JP4742331B2 - Liquefaction countermeasure structure - Google Patents

Description

  The present invention relates to a liquefaction countermeasure structure that suppresses liquefaction of a ground in which a liquefied layer is deposited on a non-liquefied layer or suppresses ground deformation due to liquefaction.

  When shear stress is repeatedly applied to the liquefied layer, the liquefied layer becomes liquid and loses the support function as the ground. In order to avoid such a situation, as shown in FIG. 1, a liquefaction countermeasure structure is known in which an entirely grounded ground improvement body 103 is constructed in a liquefied layer 102. The entire bottom-type ground improvement body 103 is bottomed or embedded in the non-liquefied layer 101 and is constructed over the entire vertical direction of the liquefied layer 102 (see, for example, Patent Document 1). However, when the liquefied layer 102 is thick, a large cost is required.

  On the other hand, as shown in FIG. 2, a liquefaction countermeasure structure in which the ground improvement body 203 is constructed so as to float on the liquefaction layer 202 has been proposed in order to reduce costs. This liquefaction countermeasure structure sets the thickness of the ground improvement body 203 so that the subsidence amount of the structure is less than an allowable value on the premise that the unimproved portion below the ground improvement body 203 is liquefied. There is no need for the improvement body 203 to be grounded or embedded in the non-liquefied layer 201, nor is it required to be constructed over the entire vertical direction of the liquefied layer 202. For this reason, even if the liquefied layer 202 is thick, the cost can be reduced (see, for example, Patent Document 2).

Japanese Examined Patent Publication No. 4-54004 JP 2005-83174 A

  However, in the liquefaction countermeasure structure described in Patent Document 2, when an earthquake occurs, the constructed ground improvement body 203 is displaced in the horizontal direction due to the influence of the liquefaction layer 202 outside the liquefaction countermeasure area. Then, the displacement difference between the horizontal displacement of the non-liquefaction layer 201 and the horizontal displacement of the ground improvement body 203 becomes large. And a shear stress acts on the liquefied layer 202 between the non-liquefied layer 201 and the ground improvement body 203, and it liquefies depending on an acceleration level.

  The present invention has been made in view of the above, and suppresses the liquefaction of the ground in which the liquefied layer is deposited on the non-liquefied layer or suppresses the ground deformation due to the liquefaction without incurring great expense. An object is to provide a liquefaction countermeasure structure that can be used.

In order to solve the above-described problems and achieve the object, the present invention applies to the ground where the liquefied layer is deposited on the non-liquefied layer, and is located at a position separated from the non-liquefied layer in the liquefied layer. In the liquefaction countermeasure structure in which the ground improvement body is constructed, the side surface of the ground improvement body with respect to the earthquake direction so that the ground improvement body is settled or embedded in a non-liquefaction layer and the ground improvement body can be relatively displaced in the vertical direction. a contact to secure the opening in a plan view, a structure of the vibration of the non-liquefied layer has a width which reduces the displacement difference in the horizontal direction by transmitting the soil improvement material non liquefaction layer and ground improvement body characterized in that it was constructed.

In the liquefaction countermeasure structure, the present invention is characterized in that the width of the structure is set to 0.4 times or more of the height of the structure in the liquefaction layer.

In the liquefaction countermeasure structure according to the present invention, the open portion is provided on the outer periphery of the remaining liquefied layer left in the lower region of the ground improvement body, and the behavior of the remaining liquefied layer during the assumed earthquake is It has the size transmitted to the side area of the ground improvement body.

In the liquefaction countermeasure structure, the present invention is characterized in that the size of the open portion is ¼ or more of the total extension of the outer periphery of the ground improvement body in plan view.

The liquefaction countermeasure structure according to the present invention is flat on the side surface of the ground improvement body with respect to the earthquake direction so that the ground improvement body is bottomed or embedded in the non-liquefaction layer and the ground improvement body can be relatively displaced in the vertical direction. A structure that has a width that secures an open portion by visual contact and transmits the vibration of the non-liquefiable layer to the ground improvement body to reduce the horizontal displacement difference between the non-liquefaction layer and the ground improvement body was constructed . Therefore, even when an earthquake occurs, the horizontal displacement of the ground improvement body is suppressed, and the horizontal displacement difference between the non-liquefied layer and the ground improvement body can be small. Accordingly, the shear strain of the liquefied layer between the non-liquefied layer and the ground improvement body is suppressed, and the ground deformation can be suppressed even if the ground liquefaction is suppressed or liquefied without incurring a great expense.

