JP2002257940A - Method and apparatus for measuring and visualizing liquefaction phenomenon using specific resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a measuring method capable of directly measuring the porosity and the relative density of a layer of saturated sand in real time and visualizing a liquefaction phenomenon and a measuring apparatus for use. SOLUTION: The method for measuring and visualizing liquefaction phenomenon using specific resistance visualizes the liquefaction phenomenon within a layer of saturated sand by making a graph of the relationship between a value of specific resistance and time, at each of measuring positions at each of plural points within the layer of saturated sand to which vibration was imparted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for visualizing, grasping and evaluating the liquefaction phenomenon of the ground, and an apparatus used for the method.

[0002]

2. Description of the Related Art The ground liquefaction phenomenon is considered to be a phenomenon that occurs during a process in which a loose sand layer is repeatedly subjected to a shearing force due to seismic motion, and particles are gradually disengaged and the particle structure is destroyed. In addition, a phenomenon in which a sand layer is liquefied by an upward seepage flow is known as a boiling phenomenon. In order to observe these liquefaction states, measurement of pore water pressure and shear stress in a saturated sand layer, and measurement of the amount of settlement on the surface of the sand layer have been performed. These methods monitor the contraction of the sand layer due to liquefaction based on pore water pressure, shear stress and the amount of settlement of the surface layer.However, while these methods are effective for sand layers having a uniform structure, they It is not enough to grasp the liquefaction phenomenon of the sand layer with unevenness due to the difference of the consolidation method and the influence of the underground structure. In addition, there is a problem that it is difficult to obtain data measured at multiple points at almost the same time. Also,
The liquefaction phenomenon is thought to be caused by changes in the particle structure in the sand layer.If the distribution of the relative density in the sand layer linked to this particle structure and the change can be measured, the mechanism of the liquefaction phenomenon should be understood in more detail. Becomes possible. Therefore, there has been a demand for the development of a method capable of directly monitoring the porosity and relative density of the sand layer in real time and visually grasping the liquefaction phenomenon.

[0003]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and can directly observe the porosity and relative density of a saturated sand layer in real time, and can visualize a liquefaction phenomenon. An object of the present invention is to provide a measuring method and a measuring device used for the method.

