JP6365979B2 - How to prevent ground liquefaction - Google Patents

Description

  The present invention mainly relates to a ground liquefaction prevention method for preventing liquefaction of ground such as sandy ground under a structure.

  In the ground such as sandy ground that is saturated (hereinafter referred to as liquefaction layer), the pressure between water (pore water) in the ground suddenly increases due to the impact of earthquake motion, etc. The phenomenon that the balance is broken and the earth and sand behave like a liquid, that is, the liquefaction phenomenon of the ground is regarded as a problem.

  Conventionally, from the viewpoint of disaster prevention, it is necessary to take measures to prevent liquefaction of the ground in advance from the viewpoint of disaster prevention, and as a countermeasure against liquefaction, the liquefied layer is surrounded by a water shielding wall, There is a known groundwater level lowering method that prevents ground liquefaction by pumping the groundwater in the impermeable wall to lower the groundwater level of the liquefied layer and making the upper layer unsaturated from the lowered groundwater level. (For example, refer to Patent Document 1).

  In this groundwater level lowering method, a water collection pipe with a well point at the lower end is embedded in the liquefied layer, and the groundwater is pumped up by a pump installed on the ground through this water collection pipe. There is a deep well method in which a water well is formed, a pumping means is installed at the bottom of the catchment well, and groundwater is discharged to the ground by this pumping means.

  On the other hand, in this type of groundwater level lowering method, the lower the groundwater level, the higher the effect of preventing liquefaction.However, as the groundwater level decreases, the effective stress increases and the mounting pressure increases. When a clay layer is present, there is a problem that consolidation settlement occurs when the water level is greatly lowered.

  Therefore, in the past, in view of the fact that the liquefaction strength is improved by increasing the confining pressure for the ground below the lowered groundwater level, the groundwater level is set to a predetermined depth from the ground without significantly lowering the groundwater level. By maintaining in a lowered state, both the effect of preventing liquefaction and the prevention of land subsidence are achieved.

Regarding the amount of groundwater level reduction (how many meters below natural groundwater level) in this type of groundwater level lowering method, the safety factor against liquefaction of each layer at a given depth from the ground surface based on the ground survey results ( liquefaction resistance; hereinafter, to calculate the F _L value hereinafter), to evaluate the safety against ground liquefaction layers on the basis of the F _L value (hereinafter, taken in principle be determined by F _L method) Yes.

Specifically, a layer of F _L value is 1 or less is determined that the layer of potential liquefaction, so that the layer is determined that there is a possibility of liquefaction is included in the scope of groundwater drawdown method Determine the amount of groundwater level drop.

However, the magnitude of F _L value, the depth, even though the different effects the degree and ground structures liquefaction in each by a layer thickness and the like, F _L value based only on the F _L method all of 1 or less by setting the amount of decrease in groundwater level as the layer is improved, it is inevitable to reduce significantly the groundwater level, since the consolidation settlement of the problems described above can occur, lowering of groundwater level based only on F _L Law It was difficult to determine the amount.

Therefore, in general, index indicating the degree of impact of liquefaction (liquefaction index; hereinafter, P _L value of) introduced, the magnitude of F _L values, depth, degree of liquefaction in each by layer thickness, etc. and ground the influence the structure so as to comprehensively evaluate (hereinafter, referred to as P _L method), while setting the amount of decrease in groundwater level as P _L value is 5 or less, lowering of the groundwater level By suppressing the amount from the ground surface to a desired depth, both the effect of preventing liquefaction and the prevention of land subsidence are achieved.

JP 2004-263379 A

  However, in a liquefied layer where the pore water pressure has increased due to a large earthquake (hereinafter referred to as the main shock) and the risk of liquefaction or liquefaction has increased, the pore water pressure remains even after the liquefaction phenomenon due to the shock of the main shock is stopped or avoided. Since it is in a high state, there is a risk of liquefaction due to small aftershocks that occur frequently thereafter, and liquefaction due to this aftershock has become a major problem that causes damage to spread.

Therefore, in the conventional groundwater drawdown method for determining the amount of decrease in the groundwater level on the basis of the above-mentioned P _L method, F _L value is 1 or less liquid layer is left in a lower layer than the groundwater level was lowered, its liquefaction There was a risk that the stratum would be liquefied by an impact smaller than that assumed by the aftershocks that occur frequently after the main shock.

  Therefore, in view of such conventional problems, the present invention provides a ground liquefaction prevention method capable of minimizing the damage of reliquefaction due to aftershocks while achieving both a liquefaction prevention effect and ground subsidence prevention. It was made for the purpose of providing.

