JP2004257745A - Simple determination method of flow direction and flow speed of underground water by ground freezing technology - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of easily determining the flow direction and flow speed of the underground water by using the ground freezing technology. <P>SOLUTION: A boring hole for ground freezing is dug in the ground as an object of examination, and a frozen duct is installed in the hole. A plurality of temperature measuring holes are dug in the ground in a state of surrounding the frozen duct at almost equal distances from the frozen duct, and a plurality of temperature sensors are respectively mounted in the depth direction of each measuring hole. The ground is cooled and frozen through the frozen duct, and the change of the ground temperature is measured by the temperature sensor in each temperature measuring hole with time. The distribution of temperatures in the depth direction of the temperature measuring holes at the same time is determined, so that it is determined that the flow speed of the underground water is relatively high at a measuring point where a temperature lowering degree is small by every depth, and the direction of the measuring point of highest temperature lowering degree by every depth with respect to the frozen duct as a center, is determined as the flow direction of the underground water. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention belongs to the technical field of a method of easily determining groundwater flow direction and flow velocity by cooling and freezing a surveyed ground by a ground freezing method and measuring a temperature distribution at each depth in the surrounding ground. .
[0002]
[Prior art]
In recent years, the necessity of specifically grasping the state of movement of groundwater in the ground has been increasing. For example, in recent years, interest in environmental pollution has increased, and attention has been paid to techniques for purifying harmful substances present in the ground. When purifying harmful substances in the ground, there is a great need to take into account that harmful substances move together with groundwater, and it is extremely important to understand the state of movement of groundwater specifically and precisely. is important. In addition, in the geological disposal of high-level waste that will be put into practical use in the future, various types of advection and diffusion are evaluated in advance with high accuracy in consideration of the movement of groundwater in the ground, and the impact on the environment is grasped. It is important to.
[0003]
In the above-mentioned necessity, it is particularly important to accurately and precisely grasp the state of movement of groundwater in the ground, more specifically, the “flow direction and velocity of groundwater” in the ground. Since the ground has been formed over a long period of time, the properties of the ground often differ depending on the depth. For this reason, it is important to grasp the “groundwater flow direction and flow velocity” not only at a specific ground depth but also over a wider range, especially in a wider range in the depth direction.
[0004]
2. Description of the Related Art Conventionally, as a technique for grasping the “groundwater flow direction and flow velocity” in the ground, the following techniques are generally known and implemented.
{Circle around (1)} As shown in the conceptual diagram of FIG. 4, a boring hole a is excavated in the ground, an iron pipe b is built in the boring hole a, and the gap between the two is stopped with grout c. Thereafter, a slightly smaller diameter lower hole d is further excavated below the iron pipe b, and water is injected into the iron pipe b at a level higher than the level of groundwater (water level difference h). With the passage of time, the water escapes from the lower hole d, and the water level difference h is observed to decrease. This is a method of measuring the temporal change (h / t) of the decrease in the water level and theoretically calculating and evaluating the ease of flow of water (water permeability). This method is standardized by the Japan Geotechnical Society and is widely practiced.
[0005]
{Circle around (2)} As shown in the conceptual diagram of FIG. 5, a boring hole a is also excavated in the ground, and packers e and e are installed above and below a required depth in the boring hole a to block the groundwater flow in the vertical direction. I do. Then, a flow direction and a flow velocity meter f are installed in places separated by the upper and lower packers e, e, and the horizontal flow direction and flow velocity of the groundwater are measured and evaluated. The measuring methods and measuring devices disclosed in Patent Documents 3 and 4 below belong to this method.
[0006]
(3) Although the measuring methods and measuring instruments disclosed in Patent Documents 1 and 2 below are not shown, a measuring instrument is installed at a certain point in the ground, and the groundwater is heated by a heater installed in the measuring instrument. In this method, one point is heated, the temperature of groundwater is measured at a slightly distant position, the temperature change of the groundwater temperature is measured, and the flow direction and flow velocity are measured.
{Circle around (4)} Patent Document 5 discloses a technique relating to a method of freezing the ground.
[0007]
[Patent Document 1]
Japanese Patent Publication No. 6-64082 [Patent Document 2]
JP-A-59-160788 [Patent Document 3]
Japanese Patent No. 2586735 [Patent Document 4]
JP-A-6-273538 [Patent Document 5]
JP 9-41356 A
[Problems to be solved by the invention]
(1) The method (1) above is a test method for evaluating the ease of flow of water (permeability) in the ground limited to the lower hole d below the iron pipe b, and evaluates the flow direction and flow velocity of groundwater. This is impossible in principle.
