JP6919368B2 - Storage structure for heavy metal contaminated soil - Google Patents

Description

The present invention relates to a storage structure for storing contaminated soil containing heavy metals.

The shavings generated during excavation may contain heavy metals specified by soil environmental standards (eg, cadmium, lead, hexavalent chromium, mercury, arsenic, selenium, fluorine, boron). If the soil elution amount standard or content standard is not met, it is necessary to take measures such as containment measures for heavy metals.

As an example of this measure, there is an adsorption layer construction method. The adsorption layer method is to elute the heavy metals in the contaminated soil layer into the permeated water that has permeated into the embankment due to rainfall or the like and leach it into the adsorption layer, and while the leachate passes through the adsorption layer, the heavy metals are adsorbed on the adsorbent. This is a construction method in which the concentration of heavy metals in leachate (hereinafter, also simply referred to as the contamination concentration) is reduced to a level that does not affect the environment and discharged. As a general embankment structure used in this type of adsorption layer construction method, for example, in Patent Documents 1 to 3, an adsorption layer in which a base material such as crushed stone or sand and an adsorption material are mixed is placed under the contaminated soil layer. A structure provided over substantially the entire surface is disclosed.

Japanese Unexamined Patent Publication No. 2011-240289 Japanese Unexamined Patent Publication No. 2012-110852 Japanese Unexamined Patent Publication No. 2014-226589

By the way, in order to effectively reduce the pollution concentration of leachate, the contact frequency between the adsorbent mixed in the adsorption layer and the heavy metal contained in the leachate (the opportunity for the heavy metal to come into contact with the adsorbent and be adsorbed) should be determined. It is desirable to increase it.

In the conventional general embankment structure, since the adsorption layer is provided over a wide area under the contaminated soil layer, the volume of the adsorption layer is inevitably large. Therefore, if an attempt is made to increase the contact frequency between the adsorbent and the heavy metal by simply increasing the mixing amount of the adsorbent, the amount of the adsorbent becomes excessive with respect to the contamination concentration, which wastefully increases the cost. There is a problem such as. On the other hand, if an appropriate amount of adsorbent for the contamination concentration is mixed with the adsorbent layer having a large volume, the amount of the adsorbent per unit volume decreases, which leads to a decrease in contact frequency, and the contamination concentration cannot be effectively reduced. There are also challenges.

In addition, if the leaching of heavy metals from the contaminated soil layer is prolonged and the adsorption capacity of the adsorption layer deteriorates due to this, it is necessary to excavate and remove the adsorption layer and reconstruct the embankment to be newly replaced. In the above-mentioned conventional embankment structure, since the adsorption layer is laid over the entire lower part of the contaminated soil layer, it is necessary to excavate and remove the entire embankment including the contaminated soil layer in the upper layer when replacing the adsorption layer. There are also problems such as lengthening the construction period required for re-construction and increasing the construction cost.

An object of the present disclosure technique is to provide a storage structure for heavy metal contaminated soil capable of effectively increasing the frequency of contact between an adsorbent and a heavy metal in the adsorption layer while facilitating reconstructability.

In the technique of the present disclosure, at least an impermeable layer and a contaminated soil layer containing heavy metals are laminated from the ground side, and leachate flowing from the contaminated soil layer onto the upper surface of the impermeable layer is directed toward the side surface of the impermeable layer. The embankment main body and the outside of the embankment main body are provided at least in contact with the side surface of the impermeable layer, and the leachate that seeps out from the embankment main body is circulated and included in the leachate. It is characterized by comprising an adsorption layer in which an adsorbent for adsorbing the heavy metal is mixed.

Further, a drainage layer having a higher hydraulic conductivity than the impermeable layer is provided between the impermeable layer and the contaminated soil layer of the filling body, and the leachate flowing down from the contaminated soil layer is the drainage layer. It is preferable that the water flows through the inside and is guided toward the side surface of the impermeable layer.

Further, it is preferable to further include a water-impervious member that is embedded in the adsorption layer and partially blocks the flow of leachate in the adsorption layer to lengthen the leachate flow path in the adsorption layer.

Further, the water-impervious member may be a plurality of water-impervious sheets that are alternately arranged in the adsorption layer at intervals above and below to meander the leachate flow path.

Further, the water-impervious sheet may be embedded in the adsorption layer by inclining the sheet surface so as to have an upward slope from the upstream side to the downstream side in the flow direction of the leachate.

Further, the water-impervious member is a water-impervious sheet embedded in the adsorption layer in the vertical direction, and leachate temporarily flows between the sheet surface of the water-impervious sheet and the side surface of the water-impervious layer. It may be blocked and stay.