FIG. 1 is a diagram showing the concept of the liquefaction countermeasure structure of Patent Document 1. In FIG. FIG. 2 is a diagram showing the concept of the liquefaction countermeasure structure of Patent Document 2. FIG. 3 is a conceptual diagram showing the liquefaction countermeasure structure according to the first embodiment of the present invention. FIG. 4 is a diagram showing what kind of lattice is effective and economical in the lattice wall body. FIG. 5 is a conceptual diagram showing a liquefaction countermeasure structure according to a modification of the first embodiment of the present invention. FIG. 6 is a conceptual diagram showing a liquefaction countermeasure structure according to the second embodiment of the present invention. FIG. 7 is a conceptual diagram showing a liquefaction countermeasure structure according to Embodiment 3 of the present invention. FIG. 8 is a diagram illustrating the relationship between the time of the scenario wave and the acceleration. FIG. 9 is a diagram showing an outline of the centrifugal model experiment. FIG. 10 is a diagram showing the results of a centrifugal model experiment. FIG. 11 is a diagram showing the relationship between the excess pore water pressure ratio of the lower ground and the maximum acceleration due to the step excitation. FIG. 12 is a diagram showing the relationship between the amount of subsidence of the surface ground due to stepwise excitation and the maximum acceleration. FIG. 13 is a diagram illustrating an FEM analysis result obtained when an earthquake wave is applied to the ground of the liquefaction countermeasure structure according to the first embodiment. FIG. 14 is a diagram showing an FEM analysis result obtained when a seismic wave is applied to the ground having a whole floating structure as a comparative control. FIG. 15 is a diagram showing the relationship between the amount of subsidence at the center of the top of the ground improvement body after final excitation by stepwise excitation and the ratio of the thickness of the ground improvement body to the thickness of the liquefied layer. FIG. 16 is a plan view showing another embodiment in which the contact portion between the ground improvement body and the bottom structure is an engagement structure. FIG. 17 is a conceptual diagram showing an analysis model for performing FEM analysis. FIG. 18 is a diagram illustrating an analysis result 20 seconds after the seismic wave is applied to the analysis model illustrated in FIG. FIG. 19 is a diagram showing the relationship between the dimensional ratio of the bottomed structure obtained from the result of the FEM analysis of FIG. 18 and the behavior of the ground improvement body. FIG. 20 is a contour diagram showing the excess pore water pressure ratio obtained from the result of the FEM analysis of FIG. FIG. 21 is a diagram showing the amount of settlement of the liquefied layer at each maximum acceleration in FIG. FIG. 22 is a diagram showing the volume strain of the liquefied layer at each maximum acceleration in FIG.

  Hereinafter, embodiments of a liquefaction prevention structure according to the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

(Embodiment 1)
Based on FIG. 3, the liquefaction countermeasure structure which is Embodiment 1 of this invention is demonstrated. FIG. 3 is a conceptual diagram showing the liquefaction countermeasure structure according to the first embodiment of the present invention. The liquefaction countermeasure structure according to the first embodiment is applied to the ground where the liquefied layer 2 is deposited on the non-liquefied layer 1. The non-liquefiable layer 1 is a hard layer that does not liquefy, for example, and the liquefied layer 2 is a layer that may lose its support function due to liquefaction, like a loose sandy ground.

  Liquefaction means losing the support function as the ground. When the liquefied layer 2 (for example, sand ground saturated with groundwater) is repeatedly subjected to shear stress such as earthquakes, it is liquefied. Note that the excess pore water pressure ratio ru is used as an indicator of liquefaction. The excess pore water pressure ratio ru is a ratio between the excess pore water pressure Δu and the effective earth covering pressure σv ′, and is in a liquefied state when the excess pore water pressure ratio ru is theoretically 1.0.

  As shown in FIG. 3, the liquefaction countermeasure structure according to the first embodiment is a structure that should be referred to as a one-end bottom floating type, and is in contact with one side surface as a part of the side surface of the ground improvement body 3. A bottoming structure 4 as a displacement suppressing structure is constructed.

  The ground improvement body 3 is constructed in a position separated upward from the non-liquefied layer 1 in the liquefied layer 2. In the example shown in FIG. 3, the ground improvement body 3 is constructed so as to float on the liquefied layer 2. The ground improvement body 3 has a rectangular outer shape in plan view, and the direction in which the earthquake direction (vibration direction) (left and right direction in FIG. 3) coincides with the longitudinal direction of the ground improvement body 3 and intersects the earthquake direction. It is constructed so that the short direction of the improved body 3 matches. Further, the ground improvement body 3 is a lattice wall body having a lattice shape in plan view, and an upper portion is opened and a lower portion is also opened. In FIG. 3, the ground improvement body 3 is an economical ground improvement body with a lattice wall body having a lattice shape in plan view, but a ground improvement body of a whole surface improvement type may be used.

  The bottoming structure 4 as a displacement suppressing structure is bottomed or embedded in the non-liquefaction layer 1 and the ground improvement body 3 abuts against the one side surface 4a. The displacement is suppressed. In the example shown in FIG. 3, it is adjacent to the ground improvement body 3 and is constructed over the entire vertical direction of the liquefied layer 2, and the lower end is attached to the non-liquefied layer 1. The bottomed structure 4 has a rectangular outer shape in plan view, and one outer surface in the longitudinal direction is in contact with one outer surface in the short direction of the ground improvement body 3. The bottoming structure 4 is a lattice wall having a lattice shape in plan view, and the upper side is opened and the lower side is closed by bottoming on the non-liquefied layer 1. In the example shown in FIG. 3, the bottoming structure 4 is constructed so that the length in the longitudinal direction of the bottoming structure 4 matches the length in the short direction of the ground improvement body 3. The length in the longitudinal direction of the bottom structure 4 need not match the length in the short direction of the ground improvement body 3. That is, the length in the longitudinal direction of the bottom structure 4 may be shorter than the length in the short direction of the ground improvement body 3 or may be longer than the length in the short direction of the ground improvement body 3. Similarly to the ground improvement body 3, the bottom structure 4 is not limited to a lattice wall body having a lattice shape in plan view.

  It is clear from existing literature (Japanese Patent No. 2568115) what kind of lattice is effective and economical in a lattice wall body in a plan view lattice that constitutes the ground improvement body 3 and the bottom structure 4 described above. Has been. According to this document, as shown in FIG. 4, the ratio (L / H) between the inner frame width L of the lattice and the thickness H of the liquefied layer is required to be 0.8 or less, and L / H A range of 0.5 to 0.8 is considered appropriate in design.