[0009]
(2) The method (2) allows the flow direction and flow velocity of the groundwater to be evaluated only at the place where the flow direction and the flow velocity meter f are installed. However, as described in paragraph [0003] above, the grasp of the “groundwater flow direction and flow velocity” aimed at by the present invention is not performed only at a specific ground depth, but rather over a wider range, especially the depth. Considering that it is important to grasp in a wide range in the direction, the method (2) has a disadvantage that it can grasp only in a too narrow range. In order to grasp over a wide range, especially over a wide range in the depth direction, it is necessary to excavate and measure a large number of boring holes a, and the time and cost required for the drilling are enormous. I can't reach it. In the case of the method (2), the packer e separates the upper and lower portions in the borehole a to block the vertical groundwater flow, but it is considered that the original ground has the vertical groundwater flow. Therefore, the method (2) measures the flow of groundwater different from the original ground, and has problems in accuracy and reliability. In addition, since the boring holes a are present above and below the packer e, the groundwater in the boring holes in that portion is clearly under the condition that the groundwater is easily moved as compared with the surrounding ground. For this reason, depending on the case, the presence of the high-permeability boring hole may cause the situation of movement of groundwater in the ground around the boring hole to be different from the original one. Eventually, the results measured by the flow direction and the velocity meter f may be different from the actual groundwater movement state, and the accuracy and reliability are questioned.
[0010]
(3) The method of (3) above can evaluate the point focusing on the temperature change of the groundwater. However, when heating one point of the groundwater and measuring the water temperature at a position slightly away from it, how much heat is required In addition, there are great questions about the effectiveness, accuracy, and reliability of how clearly the effects can be grasped as changes in water temperature.
[0011]
The object of the present invention is a method for freezing the ground, known as a method for freezing the ground, which involves extensive cooling of the ground and accompanying freezing, by measuring the temperature distribution at each depth in the surrounding ground, to provide a wide range, especially in the depth direction. An object of the present invention is to provide a simple determination method capable of grasping “groundwater flow direction and flow velocity” over a range.
An object of the present invention is to provide a simple determination method capable of accurately and precisely grasping the direction and velocity of groundwater flow without obstructing the flow and movement of the groundwater, which is the original ground, and relatively simple and inexpensive. It is to be.
[0012]
[Means for Solving the Problems]
As a means for solving the above-mentioned problems, a simple determination method of the groundwater flow direction and the flow velocity by the ground freezing method according to the invention described in claim 1,
Drilling a borehole for ground freezing in the ground to be surveyed and installing a freezing pipe in the borehole;
Excavating a plurality of temperature measurement holes in the ground in an arrangement surrounding the freeze tube at substantially the same distance around the freeze tube, and installing a plurality of temperature sensors in the depth direction in each measurement hole,
Cooling and freezing the ground through the freezing tube, and measuring the temperature change of the ground due to the cooling and freezing of the ground over time with a temperature sensor in each of the temperature measurement holes,
The temperature distribution in the depth direction of each temperature measurement hole at the same time is obtained, and a measurement point having a small degree of decrease in temperature at each depth is determined to have a relatively high flow rate of the groundwater. Determining the direction of the measuring point having the largest degree of temperature decrease as the direction of groundwater flow.
[0013]
According to a second aspect of the present invention, there is provided a method for easily determining groundwater flow direction and flow velocity by a ground freezing method according to the first aspect,
The depth of the borehole and the freezing pipe installed in the borehole, and the depth of each temperature measurement hole and the temperature sensor installed in the depth direction are the depths at which the state of movement of the groundwater in the surveyed ground is to be grasped. It is characterized in that it is located at a deeper position.
[0014]
The simple determination method of the present invention is based on applying and utilizing the following phenomena.
It is empirically or theoretically recognized by the implementation of the ground freezing method that a place where the flow rate of groundwater is high in the ground (ground) is clearly harder to freeze than the ground in other places. This is because the ground with a high flow velocity generally has a large void and therefore a large water content per unit volume, and requires a large amount of cold energy required for freezing. In addition, if the flow rate of the groundwater is high, even if cold water is supplied, the cold heat flows downward due to the flow of the groundwater, and is not easily frozen. Therefore, it can be determined that the smaller the degree of temperature decrease during cooling and freezing (the lower the temperature is, the lower the temperature is), the relatively faster the flow rate of groundwater is (hereinafter, this is referred to as the basis for evaluating the flow velocity).
