JP4903467B2 - Improved ground structure considering ground flow countermeasures during liquefaction - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an improved ground having a more rational structure against ground fluidization. <P>SOLUTION: The improved ground is constructed in such a way that the left/right of a wall facing an estimated direction of fluidization of the ground has slopes exceeding 90 degrees in a plan sectional view. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention relates to an improved ground structure for improving a ground where liquefaction due to an earthquake is likely to occur.

  Typically, sandy ground is well known to undergo ground liquefaction during an earthquake. It is also known that when such a ground is located near the shoreline of a coastal area or on a gentle slope, fluidization of the ground accompanying such liquefaction occurs.

  For example, when the ground is liquefied, the flow shown by (a) on the slope as shown in FIG. 6, and the displacement shown by (b) on the side opening surface (represented by the movement of the trees, respectively). Occurs.

In fact, in the Hyogoken-Nanbu Earthquake, residual displacement has occurred in the pier foundations, etc. due to fluidization of the ground, and various guidelines indicate that countermeasures against this phenomenon are necessary. As an example, in the Road Bridge Specification V1 Seismic Design H14.3 by the Japan Road Association, the foundation structure within a certain range from the shoreline is designed in consideration of the external force generated by the flow as shown in FIG. It is obligatory.
Road Bridge Specification V Seismic Design Edition Japan Road Association March 2002

Various improved grounds for coping with liquefaction of the ground have been proposed. As an example, the improved ground construction method (TOFT construction method) by the present applicants (refer to Japanese Patent Laid-Open No. 59-96320: Japanese Patent No. 2568115), the plane cross-section becomes a grid as shown in FIG. In this case, the ground with a grid width 0.5 to 0.8 times the height of the wall is used as an improved ground, which restrains the shear deformation of the liquefied ground during an earthquake and reduces excess pore water pressure. To prevent the occurrence and, as a result, protect the structure, prevent large deformation, and at the same time, transmit the load of the structure to the lower non-liquefied ground through the improved body so as to support it stably. ing.
JP 59-96320 A

  Such an improved ground is designed to have a size as shown in FIG. 9 when viewed in plan view in order to cope with not only liquefaction but also the flow of the liquefied ground as described above. In FIG. 9, 1 is a structure (the basis), 2 is a range to deal with liquefaction, and 3 is an improved composition that fully considers the ground flow. That is, the flow force indicated by the arrow on the left side is received by one side wall of the structure having a lattice-like structure in a plane cross section, and the frictional force on the bottom surface (by placing on the non-liquefied layer at the bottom, or by rooting, etc.) ) Is designed to be particularly long in the direction of the flow force.

The reason for this is that in addition to solid mechanical elements such as active earth pressure, passive earth pressure, hydrostatic pressure, residual water pressure, and inertial force during earthquake, the external force assumed when designing flow countermeasures is This is because the hydrodynamic pressure is taken into consideration in consideration of the behavior of the liquefaction (see, for example, “liquefaction countermeasure design / construction manual (draft)”).
Therefore, when implementing ground flow countermeasures by the TOFT method, the state of completely liquefied ground is considered as fluid out of the external force acting on the improved body, and the hydrodynamic pressure component due to the flow is added. In order to resist it, the width of the improved body becomes large in order to ensure the frictional resistance and the like of the bottom surface, resulting in an uneconomical section.
This is based on the current design method that allows 100% of the hydrodynamic pressure acting on the wall of the improved body, and there are no examples or experimental data that measure the hydrodynamic pressure that acts on the design, so this design on the safe side is possible. This is because it must be.
However, hydrodynamic pressure is a pressure generated by hydrodynamic behavior, and is a different kind of pressure from the external force based on solid mechanics that has been considered in the design of structures so far. Not.
Liquefaction countermeasure design / construction manual (draft)

In addition, the ground is in a completely liquefied state and the time for acting as a fluid is finite, and the ground gradually returns to a solid state due to the dissipation of pore water pressure within a few tens of minutes after the earthquake. However, in the current design, these behaviors are not considered at all.
According to the research so far, the duration of the fluid behavior when the ground with a liquefied layer thickness of 5 m and the initial gap ratio indicating ground density of 0.75 is completely liquefied is about 60 seconds, The flow behavior ends linearly from the bottom of the liquefied layer and returns to solid within time (i.e., the bottom of the liquefied layer returns to solid immediately after the earthquake, and the thickness of the solidified layer is approximately proportional to time toward the ground surface. Will increase).
In the current design, this phenomenon is not taken into consideration at all, and 100% hydrodynamic pressure is applied to the entire liquefied layer thickness.
Other countermeasure methods for ground fluidization due to liquefaction have specifications that are more or less uneconomical, similar to the TOFT method as described above.

  The present invention has been made in view of the above-described problems of the conventional example, and an object of the present invention is to show a rational and economical ground improvement specification and its design method as a countermeasure against ground flow. To this end, we solve the unreasonableness caused by applying 100% of the hydraulic pressure acting as a fluid, and propose a shape that allows rational design by relaxing the hydraulic pressure with a shape that takes fluid characteristics into consideration. .

According to the first aspect of the present invention, the structure is arranged on the upper side to support the structure, and the area viewed in a plane cross section is sized to include the entire improved range for dealing with liquefaction required for the structure. In addition, the right and left sides of the wall to be opposed to the expected flow direction of the ground are inclined at an angle of more than 90 degrees and less than 135 degrees when viewed in a plane cross section, with the sliding resistance of the bottom part characterized by Improved ground is provided to counteract fluid forces.
As a result, the improved ground has a reasonable plane cross section and can resist the ground flow. In addition, the improved ground does not impair the cross-sectional area that should deal with liquefaction.

