JP5822200B2 - Structure for reducing liquefaction damage of structures - Google Patents

Description

  The present invention relates to a liquefaction damage reducing structure intended for a structure constructed on a liquefied ground.

  In the Great East Japan Earthquake, small-scale structures such as detached houses and incidental facilities were seriously damaged due to ground liquefaction. When constructing large-scale structures and important structures on liquefied ground, measures to prevent liquefaction are becoming common, but small-scale structures cannot implement sufficient measures due to cost constraints. In many cases, it was thought that this was one of the causes of the major liquefaction damage.

  As a liquefaction prevention measure proposed so far, for example, the construction methods disclosed in Patent Document 1 and Patent Document 2 are known.

Japanese Patent Laid-Open No. 9-95954 Japanese Unexamined Patent Publication No. 2007-191984

  The conventional liquefaction prevention methods as shown in Patent Document 1 and Patent Document 2 are all aimed at preventing the occurrence of liquefaction itself, and therefore in the implementation, large-scale construction for the entire lower ground of the structure Therefore, it is not realistic to implement it for small-scale structures that have been damaged by the recent earthquake.

  Therefore, the actual situation is that it is desired to develop an effective and appropriate structure that can effectively reduce the liquefaction damage particularly for small-scale structures constructed on the liquefied ground.

鉛直ドレーンの透水係数ｋ _２ （＝水平ドレーンの透水係数）、鉛直ドレーンの断面積Ａ _ｗＶ （設置本数の総和）の値は、液状化後の地盤沈下量Ｄ _ｓ による必要排水量ΔＶ _ｗ 、排水ゾーンからの必要排水量ｑ _ｗ１ 、沈下時間Δｔとすると、下記式を満たすように決定される

ことを特徴とする。 In view of the above circumstances, the present invention is a structure for reducing liquefaction damage of a structure for reducing damage to the structure when the liquefied ground is liquefied for a structure constructed on the liquefied ground. A horizontal drain that horizontally crosses the lower part of the structure in the liquefied ground below the structure, and a vertical drain whose upper end communicates with the ground surface in the liquefied ground around the structure. By connecting both ends of the horizontal drain to the vertical drain, excess horizontal water generated in the liquefied ground when the liquefied ground below the structure is liquefied is removed from the horizontal drain. and wherein Ri a drainable drainage zones to the surface portion through the vertical drain greens formed in liquefaction of soil beneath the structure, the horizontal drain permeability k _2, the horizontal drain hitting set pitch B, the depth D of the horizontal drain strokes set pitch value of the cross-sectional area A _wH horizontal drains, requires wastewater q _w1 from the drainage zone _and to allow wastewater q _w2 by the horizontal drain, is determined so as to satisfy the following equation ,

Permeability coefficient of vertical drain k ₂ (= permeability coefficient of horizontal drain) and vertical drain cross-sectional area A _wV (total number of installations) are required drainage amount ΔV _w due to ground subsidence amount D _s after liquefaction , drainage zone When the required amount of drainage _qw1 from the water and the settlement time Δt are determined, the following equation is satisfied.

It is characterized by that.

  In the present invention, instead of the vertical drain, it is also conceivable to provide a groove-shaped vertical trench whose upper part leads to the ground surface, and to connect both ends of the horizontal drain to the vertical trench.

  According to the present invention, the liquefaction of the liquefied ground itself cannot be completely prevented, but when liquefaction occurs, excess pore water is drained from the drainage zone to the ground surface, and at least in the drainage zone. Therefore, it is possible to prevent or suppress the collapse and excessive deformation of the structure, and to effectively suppress the settlement and inclination of the structure installed there, thereby sufficiently reducing the liquefaction damage.

  In addition, the present invention only requires a horizontal drain directly under the structure and a vertical drain or vertical trench around the structure to form a drainage zone. In order to prevent the occurrence of liquefaction itself, no large-scale construction is required for the entire lower ground of the structure, and therefore it can be easily carried out at low cost. It is most suitable for application to small-scale structures such as existing or newly built detached houses and incidental facilities that were difficult.

It is an elevation sectional view showing an embodiment of the present invention. FIG. It is an elevation sectional view and a principal part enlarged view. It is an elevation sectional view showing other embodiments of the present invention. FIG. It is a figure for demonstrating the principle of this invention. It is a figure which shows the analysis experiment for demonstrating the effectiveness of this invention, and its result (the situation of the raise of pore water pressure). It is a figure which shows an analysis result (situation of the inclination of a structure). It is a system design block diagram for a design same as the above.