Next, when groundwater flows in the ground, the downstream side is more likely to freeze than the upstream side of the flow, and the freezing range is greatly expanded on the downstream side. This is because the cold heat flows downstream due to the flow of groundwater. Therefore, the direction of the measurement point having a large degree of temperature decrease can be determined as the direction of groundwater flow, that is, the flow direction (hereinafter, this is referred to as a flow direction evaluation basis).
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention shown in the drawings will be described.
FIG. 1 and FIG. 2 show an embodiment of a simple flow direction and flow velocity determination method according to the present invention.
First, a boring hole 2 (also referred to as a cooling hole) for freezing the ground is excavated in the ground 1 to be surveyed, and a freezing pipe 3 is installed in the hole. Although illustration is omitted, a refrigerant supply device that prepares a refrigerant such as liquid nitrogen or a dry ice solution and supplies the refrigerant in a circulating manner is connected to the upper end of the freezing tube 3.
[0016]
Next, as shown in FIG. 2, a plurality of temperature measurement holes 4 are excavated in the ground 1 around the freezing pipe 3 at substantially equal distances R around the freezing pipe 3. Then, a plurality of temperature sensors 5 are installed in the depth direction in each measurement hole 4. For example, the temperature sensor 5 is attached to the detection rod at regular intervals (measurement point pitch), and the temperature sensor 5 is inserted into the temperature measurement hole 4 and fixed. Of course, although illustration is omitted, a bundle of signal lines for guiding the detection signals of the temperature sensors 5 is guided to the ground along the detection rod or in the temperature measurement hole 4 and is not shown. Connected to the measured and recorded device (so-called personal computer, etc.). 1 and 2, the distance R is within a range where the ground 1 can be cooled and frozen by the freezing tube 3 (see a freezing range 6 indicated by a dotted line). The distance is within the range of the ground for which the simple determination of the flow direction and the flow velocity is required.
[0017]
The depth of the boring hole 2 and the freezing tube 3 installed in the hole, the depth of each temperature measurement hole 4 and the installation depth of the temperature sensor 5 installed therein in the depth direction are respectively described. The position is set to a position that is deep enough to be used for determination and evaluation than the depth at which the groundwater movement state of the survey target ground 1 is to be grasped (the invention described in claim 2).
[0018]
When the above preparation is completed, a cooling medium is supplied through the freezing tube 3 to cool and freeze the ground 1. The method is the same as the existing ground freezing method (for example, see Patent Document 5). Then, the temperature change of the ground due to the cooling and freezing of the ground 1 is measured over time by the temperature sensors 5 in the temperature measuring holes 4 described above, and the measurement result is obtained every predetermined time. It is taken into a measurement and recording device (a so-called personal computer or the like), recorded and stored, and also evaluated for tabulation and examination.
That is, the temperature distribution in the depth direction of each of the temperature measurement holes 4 at the same time is obtained, and the magnitude of the degree of temperature drop is indicated for each depth, and an isotherm distribution diagram 7 is created as shown in FIG. 3, for example. According to this isotherm distribution diagram 7, according to the evaluation basis of the flow velocity described in the above paragraph number [0014], the measurement points with a small degree of temperature decrease, for example, D1 and D2, determine that the flow velocity of the groundwater is relatively high. . Conversely, measurement points with a large degree of temperature decrease, for example, D3 and D4, determine that the flow rate of groundwater is relatively slow.
[0019]
In addition, in accordance with the grounds for evaluating the flow direction described in the above paragraph [0014], the distribution map is searched for the direction of the measuring point where the degree of temperature decrease is largest at each depth centering on the freezing tube 3. For example, as shown by an arrow K in FIG. 2, the direction of the measurement point where the temperature decreases the most is determined as the direction of groundwater flow.