Further, a slope reinforcing layer may be formed at least at a portion of the embankment main body in contact with the adsorption layer.

According to the storage structure of heavy metal contaminated soil of the present disclosure, the frequency of contact between the adsorbent and the heavy metal in the adsorption layer can be effectively increased while facilitating reconstructability.

It is a schematic vertical sectional view which shows the embankment which concerns on this embodiment. It is a schematic vertical sectional view which shows the main part of the embankment which concerns on this embodiment. It is a schematic perspective view which shows the catchment structure which concerns on this embodiment. It is a schematic diagram explaining the construction procedure of the embankment which concerns on this embodiment. It is a schematic diagram explaining the reconstruction procedure of the 2nd embankment part which concerns on this embodiment. (A) is a vertical cross-sectional view schematically explaining the flow of leachate in the embankment according to the present embodiment. (B) is a schematic vertical sectional view showing a general embankment. It is a schematic vertical sectional view which shows various deformation examples of the adsorption layer which concerns on this embodiment. It is a figure which explains typically the test apparatus for the water flow test. (A) is a figure explaining the material used for the water flow test. (B) is a figure explaining a test case of a water flow test. It is a line graph explaining the test result of the water flow test. It is a schematic vertical sectional view which shows the embankment which concerns on other embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embankment 1 shown in FIG. 1 shows an example of a storage structure for heavy metal contaminated soil according to the present invention. The embankment 1 is formed in a substantially trapezoidal cross section having a slope on the side surface, and is provided adjacent to the first embankment portion 1A as the embankment main body portion and the lower slope of the first embankment portion 1A. It has 2 embankment parts 1B.

[1st embankment]
The first embankment portion 1A is formed by laminating an impermeable layer (impermeable layer) 2, a drainage layer 3, a contaminated soil layer 4, and a first soil covering layer 5 in order from the ground G.

The impermeable layer 2 blocks the permeation of the leachate that has exuded from the contaminated soil layer 4 into the drainage layer 3 into the ground G, and is laid on the lowest layer in the first embankment portion 1A, specifically, the surface of the ground G. The drainage layer 3 is laminated on the upper surface thereof. The upper surface of the impermeable layer 2 is formed in a horizontal plane, or is formed in an inclined surface slightly downward toward the outer peripheral edge (hoto) thereof, from the contaminated soil layer 4 to the drainage layer 3. The leachate that has leached out is guided toward the outer periphery of the first embankment portion 1A. The thickness of the impermeable layer 2 is not particularly limited, but is, for example, about 30 cm to 50 cm.

The impermeable layer 2 is laid on the surface of the ground G wider than the outer circumference of the lower surface (the tail) of the drainage layer 3 in a plan view, and a slope 2A inclined at a predetermined gradient angle is provided on the side surface thereof. ing. The slope of the slope 2A is not particularly limited, but is set to, for example, about 1: 0.5. In the present embodiment, the slope 2A is preferably provided with a terracell (registered trademark) having a honeycomb lattice structure in a plan view, or a slope reinforcing layer 2B formed by a sheet-shaped geotextile, a geogrid, or the like.

The hydraulic conductivity P1 of the impermeable layer 2 ^{is set to 1.0 × 10-7} m / s or less, which is the impermeable standard of the pond. In the present embodiment, a water-impervious layer 2 having ^{a hydraulic conductivity P1 of 9.20 × 10-8} m / s is formed by compacting a mixed soil of 70% mountain sand and 30% cohesive soil. As the cohesive soil, for example, the trade name "Tochikure" of Kanto Kasei Co., Ltd. can be used.

The drainage layer 3 guides the leachate leached from the contaminated soil layer 4 toward the outer periphery of the first embankment portion 1A along the upper surface of the impermeable layer 2, and is formed by the contaminated soil layer 4 and the impermeable layer 2. It is formed between layers. That is, the lower surface of the drainage layer 3 is in contact with the upper surface of the impermeable layer 2, and the upper surface of the drainage layer 3 is in contact with the lower surface of the contaminated soil layer 4. Further, in a plan view, the drainage layer 3 is laid on the upper surface of the impermeable layer 2 inside the outer circumference (the head) of the upper surface of the impermeable layer 2, and the side surface thereof is inclined at a predetermined gradient angle. Surface 3A is provided. The slope of the slope 3A is not particularly limited, but is preferably about 1: 0.5, which is equivalent to the slope of the slope 2A. The slope 3A is preferably provided with a slope reinforcing layer 3B formed of teracell (registered trademark), geotextile, geogrid, or the like.

