Method for measuring the content of swellable clay

Abstract

This invention provides a method for measuring the content of swelling clay in soils containing sand or gravel in addition to swelling clay, which allows for obtaining a value close to the true value while ensuring representativeness. [Solution] The measurement method of the present invention comprises a stirring and mixing step of stirring and mixing 300g or more of a sample of soil taken from a target soil containing swelling clay and at least one of sand and gravel, with water weighing 5 to 20 times the weight of the soil, and a measurement step of measuring the suspension, which is the portion of the mixture of soil and water obtained in the stirring and mixing step from which the sand and gravel have been removed, to determine the content of swelling clay. In this method, by stirring and mixing the soil with water, the swelling clay adhering to the surface of the sand or gravel is reliably separated from the sand or gravel and suspended in water.

Description

[Technical Field]

[0001] This invention relates to a method for measuring the content of swellable clay. [Background technology]

[0002] Mixed soil, consisting of swellable clay such as bentonite mixed with sand and gravel, is an excellent waterproofing material and is used in final disposal sites and radioactive waste disposal facilities. The waterproofing performance of the mixed soil depends on the content of swellable clay, so the content of swellable clay must meet the designed content.

[0003] By utilizing the property of swelling clay to adsorb methylene blue, the amount of swelling clay in any given soil can be measured. The amount of adsorbed methylene blue is measured according to JIS Z2451:2019 "Method for measuring the amount of methylene blue adsorbed by bentonite, etc." This measurement uses only 0.5 g of swelling clay as a dry sample.

[0004] It is assumed that this measurement method for determining the content of swellable clay is intended for soils with small particle sizes and a narrow particle size distribution. This is because, in soils with large particle sizes or a wide particle size distribution, the measurement results from the 0.5g dry sample cannot be said to guarantee representativeness of the entire soil containing swellable clay.

[0005] A method for measuring the bentonite content in mixed soil containing sand and gravel of varying particle sizes is known (for example, Patent Document 1). Patent Document 1 discloses measuring the bentonite mixture to determine the bentonite content in order to obtain a bentonite mixture with desired water-impermeable properties. In this method, 500g to 1000g of soil is used for measurement, and after removing sand and gravel using a sieve with a nominal size of 2mm, the measurement is performed using methylene blue. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 7424068 [Overview of the project] [Problems that the invention aims to solve]

[0007] In Patent Document 1, swelling clay such as bentonite may be attached to the surface of sand or gravel removed by sieving, in which case the measurement result for bentonite-mixed soil will be lower than the true value. In view of this situation, the present invention aims to provide a method for measuring the content of swelling clay that can obtain a value close to the true value while ensuring representativeness, for soils that contain sand or gravel in addition to swelling clay. [Means for solving the problem]

[0008] The present invention provides a method for measuring the content of swellable clay, comprising: a stirring and mixing step of stirring and mixing 300 g or more of a sample of soil taken from a target soil containing swellable clay and at least one of sand and gravel with water weighing 5 to 20 times the weight of the sample soil; and a measurement step of measuring the suspension, which is the portion of the mixture of sample soil and water obtained in the stirring and mixing step from which the sand and gravel have been removed, to determine the content of swellable clay.

[0009] By stirring and mixing the soil sample with water, the swellable clay adhering to the surface of the sand or gravel is reliably separated from the sand or gravel and suspended in the water. By measuring this suspension, the amount of swellable clay contained in the soil sample can be determined. Furthermore, a sample size of 300g or more is sufficient to ensure that the particle size distribution of the soil sample closely matches that of the target soil. Therefore, according to this invention, it is possible to obtain a value close to the true value while ensuring representativeness, even for soils containing sand or gravel in addition to swellable clay.

[0010] For the measurement, multiple sample soils collected from multiple locations within the target soil may be used. Each of these sample soils may undergo a mixing and measurement process, and the results obtained from the measurement process may be compared among the sample soils. By comparing the measurement results of multiple sample soils collected from multiple locations within the target soil, the homogeneity of the target soil can be determined.

[0011] In the measurement process, it is preferable to perform the measurement using the adsorption of methylene blue onto the swollen clay in the suspension (JIS Z2451:2019).

[0012] The suspension may be the supernatant of elutriation. This method is simple because it does not require any further special procedures for collecting the sample for measurement.

[0013] Before measuring the suspension, it may be subjected to ultrasound. The ultrasonic vibrations can loosen any aggregated swelling clay, further improving the reliability of the measurement.

