JP2018150772A - Liquefaction countermeasure structure of underground structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flotation countermeasure or a subsidence countermeasure of an underground structure when a natural ground in the periphery of the structure is liquefied by quake such as earthquake, and to quickly remove underground water having an excess pore water pressure.SOLUTION: A liquefaction countermeasure structure of an underground structure has a water collecting frame 3 projecting to the lower part from a peripheral edge part on a lower end face of a foundation part of an underground structure in the underground structure or the peripheral edge part thereof, efficiently collects underground water having an excess pore water pressure from a ground of the lower part of the underground structure by a combination of a water collecting non-permeable surface 4 and a water collecting permeable layer 42, and drains the underground water by a tubular vent 5 of which an upper part reaches a peripheral underground water level or higher.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an underground structure such as an underground water storage tank or fire prevention water tank, and the underground structure around the structure is liquefied due to an earthquake or the like, and the underground structure is countermeasures against levitation or settlement. .

It is known that in ground with high groundwater level and insufficient compaction, the ground liquefies due to shaking such as an earthquake. Such underground structures in the ground have a risk of rising and sinking due to generation of excess pore water pressure accompanying liquefaction, reduction of the supporting capacity of the underground structure and frictional resistance of the side surfaces.
Patent Document 1 proposes a groundwater ejection treatment method as a countermeasure for the floating of a structure caused by groundwater when constructing an underground structure.
In Patent Document 2, as an countermeasure against groundwater, an opening that guides groundwater to the bottom part of the underground structure and a hollow column that is higher than the surrounding groundwater level standing from that part are provided. Complements
In Patent Document 3, as a protection method for underground structures in a liquefied sand ground, a horizontal gravel layer is provided below the bottom of the structure, and a drain pipe extends from the gravel layer to the top through the structure. And a method of draining excess pore water pressure on the ground or in the structure has been proposed.
In Patent Document 4, as a countermeasure against levitation due to liquefaction of an underground buried water tank, a stone paving layer and a concrete casting layer are provided below the water tank bottom plate, and the top of the structure is formed from the center of the bottom plate. Excess pore water can be discharged to the ground by the drainage pipe extending to the center of the plate and the guide passage provided in the concrete casting layer, and the floating due to excess pore water pressure can be prevented.

  The total stress acting on the soil where the gap is saturated with water is the sum of the pore water pressure and the effective stress. The effective stress is governed by the skeletal structure of the soil particles. In the case of loosely-packed sand saturated with water, the cause of the risk of the underground structures such as underground water tanks such as floating, subsidence, and tilting is as follows. Trying to reduce the volume (gap ratio)-Omitted-However, in reality, repeated shearing due to earthquakes occurs in a short time of several tens of seconds, and drainage is not in time (undrained), "(4th Edition Civil Engineering Handbook Volume 1 P.383, edited by Japan Society of Civil Engineers), the original skeletal structure of the sand ground changes in a non-drained state, so that effective stress is lost and pore water pressure is generated to compensate for it. This pore water pressure is called excess pore water pressure. While the skeletal structure between the original sand particles is changing, it is in a muddy state where the sand particles are floating in the water. Therefore, as for the structure in the natural ground, the structure whose specific gravity is smaller than that of muddy water rises, and the heavy structure sinks. Thereafter, excess pore water rises and appears as a sandblast with sand on the ground surface. The conventional technology leads groundwater having excess pore water pressure generated below a structure to the ground surface by a drain pipe, and has a certain result. However, in order to avoid a non-drainage state in a short time, it is necessary to collect water up to the drain pipe more quickly to minimize the influence of liquefaction.

JP 52-15111 A JP-A-55-4455 JP-A-1-239217 JP2013-189801A

  Although removal of groundwater having excess pore water pressure can be expected by the conventional technology, further prompt removal is required. The issue is to quickly and reliably collect and drain water that exceeds the drainage of groundwater with excessive pore water pressure of the prior art. In addition, the technologies that have been proposed so far are countermeasures that are carried out by digging deeper into the ground with high groundwater levels, such as the need to drain groundwater more than before during the construction. There was a problem of implementation difficulty.

