquefaction is more likely to occur. Then, after the liquefaction, the saturated sand layer 4 sinks, but since the structure 10 is supported by the ground improvement body 2 formed below the structure 10, the settlement of the structure 10 and the differential settlement are suppressed. be able to.
Further, even after the liquefaction of the saturated sand layer 4, the saturated sand of the saturated sand layer 4 does not flow out and move to the outside of the pit portion 5, and the gap between the lower surface 10 a of the structure 10 and the support layer 6 is prevented. Liquefied sand comes around. Therefore, no gap is formed below the structure 10, the structure 10 is stably supported even after the earthquake ends, and the structural stability can be continued.

  Although the embodiments of the liquefaction-based seismic isolation structure according to the present invention have been described above, the present invention is not limited to the above-mentioned embodiments, and can be appropriately modified without departing from the spirit of the present invention.

  For example, in the present embodiment, as the ground improvement body 2 provided in the liquefaction layer, a plurality of columnar improvement portions 2A are arranged vertically and horizontally in a plan view, but the present invention is not limited to this and other It is also possible to adopt an arrangement pattern. For example, as in the first modified example shown in FIG. 7, in the lower region (two-dot chain line in FIG. 7) of the structure 10, only the outer peripheral portion in the plan view may be the outer peripheral improved portion 2B in which the ground is improved. Further, as in a second modified example shown in FIG. 8, in addition to the outer circumference improved section 2B of the first modified example, a grid improved section 2C in which a ground is improved in a grid may be formed inside thereof. Further, as in the third modified example shown in FIG. 9, in the lower region (two-dot chain line in FIG. 9) of the structure 10, only the corner portion is improved into a L-shaped corner improving portion 2D in plan view. It is also possible to do so. Furthermore, the ground improvement body is not limited to being partially arranged in the lower region of the structure 10 in a plan view, but of course, it may be provided over the entire surface of the lower region although not shown.

  In addition, it is possible to appropriately replace the components in the above-described embodiments with known components without departing from the spirit of the present invention.

1, 1A Liquefaction seismic isolation structure 2 Ground improvement body 3 Side wall 4 Saturated sand layer (liquefaction layer)
5 Pit area 6 Support layer (support ground)
10 Structure G Soft ground G1 First non-liquefied layer G2 Saturated sand layer (liquefied layer)
G3 Second non-liquefied layer G4 Support layer

Claims (3)

A liquefaction base isolation structure used as a base isolation structure for structures,
The structure is provided above the liquefied layer,
The liquefaction layer in the lower region of the structure, a ground improvement body is formed over the entire vertical direction,
The ground improvement body, which is obtained by infiltrating a special silica-based agent into sand is used, and is provided with a height dimension of the entire layer thickness from the upper layer surface to the lower layer surface of the liquefied layer,
A liquefaction seismic isolation structure characterized by exhibiting a behavior that combines the repeated characteristics of the liquefied layer and the repeated softening characteristics of the ground improvement body during an earthquake. On the support ground, the structure is provided via the liquefaction layer,
Side walls are provided around the entire structure at intervals around the structure,
The liquefaction isolation system according to claim 1, wherein the liquefaction layer in which saturated sand set to a saturated state is cast is formed in a region surrounded by the support ground and the side wall. Construction.   The liquefaction seismic isolation structure according to claim 1 or 2, wherein the ground improvement body is provided in a part of a lower region of the structure.

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