  The feature of the invention according to claim 1 for solving the conventional problems as described above is that a water collecting body is installed in the liquefied layer, and the water flowing into the water collecting body is pumped by a pumping means, and the liquefied layer In the ground liquefaction prevention method for preventing liquefaction of the ground by lowering the groundwater level, a pumping control means for controlling the pumping amount of the pumping means and a seismic motion detection means for detecting seismic motion are used. The groundwater of the liquefied layer is pumped by means, and the groundwater level is lowered to a predetermined normal water level, and then the normal water level is controlled by controlling the pumping amount of the pumping means, while the ground motion detection means Is detected, the amount of pumping of the pumping means is increased and the groundwater level is lowered in advance to a level lower than the normal water level, which is lower than the normal water level, and the water level during the earthquake is maintained for a certain period of time. The water level during the earthquake Further, the present invention is in a ground liquefaction prevention method for returning to the normal water level.

  A feature of the invention described in claim 2 is that, in addition to the structure of claim 1, a water shielding wall is provided around the liquefied layer.

  The feature of the invention described in claim 3 is that, in addition to the configuration of claim 1 or 2, the normal water level is determined by repeating the operation and stop of the pump constituting the pumping means as appropriate while sequentially measuring the groundwater level. When the ground motion is detected, the operation of the pump is continued for a certain period of time, and the groundwater level is lowered to the earthquake water level that is lower than the normal water level that has been lowered in advance. The pump is stopped until the groundwater level returns to the normal water level, and then the normal water level is maintained by appropriately repeating the operation and stoppage of the pump.

  The feature of the invention described in claim 4 is that, in addition to the structure of any one of claims 1 to 3, the period during which the water level during the earthquake is maintained is an aftershock frequent period.

  In the ground liquefaction prevention method according to the present invention, as described above, a water collecting body is installed in the liquefied layer, the water flowing into the water collecting body is pumped by the pumping means, and the groundwater level of the liquefied layer is lowered. In the ground liquefaction prevention method for preventing ground liquefaction, a pumping control means for controlling the pumping amount of the pumping means and a seismic motion detecting means for detecting earthquake motion are used, and the liquefaction by the pumping means is used. After pumping up the groundwater of the stratum and lowering the groundwater level to a predetermined normal water level, while controlling the pumping amount of the pumping means and maintaining the normal water level, when detecting the ground motion by the ground motion detection means Increases the pumping amount of the pumping means and lowers the groundwater level to an earthquake level lower than the normal water level that has been lowered in advance, and maintains the water level during the earthquake for a certain period of time, and then changes the groundwater level to the water level during the earthquake. More normal By returning to the water level, it is possible to achieve both the liquefaction prevention effect due to the groundwater level drop and the consolidation settlement prevention effect at normal times, and at the time of an earthquake, quickly improve the liquefaction prevention effect due to the groundwater level drop. Can do. Moreover, long-term consolidation settlement can be prevented by limiting the period during which the water level during an earthquake is maintained.

  Moreover, in this invention, the fall of the groundwater level by pumping can be efficiently performed by installing a water-impervious wall around the said liquefied layer.

  Furthermore, in the present invention, while continuously measuring the groundwater level, maintaining the normal water level by repeating the operation and stopping of the pump constituting the pumping means as appropriate, and detecting the ground motion, the operation of the pump The groundwater level is lowered to the level at the time of the earthquake that is lower than the normal water level that has been lowered in advance for a certain period of time, and after the certain period of time, the pump is turned on until the groundwater level returns to the normal water level. By stopping and then maintaining the normal water level by repeating the operation and stop of the pump as appropriate, complicated control is not required, and the water level that has been lowered in advance is preferably lower when an earthquake occurs Can be lowered into position.

  Furthermore, in the present invention, liquefaction due to aftershocks can be efficiently prevented by setting the period during which the water level during the earthquake is maintained as a frequent aftershock period.

(A) The flowchart which shows an example of the ground liquefaction prevention method which concerns on this invention, (b) is a flowchart which shows an example of a normal groundwater level fall process. It is sectional drawing which shows the state of the normal time same as the above. It is sectional drawing which shows the state at the time of the earthquake occurrence same as the above.

  Next, an embodiment of the ground liquefaction prevention method according to the present invention will be described based on the examples shown in FIGS.