[0004]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in view of the above-mentioned problems, and as a result, have taken advantage of the relationship established between the resistivity and the porosity of the sand layer to determine the gap at a plurality of measurement points of the sand layer. The porosity of the sand layer can be measured non-contactly and in real time in a two-dimensional or three-dimensional manner, by being able to measure the resistivity by resistivity and by imaging the distribution of resistivity during vibration two-dimensionally or three-dimensionally. Alternatively, it is possible to measure the distribution and change of the relative density, and furthermore, it has been found that by imaging this, the occurrence state of the liquefaction phenomenon can be visually grasped and evaluated, and the present invention is made based on this finding. Reached. That is, the present invention (1) visualizes the occurrence of the liquefaction phenomenon in the saturated sand layer by graphing the relationship between the specific resistance value and the time at each of a plurality of measurement positions in the saturated sand layer subjected to the vibration. A method for measuring and visualizing a liquefaction phenomenon using a specific resistance, (2) a measurement method according to (1), wherein the occurrence of the liquefaction phenomenon is imaged, and (3) an occurrence of the liquefaction phenomenon Image and visualize in real time (1)
Or (4) visualizing the liquefied shrinkage surface indicated by a rapid change in specific resistance due to the liquefaction phenomenon at a predetermined measurement position by image display (1), (2) or (3) The measurement method described in (3), (5) a plurality of potential electrodes are provided at positions inside or on the side of the sand layer of a saturated sand layer container having a vibration means, a power supply and a pair of current electrodes, and A device for measuring liquefaction phenomena, characterized by having means for continuously measuring the specific resistance value. (6) A plurality of potential electrodes are provided inside the sand layer or on a straight line on the side surface at only different vertical positions. (5)
(7) The liquefaction phenomenon measuring apparatus according to (5), wherein the plurality of potential electrodes are provided on two parallel surfaces which are inside or on the side surfaces of the sand layer. Is provided. In the present invention, the saturated sand layer refers to a sand layer saturated with water.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, measurement of the liquefaction phenomenon of the present invention
The method will be described. Where rocks and soil are saturated with water
In the case of Archie's law, the relationship
It is known to stand. F = ψ_t/ Ψ_w= an^-m  Here, F is called a formation factor, and ψ_t
Is the specific resistance of rock and soil, ψ_wIs the specific resistance of the pore water, n is the gap
Rate. a and m are constants determined experimentally, and
Constants have been obtained in laboratory experiments
(Mogi et al., Wednesday Journal, Vol. 20, No. 1, pp100-108, 1
983). Pore water resistivity is known and does not change during measurement
Can be assumed, and
If a constant between the data and the porosity is determined,
The porosity n of the sand layer can be obtained from the specific resistance value of.
For sand layers containing silt other than clean sand
What is the relationship between the formation factor and the porosity?
The porosity-resistivity curve was determined by an initial experiment, and the resistivity value was calculated.
The approximate curve for converting porosity into porosity
No. If the porosity can be obtained from the specific resistance value, the porosity is calculated
Using the maximum and minimum void ratios of the sand
Can be converted to relative density. However, such as the ground
Occurrence of liquefaction during vibration and its movement from bottom to top
Can be predicted and tracked only by measurements like the top
Can not. As shown in FIG.
The sand layer contracts from the bottom of the sand layer container immediately after excitation.
And rises toward the top, where the shrinkage of the sand layer
The boundary surface that is occurring will be referred to as the liquefaction contraction surface. Exchange
In other words, this liquefaction shrinkage surface is the specific resistance value
(Correlated with relative density)
Yes, indicating a liquefaction front. Liquefaction shrinkage surface rising speed
Can be regarded as almost constant, and the rise of the liquefied shrinkage surface
Knowing the speed and starting depth predicts the duration of liquefaction
It is possible to To explain this in more detail,
2 tracks changes in the relative density of the sand layer during the liquefaction process
It is an imaged drawing. Measurement step is 10 seconds
When it is considered that liquefaction has been completed for about 1 minute after vibration
The change in resistivity was recorded continuously up to the point. In Figure 2, the liquid
Changes in resistivity, that is, changes in relative density
The sand layer was collected when the relative density increased.
If it contracts and decreases, the sand layer has expanded. Figure
From 2 immediately after the start of the excitation,
The depth of the sand corresponds to the excitation force) The relative density of the sand layer increases rapidly
A region that occurs and progresses toward the top
I understand. The area where this sand layer shrinks is very small.
It is called the sand layer contraction surface. The sand layer above this sand layer contraction surface,
It seems that the surface layer is obviously boiling
The relative density has not changed except for the region where That
Therefore, pore water drained from the sand layer contraction surface
Upward and seep to the top
The flow disrupts the mechanical balance of the sand particles that make up the sand layer.
Liquefaction occurs due to loss of shear force
It is thought that. This phenomenon occurred during the simultaneous measurement.
Consistent with the interpretation based on the pore pressure gauge results. Also liquid
This phenomenon persists until this sand layer contraction surface reaches the top
However, the rate of rise of the sand layer shrinkage surface depends on the nature of the sand.
Depends on conditions such as quality and relative density.
Important parameters for understanding predictions and properties
Becomes The liquefaction visualization method of the present invention uses the sand layer shrinkage surface
The rate of rise of the water and changes in the relative density directly
To make things possible.