  A frame continuously closed in a strip shape along the peripheral edge of the lower end surface of the base part of the underground structure or the underground structure, and a water collecting frame projecting downward from the lower end surface, and a lower part of the lower end surface Impervious surface that divides the ground up and down (concept that includes the lower surface of impermeable layer, which is a concept that includes impermeable layer and hardly permeable layer), and has one or more apexes depending on the vertical inclination And a peripheral edge of the water collecting impermeable surface reaching the water collecting frame, a water collecting permeable layer provided below and in contact with the water collecting impermeable surface, and an upper end at least equal to or higher than a surrounding ground water level of the underground structure. An underground structure including a tubular vent that is open at the bottom and penetrates the water-impermeable impermeable surface at a location including the apex portion, and has a lower end that reaches the upper surface of the water-permeable impermeable layer and is opened. Liquefaction countermeasure structure.

  A liquefaction countermeasure structure for an underground structure, wherein the water impermeable surface is a lower surface of an impermeable plate having rigidity.

  A liquefaction countermeasure structure for an underground structure provided with a water collection frame groundwater inflow hole which is a hole provided in the water collection frame and communicates with the ground outside the water collection frame and the water collection permeable layer or vent. object.

  In the ground below the lower end surface of the underground structure or the foundation of the underground structure, sand particles float in groundwater and begin to sink due to liquefaction caused by shaking such as an earthquake. Contrary to sand particles, groundwater with excess pore water pressure will remain above. Below the catchment impermeable surface, the groundwater remaining immediately below the catchment permeable layer quickly passes through the highly permeable catchment layer and reaches the catchment impermeable surface. According to the gradient of the impervious surface, it reaches the release surface of the lower end of the vent existing at the apex portion through the gradient flow route to the vent, and is quickly drained from the surrounding groundwater level through the vent. In particular, when the water collection impermeable surface is a smooth material, the groundwater that has reached the impermeable surface flows at a speed exceeding the flow in the gaps between the sand particles, and the effect is further increased.

  When the water-impermeable impermeable surface is a rigid impermeable material, for example, the lower surface of a steel plate, and when using a crushed stone or crushed stone (hereinafter also referred to as a crushed stone) as the water-collected permeable layer, construction in water is easy. After installing the water-collecting permeable layer, it can be installed by dropping the assembled steel sheet as it is near the site, and it can be constructed without lowering the groundwater level to provide a water-impermeable impermeable surface during construction. . It can be adopted as an extremely effective construction method in places where forced drainage of groundwater by submersible pumps (hereinafter referred to as water change) is difficult.

  The liquefaction of the ground has an adverse effect on the stability of the underground structure in the form of buoyancy and reduced bearing capacity below the underground structure, but liquefaction of the ground on the side of the underground structure and coverage on the lower side of the underground structure. When pressurized water occurs, the boundary between the natural ground and the structure tends to be a route for rising groundwater. Therefore, a water collection frame groundwater inflow hole, which is a hole provided in the water collection frame that communicates the ground near the boundary and the water collection permeable layer, is provided to guide the groundwater that may rise near the structure boundary to the vent. To achieve the effect.

Overall view of liquefaction countermeasure structure Structural drawing (1 vent) Structural drawing (3 vents) Structural drawing (4 vents) Catchment conceptual diagram (vertical direction) Catchment conceptual diagram (horizontal direction) Drainage impermeable surface panel assembly drawing Catchment frame groundwater inflow hole map Temporary figure

  As a typical underground structure, an embodiment will be described for the underground water tank 2. As shown in FIG. 1, the underground water storage tank 2 is a cylindrical structure, and the side plate 23 is formed by assembling and joining steel segments 24 by welding and bolts, and the bottom and top are made of steel. Concrete is placed on top of the bottom plate 27 and the top plate 21 to construct. In the bottom portion, a water collecting pit 28 (shown in FIG. 3) is provided in the center, and a suction pipe introduction hole / inspection hole 22 is provided in the top portion. Although not shown in the drawing, the upper surface of the bottom slab concrete is provided with a draining slope so that all water can be supplied from the water collecting pit 28. In the underground water tank 2, foundation work using crushed stone or crushed stone 13 (hereinafter also referred to as foundation crushed stone, etc.) as a foundation and an upper surface of the foundation are leveled with leveling concrete 14, and a steel bottom plate 27 And place bottom slab concrete. The water collecting pit position may be provided with a pottery 12 (shown in FIG. 9) at the time of water change necessary for construction.