[0014] The water used may be deionized water or distilled water. Since these types of water do not contain substances that could cause measurement errors, no measurement errors will occur due to the water used.

[0015] The target soil may be a soil prepared by mixing swellable clay with at least one of sand and gravel. In this case, the amount of adsorbent substances contained in the components other than swellable clay in the target soil may be measured in advance, and the measurement results of the measured soil may be corrected using these results.

[0016] In this invention, the target soil may be for water-impermeable purposes. The swellable clay contained in the target soil absorbs water and swells, thus providing high water-impermeability. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a method for measuring the content of swelling clay in soils that contain sand or gravel in addition to swelling clay, while ensuring representativeness and obtaining a value close to the true value.

Brief Description of the Drawings

[0018] [Figure 1] It is a graph showing the results of Tests 1 to 4.

Modes for Carrying Out the Invention

[0019] The present invention is a method for determining the content ratio of swelling clay for soil containing at least one of swelling clay, sand, and gravel. Soil containing swelling clay is effective as a water barrier material for preventing harmful substances from reaching the living area at final disposal sites and radioactive waste disposal facilities. In order to exhibit effective water barrier performance, it is preferable that the content ratio (or mixing ratio) of swelling clay is within a predetermined range.

[0020] The mixing ratios of swelling clay, sand, and gravel may be designed so that the content ratio of swelling clay is within a predetermined range, and these may be mixed to obtain mixed soil. In this case, when storing, transporting, etc. and using the mixed soil, it is necessary to confirm whether the content ratio of swelling clay is the designed value. According to the measurement method of the present invention, it is possible to obtain the content ratio of swelling clay as a value close to the true value.

[0021] Hereinafter, preferred embodiments of the present invention will be described in detail. In this embodiment, the "content ratio of swelling clay" means the weight ratio of swelling clay contained in any soil. Also, in the embodiments described below, as the measurement of the content of swelling clay, measurement based on JIS Z2451:2019 "Method for Measuring Methylene Blue Adsorption Amount of Bentonite, etc." is adopted. Among these measurement methods, the "spot method" is performed.

[0022] <Implementation Procedure> The measurement method of this embodiment targets soil containing swellable clay and at least one of sand and gravel (hereinafter referred to as "target soil"). The target soil weighs several tons to over ten tons, and even several hundred tons. In this embodiment, the target soil is soil prepared by artificially mixing swellable clay, sand, and gravel. The swellable clay, sand, and gravel used are each subjected to a particle size distribution test in advance to obtain the particle size distribution. In this embodiment, sand refers to particles with a diameter of 0.075 mm to 2 mm, and gravel refers to particles with a diameter of 2 mm or more.

[0023] A sample for measurement (hereinafter referred to as "measurement soil") is collected from the target soil. The amount of measurement soil collected should be 300g or more. This amount may be 500g or more, 1000g or more, or 1500g or more. There is no particular upper limit on the weight, but it is preferable that it be within a range that allows the following procedure to be easily carried out, for example, it may be 10000g, 8000g or 5000g.

[0024] Here, the wet weight and water content of the soil sample are measured. The wet weight is measured using a scale. The water content is measured according to "JIS A1203 Method for Measuring Water Content of Soil".

[0025] Prepare water for mixing with the soil sample. Preferably, the water does not contain substances that may cause measurement errors, as described later; for example, it is preferably deionized water or distilled water. The amount of water may be 5 to 20 times, 8 to 18 times, or 10 to 16 times the amount of soil sample.

[0026] The prepared soil sample and water are thoroughly stirred and mixed in any container to obtain a mixture (stirring and mixing step). The stirring method is arbitrary. In this embodiment, sand and gravel are separated by elutriation. The mixture is left to stand for 30 to 60 minutes to allow the sand and gravel to settle, and after this time has elapsed, the settlement of the sand and gravel is visually confirmed. At this point, the water on top of the container is a suspension containing swellable clay. Depending on the settling of the sand and gravel, the standing time may be 10 seconds to 1 minute after stirring.

[0027] Next, a small amount of the supernatant liquid in the container is collected in a small container. The amount collected should be small enough for measurement. A dispersant such as sodium pyrophosphate or sodium hexametaphosphate is added to the collected supernatant liquid. The amount of dispersant added may be 1% to 2% by weight. Then, ultrasound is applied to the small container using an ultrasonic device. The settings for the ultrasonic device should be, for example, a frequency of 25kHz to 30kHz and an output of 150W to 200W.

