JP2013194362A - Ground improvement method for liquefaction countermeasure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ground improvement method for liquefaction countermeasures that can suitably and securely improve liquefaction strength at a low cost.SOLUTION: A ground improvement method includes the steps of: inserting an air injection pipe 21 into the ground to be improved and injecting air bubbles; installing a transmitter 31 which transmits a sound wave into the ground including the sandwiched ground to be improved and a plurality of receivers 32 arrayed in a vertical direction, and receiving the sound wave transmitted by the transmitter 31 by the respective receivers 32 so as to find distributions of elastic wave speeds and energy attenuation on the basis of arrival times and amplitudes of sound waves; and estimating a liquefaction strength distribution of the ground to be improved on the basis of the distributions of elastic wave speeds and energy attenuation. Further, the method includes a step of injecting air bubbles into a part of the ground to be improved that does not reach predetermined liquefaction strength according to the estimated liquefaction strength distribution.

Description

  The present invention relates to a ground improvement method capable of improving liquefaction strength appropriately and reliably.

  In sandy ground with a high groundwater level, such as landfills, liquefaction occurs due to earthquake vibration, and manholes and sewer pipes are pushed up and protrude from the ground surface, and damage to the building may occur. ing. In order to prevent such a liquefaction phenomenon, improvement work must be performed on the ground where liquefaction is a concern.

  The conventional liquefaction countermeasure methods include a method of increasing the strength of the ground by injecting a chemical into the target ground, a method of reducing the groundwater level, etc., and many techniques have been proposed (for example, , See Patent Document 1 and Patent Document 2).

  The technique described in patent document 1 (Unexamined-Japanese-Patent No. 2012-21269) is a technique which produces a several pile improvement body with respect to the liquefaction layer in a liquefaction countermeasure ground. And the pile type improvement body has reached the substrate layer beyond the liquefied layer, and a plurality of reinforcement improvement bodies that are separated in the depth direction are formed between adjacent pile type improvement bodies. . Moreover, the reinforcement improvement body has connected adjacent pile type improvement bodies.

  The technique described in patent document 2 (Unexamined-Japanese-Patent No. 2008-208631) arrange | positions the improvement body created by the solidification method adjacent to a pile in the ground of a pile foundation at intervals in the depth direction. Technology. Moreover, the drain material which has water permeability is embed | buried in the surrounding ground of a pile foundation, and the drain material is arrange | positioned in the outer peripheral side vicinity of the improved body.

JP 2012-21269 A JP 2008-208631 A

  By the way, the chemical injection method for countermeasures against liquefaction not only requires a permanent grout material but also requires a large chemical injection device, resulting in a high cost. Furthermore, verification of the long-term durability of the injection material and verification of the reliability of the injection ground are also necessary.

  In addition, the groundwater level lowering method for liquefaction countermeasures has a problem of long-term land subsidence, and when it covers a wide area, all residents need to agree. In addition, the individual measures require a water shielding wall, and there is a problem that the construction of the water shielding wall is expensive. Furthermore, long-term maintenance of drainage is necessary, and verification of the effect of countermeasures against liquefaction is essential in order to achieve a water level drop of a few meters (part of the liquefaction target layer).

  As described above, various methods related to liquefaction countermeasures have advantages and disadvantages, respectively, but they have not yet been developed as a technology capable of preventing liquefaction at an appropriate, effective, and inexpensive price. Various technologies have been developed to compensate for the shortcomings of each method.

  This invention is proposed in view of the situation mentioned above, and it aims at providing the ground improvement construction method in the countermeasure against liquefaction which can improve liquefaction intensity | strength appropriately and reliably and cheaply.

  The ground improvement method in the countermeasure against liquefaction of the present invention is a ground improvement method for improving the liquefaction strength of the improvement target ground, and includes the following steps. First, an air injection pipe is inserted into the improvement target ground, and bubbles are injected into the improvement target ground from the tip of the air injection pipe. Subsequently, a transmitter that emits sound waves and a plurality of receivers arranged in the vertical direction are installed in the ground sandwiching the ground to be improved, and the sound waves emitted from the transmitters are received by each receiver. Based on the arrival time and the amplitude of the sound wave, the elastic wave velocity and the energy attenuation distribution are obtained. Subsequently, the liquefaction strength distribution of the ground to be improved is estimated based on the elastic wave velocity and the energy attenuation distribution.

