JP3800525B2 - Ground saturation measurement method and liquefaction prevention method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for measuring the degree of saturation of the ground, and a method for preventing liquefaction of the ground using the same.
[0002]
[Prior art]
As is well known, the liquefaction of the ground means that the water pressure of the pore water that exists in a saturated state between the soil particles suddenly increases as the ground with a high water content undergoes deformation due to impact or vibration due to an earthquake. As a result, the friction resistance between soil particles disappears, and the ground behaves like a liquid and loses its proof strength.
[0003]
In order to prevent such liquefaction, techniques such as increasing the ground strength or injecting fine particles into the ground are known, but recently, a liquefaction prevention method by pumping groundwater is also effective. It is believed that. This is because, as shown in FIG. 6, the well 1 having the screen 1a is provided on the target ground, and the groundwater is continuously pumped from the well 1 to keep the groundwater level low. It is to eliminate pore water from the ground.
[0004]
However, in the liquid prevention method by pumping groundwater as described above, groundwater must be continuously and permanently pumped in order to keep the groundwater level low at all times. There is a concern that it will become enormous and consolidation settlement will occur due to a drop in groundwater level.
[0005]
Therefore, a method of preventing liquefaction by temporarily lowering the groundwater level and lowering the saturation level of the ground instead of constantly pumping up the groundwater and maintaining the groundwater level constantly low has been proposed.
[0006]
Here, the degree of saturation of the ground is an index indicating the ratio of the volume of pore water existing in the gap between the soil particles (volume of groundwater in the ground / volume of the gap in the ground). A completely filled state has a saturation degree of 100%, and in that state, liquefaction easily occurs, and it is considered that a normal liquefied ground is in such a saturated state. However, if a gas phase such as bubbles is mixed in the gaps between the soil particles, the saturation level is reduced to less than 100%. In such an unsaturated state, liquefaction is much less likely to occur. It is considered that the strength against liquefaction increases to about 1.5 times when the saturation level is 95%, and about twice as high when the saturation level is 90%. Moreover, if the gas phase such as bubbles is positively mixed in the gaps between the soil particles to lower the saturation, the gas phase parts such as the bubbles are easily trapped in the gaps between the earth particles. Therefore, it has the property that it stays there for a long period of time, so that the degree of saturation once lowered is maintained for a long period of time.
[0007]
Therefore, if groundwater is pumped from the ground where liquefaction should be prevented and the groundwater level is lowered temporarily, gas phase such as bubbles is mixed with pore water in the ground, and the degree of saturation is lowered. It is possible to increase the strength against liquefaction and prevent liquefaction, and if the degree of saturation is lowered once in this way, the unsaturated state is maintained for a long time, so even if pumping is stopped thereafter There is no hindrance, and as shown by the broken line in FIG. 6, even if the groundwater level naturally recovers, the effect of preventing liquefaction can be maintained as it is for a long time, which is considered to be an extremely effective method.
[0008]
[Problems to be solved by the invention]
By the way, in the method of preventing liquefaction by reducing the saturation of the ground as described above, it is naturally preferable to suppress the rise of the groundwater level in order to keep the saturation of the ground low over a long period of time. Therefore, it is preferable to surround the target ground with the water blocking wall 2 to suppress the natural rise of the groundwater level. However, in such a case, the groundwater level may eventually recover and return to saturation again. Therefore, in order to make such a liquefaction prevention method truly effective, the degree of saturation is regularly monitored, and when there is a concern that the degree of saturation increases and liquefaction occurs. Need to reduce the saturation again by re-pumping and lowering the groundwater level again.
[0009]
In that case, it is necessary to measure the degree of saturation, but at present there is no effective and appropriate method for measuring the degree of saturation. Therefore, sampling from the ground periodically and measuring the degree of saturation by laboratory tests, or As shown in FIG. 7, it is considered that the relative permittivity of the ground is measured by the relative permittivity measuring probe 3 and the measuring device main body 4 and the saturation is estimated therefrom, but both are troublesome labor and time. In addition, it is not always possible to measure with high accuracy, and the development of an effective and appropriate method for measuring the degree of saturation has been desired.
[0010]
In view of the above circumstances, an object of the present invention is to provide a method for easily measuring the degree of saturation of the ground, and an effective liquefaction prevention method using the measurement method.
[0011]
[Means for Solving the Problems]
The invention according to claim 1 is a method for measuring the degree of saturation as an index of strength against liquefaction of the ground, and measures the P-wave propagation velocity at each depth of the ground and obtains the degree of saturation from the measured value. Basic .
[0012]
In the invention of claim 1, the probe for measuring the P wave propagation velocity is inserted into the measurement hole provided in the ground, and the P wave propagation velocity at each depth is measured. In the measurement hole, A bellows-shaped well pipe made of a flexible material having a shape-retaining ring mounted at intervals is provided.
[0013]
The liquefaction prevention method of the invention of claim 2 measures the degree of saturation of the ground by the method of the invention of claim 1 , and according to the result, pumps water from the ground to lower the groundwater level, thereby saturating the ground. It is characterized by increasing the strength against liquefaction by decreasing the degree.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the saturation measurement method of the present invention will be described with reference to FIGS. The saturation measurement method of this embodiment measures the P wave propagation velocity (hereinafter referred to as V _P ) at each position below the groundwater surface, and obtains the saturation from the measured value.