  In the embodiment, the number of the vents 5 is 1, 3, and 4, but the groundwater collection having the excess pore water pressure up to the vents 5 depends on the flow of the groundwater below the water collection impermeable surface 4. In order to drain the ground water below the underground water storage tank 2 in a short time and in a well-balanced manner, it is desirable to collect water efficiently and evenly in a plane position. In each case, the maximum flow length of the groundwater below the water collection impermeable surface 4 up to each vent is approximately the radius of the circle surrounded by the water collection frame 3, and in the case of multiple vents, to each vent 5 The water collection area is divided equally.

FIG. 2 shows a front sectional view and a bottom view of the installation of the structure according to the present invention in which one vent 5 is provided at the center.
As shown in FIG. 2 (2), the water collecting frame 3 forms a continuous frame in the periphery of the bottom of the base part of the underground water storage tank 13, as shown in FIG. 2 (1). It has a structure projecting downward from the base part Kuriishi 13 and the like. The water collecting frame 3 in FIG. 2 is a structure formed by placing concrete. The lower end surface of the underground structure or the base portion of the underground structure is a surface that is a target of a supporting force in the case of subsidence of the underground structure or a lifting pressure in the case of ascent. The water collecting frame 3 is provided on the periphery of the lower end surface. This is because it is preferable that the entire lower part of the underground structure is within the groundwater collection range.

  As shown in FIG. 2A, the water collection impermeable surface 4 has a conical shape with the lower surface released, and the upper part is also released by the vent 5 passing through the apex 41 of the cone. The water impermeable surface 4 in the case of FIG. 2 is obtained by processing a steel plate. FIG. 2 (2) shows a bottom view, and the steel plate and the vent 5 of the water collection impermeable surface 4 correspond to the cone portion of the funnel opened downward and the foot portion extending upward. As shown in FIGS. 2 (1) and 2 (2), the peripheral edge of the water collection impermeable surface 4 reaches the water collection frame 3. On the steel plate forming the water collection impermeable surface 4, the soil at the site is backfilled as backfill soil 17 and compacted.

  As shown in FIG. 2 (1), the water collection permeable layer 42 has an upper surface portion in contact with the water collection impermeable surface 4, and a bottom surface portion of the water collection impermeable surface 4 below the water collection impermeable surface 4. Is provided. As shown in FIG. 2 (1), the water collecting / permeable layer 42 is constructed of chestnut stone or crushed stone (hereinafter referred to as chestnut stone or the like). In general, it is considered that the excess pore water pressure caused by liquefaction becomes the same level at the same height in the ground. This is because, as described above, the excess pore water pressure supplements the effective stress of the soil, and the effective stress is proportional to the depth in the ground. On the other hand, the flow rate of groundwater flowing through the water-collecting permeable layer 42 is considered to be proportional to the hydraulic gradient related to the magnitude of excess pore water pressure when the hydraulic conductivity is the same, and the lower surface of the water-collecting permeable layer 42 Is substantially horizontal, there is no significant difference in the flow rate of the upward flow of the bottom surface of the water-collecting permeable layer, and the flow of biased groundwater can be avoided.

  As shown in FIG. 2 (1), the vent 5 reaches a height above the groundwater level 11 from the center position of the base portion to the periphery of the underground water storage tank 2. The lower part penetrates the structure base part, further penetrates the apex part 41 of the water collection impermeable surface 4 and reaches the water collection / permeable layer 42.