  Further, in the process of estimating the liquefaction strength distribution of the improvement target ground, the liquefaction strength distribution is calculated in advance using the bubble mixing rate and the ground hardness in the improvement target ground obtained from the elastic wave velocity and energy attenuation distribution. It is estimated based on the measured bubble mixing rate and the correlation between ground hardness and liquefaction strength.

  Furthermore, according to the estimated liquefaction strength distribution, the method further includes a step of injecting air bubbles into the improvement target ground portion that has not reached the predetermined liquefaction strength.

  In the ground improvement method in the liquefaction countermeasure having such a configuration, the ground is desaturated by mixing bubbles in the ground to be improved (saturated ground). And when an earthquake motion acts on the ground, due to the compressibility of the air existing in the ground, the increase in excess pore water pressure is reduced and the liquefaction strength is increased. At this time, if the ground is unsaturated unevenly, the liquefaction strength cannot be improved in the uneven portion, and sufficient liquefaction countermeasures cannot be taken.

  Therefore, in the present invention, the sound wave emitted from the transmitter is received by a plurality of receivers for the ground after the bubble is injected, and the elastic wave velocity and energy attenuation are determined based on the arrival time and amplitude of the sound wave. By obtaining the distribution, it is possible to estimate the liquefaction strength distribution of the ground to be improved and know whether or not the countermeasure for liquefaction has been appropriately and effectively performed.

  Furthermore, according to the estimated liquefaction strength distribution, it is possible to inject more bubbles into the improvement target ground portion that has not reached the predetermined liquefaction strength, so that countermeasures for liquefaction can be taken more appropriately and effectively. be able to.

  According to the ground improvement method in the countermeasure for liquefaction of the present invention, liquefaction strength can be improved by injecting bubbles into the ground to be improved, and liquefaction due to earthquake motion can be prevented beforehand. At this time, by estimating the liquefaction strength of the ground to be improved using a so-called acoustic tomography technique, the liquefaction strength can be improved appropriately, reliably and inexpensively.

  In addition, when there is an improvement target ground portion that does not reach the predetermined liquefaction strength, by taking further measures against liquefaction on the relevant portion, the entire improvement target ground can be appropriately, reliably and inexpensively liquefied. It is possible to improve the forming strength.

The schematic diagram of the air injection apparatus used with the ground improvement construction method in the countermeasure against liquefaction concerning the embodiment of the present invention. The schematic diagram of the acoustic tomography apparatus used with the ground improvement construction method in the countermeasure against liquefaction concerning the embodiment of the present invention. Explanatory drawing of the apparatus and function means which are used with the ground improvement construction method in the countermeasure against liquefaction concerning the embodiment of the present invention.

  Hereinafter, an embodiment of a ground improvement construction method (hereinafter abbreviated as a ground improvement construction method) in the liquefaction countermeasure of the present invention will be described with reference to the drawings. 1 to 3 illustrate a ground improvement method for liquefaction measures according to an embodiment of the present invention. FIG. 1 is a schematic diagram of an air injection device, FIG. 2 is a schematic diagram of an acoustic tomography device, and FIG. It is explanatory drawing of the apparatus and functional means which are used by the improved construction method.

<Outline of ground improvement method>
In Japan, liquefaction of the ground occurs due to the occurrence of a massive plate boundary earthquake, and large scale damage is expected due to this phenomenon. In fact, during the Great East Japan Earthquake, liquefaction occurred in landfills, embankments, banking, and revetments, causing various damage. For this reason, it is desirable to take measures against liquefaction as soon as possible in preparation for a large-scale earthquake that is expected to occur in the near future. The ground improvement method according to the embodiment of the present invention is a technique for improving the liquefaction strength of the ground to be improved appropriately, reliably and inexpensively without using a large-scale facility and machine. In other words, the ground improvement method according to the embodiment of the present invention improves the liquefaction strength by injecting air into the improvement target ground and reducing the increase in excess pore water pressure caused by the earthquake motion, and at the original position. Thus, the liquefaction strength after ground improvement is estimated, so that there is no insufficient improvement in the ground to be improved.