According to the second aspect of the present invention, there is provided an improved ground characterized in that the area of the inclined portion is gradually increased in the height direction in order to cope with the temporal behavior of the excess pore water pressure. .
As a result, the improved ground has a structure that matches the temporal behavior of excess pore water pressure in the height direction.

ここにＰ：動水圧
ρ：流体の密度
Ｑ：流量
υ_Ｏ：流速
θ：動作水圧（線）の上流方向と固体壁がなす角度
ここで、今までは図９に示したように動水を直角（θ＝９０度）な壁部で対抗させる設計がなされているが、この式から固体壁の角度を９０度を越えて１８０度の範囲とすると、このような固体壁に作用する動水圧は、直角な固体壁に作用する動水圧に比べて低減されていくことが明らかである。
したがって、例えば図１に示すように、改良地盤の動水圧側の壁を平面視で１３５度（動水圧の仮想の延長線からすると４５度）とすると、仮に流動化した土砂の流れを完全に流体であるとすると、動水圧の低減は、従来のように直角の壁によって対抗する場合に比べて動水圧は約３割低減され、改良地盤構造は材料の無駄の少ない構造となることが明らかである。
永井壮七郎著「水理学」コロナ社発行 Study on reduction of hydrodynamic pressure The hydrodynamic pressure exerted on a stationary solid wall is expressed by the following equation ("Hydrophysics", published by Corona, page 286).

Where P: dynamic water pressure
ρ: Fluid density
Q: Flow rate
υ _O : flow velocity
θ: Angle formed by the upstream direction of the operating water pressure (line) and the solid wall Here, up to now, as shown in FIG. 9, the design is made to counteract the dynamic water with a wall portion having a right angle (θ = 90 degrees). However, if the angle of the solid wall exceeds 90 degrees and is in the range of 180 degrees from this equation, the hydrodynamic pressure acting on such a solid wall is reduced compared to the hydrodynamic pressure acting on the solid wall at right angles. It is clear.
Therefore, for example, as shown in FIG. 1, if the wall on the hydrodynamic pressure side of the improved ground is set to 135 degrees in plan view (45 degrees from the virtual extension line of the hydrodynamic pressure), the fluidized sediment flow is completely removed. If it is a fluid, it is clear that the hydraulic pressure is reduced by about 30% compared to the conventional case where it is opposed by a right-angle wall, and the improved ground structure is a structure with less waste of material. It is.
Published by Corona Co., Ltd.

As described above, the dynamic hydraulic pressure can be reduced as the inclination angle on the dynamic hydraulic pressure side in the plane cross section exceeds 90 degrees and approaches 180 degrees. On the other hand, from the viewpoint of securing the planar range of the region that prevents liquefaction. The planar inclination angle should be as small as possible. Therefore, it is considered that a range exceeding 90 degrees and not more than 135 degrees is a reasonable range.
In addition, when there are a plurality of assumed flow directions, it is possible to reduce the dynamic water pressure by installing a plurality of inclined portions in a plane cross section as shown in FIG.

  Moreover, as shown in FIG. 3, although the improvement range required for a structure may be larger than the improvement range for hydrodynamic pressure (the improvement range required for the structure 1 is indicated by a dotted line 2), in this case, Conventionally, by giving an angle of, for example, 135 degrees only to the corners on both sides of the hydrodynamic pressure side of the improved ground, which has been rectangular in plan view, it is rational and wasteful in terms of hydrodynamic pressure and the scope of improvement. A structure without any will be obtained.

Considering the time course of excess pore water pressure The liquefied ground gradually transforms from fluid to solid as the excess pore water pressure dissipates, but this process proceeds from the bottom of the liquefaction layer. Since the layer showing the behavior gradually becomes thinner in the surface layer direction, it is considered that the inclined portion in the plane cross section as described above is not necessarily uniform over the entire improved depth. For example, considering that the time for fluid behavior is short and the action time of hydrodynamic pressure is short at the bottom of the liquefied layer, the inclined portion in the plane cross section may not be large.
From these, the notch of the slope for coping with the hydrodynamic pressure in the plan view of the above-mentioned improved ground is stepped toward the upper surface of the liquefied layer having a long fluid behavior as shown in FIG. 4 or FIG. It is conceivable that the design is made larger or the inclination angle itself is increased toward the upper surface of the liquefied layer.
In any case, this concept is more rational for the TOFT method where a large contact surface is desirable in order to increase sliding resistance in the deep part, and gives a structure that does not cause safety problems. it is conceivable that.
Proceedings of the 26th Earthquake Engineering Research Presentation “Particle Behavior in Liquefied Ground” (August 2001)

It is a plane section of one example of the improved ground by the present invention. It is a plane cross section of another Example of the improved ground by this invention. It is a plane cross section of another Example of the improved ground by this invention. It is a perspective view of another Example of the improved ground by this invention. It is a perspective view of another Example of the improved ground by this invention. It is a figure which shows the flow of the ground by liquefaction. It is a figure which shows what should cope with the flow of a liquefied layer. It is an improved ground in the TOFT method. It is plane sectional drawing which shows the design of the improved ground of a prior art example.

Claims (2)

Place the structure above to support the structure, and make the area seen in the plane cross section to include the entire improved range to cope with liquefaction required for the structure, and in the expected flow direction of the ground An improved ground that counteracts the fluid force of the ground by sliding resistance at the bottom, wherein the right and left sides of the walls to be opposed are inclined at an angle of more than 90 degrees and not more than 135 degrees when viewed in plan. 2. The improved ground according to claim 1, wherein the area of the inclined portion is gradually increased in the height direction so as to correspond to the temporal behavior of the excess pore water pressure.

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