Water injection pressure determination method

Abstract

To provide an injection pressure determination method for setting such an injection pressure as to prevent outflow of water from a ground surface, in a site permeability testing method using a pressurization-infiltration accelerating method for shortening test time.SOLUTION: An injection pressure determination method includes: a time determination step of determining testing time from start to end of testing; a flow rate calculation step of calculating an injection flow rate required for making water injected to the ground in the testing time reach a ground surface; a permeability coefficient assumption step of assuming a permeability coefficient of a ground; and a pressure determination step of determining an injection pressure using the injection flow rate and the permeability coefficient.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for determining a water injection pressure in an in-situ permeability test using a pressurized permeation acceleration method. [Background technology]

[0002] As part of quality control in the construction of embankments such as dams and levees, it is necessary to periodically conduct in-situ permeability tests. In these tests, the saturated permeability coefficient of an unsaturated embankment is evaluated. One method for on-site permeability testing of unsaturated ground is the "Permeability Test Method for Compacted Ground (JGS1316)," specified by the Geotechnical Society of Japan. This method uses a Marriott siphon to inject water into the ground while maintaining a constant level of water in the hole, and calculates the saturated permeability coefficient from the amount of water injected when saturation is reached and the water level maintained constant in the test hole. The amount of water injected is greatest at the beginning, and converges to a constant amount as the ground around the test hole approaches saturation. However, it takes a long time to inject water until the ground is saturated, which makes the test time long. For this reason, the time required to reach a saturated state may be shortened by injecting water into the ground, as shown in Patent Document 1. When injecting water into the ground, it is necessary to adjust the water injection pressure so that the water does not overflow from the ground surface. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-127832 Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention aims to propose a method for determining a water injection pressure that will prevent water from flowing out of the ground surface in an in-situ permeability test using a pressurized infiltration promotion method to shorten the test time. [Means for solving the problem]

[0005] The inventors discovered that by setting a test time, assuming how the saturation state of the ground will spread, determining the water injection flow rate at which the saturation of the ground will reach the ground surface at the end of the test time, and calculating the required pressurized head by assuming the permeability coefficient of the ground, it is possible to set a water injection pressure that will prevent water from overflowing from the ground surface. The water injection pressure determination method of the present invention, based on this finding, comprises a time determination step of determining the test time from the start to the end of the test, a flow rate calculation step of calculating the water injection rate required for water injected into the ground to reach the ground surface during the test time, a permeability coefficient assumption step of assuming the permeability coefficient of the ground, and a pressure determination step of determining the water injection pressure using the water injection rate and permeability coefficient. According to this method for determining the water injection pressure, an on-site permeability test can be carried out quickly at a pressure that prevents the injected water from overflowing the ground surface, and the permeability coefficient can be obtained with high accuracy. In the flow rate calculation step, it is desirable to calculate the water injection flow rate using Equation 1. In addition, in the pressure determination step, it is desirable to calculate the pressurized water head using Equation 2, and to determine the water injection pressure based on the pressurized water head. If the ground is a dam body, the assumed value of the hydraulic conductivity k a is preferably set to a value greater than the design standard value of the hydraulic conductivity used in the design of the cut-off wall.

[0006]

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[0007] According to the water injection pressure determination method of the present invention, in an on-site permeability test method using a pressurized infiltration promotion method to shorten the test time, it is possible to set a water injection pressure that will not cause water to flow out from the ground surface. [Brief explanation of the drawings]

[0008] [Figure 1] This is a cross-sectional view showing the installation status of a water injection pipe for an on-site permeability test. [Figure 2] This is a cross-sectional view showing the water injection status during the on-site permeability test. [Figure 3] 3 is a flowchart showing the steps of a water injection pressure determination method according to the present embodiment. [Figure 4] FIG. 1 is a cross-sectional view showing an outline of the dam body. [Figure 5] FIG. 10 is a distribution diagram of injected water in the simulation of Example 1. [Figure 6] 1 is a graph showing the change over time in estimated hydraulic conductivity in Example 1 and Comparative Example 1. [Figure 7] FIG. 10 is a distribution diagram of injected water in the simulation of Example 2. [Figure 8] 1 is a graph showing the change in ke / kg over time in Examples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0009] In this embodiment, we will explain the in-situ permeability test of the dam body, which is periodically conducted for the purpose of quality control during the construction of a dam. For the in-situ permeability test of unsaturated ground, the method "Permeability test method for compacted ground (JGS1316)" specified in the standard of the Geotechnical Society of Japan is adopted. In-situ permeability tests measure the saturated hydraulic conductivity of unsaturated ground (cutoff wall) located above the groundwater level, so it is necessary to inject water into the ground to saturate it. In this embodiment, water is injected to shorten the water injection time required to saturate the ground.