<Air injection>
It is known that when ground motion acts on the saturated ground, when the excess pore water pressure increases and reaches the effective upper pressure, the strength of the ground decreases and liquefaction occurs. In this regard, in the ground where air (bubbles) is present in the gap, it is also known that when seismic motion acts, the increase in excess pore water pressure is reduced and the liquefaction strength is improved due to the compressibility of air. Therefore, a technique for increasing the liquefaction strength by injecting air into the saturated ground where the occurrence of the liquefaction phenomenon is a concern has attracted attention. The ground improvement method of the present embodiment uses such a liquefaction countermeasure method by air injection, and estimates the liquefaction strength in the ground after air injection in-situ, and performs appropriate, reliable and inexpensive ground improvement. This is a technology that can be used.

  In order to inject air into the improvement target ground, as shown in FIG. 1, an air injection pipe 21 is inserted into the improvement target ground, and air bubbles are injected into the improvement target ground from the tip of the air injection pipe 21. A specific device includes an air injection pipe 21 to be inserted into the ground to be improved, an air supply pipe 22 continuously connected to the air injection pipe 21, and air to the air injection pipe 21 through the air supply pipe 22. A compressor 23 to be sent out is a main component device, and as an accompanying device, a pressure gauge 24 for measuring air pressure and an opening / closing valve 25 for controlling air supply to the air supply pipe 22 are provided.

  In addition, the apparatus for performing air injection is not restricted to what is shown in FIG. 1, A suitable apparatus can be used according to the soil quality of an improvement object ground, an area, the situation of the field, etc. For example, instead of using the compressor 23, an air cylinder filled with compressed air may be used, and air having an appropriate pressure by a pressure reducing valve may be supplied to the air injection pipe 21. The pressure of the air to be supplied and the size of the bubbles are appropriately selected according to the soil quality of the improvement target ground, but it is necessary to set the pressure and the size so that the bubbles are evenly distributed in the saturated ground.

<Acoustic tomography>
Generally, the strength of the ground after the improvement can be obtained by performing core sampling and performing a uniaxial compression test, a triaxial compression test, or the like in a laboratory. However, when performing such a sampling test, a boring operation is indispensable, which not only increases the cost of the test, but also cannot be performed at a site where there is no space for installing the boring machine. Furthermore, it is impossible to perform sampling (boring) on the entire ground after the improvement, and it is only possible to grasp the ground strength at the position where the sampling was performed. There is no assurance that it could have been done.

  Therefore, in the ground improvement method of the present invention, the so-called acoustic tomography technique is used to estimate the liquefaction strength in the ground after the improvement in situ. In this acoustic tomography, as shown in FIG. 2, a transmitter 31 that emits sound waves and a plurality of receivers 32 arranged in the vertical direction are installed in the ground sandwiching the ground to be improved. And the sound wave emitted from the transmitter 31 is received by each receiver 32, and the distribution of elastic wave velocity and energy attenuation can be obtained based on the arrival time and amplitude of the sound wave.

  That is, as shown in FIG. 2, two measurement holes 33 are drilled across the ground to be improved, a transmitter 31 (for example, a piezoelectric ceramic speaker) is installed in one measurement hole 33, and the other measurement hole 33. A plurality of receivers 32 (for example, microphones) are arranged in the vertical direction. Then, an elastic wave is transmitted from the transmitter 31 and the cross section of the ground to be improved is divided into a plurality of grids based on the arrival time and amplitude of the elastic wave received by the receiver 32. Create a diagram. The hardness and type of the ground can be estimated from the velocity distribution map, and the properties of the gap can be estimated from the attenuation rate distribution map.

  Furthermore, the liquefaction strength distribution of the improvement target ground is estimated based on the velocity distribution map and the attenuation rate distribution map. At this time, the correlation between the bubble mixing rate, the hardness of the ground and the liquefaction strength is measured in advance. Then, using the bubble mixing rate and ground hardness in the ground to be improved obtained from the distribution of elastic wave velocity and energy attenuation, the correlation between the bubble mixing rate and ground hardness and liquefaction strength measured in advance. Based on the above, the liquefaction strength distribution of the ground to be improved can be estimated.

  It should be noted that the creation of the velocity distribution map and the attenuation rate distribution map and the estimation of the liquefaction strength distribution of the ground to be improved can be performed by computer analysis using a predetermined program. Commercially available equipment can be used for such acoustic tomography.