[0028] Subsequently, the amount of methylene blue adsorbed onto the swelling clay is measured in the supernatant water in the small container according to JIS Z2451:2019 "Method for measuring the amount of methylene blue adsorbed by bentonite, etc." and the MBC value is determined (measurement step). The amount of methylene blue adsorbed onto the swelling clay alone used in the preparation of the target soil is also measured in advance. The swelling clay content of the soil being measured can be calculated using the following formula (1).

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[0029] If the sand used to prepare the target soil contains an adsorbent substance that adsorbs methylene blue, the content of swollen clay in the measured soil can be determined using the following correction formula (2). The amount of methylene blue adsorbed by the sand alone used to prepare the target soil should also be measured beforehand.

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[0030] In addition, when the gravel used for preparing the target soil contains an adsorbent substance that adsorbs methylene blue, it can be corrected by the same principle as above.

[0031] The ponded water can be collected multiple times from the container in which the mixture of the measured soil and water is obtained, the adsorption amount of methylene blue can be measured for each ponded water, and the average of them can be used as the value in the above calculation formula.

[0032] In this embodiment, instead of using only one measured soil, a plurality of measured soils collected from multiple locations of the target soil are used as the measurement targets. The above stirring and mixing process and measurement process are performed for each of the plurality of measured soils, and the obtained results are compared among the measured soils. The homogeneity of the target soil is judged from the magnitude of these differences.

[0033] <Effect> If sand and gravel are removed by sieving before measuring the swellable clay content, there is a possibility that swellable clay may be adhering to the surface of the sand and gravel. Also, the swellable clay may aggregate, increasing in particle size and preventing it from passing through the sieve. In these cases, the calculated swellable clay content will be smaller than the true value. In contrast, according to the measurement method of this embodiment, by stirring and mixing the soil to be measured with water, not only is the small particle size of swellable clay suspended in the water, but the swellable clay adhering to the surface of the sand and gravel is reliably separated from the sand and gravel and suspended in the water. By measuring this suspension, the amount of swellable clay contained in the soil to be measured can be accurately determined. Furthermore, in this embodiment, since 300g or more of the soil to be measured is used, it can be said that the amount of soil to be measured is sufficient to such an extent that the particle size distribution of the soil to be measured almost matches the particle size distribution of the target soil, considering the normal particle size distribution of the target soil containing swellable clay, sand, and gravel. Therefore, according to the measurement method of this embodiment, it is possible to obtain a value close to the true value while ensuring representativeness, even for soil containing sand and gravel in addition to swellable clay.

[0034] In general, when mixing swellable clay, sand, and gravel, it can be difficult to achieve a uniform mixture. Even if a uniform mixture is achieved, material separation may occur during transportation. Therefore, it is important to have a means of confirming the homogeneity of the mixed soil. In this embodiment, measurements are taken on multiple sample soils collected from multiple locations on the target soil, and the results are compared. If the differences between these samples are small, it can be said that the homogeneity of the mixture when preparing the target soil by mixing swellable clay, sand, and gravel is ensured. Conversely, if the differences are large, it can be seen that the mixing during the preparation of the target soil was insufficient, or that material separation occurred during subsequent handling. In other words, the homogeneity of the target soil can be determined by comparing sample soils collected from multiple locations on the target soil.

[0035] Furthermore, in this embodiment, since the suspension used for measurement is the supernatant of elutriation, no further special procedures are required for collecting the measurement sample, making it convenient. As the particle size of swellable clay is smaller than that of sand and gravel, and as is clear from Stokes' equation, the swellable clay remains suspended in the supernatant for a long time. Therefore, a large-scale classification operation is unnecessary to obtain a fraction containing swellable clay.

[0036] Furthermore, since the suspension is subjected to ultrasound before measurement, any aggregated swelling clay is loosened, making it easier for methylene blue to adhere. This further improves the reliability of the measurement.

[0037] Furthermore, when deionized water or distilled water is used as the water source, these waters do not contain substances that could cause measurement errors, thus eliminating measurement errors caused by the water used. For example, if tap water is used, the metal ions and other substances it contains may cause the swelling clay to coagulate and settle, potentially resulting in some of the swelling clay not being included in the supernatant of the elutriation.

[0038] In this embodiment, the target soil may be for water-impermeable purposes. The swellable clay contained in the target soil absorbs water and swells, thus providing high water-impermeability.

[0039] Although preferred embodiments of the present invention have been described above, the present invention is not limited in any way to the above embodiments. For example, in the above embodiments, after the stirring and mixing step, sand and gravel were separated by elutriation and the supernatant was used as the target of measurement. However, instead, the mixture may be filtered using a sieve that can remove sand and gravel and is sufficient to allow particles with a particle size of 75 μm or less to pass through, and the obtained filtrate may be used as the target of measurement.