  Fig. 5 shows a conceptual diagram of water collection. In the natural ground below the catchment permeable layer 42, liquefaction generates groundwater having an excess pore water pressure immediately below the catchment permeable layer 42. This groundwater flows upward at a low pressure. The upward arrow shown in the front cross-sectional view of FIG. 5 (2) indicates the flow of groundwater in the water catchment permeable layer 42. On the other hand, the groundwater that has passed through the water-collecting permeable layer 42 and has reached the water-impermeable impermeable surface 4 flows as indicated by an arrow directed obliquely upward in the figure, and reaches the open portion of the vent lower end 52 in the center. The bottom view of FIG. 5 (1) shows the flow of groundwater immediately below the water impermeable surface 4. In this case, the slope of the water collection impermeable surface leads to the central vent opening by the shortest route, reaches the vent with an arrival time corresponding to the distance from the vent, and is drained. The arrival time varies depending on the distance from the vent, but is not affected by the direction. The speed of the groundwater flow on the surface is larger than the flow of the water-impervious surface having no gradient due to the vertical gradient of the water-impermeable impermeable surface 4, and the groundwater below the underground water storage tank is efficient in a short time. Drainage is possible.

  FIG. 6 (1) shows a processing development view of the steel plate forming the water collecting impermeable surface 4. FIG. 4 is a development view of a truncated cone having a bent penetrating portion and a bottom surface of a vertex portion 41 shown in FIG. 2. In the case of forming a hexagonal pyramid shape as the shape of the water-impermeable impermeable surface 4 that joins the side M and the side L in FIG. A panel assembly drawing is shown in FIG. By joining such six fan-shaped flat panels, a groundwater flow toward the vent can be generated by a route substantially similar to that of the cone. Comparison of groundwater flow is shown in (1-a) and (1-b) of FIG.

  FIG. 1 and FIG. 3 show an embodiment in which three vents 5 are arranged at the peripheral edge portion of the lower end surface of the base portion. The water collecting frame 3 is the same as that in the first embodiment. In FIG. 1, the water collection impermeable surface 4 has a shape in which approximately one-third cones having b, d, and f as apexes 41 are combined. Accordingly, the flow of groundwater under the water impermeable surface 4 has a route as shown by the arrow line in FIG.

  The water collecting impermeable surface 4 of FIG. 3 is formed by a combination of steel fan-shaped panels of 60 degrees on the bottom view as shown in the bottom view of FIG. Groundwater from the water collection area formed by two panels, abo and bco, cdo and deo, or efo and fao, is discharged from one vent 5 in b, d, or f. oa, oc, and oe form sides of the same height (solid line in the figure), and ob, od, and of sides rise from the center toward the periphery (dotted line in the valley figure). Form. For the groundwater that rises below these water impermeable surfaces 4, oa, oc and oe become ridgelines of the same height, and when a vertical flow is produced from the ridgelines and ob, od or of is reached From the center to the peripheral vertex 41. In addition, the flow that has reached abc, cde, or efa goes to the apex 41 along the side. These flows are shown in FIG. 7 (2-b). In this example, as in the case of 1 vent, FIG. 7 shows an easy example of the flow processing similar to FIG. 7 (2-a) of the flow form by the water collecting impermeable surface 4 formed by the curved surface of FIG. Shown in (2-b).

  As shown in FIG. 3, vents 5 are arranged at b, d, and f where the apex portion 41 is formed in the peripheral portion of the water collection impermeable surface 4. As a result, the groundwater flowing under the impermeable surface of the water collection and having an excess pore water pressure going upward reaches the vent lower end 52 and is drained in an arrival time corresponding to the distance from the vent lower end 52.

  As the water collection impermeable surface 4, a steel plate can be used as a rigid material, and a soft material such as a rubber sheet or a water shielding sheet (hereinafter referred to as a rubber sheet) can be used. As the water-collecting water-permeable layer 42, a water-permeable mat or the like can be used in addition to the chestnut stone described above. Although the Example constructed | assembled with the concrete structure was shown as the water collection frame 3 above, impermeable materials, such as steel plates, can be used. The water collecting frame 3 can be constructed integrally with the water collecting impermeable surface 4 depending on the material of the water collecting impermeable surface 4. Below, the construction procedure of this invention in the case where the water collection frame 3 and the water collection impermeable surface 4 are made of steel and this underground water storage tank is carried out by construction by the well-flooring method is shown.