  In the figure, reference numeral 1 denotes a structure such as an oil tank, reference numeral 2 denotes a ground liquefaction layer, reference numeral 3 denotes a clay layer under the liquefaction layer 2, and reference numeral NL denotes a natural groundwater level. Further, in the present embodiment, the liquefied layer 2 is a layer that is in a saturated state such as water-containing sandy ground and is likely to cause a liquefaction phenomenon.

  The ground below the structure 1 has a liquefied layer 2 made of sandy ground or the like. In the ground liquefaction prevention method according to the present invention, a plurality of water collecting bodies 4 are directed vertically to the liquefied layer 2. , 4 is pumped up, the water flowing into the water collectors 4, 4 is pumped up by the pumping means 5, and the groundwater level of the liquefied layer 2 is lowered to make the upper layer above the groundwater level unsaturated, thereby liquefaction of the ground It comes to prevent.

  Further, in this ground liquefaction prevention method, the pumping control means 6 for controlling the pumping amount of the pumping means 5 and the seismic motion detecting means 7 for detecting the seismic motion are used, and the groundwater level is greatly reduced in normal times. By maintaining the groundwater level at a predetermined depth (normal water level WL1) from the ground without causing the earthquake to occur, both earthquake prevention and ground subsidence can be achieved while preparing for an earthquake. Has been designed to prevent liquefaction due to the mainshock and reliquefaction due to aftershocks by rapidly and significantly lowering the groundwater level to the water level WL2 during an earthquake.

  The water collectors 4, 4 are wells having a depth reaching the lower end of the liquefied layer 2, and groundwater that has flowed into the water collectors 4, 4 by the pumping means 5 is discharged to the ground.

  The water collectors 4, 4 are provided with strainer portions (not shown), and groundwater flows into the water collectors 4, 4 through the strainer portions.

  The pumping means 5 includes a pump 5a such as one or a plurality of submersible pumps installed at the bottom of the water collecting bodies 4 and 4, and a pumping pipe 5b inserted into the water collecting bodies 4 and 4, and a lower end of the pumping pipe 5b. Is connected to the pump 5a, and the pump 5a is operated to pump the water flowing into the water collecting bodies 4 and 4 through the pumping pipe 5b to the ground.

In addition, the installation position of the pump 5a is appropriately determined based on the pump performance, ground conditions, and the like. For example, when the pump 5a capable of pumping about 1000 L / min is used, it is usually one place in 2000 to 4000 m ^2. Install at the rate of.

  In addition, the pumping means 5 can control the pumping amount by controlling the pump operation by the pumping control means 6.

  The pumping control means 6 is composed of computer equipment including an arithmetic unit, and the seismic motion detection means 7 and the groundwater level measurement means 8 are connected by wire or wirelessly, and transmitted from the seismic motion detection means 7 and the groundwater level measurement means 8. The pumping means 5 can be controlled based on the measured data.

  The groundwater level measuring means 8 uses a groundwater level gauge and transmits the measured groundwater level data to the pumping control means 6 as needed.

  The seismic motion detection means 7 includes, for example, an accelerometer installed on the ground, measures acceleration caused by seismic motion and the like, and transmits it to the pumping control means 6 as needed.

  The water-impervious wall 9 is formed in a continuous shape so that the wall embedded up to a position deeper than the bottom of the liquefied layer 2 surrounds the liquefied layer 2, and the flow of water is blocked between the ground inside and outside the wall. Yes.

  Although the aspect of the wall body is not particularly limited, for example, sheet piles such as steel sheet piles and steel pipe sheet piles driven deeper than the bottom of the liquefied layer 2 are connected to each other, and the joints between the sheet piles are water-stopped with a water-stop material. It may be constructed by doing so, and may be a precast concrete or a concrete structure by casting in place.

  Next, a specific ground liquefaction prevention method using the above-described configuration will be described.

First, as a pre-stage of the method of the present invention, the state of the liquefied layer 2 is investigated by a pumping test or the like. evaluation method (hereinafter, referred to as P _L method) by thinning index sets the normal time water level WL1 used.

Specifically, the safety factor for the liquefaction of each layer from the surface to a predetermined depth (liquefaction resistivity; hereinafter, F _L value hereinafter) and F _L index indicating the degree of influence by liquefaction based on value (liquid exponent; hereinafter investigated using P _L value of), when lowering the groundwater level can be secured below P _L value of 5, and lower than the natural groundwater level NL, in view of the effect on surface displacements The normal water level WL1 is set to a depth that is a certain depth (distance) away from the ground surface and is suppressed to such a level that an increase in effective pressure due to a decrease in groundwater level does not lead to significant consolidation settlement.