Liquefaction of the ground means that the excess pore water pressure in the ground (especially sand ground) suddenly increases due to an earthquake, so conventional countermeasures to prevent liquefaction prevent the sudden increase in excess pore water pressure. However, the structure for reducing liquefaction damage according to the present invention does not directly prevent or suppress such liquefaction of the ground (that is, rapid increase in excess pore water pressure). The main purpose is to control the behavior of the ground by draining excess pore water efficiently after the liquefaction has occurred and after the liquefaction has ended.
That is, in the present invention, when liquefaction occurs, excess pore water is drained from the lower ground of the structure through the drainage zone, thereby suppressing the ground collapse and excessive deformation there, and stable “post-liquefaction state ( This will be described in detail later)), and the liquefaction damage is reduced by suppressing the subsidence and inclination of the structure on the ground. This is in principle different from various conventional liquefaction countermeasure methods and liquefaction prevention structures, which are mainly aimed at preventing and suppressing the liquefaction.

  1 to 3 show a specific embodiment of the present invention. This is intended for the liquefied ground 2 where the existing small-scale and lightweight structure 1 is installed. When the liquefied ground 2 is liquefied at the time of an earthquake, at least the lower ground of the structure 1 collapses. The behavior is controlled so as to prevent or suppress excessive deformation, thereby suppressing the settlement and inclination of the structure 1 to reduce liquefaction damage.

Specifically, in the liquefied ground 2 below the structure 1, a horizontal drain 3 that horizontally traverses the lower part of the structure 1 is provided, and an upper end portion is provided in the liquefied ground 2 around the structure 1. A drainage zone 5 is formed immediately below the structure 1 by providing a vertical drain 4 that communicates with the ground surface and connecting both ends of the horizontal drain 3 to the vertical drain 4.
In the illustrated example, four vertical drains 4 are provided on both sides of the structure 1 at intervals, and the horizontal drain 3 is interposed between a pair of vertical drains 4 that are opposed to each other across the structure 1. Are provided in two stages, so that the drainage zone 5 is formed by all the eight vertical drains 4 and all the eight horizontal drains 3.

The planar size of the drainage zone 5 may be slightly larger than the planar shape of the structure 1 within a range in which the vertical drain 4 can be installed around the structure 1, and the depth H of the drainage zone 5 is 2 below the groundwater level. What is necessary is just to set so that it may be set to about ~ 4m.
The structure of the horizontal drain 3 and the vertical drain 4 may be appropriate, but a hole is formed in the liquefied ground 2 by boring and a water-permeable material such as gravel is filled therein, and clogging is caused by fine particles such as sand. It is realistic to have a structure that prevents this.
When constructing the horizontal drain 3, it is only necessary to form a substantially horizontal drilling hole below the structure 1 by performing control boring capable of controlling the drilling direction from the ground surface. It can be easily constructed and can also be constructed on the lower ground of the existing structure 1 without hindrance.
Further, instead of installing a plurality of vertical drains 4 at predetermined intervals as described above, a series of groove-like vertical trenches 6 are respectively installed on both sides of the structure 1 as shown in FIGS. It is also possible to connect the tip of the horizontal drain 3 to this, and this also functions in the same manner as in the above embodiment and the same effect is obtained.

  By forming the drainage zone 5 in the lower ground of the structure 1 in this way, when the liquefied ground 2 below the structure 1 is liquefied, excess pore water generated in the drainage zone 5 is shown in the figure. As indicated by the arrows, the water is quickly drained to the surface through the horizontal drain 3 and the vertical drain 4, and therefore, at least in the drainage zone 5, it is possible to prevent or suppress the collapse or significant deformation of the liquefied ground 2. Therefore, the settlement and inclination of the structure 1 supported by the structure 1 can be sufficiently suppressed, and the liquefaction damage to the structure can be effectively reduced.

In addition, as a conventional general liquefaction countermeasure method, a method of draining groundwater from the entire liquefied ground that is the original ground to prevent the occurrence of liquefaction itself is known, but in that case, from the entire original ground It is necessary to drain a large amount of groundwater permanently. To that end, it is necessary to construct a large-scale drainage facility for the entire raw ground, and large-scale construction using large heavy machinery is indispensable.
On the other hand, the present invention is different in principle from the conventional liquefaction countermeasure construction method as will be described in detail below, and it eliminates the need for positive drainage from the entire raw ground, and the foundation of the structure 1 during liquefaction. It is sufficient to promote drainage from a limited area around it. For that purpose, a sufficient drainage zone 5 can be obtained just by providing a small drainage zone 5 around the foundation as in the above embodiment. The small drainage zone 5 provided for this purpose can be constructed simply and inexpensively by simply constructing the horizontal drain 3 and the vertical drain 4 (or the vertical trench 6), and large heavy machinery is also required for the construction. Therefore, the present invention can be applied to small structures such as houses without any problem, and is particularly effective as a liquefaction prevention measure for such small structures.