In the present invention, the measurement of the specific resistance for obtaining the liquefaction shrinkage surface by obtaining the porosity or relative density of a plurality of points in the sand layer is performed at a plurality of points inside or on the side of the saturated sand layer. As this mode, in addition to measurement at a plurality of points where only the depth (vertical position) or only the horizontal position is different, there is a method of measuring at a plurality of points where the depth and the horizontal position are different. Measurement is possible. The number of measurement points can be appropriately selected depending on the measurement mode, required measurement accuracy, the type of sand layer, etc., but the obtained specific resistance value is converted into a porosity or relative density as described above, and this is measured. By graphing the position, or the relationship between the measurement position and time, the liquefaction phenomenon occurring in the sand layer can be visualized and grasped.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. First, an aspect in which measurement is performed at a plurality of points that differ only in depth and an apparatus used therefor will be described with reference to FIG. In the figure, reference numeral 1 denotes a cylindrical container having potential electrodes 2 on the side surface at regular intervals. In the example shown in FIG. 1, the cylindrical container has a diameter of 200 mm and a height of 600 mm, and 30 potential electrodes are arranged every 2 cm. Reference numeral 4 denotes a copper mesh current electrode plate provided on the upper and lower surfaces of the container 1, and 3 denotes a current electrode. Reference numeral 5 denotes a saturated sand layer filled in the container 1. FIG. 1 shows a cylindrical container for the measuring earth tank, but the shape is not particularly limited as long as the wall is insulated, and may be, for example, rectangular. The specific resistance distribution in the depth direction of the measurement earthen tub is measured by an electric potential distribution by a large number of electrodes 2 provided on the wall surface. A power supply, a measuring device for specific resistance, and a vibrating means are not shown.

FIG. 2 shows a measurement result of a porosity change performed using the apparatus shown in FIG. 1 and using Toyoura standard sand as a sample. The relative density is obtained from the porosity calculated from the porosity. Is calculated. When the porosity is φ, the gap ratio e is: e = φ / (1−φ) The relative density Dr (%) is: Dr = (e _max −e) / (e _max −e _min ) × 1
00. The measurement was performed during the process of saturating the saturated sand layer and during a series of liquefaction processes until the liquefaction state was completed and the rigidity of the saturated sand layer was restored. A dispersion plate (Pearlcon) was installed below the cylindrical container 1, and the relative density of the sand layer was adjusted by boiling to make the thickness of the sand layer 50 cm.
As a measuring device, a 15-channel real-time continuous resistivity measuring device was used. This measuring instrument has a sampling speed of 250m
With s, it is possible to measure the specific resistance value of 15 channels in real time. The specific resistance of the saturated sand layer was measured by passing a constant alternating DC current between the electrode plates 4 and continuously measuring the potential between the electrodes 2 installed in the vertical direction. As the vibration means, an electromagnetic vibration vibrator SSV-725 (manufactured by San-S) was used, and a vibration of 600 gal, 5 Hz, and 4 seconds was applied. Two pore water pressure gauges (P1, P2) PGM-02KG (manufactured by Kyowa Electric Industry Co., Ltd.) were arranged at the center of the cylindrical container 1 at intervals of 10 cm.

FIG. 2 shows the distribution of relative density (%) with time on the horizontal axis and depth (distance from the upper surface of the sand layer) on the vertical axis. In this case, simultaneously with the vibration, the pore water pressure rises and reaches a liquefied state. As can be seen from FIG. 2, it can be confirmed that the front where the relative density increases from the lower part of the sand layer toward the upper part rises. The relative density rapidly increases on the liquefied shrinkage surface where the relative density increases, but before reaching the liquefied shrinkage surface, there is no change in the relative density except for the vicinity of the surface layer where the relative density increase due to boiling is observed. . In FIG. 2, the image has been divided into several regions for explanation of the relative density. The area A is about 35%, the area B is about 38 to 40%, and the area C is over 40% and 55%.
And D is an area of about 37% or less. E is assumed to be 0% in the aqueous phase. Further, in the experiment of FIG. 2, looking at the relationship between time and pore pressure after the start vibrating, simultaneously pore pressure and the arrival of the liquefaction contraction surface L _f is found to be reduced. Dissipation of pore water pressure due to liquefaction is regarded as a one-dimensional consolidation problem due to its own weight.However, from the results of previous laboratory experiments, the excess pore water pressure gradient of It has been found that a critical hydraulic gradient is reached. Therefore, the differential pressure corresponding to the critical hydrodynamic gradient between the sand layers 10 cm from the above measurement conditions (sand specific gravity 2.7, water density 1, sand layer thickness between pore pressure gauges 10%, porosity 50%) Is 8.5g
/ Cm ² . From this value, it can be seen that the upward seepage flow has reached the critical hydraulic gradient, which indicates that the sand layer has reached the liquefied state.