Incidentally, F _L value, as shown in the following equation is defined by the ratio of the equivalent cyclic shear stress ratio L generated in each depth in the soil examination point and liquefaction resistance ratio R, F _L value If the value is smaller, the resistance to liquefaction is smaller, and if the _FL value is 1 or less, it is determined that there is a possibility of liquefaction.

Here, τ _l is the dynamic shear strength (kN / m ² ), τ _d is the equivalent constant repeated shear stress amplitude (kN / m ² ) generated on the horizontal plane, and σ ′ _z is the effective earth cover pressure (kN / m). ² ).

On the other hand, P _L value, as shown in the following equation, multiplied by the weight function w (z) relating to the depth from the ground surface to F _L value in each layer, which it is defined by integrating in a desired depth range , if the comparison with the past damage case P _L value is 5 or less, it is determined that a low risk of liquefaction.

Where F = 1−F _L (F _L <1.0), F = 0 (F _L ≧ 1.0), D is the depth (m), and w (z) = 10 for the weight function. -0.5z is used.

  The normal water level WL1 has an upper limit and a lower limit in the depth direction, and vertical movement is allowed within a certain range between the upper limit and the lower limit.

  Next, the normal groundwater level lowering step (S1) is actually started.

  First, the pumping means 5, that is, all pumps 5 a are fully operated (S 11), and the pumping of groundwater flowing into the water collectors 4 and 4 is started. As shown in FIG. 2, the normal water level from the natural groundwater level NL is started. Reduce to WL1.

  At that time, the groundwater level is sequentially measured by the groundwater level measuring means 8 (S12), and the operation of all the pumps is continued until the groundwater level reaches the normal water level WL1 (L11). S13), the pumping control means 6 controls the amount of pumping by the pumping means 5 based on the measurement data from the groundwater level measuring means 8 (S14), and maintains the groundwater level at the normal water level WL1.

  In order to maintain the groundwater level at the normal water level WL1, for example, as shown in FIG. 1B, the step of stopping the pump 5a when the groundwater level falls below the normal water level WL1 (S14), and the groundwater level When the water level rises above the normal water level WL1, the step of operating the pump 5a (S11) is repeated. In order to maintain the groundwater level at the normal water level WL1, control may be performed by adjusting the output of the pump 5a.

  Then, the normal groundwater level lowering process is repeated until an earthquake motion is detected (L1), and the groundwater level is maintained at the normal water level WL1 as shown in FIG.

  In the state where the groundwater level is maintained at the normal water level WL1, since the groundwater level is maintained lower than the natural groundwater level NL by a desired depth, the ground 2a above the normal water level WL1 is Since the liquefaction strength of the lower layer 2b below the normal water level WL1 is improved by increasing the restraining pressure, even if a certain-scale earthquake (hereinafter referred to as the mainshock) occurs. Liquefaction can be suitably prevented.

  In this state, when a mainshock occurs, the ground motion is detected by the ground motion detection means 7 (S2), the detection signal is transmitted to the pumping control means 6, and the pumping control means 6 increases the output of the pumping means 5. That is, all pumps 5a are fully operated to increase the amount of pumped water (S3), and as shown in FIG. 3, the groundwater level is lowered to an earthquake level WL2 that is lower than the normal level WL1.

Incidentally, seismic water level WL2, the evaluation method based on F _L value (hereinafter, referred to as F _L method) using, in the depth was significantly reduced from normal time water level WL1, remaining lower than normal time water level WL1 It is preferable that all layers having a _FL value of 1 or less are set so as to become unsaturated due to a decrease in groundwater level (so as to be included in the improved range), and further, the lower end of the liquefied layer 2 It is preferable that the depth reaches the depth.

  At that time, since the groundwater level is lowered to the normal water level WL1 which is lower than the natural groundwater level NL in advance, it can be lowered from the occurrence of the main shock to the water level WL2 during the earthquake in a short time, and the pore water pressure increases due to the groundwater level drop Since the water in the liquefied layer 2 is removed and saturated, liquefaction due to aftershock impacts can be prevented.

Specifically, when a pump 5a having a pumping performance of 1000 L / min is used in a sand ground with a porosity n = 40%, if the pump 5a is fully operated for 24 hours from the occurrence of an earthquake, 1440 m ³ per day. Since groundwater can be excluded, the amount of groundwater level fall is calculated as groundwater level drop = 1440 / (operating range area per pump × porosity n), and a single pump was installed at 2000 m ² . in the case, the normal time the water level WL1 1.8m (= 1440 / (2000 × 0.4) can be lowered groundwater level in 1 day, the case of installing the one place pump 4000 m ^2, normal time in one day The groundwater level of 0.9 m (= 1440 / (4000 × 0.4) can be lowered from the water level WL1.