The basic principle of the present invention, its superiority, and a specific design method will be described in detail with reference to FIGS.
In the design method for normal liquefaction, the state where the excess pore water pressure ratio of the ground has reached 1 is not considered as the state after this as a completely liquefied state (a state in which it has become liquid). Then, pay attention to the fact that the rigidity is restored by shear deformation after the excess pore water pressure ratio reaches 1 (hereinafter referred to as “post-liquefaction state”), and the post-liquefaction state is continued stably. Therefore, it is based on the design philosophy of maintaining and securing the supporting force for the structure.
That is, as shown in FIG. 6, if a shear force continues to act on the ground that has reached the post-liquefaction state without draining, the reversible due to the directivity is applied to the irreversible plastic volume strain (compression side). Plastic volume strain (expansion side) cannot catch up, and the ground becomes completely liquid. This state is the state that reached the destruction of the ground where the undesired subsidence of the sand and structures.
On the other hand, if the above shear force is applied while draining properly, the irreversible plastic volume strain compression side) and the reversible plastic volume strain (expansion side) are always balanced, and the post-liquefaction state continues stably. In the present invention, by maintaining such a stable post-liquefaction state, the supporting force of the structure is secured, and liquefaction damage such as settlement or inclination of the structure is reduced.

Analysis experiments and results thereof for demonstrating the effectiveness of the present invention are shown in FIGS.
As shown in FIG. 7, a drainage layer a (corresponding to the drainage zone 5 in the above embodiment) is formed on the surface layer of a liquefied layer having a relative density of 35%, and a simulation foundation b (same structure) is formed thereon. The water pressure was measured at 4 points (PP1 to PP4) just below the drainage layer a and 2 points (PP5 to PP6) on the side. The drainage layer a was a gravel layer made of gravel material, and the layer thickness was two patterns of 0.3 m and 0.6 m.
As a result, the increase in pore water pressure at the same level as the surrounding ground was observed at the deep measurement points PP1 (depth -4.95m) and PP2 (depth -2.85m), but at the shallow measurement point pp3 (depth -1.35). In m) and PP4 to PP6 (depth -0.6m), it was confirmed that the increase in pore water pressure was sufficiently suppressed compared with the surrounding ground, and there was substantially no liquefaction there.

FIG. 8 shows the measurement result of the inclination of the simulation foundation b in the case of the above analysis experiment. In the case of no measures (without drainage layer a), a large slope of up to 0.2 rad is generated in the simulated foundation b, but by providing the drainage layer a, the maximum slope is greatly suppressed to about 0.02 rad. The effectiveness of providing the drainage layer a (drainage zone 5 in the above embodiment) was demonstrated.
Moreover, such an effect does not depend much on the layer thickness of the drainage layer a, and as shown in FIG. Even if it is changed within the range of 1/8 to 1/2 of B, and the extension width dimension around the simulated foundation b is changed within the range of 30 cm to 1.2 m as shown in (b) It was confirmed that almost the same effect was obtained. Therefore, it is sufficient that the layer thickness of the drainage zone 5 in the above-described embodiment is at least 1/8 or more of the width dimension B in the short side direction of the foundation of the structure 1 and extends around the structure 1. A width dimension of at least 30 cm is sufficient.

  By the way, in the present invention based on the above principle, it is necessary to reliably drain excess pore water from the drainage zone 5 at the time of occurrence of liquefaction and thereafter, and for this purpose, the horizontal drain 3 is disposed in the drainage zone 5. Since it is necessary to install the vertical drain 4 appropriately, a specific design method for that purpose will be described below.

"Horizontal drain design"
The installation interval of the horizontal drain 3 is determined as follows.
As shown in FIG. 3 (b), when the water collection range of one horizontal drain 3 is B × D, the required drainage amount q _w1 from the drainage zone 5 and the possible drainage amount q _w2 from the horizontal drain 3 are

Therefore, the water collection condition of horizontal drain 3 is

Thus, the values of k ₂ , B, D, and A _WH are determined so as to satisfy the above formula.