As described above, according to the method and apparatus of the present invention, the distribution of the relative density (or porosity) of the sample sand layer and its change can be visualized by graphing the liquefaction phenomenon, and the liquefaction shrinkage surface can be visualized. the presence of L _f can be visually clearly capture. The liquefaction shrinkage surface is generated at a specific depth or more corresponding to the excitation force and rises from there. The time required for the liquefaction shrinkage surface to reach the upper surface of the sand is considered to be the duration of the liquefaction phenomenon. Such information necessary for performing the property evaluation of the liquefied ground and leading to the mechanism of the liquefaction phenomenon can be visualized and obtained.
The method of the present invention can be carried out in an indoor experiment using a small earthen vessel, and the liquefaction property of the sand layer can be measured and visualized. it can. That is, it is possible to cause liquefaction of the actual ground by applying a strong impact to the ground using blasting. Specifically, a potential electrode and a current electrode are provided on the actual ground of the landfill in the same way as in Fig. 1, and the specific resistance change of the saturated sand layer when blasting and liquefaction is caused is measured and visualized, and this is compacted. This is a method of comparing with data obtained in an earth tank experiment.

The following findings were obtained by measuring and visualizing the liquefaction phenomenon. 1) The liquefaction phenomenon occurs at a depth shallower than the generation depth of the liquefaction shrinkage surface, and changes in relative density (resistivity) occur when the liquefaction shrinkage surface reaches, except in a shallow region where boiling occurs. 2) The depth of occurrence of the liquefaction shrinkage surface corresponds to the excitation force, and the stronger the excitation force, the deeper the generation depth of the liquefaction shrinkage surface. 3) In the sand layer shallower than the liquefaction shrinkage surface, upward osmotic flow is generated by pore water discharged upward due to the contraction of the sand layer on the liquefaction shrinkage surface. Present state. 4) After passing through the liquefaction shrinkage surface, the pore water pressure sharply decreases and the liquefaction state ends.
5) The temperature at which the liquefied shrinkage surface rises depends on the ground constant such as the permeability coefficient and relative density of the sand layer.

Next, one mode of two-dimensional measurement in which the resistivity distribution is measured at a plurality of points on a certain plane will be described. FIG. 3A is an explanatory diagram showing an example of the apparatus of the present invention capable of measuring the specific resistance at a plurality of points on the same surface using a parallel current. The box for measuring by applying a parallel current between the copper or lead plates 20 and 21 at both ends is divided into three by two plates 22 and 23. The plates 22 and 23 are in the form of a matrix in which a large number of electrode plates 25 are attached to an insulating base 24 (FIG. 3B).
2 is an enlarged sectional view of a part of the plates 22 and 23).
A sample consisting of sand and water is placed in the central box 26, and water is placed in the outer boxes 27 and 28. Since the parallel current flows through the electrode plate 25 of the plate 22 via the water in the boxes 27 and 28, the potential difference between the parallel electrode and the corresponding electrode plate of the other plate 23 is measured. The specific resistance between them can be measured (a parallel power supply and the like are not shown). The matrix electrodes are switched at a high speed by the electrode scanner 29 and are recorded on the resistivity recording device 30.
Thereby, it is possible to obtain a planar image of the specific resistance distribution in which data corresponding to the number of the electrode plates 25 on the plate 22 is plotted. The switching time of the electrode matrix depends on the sampling time of the A / D converter. For example, when using 225 matrix electrodes of 15 points each in the vertical and horizontal directions, even if the sampling time is 1 ms, the entire image capture is 0.225 s. Will end. Since these parameters are variable amounts that can be set by computer control, they can be set according to actual measurement conditions and the like.