(Construction procedure when steel material is used as the water impermeable surface 4)
(1) Topsoil excavation. If necessary, the groundwater level 11 is lowered by forced drainage using a pump from this stage.
(2) The segment 24 is assembled on the floor excavation surface 16 on the lowermost cutting edge 29 (completed well) (FIG. 8).
(3) Excavation from the outside of the well with a clamshell or from the inside of the well with a backhoe while the groundwater level 11 is lowered by forced drainage by a pump from Kamba 12, and the blade edge 29 is excavated manually.
(4) At the stage where the well has settled down to a predetermined position, the blade 29 is left so as not to sink the well body, and the inside of the well is further excavated to a certain depth.
(5) As the water collection permeable layer 42, the chestnut or crushed stone is thrown to a predetermined height, and the upper surface is formed.
(6) The steel water collecting frame 3 and the water collecting impermeable surface 4 which are assembled in advance are lowered from the upper portion of the well and installed on the water collecting and permeable layer 42 formed in (5).
(7) Putting earth and sand on the steel plate of the water impermeable surface 42, compacting, and laying the crushed stone or crushed stone 13 as a main construction foundation.
(8) On the upper part of the foundation work in (7), the first concrete is spread and concrete 14 is placed, and the bottom plate concrete is placed on the upper part with the side plate lower part on the side.
(9) After that, installation of manholes, placement of top slab concrete, etc., backfilling to the initial ground level, etc. will be completed, and construction will be completed.

  In general, in underground construction where grounds where liquefaction measures need to be taken, as shown in FIG. 1, the groundwater level 11 is high, and from the topsoil excavation (1) above, It is necessary to perform a step of reducing the pressure, and forced drainage by the pump is difficult at the stage after (4) above. Even in such a situation, an effective liquefaction countermeasure can be easily implemented by performing the construction according to the procedure shown above. Moreover, when performing concrete placement as the water collection frame 3, after the above (5), construction can be performed by placing underwater concrete after installation of the steel plate of the water collection impermeable surface 4 assembled in advance. . In addition, when forced drainage is possible in the stage after (4), a construction procedure using a rubber sheet or the like as the water collection impermeable surface 4 can be adopted as shown below.

(Construction procedure when using a rubber sheet, etc. as the water impermeable surface 4)
Concerning (4) above, forced drainage with a pump is further performed to lower the groundwater level, excavate natural ground, and form.
Regarding (5) above, a water-permeable mat is installed as the water-collecting water-permeable layer 42 on the natural ground formed in (4), and a rubber sheet or the like is installed on the upper surface.
There is no process of (6) above, and the process continues from (7) onwards.

  FIG. 4 shows an embodiment in which four vents 5 are arranged on the peripheral edge portion of the lower end surface of the base portion. FIG. 4 (2) shows a bottom view of the water impermeable surface 4 in which 8 panels are combined. A conceptual diagram of water collection is shown in Fig. 7 (3-b).

  FIG. 9 shows an example in which a water collection frame groundwater inflow hole 31 is provided in the water collection frame 3 around the vent 5. It is desirable to take measures to prevent the inflow of earth and sand from the surrounding ground such as a water permeable mat and a chestnut maker on the outer ground side of the water collecting frame groundwater inflow hole 31 installed in the water collecting frame 3. With such an arrangement of the water collecting frame groundwater inflow hole 31, the groundwater flowing into the vent 5 through the water collecting impermeable surface 4 from the water collecting permeable layer 42 and the water collecting frame groundwater inflow hole 31 from the lower side of the underground structure side surface. However, the latter is intended for groundwater that may rise near the boundary of the structure and is not intended for draining in a short time like the former. .

  As described above, the position of the water collecting frame 3 overlaps with the periphery of the entire structure or the foundation of the structure, and the water collecting frame 3 and the water impermeable surface 4 cover all the lower part of the structure. However, depending on the construction method, it may be difficult. The method of dropping the pre-assembled water collection frame and water collection impervious surface with the pipe subsidence method hinders the main girder part of the side plate segment and the edge of the bottom plate, etc. It must be installed inside. FIG. 9 of the water collection frame groundwater inflow hole of Example 5 is illustrated as such a case.