[0010] Figures 1 and 2 show the on-site permeability test. As shown in Figure 1, the on-site permeability test is carried out using a test hole 6 formed by drilling ground G. Specifically, water W is injected into ground G through a test device 1 installed in test hole 6. Test device 1 comprises a water storage section 2 and a water injection pipe 3. A packer 4 is installed around the lower end of the water injection pipe 3 to seal the gap between the outer surface of the water injection pipe 3 and the inner surface (hole wall) of the test hole 6. A protective pipe 5 is installed above the packer 4 inside the test hole 6 to prevent the hole wall of the test hole 6 from collapsing. Water is injected into the ground G by supplying water W stored in the water storage section 2 to the ground G (the lower end of the test device 2) via the water injection pipe 3. The water W is supplied to the ground G by pressurizing the water storage section 2 with a pressure device (not shown) such as a pump. The water W is supplied to the lower end of the test hole 6 via the water injection pipe 3, and is injected (permeates) into the ground G exposed at the wall of the test hole 6. Water is injected into the ground G until the water injection flow rate becomes approximately constant, and the permeability coefficient is calculated from the approximately constant water injection flow rate. When water is injected into the ground G, it is assumed that the injected water W will spread into a spherical area (flooded area A) with a radius r(t) centered on the test device 1 (water injection pipe 3), as shown in Figure 2. Water must be injected into the ground G with a pressure that will prevent the injected water from flowing out from the ground surface GL. In other words, the radius r(t) must be smaller than the distance L from the bottom of the packer 4 to the ground surface GL.

[0011] In this embodiment, the test time is set, the spread of the saturated state of the ground G is assumed, and the water injection flow rate at which the saturation of the ground G reaches the ground surface GL at the end of the test time is calculated. The required water injection pressure (pressurized head h f ) is calculated to prevent the injected water W from overflowing the ground surface GL. Figure 3 shows the steps of the water injection pressure determination method for determining the water injection pressure of water W in an on-site permeability test. As shown in Figure 3, the water injection pressure determination method of this embodiment comprises a time determination step S1, a flow rate calculation step S2, a permeability coefficient assumption step S3, and a pressure determination step S4.

[0012] The time determination step S1 determines the test time t u This is the process of determining the test time t u is set so that it is less than the time it takes for the injected water W to reach the ground surface GL (water injection time t0). To perform the most efficient on-site permeability test, it is necessary to increase the water injection pressure as much as possible, and the conditions for this are: u = t0. Note that the test time t u is determined for each test. The flow rate calculation step S2 is the water injection flow rate Q e This is the process of calculating the water injection flow rate Q e is the time it takes for the water W injected into the ground G to reach the test time t u Assuming that the injected water W spreads spherically (see Figure 2), the injected water flow rate Q is calculated using Equation 1. e Calculate.

[0013]

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[0014] The permeability coefficient assumption step S3 is to estimate the permeability coefficient k of the ground. a This is the process of assuming the hydraulic conductivity k a is assumed to be a safe evaluation based on the design standard values, etc. Generally, the higher the permeability of the ground, the faster the injected water W reaches the ground surface. a is set to a value greater than the design standard value of the hydraulic conductivity used in the cut-off wall design. The pressure determination step S4 is a step of determining the water injection pressure. The water injection pressure is determined by the water injection flow rate Q e and the hydraulic conductivity k a The pressure head k calculated using Equation 2 f Let's say.

[0015]

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[0016] According to the method for determining the water injection pressure of this embodiment, an on-site permeability test can be carried out quickly at a pressure that prevents the injected water W from overflowing the ground surface GL, and the permeability coefficient can be obtained with high accuracy. Therefore, the on-site permeability test can be carried out efficiently, and the labor required can be reduced. In addition, the water head above ground level (pressure head k f ), the hydraulic conductivity can be measured in a state where a wider area is saturated than when pressure is not applied. By determining the hydraulic conductivity that represents a wider area, the hydraulic conductivity of the entire structure (cutoff wall) can be evaluated with high accuracy. In addition, this method omits the depth survey that was required separately in the conventional evaluation formula (E-19 method), which relies on the position of the groundwater table deeper than the test section.

[0017] Below, assuming that an on-site permeability test is conducted to evaluate the permeability of the compacted ground in a dam body, the water injection pressure is determined using specific numerical values ​​using the water injection pressure determination method of this embodiment (Example 1). Figure 4 shows an overview of the dam body 7. Example 1 illustrates a rockfill dam. The rockfill dam consists of a core portion 8 formed from a clay-like material that is difficult for water to pass through, a filter portion 9 made of sand or gravel formed on both sides of the core portion 8 to prevent the core portion 8 from collapsing, and a rock portion 10 formed by laying rocks or the like on the outside of the filter portion 9. Assuming the core 8 of the dam body 7, the design standard value of the hydraulic conductivity is set to 10 -7 m / s, and 10 -6 m / s is assumed to be the permeability coefficient k a In addition, the porosity n e is 0.2, the test section l is 0.2-0.3 m and the width is 0.1 m. Furthermore, the distance L from the ground surface GL to the top of the test section is 0.2 m. Test time t u was set at 3 hours. Water injection flow rate Q calculated using Equation 1 e and the hydraulic conductivity k a Using Equation 2, the pressure head h f This gives us 4.34m.