  The ground improvement body 3 and the bottoming structure 4 described above are in contact with each other so as to be vertically displaceable, and the bottoming structure 4 does not sink even if the ground improvement body 3 sinks. Further, since the ground improvement body 3 and the bottom structure 4 are in contact with each other so as to be displaceable in the vertical direction, the ground improvement body is formed by bending as in the case where the ground improvement body and the bottom structure are rigidly connected. 3 is not substantially damaged. In addition, although it is contact | abutting so that a displacement is possible to an up-down direction, it may be made to contact | abut by interposing a buffer layer (after-mentioned) between the ground improvement body 3 and the bottoming structure body 4, but especially a buffer layer is used. It does not have to be provided. For example, between the ground improvement body 3 and the bottom structure 4 is a construction seam, and the portion along the contact surface side of the ground improvement body 3 on the bottom structure 4 side is weak in strength. In this case, it is conceivable that the weak part substantially serves as a buffer layer, which is deformed and can be displaced in the vertical direction. In short, it is only necessary that it can be displaced in the vertical direction.

In the liquefaction countermeasure structure described above, considering the case where there is no bottom structure 4, the liquefied layer 2A (hereinafter referred to as the remaining liquefied layer) between the non-liquefied layer 1 and the ground improvement body 3 is liquid. If it becomes, the ground improvement body 3 will sink. For example, if uniformly liquefaction leaving the liquid layer 2A, soil improvement body will sink in proportion to the thickness H ₁ of the leaving liquid layer 2A. That is, the amount of settlement of the ground improvement body 3 is proportional to the thickness H ₁ (= H−t) of the remaining liquefied layer 2A obtained by subtracting the thickness t of the ground improvement body 3 from the thickness H of the liquefied layer 2. .

  However, in the liquefaction countermeasure structure described above, the bottoming structure 4 that suppresses the displacement of the ground improvement body 3 is constructed. For this reason, even when an earthquake occurs, the ground improvement body 3 abuts against the bottomed structure body 4, thereby suppressing horizontal displacement of the ground improvement body 3, and the non-liquefaction layer 1 and the ground improvement body 3 are horizontal. The displacement difference in direction is small. Therefore, the shear strain of the remaining liquefied layer 2A is suppressed. In other words, the liquefaction of the ground can be suppressed without incurring significant costs.

  In the ground in which the liquefaction countermeasure structure described above is constructed, structures such as a pavement structure, road embankment, and building can be constructed on the ground improvement body 3.

  If the structure is constructed on the ground improvement body 3 in the ground where the liquefaction countermeasure structure described above is constructed, the subsidence does not occur due to the rigid body effect of the ground improvement body 3. In addition, when the ground improvement body 3 sinks, there is a possibility that a gap (step) is generated at the boundary portion between the ground improvement body 3 and the landing structure 4. Therefore, when a structure is constructed across the boundary portion It is necessary to consider.

  In the liquefaction countermeasure structure described above, the ground improvement body 3 may include a plate structure such as a flat concrete plate, a pavement structure, and a road bed.

  In the liquefaction countermeasure structure described above, the bottom structure 4 may be constructed by placing concrete. Moreover, it is good also as the bottoming structure 4 by installing a caisson revetment block at predetermined intervals. Moreover, it is good also as the bottoming structure 4 by installing a caisson revetment block continuously.

  Furthermore, when the existing caisson revetment needs to be liquefied, the existing caisson revetment block is regarded as a bottomed structure, and the side of the existing caisson revetment block (the side opposite to the water area) That is, the ground improvement body 3 may be constructed so as to be in contact with the back surface. As a result, the displacement of the bottom structure (caisson revetment) itself can be suppressed.

  In the liquefaction countermeasure structure described above, a buffer layer 5 may be provided between the ground improvement body 3 and the bottom structure 4. The buffer layer 5 allows relative displacement in the vertical direction between the ground improvement body 3 and the bottom structure 4 when the ground improvement body 3 is displaced in the horizontal direction, and the original ground (liquefaction layer 2). May be used as the buffer layer 5 or may be constructed of sand, bentonite, or the like. In addition, the buffer layer 5 may have a thickness that does not liquefy, for example, about 30 cm.

  As shown in FIG. 5, in the above-described liquefaction countermeasure structure, relative displacement in the vertical direction between the ground improvement body 3 and the bottom structure 4 with respect to both the ground improvement body 3 and the bottom structure 4. The approach structure 6 (sink buffer zone) may be provided so that The approach structure 6 is for relaxing a gap (step) generated between the ground improvement body 3 and the bottom structure 4 due to the settlement of the ground improvement body 3. The approach structure 6 utilizes the fact that there is a correlation between the thickness of the structure floating on the liquefied layer 2 and the amount of settlement of the structure, and the lower end of the approach structure 6 is deeper than the ground improvement body 3 and is non-liquid. It is separated upward from the chemical layer 1. Since this approach structure 6 has less subsidence than the ground improvement body 3, there is a gap (step) generated at a time as compared with the case where there is no approach structure 6 between the bottoming structure 4 and the ground improvement body 3. Alleviated. The approach structure 6 may be either the ground improvement body 3 or the bottom structure 4.

(Embodiment 2)
Based on FIG. 6, the liquefaction countermeasure structure which is Embodiment 2 of this invention is demonstrated. FIG. 6 is a conceptual diagram showing a liquefaction countermeasure structure according to the second embodiment of the present invention.