[0015]
That is, in the ground in the saturation V _P is about 1600 m / s, the V _P in soil unsaturated state less than, it is known that V _P the lower the saturation is small, hence it is possible to determine the degree of saturation of the ground from the measured value by measuring the V _P of the ground.
[0016]
Therefore, in the measurement method of this embodiment, V _P at each position of the target ground is measured to obtain the distribution of V _P as shown in FIG. 4, and the relationship between the V _{P of the} ground and the degree of saturation is investigated in advance. Aside from this, the saturation level of the ground is determined. Specifically, by providing a measurement hole 5 as shown in FIG. 1 to a subject ground, insert the probe 6 for V _P measured with a built-in exciter therein, raises and lowers the probe 6 by winding mechanism 7 Meanwhile, V _P at each depth is continuously measured to obtain the distribution of V _P as shown in FIG. Then, data analysis is performed using previously investigated data, and the saturation at each depth is obtained from the above distribution.
[0017]
At this time, a well pipe 8 is provided in the measurement hole 5 as shown in FIG. 2. When a rigid material such as a normal steel pipe or a vinyl chloride pipe is used as the well pipe 8, vibration of the exciter is generated. it is impossible to perform accurate measurements span affect the measured value of V _P because soil do not accurately transmitted. Therefore, in this embodiment, as shown in FIG. 3, a flexible material, for example, a thick cylindrical body 9 made of non-woven fabric is used as the well tube 8, and a large number of shape-retaining rings 10 are mounted in the inside thereof at intervals. As a result, the vibration of the exciter can be transmitted to the ground as it is and high-precision measurement can be performed.
[0018]
As described above, in the measurement method of the present embodiment, the saturation level of the ground can be obtained only by measuring V _P with the probe 6 for V _P measurement. It is possible to obtain the saturation level more easily than when obtaining the saturation level by measuring the relative dielectric constant or by obtaining the saturation level.
[0019]
FIG. 5 shows an embodiment of a liquefaction prevention method using the above saturation measurement method. In the liquefaction prevention method of the present embodiment, the target ground is surrounded by the water blocking wall 2 as in the case shown in FIG. 6, the well 1 is provided in the interior, and the groundwater level is temporarily set by pumping from the well 1. After lowering the saturation level to increase the strength of the ground against liquefaction and obtaining the liquefaction prevention effect, stop pumping to allow the groundwater level to rise, and then increase the saturation level. Re-pumping is performed when the degree of saturation rises above a predetermined value measured periodically. The order in this embodiment performed by the method shown in FIG. 1 the measurement of saturation, may be provided with a measurement hole 5 for V _P measured, the degree of saturation was measured regularly V _P by the probe 6 When the saturation level is equal to or higher than a preset allowable value, the groundwater level is lowered again from the well 1 to lower the groundwater level again, and the saturation level is lowered again to increase the strength against liquefaction. . Therefore, in the liquefaction prevention method of this embodiment, when the degree of saturation rises based on the measurement result of V _P , the liquefaction prevention effect can be restored by automatically performing re-pumping, and the minimum necessary The effect of preventing liquefaction can be ensured over a long period of time and cost.
[0020]
【The invention's effect】
According to the first aspect of the present invention, since the saturation is obtained from the P-wave propagation velocity, the saturation can be easily measured as compared with the case of the conventional laboratory test using sampling or the measurement of relative permittivity.
[0021]
In particular, a bellows-shaped well tube made of a flexible material with a ring for holding a shape is provided in the measurement hole for measuring the P-wave propagation velocity at an interval. Can be measured.
[0022]
The invention of claim 2 measures the degree of saturation by measuring the P wave propagation velocity, and reduces the ground saturation by lowering the groundwater level by pumping water from the ground according to the result. If the degree exceeds the preset allowable value, it is only possible to re-pump water quickly, and the groundwater level is lowered again to lower the saturation level, thereby restoring the effect of preventing liquefaction. The liquefaction prevention effect can be secured over a long period of time with the minimum necessary effort and cost.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an embodiment of a saturation measurement method of the present invention.
FIG. 2 is a view showing a measurement hole for _VP measurement.
3 is an enlarged view of a portion III in FIG.
FIG. 4 is a diagram showing the relationship between _VP and saturation.
FIG. 5 is a schematic view showing an embodiment of the liquefaction prevention method of the present invention.
FIG. 6 is a conceptual diagram of a liquefaction prevention method by lowering the groundwater level.
FIG. 7 is a conceptual diagram showing another example.
[Explanation of symbols]
1 well 2 cut-off wall 5 measurement hole 6 probe 7 winding mechanism 8 well pipe 9 the cylindrical body 10 ring V _P P-wave propagation velocity

Claims (2)

A method for measuring the degree of saturation, which is an index of strength against liquefaction of the ground, in which the P wave propagation velocity at each depth of the ground is measured, and in order to obtain the degree of saturation from the measured value, A probe for measuring the P-wave propagation speed is inserted into the P-wave to measure the P-wave propagation speed at each depth, and in the measurement hole, a bellows made of a flexible material having a shape-retaining ring mounted at intervals. A ground saturation measuring method, characterized in that a well pipe is provided. The soil saturation is measured by the method according to claim 1 and the groundwater level is lowered by pumping the ground according to the result, thereby reducing the ground saturation and increasing the strength against liquefaction. A method for preventing ground liquefaction.

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