[0020]
As described above, the simple determination method of the flow direction and the flow velocity of the groundwater by the ground freezing method according to the present invention measures the temperature change at each depth by each temperature sensor 5 in each temperature measurement hole 4. In this method, a temperature distribution map is prepared for each depth and evaluated. Therefore, a wide range of the target ground 1, that is, the depth at which the temperature measurement holes 4 are excavated, and the temperature sensors 5 installed therein are formed. It is possible to make a simple judgment of “groundwater flow direction and flow velocity” over a wide range in the depth direction according to the number and pitch of groundwater, and to specifically grasp the groundwater movement status that is extremely significant for the purpose of the survey. Can be done.
Further, in the method of the present invention, the arrangement of the temperature measurement holes 4... Equidistant R from the freezing tube 3 is not limited to the single arrangement shown in FIG. Excavation can be carried out in many places, and the movement of groundwater can be grasped more specifically and precisely.
[0021]
In carrying out the method of the present invention, it is also possible to collect ground samples at arbitrary depths in excavation of the boring hole 2 and excavation of the temperature measurement hole 4 in order to grasp the stratum composition of the ground. It is. By calculating the thermal constant from the ground sample, performing an inverse analysis using the groundwater flow velocity as a parameter using the measured value of the ground temperature distribution, and obtaining the groundwater flow velocity that reproduces the measured value best, the ground It is also possible to quantitatively evaluate the velocity of groundwater in the ground.
[0022]
According to the simple determination method of the present invention, groundwater exists between the boring hole 2 (cooling hole) and the freezing pipe 3, but the groundwater is frozen after the start of cooling and freezing of the ground. For this reason, the inside of the borehole 2 is filled with ice, and the groundwater does not move. Further, the flow rate of the groundwater evaluated by the method of the present invention is an evaluation of the flow direction and the flow velocity of the ground at the portion where each of the temperature measurement holes 4... It has high practical value as a quantitative evaluation method of the natural flow direction and velocity of groundwater.
[0023]
【The invention's effect】
The simple determination method of the groundwater flow direction and flow velocity by the ground freezing method according to the invention described in claims 1 and 2 involves cooling and freezing of a large-scale ground in a wide area which is known as a ground freezing method and has already been put to practical use. This is a method of measuring the temperature distribution at each depth in the surrounding ground, excavating the required number of temperature measurement holes to the required depth for the required arrangement, and installing temperature sensors at each temperature measurement hole to the required depth. As a result, it is possible to precisely understand the "groundwater flow direction and velocity" over a wide range that is necessary and sufficiently large, especially in the depth direction. Can be provided.
Further, according to the simple determination method of the present invention, the measuring method, the totaling method, and the determining method are also very simple, and it is possible to save labor by automatic measurement. Can be done. In particular, since it is possible to accurately and precisely grasp the flow direction and flow velocity of the groundwater as it is without disturbing the flow and movement of the groundwater, the meaningful judgment result Is obtained.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing an embodiment of a method for simply determining the flow direction and flow velocity of groundwater according to the present invention.
FIG. 2 is a plan layout view of FIG. 1;
FIG. 3 is a longitudinal sectional view showing an example of a measured temperature distribution.
FIG. 4 is a longitudinal sectional view conceptually showing an embodiment of a conventional determination method.
FIG. 5 is a longitudinal sectional view conceptually showing a different embodiment of the conventional judging method.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Ground 2 Boring hole 3 Freezing tube 4 Temperature measurement hole 5 Temperature sensor

Claims (2)

Drilling a borehole for ground freezing in the ground to be surveyed and installing a freezing pipe in the borehole;
Excavating a plurality of temperature measurement holes in the ground in an arrangement surrounding the freeze tube at substantially the same distance around the freeze tube, and installing a plurality of temperature sensors in the depth direction in each measurement hole,
Cooling and freezing the ground through the freezing tube, and measuring the temperature change of the ground due to the cooling and freezing of the ground over time with a temperature sensor in each of the temperature measurement holes,
The temperature distribution in the depth direction of each temperature measurement hole at the same time is obtained, and a measurement point having a small degree of decrease in temperature at each depth is determined to have a relatively high flow rate of the groundwater. And determining the direction of the measurement point having the largest temperature decrease as the groundwater flow direction. The depth of the borehole and the freezing pipe installed in the borehole, and the depth of each temperature measurement hole and the temperature sensor installed in the depth direction are the depths at which the groundwater movement of the surveyed ground is to be grasped. 2. The method for easily determining the direction and velocity of groundwater flow by the ground freezing method according to claim 1, wherein the position is set at a deeper position.

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