  As shown in FIG. 6, the liquefaction countermeasure structure according to the second embodiment is a structure that should be referred to as a both-ends bottom floating type, and both outer side surfaces as a part of the side surface of the ground improvement body 3 abut each other. Thus, a pair of bottoming structures 4 is constructed. Specifically, when the outer shape of the ground improvement body 3 is rectangular in plan view, the bottoming structure 4 with which the one side surface 3a abuts, and the bottoming structure body 4 with which the other side surface 3b parallel to one side surface abuts Is built. Since the ground improvement body 3 and the bottom structure 4 are not different from those described in the first embodiment, description thereof is omitted.

  In the liquefaction countermeasure structure according to the second embodiment, since the pair of bottoming structures 4 are constructed so that both outer surfaces of the ground improvement body 3 are in contact with each other, the earthquake direction (excitation direction of the ground improvement body 3) ) Is further suppressed, and the horizontal displacement difference between the non-liquefaction layer 1 and the ground improvement body 3 can be further reduced. Therefore, the subsidence amount of the ground improvement body 3 can be reduced.

(Embodiment 3)
Based on FIG. 7, the liquefaction countermeasure structure which is Embodiment 3 of this invention is demonstrated. FIG. 7 is a conceptual diagram showing a liquefaction countermeasure structure according to an embodiment of the present invention.

  As shown in FIG. 7, the liquefaction countermeasure structure according to the third embodiment is a structure that should be referred to as a center bottom floating type, and both outer side surfaces of the bottom bottom structure 4 are each of a pair of ground improvement bodies. It is constructed so as to abut one outer surface as a part of the side surface. Specifically, when the outer shape of the bottomed structure 4 is rectangular in plan view, the ground improvement body 3 with which one side surface 4a abuts and the ground improvement body 3 with which the other side surface 4b parallel to one side surface abuts. Built. Since the ground improvement body and the bottom structure are not different from those described in the first embodiment, description thereof will be omitted.

  In the liquefaction countermeasure structure according to the third embodiment, since the pair of ground improvement bodies are constructed so that both outer side surfaces of the bottoming structure body 4 are in contact with each other, there is one bottoming for the pair of ground improvement bodies. What is necessary is just to build a structure. Therefore, it is possible to take measures against liquefaction over a wide area without spending a great deal of cost.

  In order to grasp the effect of the above-described liquefaction countermeasure structure, a centrifugal model experiment was conducted that can take a full-scale response from the similarity law. The object of the experiment is the ground of the liquefaction countermeasure structure according to the first embodiment (hereinafter referred to as a single-end floating ground), and the ground of the liquefaction countermeasure structure according to the second embodiment (hereinafter referred to as both-end landing). In addition to the floating-type ground), there are non-measured grounds, and grounds with a liquefaction countermeasure structure shown in Patent Document 1 (hereinafter referred to as an entire grounded-type ground).

  As the seismic wave, the scenario wave (Yokohama assumed wave) shown in FIG. 8 is used, and the amplitude is 0.1 times (maximum acceleration about 50 gal), 0.3 times (maximum acceleration about 150 gal), 0.5 times (maximum acceleration about 260 gal). ), 0.7 times (maximum acceleration about 360 gal), 0.7 times (maximum acceleration about 360 gal), 0.7 times (maximum acceleration about 360 gal) It carried out in.

  FIG. 9 is a diagram showing a centrifugal model experiment. This experiment is performed at a centrifugal acceleration of 50G, and can reproduce the behavior of the ground with a real size 50 times larger than the model size. Although not shown in the drawing, between the contact portions of the bottomed lattice improvement body and the floating lattice improvement body, thick silica sand is used as a cushioning material so as to allow relative displacement in the vertical direction between them. About 5 mm is interposed. FIG. 10 is a diagram showing the results of a centrifugal model experiment. FIG. 11 is a diagram showing the relationship between the excess pore water pressure ratio and the maximum acceleration in the lower ground due to stepwise excitation, specifically at the center at a depth of 12 cm (actual size corresponding to 6 m) from the ground surface. FIG. 12 is a diagram showing the relationship between the subsidence amount and the maximum acceleration at the surface layer ground by stepwise excitation, specifically, at the center at a depth of 2 cm (1 m in actual size) from the ground surface. In FIG. 11, the excess pore water pressure ratio ru of the whole bottom type and the both-ends bottom type t / H = 0.75 indicates that in the lattice.

  In FIG. 11 and FIG. 12, comparing the non-measured one with the liquefaction-prevented one with the bottomed structure, the non-measured one is 150 gal after the excess pore water pressure ratio approaches 1.0. Although there is a tendency that the excess pore water pressure ratio tends to decrease to some extent due to the progress of the step excitation, the excess pore water pressure ratio is still around 1.0, while the subsidence amount δ at the top of the ground improvement body increases. is there. However, the liquefaction countermeasures are 150 gal and the excess pore water pressure ratio is close to 1.0. However, even if the stepwise excitation thereafter proceeds, the excess pore water pressure ratio ru does not increase and conversely abruptly increases. It tends to decrease. As the excess pore water pressure ratio ru decreases, the amount of subsidence at the top of the ground improvement body also tends to be extremely small, and the amount of subsidence itself is much smaller than that without countermeasures. ing.