  And, for a certain period, for example, during the aftershock frequent period, the pumping amount is increased, that is, the full operation of all pumps 5a is continued (L2), and the groundwater level is greatly lowered from the natural groundwater level NL. The water level WL2 is maintained, and the water above the water level WL2 at the time of earthquake in the liquefied layer 2 is removed to make it unsaturated, thereby preventing liquefaction due to aftershocks.

  Although the aftershock frequent occurrence period is not particularly limited, it is generally set to about 24 hours when a relatively large aftershock is expected to occur. Further, the aftershock frequent occurrence period may be changed depending on the magnitude of the earthquake, and may be appropriately selected.

  Next, after a certain period, that is, aftershock frequent period (S4), all the pumps 5a are temporarily stopped (S5), the groundwater level is returned to the normal water level WL1 (L3), and thereafter, FIG. The normal groundwater level lowering step S1 in (a), that is, the state where the groundwater level is maintained at the normal water level WL1 is returned (L4).

  In the method of the present invention configured as described above, the groundwater level is normally lowered to the normal water level WL1, and the groundwater level is not significantly lowered. Therefore, the consolidation settlement is suppressed by suppressing an increase in effective stress. This prevents both the consolidation settlement effect and the liquefaction prevention effect due to lowering of the groundwater level.

  On the other hand, when an earthquake occurs, the groundwater level is decreased from the normal water level WL1 to the water level WL2 during the earthquake by detecting the ground motion and increasing the pumped water level. Can be firmly prevented.

  In addition, when the groundwater level is lowered to the water level WL2 at the time of the earthquake, the groundwater level is lowered to the normal water level WL1 in advance, so it can be lowered to the water level WL2 at the time of the earthquake in a short period of time, and it can respond quickly to earthquake motion and aftershocks. can do.

  Furthermore, the period during which the groundwater level is significantly lowered is limited to a certain period, for example, the frequent aftershock period, and after a certain period, the normal water level WL1 is restored. Can be prevented.

  In addition, although the above-mentioned Example demonstrated the example which used the deep well which installs the pump 5a in the bottom part of the water collection bodies 4 and 4 in the pumping means 5, you may use the well point which installs the pump 5a on the ground. .

  Moreover, although it is preferable to install the impermeable wall 9 around the liquefied layer 2 like the above-mentioned Example, it is not necessarily required to provide the impermeable wall 9.

NL Natural groundwater level WL1 Normal water level WL2 Earthquake water level 1 Structure 2 Liquefaction layer 3 Clay layer 4 Water collector 5 Pumping means 5a Pump 5b Pumping pipe 6 Pumping control means 7 Seismic motion detecting means 8 Groundwater level measuring means 9 Impermeable wall

Claims (4)

In the ground liquefaction prevention method of installing a water collecting body in a liquefied layer, pumping water flowing into the water collecting body by a pumping means, and preventing ground liquefaction by lowering the groundwater level of the liquefied layer,
Using the pumping control means for controlling the pumping amount of the pumping means and the seismic motion detecting means for detecting the seismic motion,
After the groundwater of the liquefied layer is pumped by the pumping means, and the groundwater level is lowered to a predetermined normal water level, the amount of pumping of the pumping means is controlled to maintain the normal water level, and the seismic motion detection means When seismic motion is detected by the above, after increasing the pumping amount of the pumping means to lower the groundwater level to a lower earthquake level than the normal water level that has been lowered in advance, after maintaining the earthquake level for a certain period, A ground liquefaction prevention method, wherein the groundwater level is returned to the normal water level from the water level during the earthquake.   The method for preventing ground liquefaction according to claim 1, wherein a water-impervious wall is provided around the liquefied layer.   While continuously measuring the groundwater level, maintaining the normal water level by repeating the operation and stopping of the pump constituting the pumping means as appropriate, and when detecting earthquake motion, the pump operation is continued for a certain period of time. The groundwater level is lowered to the level at the time of the earthquake that is lower than the normal water level that has been lowered in advance, and after the fixed period has elapsed, the pump is stopped until the groundwater level returns to the normal water level, and thereafter The ground liquefaction prevention method according to claim 1 or 2, wherein the normal water level is maintained by appropriately repeating the operation and stoppage of the pump.   The ground liquefaction prevention method according to any one of claims 1 to 3, wherein a period of maintaining the water level during the earthquake is a frequent aftershock period.

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