<Replenishment of ground improvement>
In the ground improvement method of the present invention, air bubbles are further injected into the ground portion to be improved that does not reach the predetermined liquefaction strength according to the liquefaction strength distribution estimated by the above-described procedure. That is, by estimating the liquefaction strength after ground improvement at the original position and further injecting air into the portion where the liquefaction strength is insufficient, there remains an insufficient improvement portion in the ground to be improved. I am trying not to.

<Ground improvement device>
As shown in FIG. 3, the ground improvement device 10 used in the ground improvement method of the present invention includes an air injection means 11, a sound wave transmission means 12, a sound wave reception means 13, an elastic wave velocity / energy attenuation distribution estimation means 14, and And liquefaction strength distribution estimation means 15.

<Air injection means>
As described above, the air injecting means 11 is composed of devices such as the air injecting pipe 21, the air supply pipe 22, and the compressor 23. Using this air injection means 11, bubbles are injected into the ground to be improved.

<Sound wave sending means / Sound wave receiving means>
The sound wave transmitting means 12 is a device for transmitting an elastic wave in the ground to be improved, and includes the transmitter 31 such as a piezoelectric ceramic speaker as described above. The sound wave receiving means 13 is a device for receiving the elastic wave transmitted from the sound wave transmitting means 12, and is constituted by the receiver 32 such as a waterproof microphone as described above. Moreover, by arranging a plurality of microphones in the vertical direction and receiving the elastic wave transmitted from the piezoelectric ceramic speaker by each microphone, it is possible to recognize the elastic wave velocity and the energy attenuation distribution over the entire ground to be improved. .

<Measuring means for elastic wave velocity / energy attenuation distribution>
The elastic wave velocity / energy attenuation distribution estimating means 14 includes a program for obtaining the distribution of elastic wave velocity and energy attenuation based on the arrival time and amplitude of the sound wave, and a CPU for executing the program. It consists of a computer. The calculation result by the elastic wave velocity / energy attenuation distribution estimating means 14 can be displayed on a display screen of a display device connected to the computer, or printed by a printer.

<Liquefaction strength distribution estimation means>
The liquefaction strength distribution estimation means 15 is a computer including a program for estimating the liquefaction strength distribution of the ground to be improved based on the elastic wave velocity and the energy attenuation distribution, and a CPU or the like for executing the program. It consists of. The calculation result by the liquefaction intensity distribution estimating means 15 can be displayed on a display screen of a display device connected to a computer or printed by a printer.

DESCRIPTION OF SYMBOLS 10 Ground improvement apparatus 11 Air injection means 12 Sound wave transmission means 13 Sound wave reception means 14 Elastic wave velocity and energy attenuation distribution estimation means 15 Liquefaction intensity distribution estimation means 21 Air injection pipe 22 Air supply pipe 23 Compressor 24 Pressure gauge 25 On-off valve 31 Transmitter 32 Receiver 33 Measuring hole

Claims (3)

A ground improvement method for improving the liquefaction strength of the ground to be improved,
Inserting an air injection pipe into the improvement target ground and injecting air bubbles into the improvement target ground from a tip of the air injection pipe; and
A transmitter that emits sound waves and a plurality of receivers arranged in the vertical direction are installed in the ground sandwiching the ground to be improved, and the sound waves emitted from the transmitters are received by each receiver. Receiving and determining the elastic wave velocity and energy attenuation distribution based on the arrival time and amplitude of the sound wave;
Estimating the liquefaction strength distribution of the ground to be improved based on the elastic wave velocity and the energy attenuation distribution;
A ground improvement method for liquefaction countermeasures characterized by containing liquefaction.   The liquefaction strength distribution is determined by using the bubble mixing rate and the ground hardness in the improvement target ground obtained from the elastic wave velocity and the energy attenuation distribution, and the bubble mixing rate and the ground measured in advance. The ground improvement method for liquefaction countermeasures according to claim 1, wherein the estimation is based on a correlation between the hardness of the soil and the liquefaction strength.   3. The liquefaction according to claim 1, further comprising a step of injecting air bubbles into the improvement target ground portion that does not reach a predetermined liquefaction strength in accordance with the estimated liquefaction strength distribution. Ground improvement method for countermeasures.

Images