  From this result, if the relative horizontal displacement between the lower non-liquefied layer ground and the upper ground improvement body is suppressed by the bottoming structure, the excess pore water pressure ratio of the liquefied layer below the ground improvement body is temporarily increased. It can be seen that even if it increases, this does not significantly affect the size of the settlement, that is, it has the effect of substantially suppressing the settlement due to liquefaction. In addition, the excess pore water pressure ratio of the liquefied layer of the liquefied layer that has already been liquefied has a tendency to rapidly decrease with the progress of the step excitation, even if the ground is loose, there is a bottoming structure This contributes greatly to the suppression of the relative horizontal displacement of the ground, which is thought to reduce the excess pore water pressure ratio.

  The numerical value of FIG. 12 indicates the amount of subsidence of the liquefied layer at each maximum acceleration in FIG. 21 (the subsidence measurement location is the surface ground, but if there is a ground improvement body, the ground improvement body itself It is assumed that there is no subsidence, and is the subsidence amount of the remaining liquefied layer below.), And the subsidence amount converted into strain is the volume strain of the liquefied layer at each maximum acceleration in FIG. FIG. In FIG. 22, a maximum acceleration of 150 Gal corresponds to a horizontal seismic intensity kH = 0.15, and is used for general earthquake studies. Focusing on this 150 Gal, the volume strain of the liquefied layer is 3% or more if no countermeasure is taken here, but the volume strain of the remaining liquefied layer below the ground improvement body is 1 for the one provided with the bottom structure. %. It can be seen that the bottomed structure is effective in suppressing liquefaction.

  FIG. 13 is a diagram illustrating an FEM analysis result obtained when an earthquake wave (sin wave: frequency 0.5 Hz, maximum acceleration 131 gal) is applied to the ground of the liquefaction countermeasure structure according to the first embodiment. As shown in FIG. 13, the excess pore water pressure ratio ru of the remaining liquefied layer 2A does not increase to 1.0 even after 25 seconds have passed since the occurrence of the earthquake. Therefore, the remaining liquefied layer 2A is not liquefied. This is consistent with the experimental results described above, and the effect of the liquefaction countermeasure structure according to the first embodiment can be verified from the results of FEM analysis (finite element method analysis).

  On the other hand, FIG. 14 shows an FEM analysis in which the same seismic wave as in FIG. 13 is applied when the bottoming structure 4 in FIG. 13 is eliminated and the ground improvement body 3 is extended to the bottoming structure position to form the whole floating type. It is a result. It can be seen that the excess pore water pressure ratio of the liquefied layer 2 below the ground improvement body 3 increases with the passage of time, and the excess pore water pressure becomes around 1.0 after 6 seconds. It has been found that this excess pore water pressure ratio did not become so small over time. In addition, in the sinking amount at the top of the ground improvement body, it is expected that the entire floating type is larger than the one-end landing floating type in FIG. 11 because the increase in excess pore water pressure directly affects the sinking amount. Is done.

  FIG. 17 is a conceptual diagram showing an analysis model for performing FEM analysis. FIG. 18 is a diagram illustrating an analysis result 20 seconds after the seismic wave is applied to the analysis model illustrated in FIG. FIG. 19 is a diagram showing the relationship between the dimensional ratio of the bottom structure obtained from the result of the FEM analysis and the behavior of the ground improvement body. FIG. 20 is a contour diagram showing the excess pore water pressure ratio obtained from the result of FEM analysis.

  As shown in FIG. 17, the analysis model for performing the FEM analysis is a structure that should be called a one-end-floating floating type, like the liquefaction countermeasure structure according to the first embodiment. The bottom structure 4 is constructed so as to abut on one side as a part. This analysis model is applied to the ground where the liquefied layer 2 of 12.5 m is deposited on the non-liquefied layer 1 (liquefied layer thickness H = 12.5 m), and the ground improvement body 3 is liquefied. The layer 2 is constructed so as to float upward from the non-liquefied layer 1, more specifically, to float on the liquefied layer 2. The thickness of the ground improvement body 3 is not more than 3/4 of the thickness of the liquefied layer, specifically 7 m. The bottoming structure 4 is embedded and constructed over the entire vertical direction of the liquefied layer 2. The total height of the bottoming structure is 14 m, 1.5 m is embedded, and the height h of the bottoming structure 4 in the liquefied layer 2 is 12.5 m, which is the same as the thickness H of the liquefied layer.

  In addition, the width b in the excitation direction of the bottomed structure 4 shown in the left-right direction in FIG. 17 is analyzed for six patterns of 15 m, 10 m, 8 m, 6 m, 4 m, and 0 m (overall floating type). A buffer layer 5 having a thickness of 5 cm is provided between the ground improvement body 3 and the bottom structure 4. Further, the seismic wave applied to the ground of this liquefaction countermeasure structure is the same seismic wave (sin wave: frequency 0.5 Hz, maximum acceleration 131 gal) as in the FEM analysis shown in FIG.

  As shown in FIG. 18, here, the ratio of the width b of the bottoming structure 4 to the height h in the liquefied layer 2 of the bottoming structure 4 (hereinafter referred to as “width / height ratio (b / h)”). Focused on the excess pore water pressure ratio at the center in the height direction of the remaining liquefied layer 2A, the horizontal displacement ratio of the ground improvement body, and the vertical displacement ratio. The horizontal displacement ratio of the ground improvement body is the width b in the excitation direction of the bottom structure 4 with respect to the horizontal displacement of the ground improvement body 3 when the width b in the vibration direction of the bottom structure 4 is 4 m. It is a ratio of the amount of horizontal displacement of the ground improvement body 3 when changing. The vertical displacement ratio of the ground improvement body is the vibration direction of the bottom structure 4 with respect to the vertical displacement (sinking amount) of the ground improvement body 4 when the width b in the vibration direction of the bottom structure 4 is 4 m. It is the ratio of the amount of displacement (sinking amount) in the vertical direction when the width b of is changed.