[0018] Next, a numerical simulation was performed for the case where water was injected with a pressurized head of 4.34 m. In the numerical simulation, unsaturated seepage flow analysis was performed using TOUGH2, an analysis software that uses the integral-difference method. Figure 5 shows the distribution of water saturation after 3 hours. As shown in Figure 5, the area where water was injected after 3 hours had not reached the ground surface GL, and it was confirmed that the injected water W did not flow out from the ground surface GL. Next, the estimated permeability coefficient k of Example 1 calculated by numerical simulation e The time change of the hydraulic conductivity k is estimated by the conventional method (Comparative Example 1) in which water is injected at the same height as the ground surface (pressure head = 0.0 m). e The estimated hydraulic conductivity k e The time change of was calculated by substituting the water injection flow rate output at each time in the numerical simulation into the evaluation formula (Equation 3). The relationship between water injection time and hydraulic conductivity is shown in Figure 6.

[0019]

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[0020] As shown in Figure 6, the estimated permeability coefficient k e is the estimated hydraulic conductivity k e In Comparative Example 1 (pressure head = 0.0 m), the estimated hydraulic conductivity k obtained in Example 1 3 hours after the start of water injection was larger than that of Example 1. e It took about 200 hours to obtain the same value. Therefore, it was confirmed that the time required for the permeability test can be significantly reduced by using the water injection pressure determination method of this embodiment.

[0021] In actual ground, the permeability coefficient k g Since variations in the permeability of the ground are expected, the design standard value (10 -7 m / s) is greater than (10 -6 m / s) and smaller 10 -8 The analysis was performed assuming a pressure head of 4.34 m. -6As shown in Figure 7, the water saturation distribution after 3 hours when the ground permeability coefficient is set to the design standard value (10 -7 m / s) is greater than (10 -6 It was confirmed that even if the ground had a high permeability due to insufficient compaction, water would not overflow from the ground surface GL. In other words, it was confirmed that it is possible to carry out an on-site permeability test using the water injection pressure determined by the water injection pressure determination method of this embodiment. Furthermore, it was confirmed that even if the permeability coefficient of the ground was higher than the design standard value (10 -7 m / s) is smaller than (10 -8 It was confirmed that even at high pressures (such as 1000 m / s), the surrounding ground was sufficiently saturated and it was possible to carry out an in-situ permeability test.

[0022] analytical permeability coefficient k g to 10 -6 , 10 -7 , 10 -8 m / s and the pressure head was set to 4.34 m. The estimated permeability coefficient k e The estimated permeability coefficient k e / analytical permeability coefficient k g The change over time is shown. As shown in Figure 8, in both cases, k e / k g approaches 1, and the analytical permeability coefficient k g It was confirmed that this is required. Therefore, it was confirmed that even if the permeability coefficient of the on-site ground G varies from the design standard value, by adopting the water injection pressure determination method of this embodiment, on-site permeability tests can be carried out efficiently without the injected water W flowing out from the ground surface GL.

[0023] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and each of the above-described components can be modified as appropriate within the scope of the invention. In the above embodiment, the dam body has been described, but the ground to be subjected to the on-site permeability test is not limited to a dam. [Explanation of symbols]

[0024] 1 Test equipment 2. Water storage section 3 Water injection pipe 4. Packer 5 Protection tube 6 test holes 7 Embankment body 8 Core 9 Filter section 10 Locking section G Ground GL ground surface S1 Time determination process S2 Flow rate calculation process S3 Hydraulic conductivity assumption process S4 Pressure determination process

Claims

1. A water injection pressure determination method for determining a water injection pressure in an on-site permeability test, a time determination step of determining a test time from the start to the end of the test; a flow rate calculation step of calculating a water injection flow rate required for the water injected into the ground to reach the ground surface within the test time; a permeability coefficient assumption step of assuming a permeability coefficient of the ground; and a pressure determination step of determining the water injection pressure using the water injection flow rate and the permeability coefficient.

2. 2. The water injection pressure determination method according to claim 1, wherein the water injection flow rate is calculated using Equation 1 in the flow rate calculation step. [Equation 1]

3. 3. The water injection pressure determination method according to claim 1, wherein in the pressure determination step, a pressurized water head is calculated using Equation 2, and the water injection pressure is determined based on the pressurized water head. [Equation 2]

4. The ground is a dam body, Assumed value of hydraulic conductivity k ａ 4. The method for determining the water injection pressure according to claim 3, wherein the value of the water injection pressure is set to be greater than the design reference value of the hydraulic conductivity used in the design of the cut-off wall.

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