FIG. 4 shows a planar image of the relative density distribution measured using the apparatus shown in FIG. The sand layer sample used and the conditions of the vibration were the same as those in the measurement shown in FIG. 4 (a) shows a state before liquefaction, FIG. 4 (b) shows a state after 1 second from liquefaction, and FIG. 4 (c) shows a state after 3 seconds from liquefaction. In the figure, the vertical axis and the horizontal axis indicate the electrode numbers at the vertical position and the horizontal position, respectively (the electrode interval is 1.5 cm). In addition,
The contour shows the same part of the specific resistance. The vertical axis indicates the position in the vertical direction, and the horizontal axis indicates the position in the horizontal direction, and indicates the relative density as in FIG. Further, as described above, the present invention can be applied to actual ground outdoors as a modified embodiment. A schematic diagram of this example is shown in FIG. As shown in the figure, a measurement hole 50 is drilled outdoors, a multi-point electrode group 51 with a fixed interval is arranged inside the measurement hole 50, and the current electrode 52 and the potential electrode 53 are fixed or the pair set is switched at high speed. This is a method of measuring the vertical resistivity distribution of the entire sand layer 54. It is basically similar in principle and operation to the embodiment of FIG. High-speed switching of the pair set of current and potential electrodes in the direction of arrow a,
Measure the vertical resistivity distribution of the whole sand layer. In the figure, reference numeral 55 denotes a specific resistance measuring device. Reference numeral 56 denotes a far electrode, which schematically illustrates an example in which a two-dimensional specific resistance cross-sectional exploration is performed by a multipole method of two or more poles. The details of the visualization in this case can be performed according to the method and apparatus described in FIG. In the present invention, the same information as shown in FIG. 4 can be obtained from three-dimensional measurement. Furthermore, in a non-uniform ground or a ground where foundation piles are present, the change in relative density due to liquefaction is not uniform, and varies depending on the location. The same applies to the rising speed of the liquefied shrinkage surface. The change can be grasped from the plane image, and it becomes possible to investigate in detail the influence of the structure, the foundation pile, etc. on the liquefaction of the ground. The three-dimensional measurement is the same as the two-dimensional measurement except that the specific resistance is measured at a plurality of three-dimensionally distributed points. In each of the above embodiments, the relative density is represented by a color and graphed, but the present invention is not limited to this.

[0014]

According to the present invention, it is possible to directly measure, visualize, and image the liquefaction phenomena of the sandy ground in real time. This applies not only to the sandy soil in the earthen tank but also to the actual ground. The present invention is of great practical value from the perspective of predicting and measuring the occurrence of liquefaction of an earthquake.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing one embodiment of the present invention.

FIG. 2 shows measurement results obtained by using the apparatus shown in FIG.

3A is an explanatory view showing another embodiment of the present invention, and FIG. 3B is a partially enlarged cross-sectional view of the plate in FIG.

FIGS. 4A and 4B show measurement results obtained by using the apparatus shown in FIG. 3, wherein FIG. 4A shows a state before liquefaction, FIG.
Is 3 seconds after liquefaction.

FIG. 5 is a schematic view showing an embodiment of the present invention on actual ground outdoors.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Container 2 Potential electrode 3 Current electrode 4 Copper mesh current electrode plate 5 Saturated sand layer 20, 21 plate 22, 23 plate 24 Base 25 Electrode plate 26, 27, 28 Box 29 Electrode scanner 30 Resistivity recorder 50 Measurement hole 51 Electrode group 52 current electrode 53 potential electrode 54 sand layer 55 resistivity measuring device 56 far electrode

Claims (7)

[The claims] 1. The occurrence of liquefaction in the saturated sand layer is visualized by graphing the relationship between the specific resistance value and time at each of a plurality of measurement points in the vibrated saturated sand layer. Of liquefaction phenomenon using specific resistance. 2. The measuring method according to claim 1, wherein the occurrence of the liquefaction phenomenon is imaged. 3. The measuring method according to claim 1, wherein the occurrence of the liquefaction phenomenon is visualized in real time and visualized. 4. The measuring method according to claim 1, wherein the liquefaction contraction surface, which is indicated by a sharp change in specific resistance due to the liquefaction phenomenon at a predetermined measurement position, is visualized by image display. 5. A plurality of potential electrodes are provided at a position inside or on the side of a sand layer of a saturated sand container having a vibration means, a power source and a pair of current electrodes, and a specific resistance value between the potential electrodes is continuously measured. An apparatus for measuring liquefaction phenomena, comprising means for measuring. 6. The liquefaction phenomena measuring device according to claim 5, wherein a plurality of potential electrodes are provided inside the sand layer or on a straight line on the side surface at only different vertical positions. 7. The liquefaction phenomena measuring device according to claim 5, wherein a plurality of potential electrodes are provided on two parallel surfaces inside or on the side surfaces of the sand layer.