  As shown in FIG. 19, when the width b of the bottoming structure 4 in the vibration direction increases, the horizontal displacement of the ground improvement body 3 decreases. Further, the liquefaction of the liquefied layer 2 is based on the excess pore water pressure ratio ru = 0.8, and therefore the width / height ratio of the bottomed structure body where the excess pore water pressure ratio ru is 0.8 or less ( If b / h), liquefaction of the liquefied layer 2 is suppressed. Referring to FIG. 19, the width / height ratio (b / h) of the bottomed structure where the excess pore water pressure ratio is 0.8 or less may be 0.4 or more.

  Further, considering in detail, the horizontal displacement amount and the vertical displacement amount also vary greatly in the range of width-height ratio (b / h) = 0.5 to 0.8. From this, the width / height ratio (b / h) is more preferably 0.5 or more, and considering the economy, it is preferably 0.8 or less and at most 1.0 or less.

  Note that the width b in the excitation direction of the bottomed structure 4 described above is the width at the boundary between the non-liquefied layer 1 and the liquefied layer 2, and the width in the excitation direction at the top of the bottomed structure 4. May be narrower than b. In addition, since the actual direction of seismic motion cannot be predicted, the width of the bottoming structure in a direction substantially perpendicular to the contact direction in which the bottoming structure abuts against the side surface of the ground improvement body in plan view is defined as width b.

  In addition, the bottoming structure 4 in the analysis model described above is constructed so as to abut against one side surface of the ground improvement body 3. For example, a pair of bottoming structures 4 so that both side surfaces of the ground improvement body 3 abut each other. When a structure is constructed, the width of one bottoming structure is b1, the width of the other bottoming structure is b2, and these are added to determine the width b of the bottoming structure 4 in the above-described analysis model. do it.

  Furthermore, in the analysis model described above, the bottoming structures are continuous in the back of the sheet of FIG. 17, but a plurality of bottoming structures may be arranged at intervals in the back of the sheet. This is because the horizontal displacement of the ground improvement body is suppressed even if a plurality of bottoming structures are arranged at intervals.

FIG. 15 is a diagram showing the relationship between the amount of subsidence at the center of the top of the ground improvement body after final excitation by stepwise excitation and the ratio of the thickness of the ground improvement body to the thickness of the liquefied layer. As shown in FIG. 15, in the case of the whole floating type, according to the conventional idea of liquefying uniformly, the subsidence amount of the ground improvement body 3 constructed so as to float on the liquefied layer 2 is the residual liquefaction. This is proportional to the thickness H ₁ (= H−t) of the layer 2A. For this reason, in FIG. 12, the line connecting the settlement amount of the non-measured ground (t / H = 0) and the settlement amount of the entire bottomed ground (t / H = 1) is in the liquefied layer 2. This is an assumed line indicating the amount of settlement of the ground improvement body 3 constructed so as to float.

  On the other hand, the amount of ground subsidence of the liquefaction countermeasure structure (one-end landing floating type) shown in the first embodiment and the ground of the liquefaction countermeasure structure (both-end landing floating type) shown in the second embodiment are described. The subsidence amount suppresses the subsidence amount δ much more than the liquefaction countermeasure structure in which only the ground improvement body 3 is constructed. Moreover, even if the thickness of the ground improvement body 3 is set to about one-fourth of the thickness of the liquefied layer 2, the amount of subsidence can be made the same as that of the ground of the conventional liquefaction countermeasure structure (entire bottom structure). For this reason, the liquefaction of the ground in which the liquefied layer 2 is deposited on the non-liquefied layer 1 can be suppressed without incurring significant costs.

  Further, as shown in FIG. 15, the ratio (t / H) of the thickness of the ground improvement body 3 to the thickness of the liquefied layer 2 is ¼ (0.25) to suppress the settlement amount δ. As for the amount of settlement δ, the ratio (t / H) of the thickness of the ground improvement body to the thickness of the liquefied layer 2 is ¼ (0.25) to ½ (0.5) to ¾ ( 0.75), and more preferably, it is clearly suppressed in a quarter (0.25) to a half (0.5). The subsidence amount δ is suppressed even when the ratio (t / H) of the thickness of the ground improvement body to the thickness of the liquefied layer 2 is ¼ (0.25) or less. However, the extent to which it can be made smaller is that the ground improvement body 3 has a thickness that can maintain its function as a board-like body even if it receives a load from above.

  In the liquefaction countermeasure structure shown in the first to third embodiments, the bottom structure 4 is bottomed or embedded in the non-liquefaction layer 1 and the ground improvement body 3 is in contact with the side surface. Since the horizontal displacement of the ground improvement body 3 is suppressed, even when an earthquake occurs, the horizontal displacement of the ground improvement body 3 is suppressed, and the horizontal displacement difference between the non-liquefaction layer 1 and the ground improvement body 3 is reduced. Less. Therefore, the shear strain of the remaining liquefied layer 2A between the non-liquefied layer 1 and the ground improvement body 3 is suppressed, and the liquefaction of the ground can be suppressed without incurring a great expense.