"Vertical drain design"
The vertical drain 4 is designed to have the ability to drain pore water generated by land subsidence due to liquefaction from the drainage zone 5 to the ground surface. The required drainage amount ΔV _w by the ground subsidence amount D _s after liquefaction and the drainage zone Calculate the required amount of drainage q _w1 from 5 by the following formula.
Here, the subsidence time Δt is about one day, and the ground subsidence amount D _s may be calculated by a well-known calculation method (a method for calculating the ground subsidence amount D _s will be described later).
Here, the length L in one direction of the drainage zone 5 is the distance between the vertical drains 4 on both sides as shown in FIG. 2, and the length in the other direction is equal to that.

On the other hand, the possible drainage amount q _w2 from the vertical drain 4 is calculated by the following equation.
Here, assuming that the water pressure acting on the bottom surface of the drainage zone 5 is equal to the total stress acting on the surface at the depth H, the head h _w1 at the lower end of the vertical drain 4 is calculated by the following formula, and the water pressure at the upper end of the vertical drain 4 is calculated. Water head h _w2 = 0. Furthermore, permeability coefficient k ₂ of the vertical drain 4 is equal to the permeability coefficient k ₂ of the horizontal drain 3.

The possible drainage amount q _{w2 from} the vertical drain 4 determined above is

K ₂ and A _wv are determined so as to satisfy the conditions, and at the same time, each specification is finally determined so as to satisfy the conditions of the above expressions (1), (2), and (4).

"Calculation of land subsidence"
Calculation of ground subsidence amount D _s described above, it is preferred according to the following procedure.
For example, the method shown in Japanese Patent No. 4640671 and the method shown in Japanese Patent Application Laid-Open No. 2007-9558 can be adopted for predicting the amount of settlement, but here the system configuration block diagram is shown in FIG. Show.

That is, first, earthquake, structure, and ground motion data are input from the data input unit, and an analysis model is constructed. Next, the initial stress state of the ground is obtained by an analysis in which the weight of the ground and the structure is statically applied. The initial stress analysis can be obtained by an analysis method such as a finite element method.
Next, using the same model as the initial stress analysis, the seismic response analysis is performed for the assumed ground motion by a method that considers liquefaction of the ground, such as effective stress analysis.

From the value of the maximum shear strain γ _max of each panel element obtained as a result of the seismic response analysis, the values of the residual volume strain ε _vp and the residual shear strain γ _p after liquefaction are obtained by the following equations.

Here, e ₀ is an initial gap ratio, and e ^* _min is a true minimum gap ratio, which is expressed by the following equation.

In the above equation, e _max and e _min are the maximum and minimum gap ratios obtained from the normal maximum and minimum density tests, respectively. R ₀ ^* and m are inherent constants independent of the type and density of sand, and R ₀ ^* = 2.0 and m = 0.76. M _cs.0 is the inclination of the limit state surface near the effective _confining pressure 0. C _h is a parameter indicating the irreversible volumetric strain potential contributing percentage residual volumetric strain and residual shear strain caused by seismic response at liquefaction, it is hardly ground inclination of the ground surface is about 0.2.

The residual volume strain and residual shear strain after liquefaction of the ground elements are determined independently for each ground element using the formulas (9) and (10) from the value of the maximum shear strain during the earthquake. It does not necessarily meet the conforming condition of displacement. Therefore, the equivalent elastic modulus of the ground after liquefaction is obtained from the average restraint pressure σ _m of the ground element obtained by static weight analysis and the horizontal shear stress τ _xy using the following equation based on elasticity theory.

In the above equation, G _eq , K _eq , ν _eq , and E _eq are equivalent shear modulus, bulk modulus, Poisson's ratio, and Young's modulus, respectively.

The self-weight analysis is performed again using these elastic constants, and the deformation of the ground is calculated. As a result, the self-weight analysis is repeatedly performed by changing the ground stress and the equivalent elastic constant value until the obtained volume strain and shear strain values converge to the values of equations (9) and (10). The final deformation amount obtained as a result of the convergence calculation is the residual deformation after liquefaction of the ground, that is, the ground subsidence amount D _s .

  According to the present invention, liquefaction of the liquefied ground 2 itself cannot be completely prevented, but when liquefaction occurs, excess pore water is drained from the drainage zone 5 and at least the area of the drainage zone 5 In this case, the collapse or excessive deformation of the ground is effectively prevented or suppressed, and therefore the structure 1 supported on the upper part is effectively suppressed from sinking or tilting to sufficiently reduce liquefaction damage. Is possible.