  In the embodiment, the example in which the plurality of vents 5 are provided in the positions close to the water collecting frame 3 installed on the periphery of the underground structure in the embodiments 2 and 4, but the underground structure such as an underground water storage tank is shown. When the vent 5 is installed at the center of the object or its surroundings, there are many cases in which the use as a water storage tank is hindered, and it may be desirable to install it at the peripheral part of the underground structure for the management of the underground structure. Many. Further, regarding the installation of the water collection frame groundwater inflow hole 31, when the vent 5 is in contact with a position close to the water collection frame 3, as shown in the fifth embodiment, the water collection permeation is performed only by the water collection frame groundwater inflow hole 31. Although it is possible to communicate with the layer 42, when there is a vent 5 at the center as in the first embodiment, when the groundwater from the outer ground above the water collection frame 3 is allowed to flow, the water collection impervious surface 4 is penetrated to collect the water. A pipe that communicates the water frame groundwater inflow hole 31 and the water collection permeable layer 42 is required. Therefore, when a plurality of vents 5 can be installed, the advantage of installing them at positions close to the water collecting frame 3 is great.

  In the embodiment, regarding the ground liquefaction of the underground water tank at the time of earthquake, it can be considered mainly as a countermeasure for the levitation of the water tank, but unlike the embodiment, when the unit volume weight of the entire underground structure is a large structure Although the structure does not rise due to excessive pore water pressure, repeated shearing in an undrained state causes a muddy water state and loses its supporting force and sinks. Even for such a structure, by taking the measures of the present invention, such a non-drained state can be limited to a very short time, so the subsidence of the underground structure due to such ground liquefaction It can be a countermeasure.

  As described above, the adverse effects caused by the liquefaction of the ground are caused by the process in which loosely packed sand saturated with water attempts to shrink the volume of the sand ground by repeated shearing associated with the shaking of the earthquake. In the present invention, the repeated shearing is performed in a drained state as much as possible. This is intended to limit the volume shrinkage process in non-drainage, and in the present invention, the subsidence of the underground structure due to the volume shrinkage cannot be completely prevented. However, the subsidence of underground structures due to this volume contraction is considered to be small in scale as compared to the subsidence of the entire liquefied ground layer in the muddy water state, and it is not likely to be a major obstacle.

1 ground, 11 groundwater level, 12 Kamaba, 13 foundation crushed stone or crushed stone, 14 leveling concrete, 15 topsoil excavation surface, 16 floor digging surface, 17 backfill soil
2 underground storage tank, 21 top plate, 22 suction pipe insertion hole, 23 side plate, 24 segment, 25 segment joint, 26 girder, 27 bottom plate, 28 water collection pit 29 cutting edge
3 Catchment frame, 31 Catchment frame groundwater inlet
4 water collection impermeable surface, 41 apex, 42 water collection permeable layer
5 Vent, 51 Vent upper end, 52 Vent lower end

Claims (4)

  A frame continuously closed in a strip shape along the peripheral edge of the lower end surface of the base part of the underground structure or the underground structure, and a water collecting frame projecting downward from the lower end surface, and a lower part of the lower end surface A water impermeable surface that divides the ground vertically into two, having one or a plurality of apex portions according to an inclination in the vertical direction, the periphery of the water collecting impermeable surface reaching the water collecting frame, and the water collecting A water-collecting water-permeable layer provided below and in contact with the water-impermeable surface, and an upper end that reaches at least the groundwater level around the underground structure and is opened, and the lower part penetrates the water-collected water-impermeable surface at a location including the apex portion. An underground structure liquefaction countermeasure structure comprising: a tubular vent having a lower end surface that reaches the water-permeable permeable layer and is opened.   2. The liquefaction countermeasure structure for an underground structure according to claim 1, wherein the water impermeable surface of claim 1 is a lower surface of a rigid impermeable plate.   The liquefaction countermeasure structure for an underground structure according to claim 1 or 2, wherein a plurality of the vents according to claim 1 are installed at positions close to the water collecting frame.   A hole provided in the water collecting frame of claim 1, wherein a water collecting frame groundwater inflow hole is provided to communicate the ground outside the water collecting frame and the water permeable layer of claim 1. A liquefaction countermeasure structure for an underground structure according to claim 2 or claim 3.

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