The liquefaction countermeasure structure shown in the first to third embodiments described above is liquid even if the ratio (L / t) between the inner frame width L and the ground improvement body thickness t is 2, as shown in FIG. There is an effect of the countermeasures. That is, the even not satisfy the existing literature (Patent No. 2568115 discloses) revealed limited by what L / t = 0.5 to 0.8 as described above is an effect. Therefore, an economical and rational ground for liquefaction countermeasure structure can be provided.

  When the liquefaction countermeasure structure described in the first to third embodiments is applied to an airport, for example, the runway is constructed with the ground improvement body 3 and the landing zone on the side is constructed with the landing structure 4. do it. When this liquefaction countermeasure structure is applied to an embankment structure such as an expressway, the embankment should be constructed with the ground improvement body 3 and the glue bottoms on both sides thereof should be constructed with the bottom structure 4 in a staggered manner. That's fine. If the liquefaction countermeasure structure is applied in this way, the liquefaction of the ground can be suppressed without incurring a great expense.

  Moreover, what is necessary is just to drive a foundation pile in the lattice frame of the lattice wall body which comprises the ground improvement body 3 with respect to structures with strict subsidence restrictions, such as an abutment and a high-rise building. If a foundation pile is laid, the ground around the foundation pile can be designed so as not to liquefy. In addition, since the ground improvement body 3 is restrained from being displaced in the horizontal direction by the bottom structure 4, the bending strength of the foundation pile can be reduced.

Further, in the first to third embodiments described above, the contact portion that can be relatively displaced in the vertical direction between the ground improvement body 3 and the bottom structure 4 is formed on the side surface of the ground improvement body 3 as shown in FIG. The bottom structure 4 may be brought into contact with a part. The left side view of FIG. 16 is an engagement structure in which an engagement portion 3 c is provided on the outer surface of the ground improvement body 3. The central view of FIG. 16 is an engagement structure provided with an engaging portion 3c that fits to the inner surface of the ground improvement body 3 (in the figure, one bottoming structure 4 inside the ground improvement body 3 is provided). However, it may be provided at a plurality of locations). In this way, it is possible to cope with horizontal displacement in two directions. It should be noted that the engagement by the unevenness in plan view may be either concave or convex, or a plurality of engagements may be provided at intervals. Further, as shown in the right side view, the bottom structure 4 may have a so-called buttress type in which the lateral surface in the short side view in a plan view is abutted so as to be relatively displaceable in the vertical direction, and the bottom structure is formed along the side surface of the ground improvement body. Instead of the continuous structure shown in FIG. 3, a plurality of bottoming structures 4 may be provided at intervals along the side surface of the ground improvement body as shown in the upper and right views. Incidentally, in all the above-mentioned embodiments, it is possible to use each in combination appropriately including the presence or absence of a buffer layer, and the planar view shape of the ground improvement body 3 or the bottom structure 4 is also a polygon other than a rectangle, Various shapes such as a circle can be applied. In any case, since the vibration direction of the actual earthquake cannot be predicted, if the ground improvement body 3 is long in one direction in a plan view, it will be able to cope with the vibration in a direction substantially perpendicular to the longitudinal direction. If the structure 4 is disposed at least, and the ground improvement body 3 has a shape that is not long or short in a plan view and has little directivity, a bottoming structure that can cope with vibrations in two directions intersecting substantially perpendicular to the plan view. It is desirable to arrange the body 4. Furthermore, as can be seen from FIG. 12, it is more effective to arrange the bottom structure facing the both sides of the ground improvement body than to arrange it on one side.

  In the first to third embodiments described above, the side surfaces include the outer side surface and the inner side surface, and one side of the side surface is the ground improvement body 3 regardless of whether the ground improvement body 3 is rectangular or circular in plan view. When the length of the peripheral portion in plan view is divided in half at an appropriate position, it indicates a part or all of the peripheral portion belonging to one of them. Moreover, it is set as both sides including the part which opposes one side.

  By the way, by completely surrounding the outer periphery of the ground improvement body 3 with the bottom structure 4, the liquefied layer (residual liquefied layer 2A) located inside the bottom structure 4 and the outside of the bottom structure There is known a liquefaction countermeasure structure that blocks the liquefaction layer 2 located at the bottom and restrains the liquefaction layer (residual liquefaction layer 2A) located inside the bottomed structure 4. This liquefaction countermeasure structure allows the liquefied layer (residual liquefaction layer) located inside the bottoming structure 4 together with the ground improvement body 3 even if the bottoming structure 4 does not completely surround the outer periphery of the ground improvement body 3. If the layer 2A) is in a restrained state, it is said that liquefaction is suppressed. That is, even if a plurality of bottoming structures 4 are arranged at intervals, if the liquefied layer (residual liquefied layer 2A) located inside the bottoming structure 4 together with the ground improvement body 3 is constrained It is said that liquefaction is suppressed. Therefore, in this liquefaction countermeasure structure, in the plurality of bottoming structures 4, the gap between the bottoming structure 4 and the bottoming structure 4 is a liquefied layer (inside the bottoming structure 4 ( The size of the residual liquefied layer 2A) is limited.

  On the other hand, the liquefaction countermeasure structure according to the embodiment of the present invention does not restrain the remaining liquefied layer 2A as shown in FIG. 3, FIG. 5, FIG. 6, FIG. However, since the bottom structure 4 suppresses the horizontal displacement of the ground improvement body 3 (suppresses the relative horizontal displacement between the ground improvement body 3 and the non-liquefaction layer 1), the remaining liquefied layer 2A It is characterized in that liquefaction can be suppressed.