That is, the subsidence or inclination of the structure 1 due to liquefaction does not occur suddenly during an earthquake, but usually progresses slowly over time after the earthquake. Therefore, even if liquefaction occurs in the original ground, the structure If the liquefaction around the foundation of 1 is terminated early and stabilized early, in other words, by maintaining the state in which the “post-liquefaction state” continues stably as described above, It is possible to naturally suppress the settlement of the structure 1 and the progress of the inclination.
And, for that purpose, when liquefaction occurs, it is only necessary to install drainage zone 5 for promoting drainage of excess pore water around the foundation of structure 1 and quickly draining appropriately from around the foundation. In the present invention, it is possible to sufficiently reduce the liquefaction damage of the structure 1 only by forming such a simple drainage zone 5 around the foundation.
In addition, after the end of the earthquake, the liquefied layer remains on the entire original ground, including directly under the structure 1. Therefore, the entire original ground slowly sinks after that. Since 1 also sinks with the surrounding ground, there will be no large unevenness between the structure 1 and the surrounding ground, and there will be no hindrance to service after the earthquake.

  In addition, in the present invention, in order to form the drainage zone 5, the horizontal drain 3 is installed so as to cross right under the structure 1 and the vertical drain 4 or the vertical trench 6 is installed around the structure 1. Since it is good, as described above, it does not require large-scale construction to prevent the occurrence of liquefaction itself, unlike the conventional general liquefaction prevention method, and therefore it can be easily carried out at low cost. It is optimal for application to small-scale structures such as existing or newly built detached houses and incidental facilities that have been difficult to sufficiently prevent liquefaction in the past.

  In addition, although it seems that drainage of excess pore water is also possible in the liquefaction countermeasure construction methods shown in Patent Document 1 and Patent Document 2, these prevent the occurrence of liquefaction itself as described above. Therefore, not only is this basically different from the present invention, which is mainly intended to maintain the post-liquefaction state, but in Patent Document 1, excess pore water is collected in a water storage tank provided around the structure. In Japanese Patent Application Laid-Open No. 2004-228620, since the water is only drained directly from the horizontal drain to the ground surface, none of these elements corresponds to the vertical drain 4 or the vertical trench 6 in the present invention. 4 and the vertical trench 6 are unrelated to the technical idea of the present invention in which drainage from the drainage zone 5 is promoted. Therefore, it goes without saying that the liquefaction countermeasure method shown in Patent Document 1 and Patent Document 2 and the liquefaction damage reducing structure of the present invention are completely different technologies.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and appropriate design changes and applications can be made without departing from the gist of the present invention. Of course.
For example, a specific configuration of the drainage zone 5 required to maintain the post-liquefaction state, that is, an installation pattern such as the number of installed horizontal drains 3, the vertical drains 4, and the vertical trenches 6 and the installation interval in the above embodiment, About the specifications such as those materials and hydraulic conductivity, construction method, setting range of drainage zone 5, etc., the situation of the target liquefied ground 2 and the structure, its own weight and form of the structure 1 constructed there In consideration of various conditions such as the above, an optimum design may be made so as to have a desired drainage performance.

1 Structure 2 Liquefaction Ground 3 Horizontal Drain 4 Vertical Drain 5 Drain Zone 6 Vertical Trench

Claims (2)

A structure liquefaction damage reducing structure for reducing damage to the structure when the liquefied ground is liquefied for a structure constructed on the liquefied ground,
In the liquefied ground below the structure, a horizontal drain that horizontally traverses the lower part of the structure is provided, and in the liquefied ground around the structure, a vertical drain whose upper end communicates with the ground surface is provided. By connecting both ends of the horizontal drain to the vertical drain, excess pore water generated in the liquefied ground when the liquefied ground below the structure is liquefied is removed from the horizontal drain and the horizontal drain. Ri Na a drainable drainage zones to the surface portion formed in liquefaction of soil beneath the structure through the vertical drain,
The horizontal drain permeability coefficient k ₂ , horizontal drain placement pitch width B, horizontal drain placement pitch depth D, and horizontal drain cross-sectional area A _{wH depend} on the required drainage volume q _w1 from the drainage zone and the horizontal drain. _Assuming that the possible amount of drainage _qw2 , it is determined to satisfy the following formula,

Permeability coefficient of vertical drain k ₂ (= permeability coefficient of horizontal drain) and vertical drain cross-sectional area A _wV (total number of installations) are required drainage amount ΔV _w due to ground subsidence amount D _s after liquefaction , drainage zone When the required amount of drainage _qw1 from the water and the settlement time Δt are determined, the following equation is satisfied.

Structure for reducing liquefaction damage of structures. In the structure for reducing liquefaction damage of the structure according to claim 1,
In place of the vertical drain, a groove-like vertical trench whose upper part leads to the ground surface is provided, and both ends of the horizontal drain are connected to the vertical trench, and the liquefaction damage of the structure Reduction structure.

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