[0040] Furthermore, in the above embodiment, a measurement method in accordance with JIS Z2451:2019 was used to determine the content of swellable clay, but other measurement methods, such as a method utilizing electrical conductivity, may also be used to determine the content of swellable clay. [Examples]

[0041] The present invention will be explained in more detail below with reference to test examples. However, the present invention is not limited to the following test examples.

[0042] The following three types of soil mixtures were prepared. • Mixed soil A: Sand and bentonite were mixed in a weight ratio of 85:15. • Mixed soil B: Gravel, sand, and bentonite were mixed in a weight ratio of 50:37.5:12.5. Mixed soil C: A mixture of gravel, sand, and bentonite in a weight ratio of 35:55:10.

[0043] The MBC value was determined for the bentonite used in the above preparation according to the procedure in accordance with JIS Z2451:2019.

[0044] [Test 1] 1000g of soil was taken from soil mixture A and added to 10000g of deionized water filling a container. After thorough stirring and mixing, it was left to stand for 1 minute. After visually confirming that the sand had settled, the supernatant water was collected in three bottles. The bottles were subjected to ultrasonic vibration for 20 minutes. Subsequently, the MBC value was determined according to the procedure in accordance with JIS Z2451:2019. Using the average value of the measurement results from the three bottles, the content of swellable clay was determined according to formula (1) above.

[0045] [Exam 2] A 0.5g sample of soil was taken from soil mixture A, and the MBC value was determined according to the procedure in accordance with JIS Z2451:2019. The content of swellable clay was determined according to formula (1) above.

[0046] [Exam 3] The content of swellable clay was determined in the same manner as in [Experiment 1], except that 1000g of soil was taken from soil mixture B.

[0047] [Exam 4] The content of swellable clay was determined in the same manner as in [Experiment 1], except that 1000g of soil was taken from soil mixture C.

[0048] [Repeating each test] Tests 1 through 4 were repeated. Test 1 was performed a total of 3 times, Test 2 a total of 3 times, Test 3 a total of 7 times, and Test 4 a total of 17 times. When repeating the tests, soil samples were taken from different locations in each soil mixture.

[0049] The test results are shown in Figure 1. In tests 1, 3, and 4, the measured bentonite content was close to the designed bentonite mixing ratio (15%, 12.5%, and 10%, respectively) in all repeated tests. This also means that the degree of mixing of each soil mixture was homogeneous. On the other hand, in test 2, the measured bentonite content varied. Since it was found in [Test 1] that the degree of mixing of soil mixture A was homogeneous, the results of test 2 indicate that the measurement error (deviation from the true value) of the measurement method is large, and consequently, that the repeated test results do not reflect the actual state (mixing ratio) of soil mixture A, and the reliability of the test is low. [Industrial applicability]

[0050] This invention can be used when handling soil containing swelling clay.

Claims

1. A stirring and mixing step involves stirring and mixing 300g or more of the sample soil taken from the target soil containing swelling clay and at least one of sand and gravel, with water weighing 5 to 20 times the weight of the sample soil. A method for measuring the content of swellable clay, comprising: a measurement step of measuring the suspension which is the portion obtained by removing the sand and gravel from the mixture of the soil and water obtained in the stirring and mixing step, and determining the content of the swellable clay.

2. The measurement method according to claim 1, wherein the measurement soil is a plurality of measurement soils collected from multiple locations of the target soil, the stirring and mixing step and the measurement step are performed on each of the plurality of measurement soils, and the results obtained in the measurement step are compared among the measurement soils.

3. The measurement method according to claim 1, wherein the measurement step involves performing a measurement (JIS Z2451:2019) utilizing the adsorption of methylene blue onto the swollen clay in the suspension.

4. The measurement method according to claim 1, wherein the suspension is the supernatant of elutriation.

5. The measurement method according to claim 1, wherein ultrasound is applied to the suspension before measuring the suspension.

6. The measurement method according to claim 1, wherein the water is ion-exchanged water or distilled water.

7. The measurement method according to claim 1, wherein the target soil is soil prepared by mixing swellable clay with at least one of sand and gravel.

8. The measurement method according to claim 7, wherein the amount of adsorbent substances contained in components other than the swelling clay among the target soil is measured in advance, and the measurement results of the measured soil are corrected using the results.

9. The measurement method according to any one of claims 1 to 7, wherein the target soil is for waterproofing.

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