  Here, “without restraining the remaining liquefied layer 2A” means that an assumed earthquake of the remaining liquefied layer 2A located below the ground improvement body 3 in plan view (for example, during a general earthquake) The size (the length in plan view) that the behavior during liquefaction during the horizontal seismic intensity kH = 0.15 used in the study) is substantially transmitted to the liquefied layer 2 outside the ground improvement body 3 As shown in the side view of FIG. 16 (b) at the open position (see FIG. 16 (a)), the bottoming structure 4 is arranged between the upper ground improvement body and the lower non-liquefaction layer. (Not part) is provided along at least a part of the outer surface of the outer periphery of the ground improvement body 3.

  Examples of the opening portion include those shown in FIGS. 3, 5, 6, 7, 13, and 16 (a), and specifically, one opening between the bottoming structures. If the size of the part is about 1 m, 2 m or 3 m, the behavior of the remaining liquefied layer at the time of earthquake is transmitted to the outer liquefied layer. When the sizes of the respective open portions are accumulated, the size is ½ or more of the total length of the outer peripheral portion of the ground improvement body in plan view. Further, if the bottomed structure 4 in FIG. 16A is fitted to the inner surface of the ground improvement body 3, the open portion is 100% with respect to the total outer peripheral length of the ground improvement body 3. If such an open portion is provided, the bottomed structure does not substantially restrain the behavior of the residual liquefied layer 2A during the assumed earthquake in the open portion (when the residual liquefied layer 2A is liquefied) Is transmitted to the liquefied layer 2 outside the ground improvement body 3). Of course, there are cases where the size of the open portion on the lower side of the drawing is smaller than ½ of the total length of the outer peripheral portion of the ground improvement body as shown in the plan view of FIG. The open part of the side (one side) of the ground improvement body is so large that the landing structure does not restrain the behavior of the remaining liquefied layer during the assumed earthquake. However, it is not limited to 1/2 or more with respect to the total length of the outer periphery of the ground improvement body. However, for example, assuming a ground improvement body with a square in plan view, there is an effect even when one side of the ground improvement body is an open part, so the total size of the open part in plan view is the total extension of the outer periphery of the ground improvement body. However, it may be set to 1/4 or more.

  In the present invention, the ground improvement body and the bottom structure are in contact with each other so as to be relatively displaceable in the vertical direction, thereby blocking the vertical stress transmission therebetween.

The knowledge further developed based on the above knowledge of the present invention will be described below.
From the FEM analysis shown in FIGS. 17 to 20, it can be seen that there is a preferable ratio in the relationship between the height and the width of the bottom structure. This assumes, for example, a case where a relatively thin floating type ground improvement body 3 is set and the width of the bottoming structure is varied, and the analysis and model in the assumed earthquake in the multiple cases If an experiment is performed and the settlement, horizontal displacement, and excess pore water pressure ratio of the ground improvement body 3 are calculated and measured, a suitable width of the bottomed structure can be found from the results. In other words, for the ground improvement body that is in contact with the bottom structure so as to be relatively displaceable in the vertical direction, the allowable settlement for an assumed earthquake is set for each type of load, etc. With respect to an earthquake, it is possible to set the width of the bottomed structure so that the ground improvement body falls within the set settlement amount. Thus, it is possible to obtain a liquefaction countermeasure structure in which the width of the bottoming structure is set as described above, and this structure can be applied to all the embodiments described above. In addition, although it is the ground improvement body 3 in all the said embodiment, although it is economical that it is a ground improvement body, for example, a concrete plate may be sufficient. In short, even if there is a load such as a structure on it, it is sufficient if it is a disk-like body as a whole so that it receives the load and is transmitted to the remaining liquefied layer below. The grid-like ground improvement body of the above embodiment is also a board-like body as a whole, for example, even though it is placed discretely like a sand compaction pile, the surrounding ground is modified, so as a whole It can be said that it is a board-like ground improvement body.

1 Non-liquefied layer 2 Liquefied layer 2A Remaining liquefied layer (liquefied layer)
3 Ground improvement body 4 Bottoming structure 5 Buffer layer 6 Approach structure ru Excess pore water pressure ratio Δu Excess pore water pressure σv 'Effective soil cover pressure δ Settling amount

Claims (4)

In the liquefaction countermeasure structure in which the ground where the liquefied layer is deposited on the non-liquefied layer is applied, and the ground improvement body is constructed at a position separated from the non-liquefied layer in the liquefied layer,
In order to allow the ground improvement body to be grounded or embedded in a non-liquefaction layer and to be relatively displaceable in the vertical direction , an open portion is secured in contact with the side of the ground improvement body in the earthquake direction in plan view. liquefaction countermeasure structure, characterized in that the vibration of the non-liquefied layer were constructed a structure having a width which reduces the displacement difference in the horizontal direction by transmitting the soil improvement material non liquefaction layer and ground improvement body. The liquefaction countermeasure structure according to claim 1, wherein the width of the structure is set to 0.4 times or more of the height of the structure in the liquefaction layer. The opening part is
It is provided on the outer periphery of the remaining liquefied layer left in the lower region of the ground improvement body, and the behavior of the residual liquefied layer at the time of the assumed earthquake has a size that is transmitted to the side region of the ground improvement body. The liquefaction countermeasure structure according to claim 1 or 2. The liquefaction countermeasure structure according to any one of claims 1 to 3, wherein the size of the open portion is ¼ or more of the total extension of the outer periphery of the ground improvement body in plan view. .

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