In the present embodiment, the drainage layer 3 is formed by compacting a mixed material obtained by mixing crushed stone and sand material such as mountain sand or river sand. The drainage layer 3 is formed by, for example, spreading and compacting the drainage layer 3 so as to cover the upper surface of the impermeable layer 2 with a mixed material in which 90% of No. 7 crushed stone and 10% of mountain sand are mixed. In the drainage layer 3 formed in this way, for example, the hydraulic conductivity P2 is 1.76 × 10 ^{-3 m / s, which is 1.0 × 10 2} m / s or more higher than the hydraulic conductivity P1 of the impermeable layer 2. (P2> P1).

The contaminated soil layer 4 is formed by heaping up the contaminated soil generated by excavation or the like. The contaminated soil of this embodiment is an arsenic-containing soil. In a plan view, the contaminated soil layer 4 is laid on the upper surface of the drainage layer 3 inside the outer circumference (floor head) of the upper surface of the drainage layer 3, and the lower slope sloped at a predetermined gradient angle on the side surface thereof. 4A and the upper slope 4B are provided. The slope of the lower slope 4A is preferably about 1: 0.5, which is equivalent to the slope of each slope 2A, 3A, and the slope of the upper slope 4B is preferably about 1: 0.5, which is equivalent to the slope of each slope 2A, 3A. It is set to about 1: 1.8, which is smaller than the gradient of 4A. The hydraulic conductivity P3 of the contaminated soil layer 4 is at least higher than the hydraulic conductivity P1 of the impermeable layer 2 (P3> P1).

The first soil covering layer 5 covers the outer surface of the contaminated soil layer 4 to prevent the contaminated soil from scattering or flowing out. Crushed stone or earth and sand can be used for the first soil covering layer 5. The first soil covering layer 5 of the present embodiment is formed by using crushed stone No. 7, and covers the lower slope 4A, the upper slope 4B, and the upper surface 4C of the contaminated soil layer 4 in a series. A slope reinforcing layer 5B formed by teracell (registered trademark), geotextile, geogrid, or the like is provided at least on a portion of the first soil covering layer 5 corresponding to the lower slope 4A.

[2nd embankment]
The second embankment portion 1B is formed by laminating the adsorption layer 6 and the second soil covering layer 7 in order from the ground G.

The adsorption layer 6 adsorbs heavy metals contained in the leachate leaching from the contaminated soil layer 4 through the drainage layer 3, and is provided along the slopes 2A and 3A of the impermeable layer 2 and the drainage layer 3. Has been done. Specifically, as shown in FIG. 2, the adsorption layer 6 has a substantially trapezoidal or substantially triangular vertical cross section, and its inner side surface 6B has slopes 2A and 3A of the impermeable layer 2 and the drainage layer 3. It is formed from the slope of the drainage layer 3 (the upper end of the slope reinforcement layer 3B) to the slope of the impermeable layer 2 (the lower end of the slope reinforcement layer 2B) so as to be in contact with the water shield layer 2. The gradient of the outer surface (slope) 6A of the adsorption layer 6 is not particularly limited, but is preferably about 1: 1.8, which is equivalent to the gradient of the upper slope 4B of the contaminated soil layer 4.

The adsorption layer 6 is formed by compacting a mixed material obtained by mixing crushed stone, sand material such as mountain sand or river sand, and an adsorbent material for heavy metals. In this embodiment, since arsenic is targeted as a heavy metal, an arsenic adsorbent is used. As the arsenic adsorbent, for example, the trade name "Fixall (registered trademark) FB" of Ishihara Sangyo Co., Ltd. can be used. This fixall FB contains iron oxide and calcium sulfate as main components, and the specific surface area of iron oxide particles is 180 m ² / g or more.

The adsorption layer 6 is prepared by adding ^{an arsenic adsorbent in a required amount (for example, 30 to 50 kg / m 3} ) suitable for the contamination concentration to a base material obtained by mixing 90% of No. 7 crushed stone and 10% of mountain sand. , It is formed by leveling and compacting so as to cover the surface of the ground G and the slopes 2A and 3A of the impermeable layer 2 and the drainage layer 3. In the adsorption layer 6 formed in this way, for example, the hydraulic conductivity P4 is 1.76 × 10 ^-3 m / s, which is equivalent to the hydraulic conductivity P2 of the drainage layer 3, which is 1. The value is higher than 0 × 10 ² m / s (P4≈P2> P1).

The second soil covering layer 7 functions as a slope reinforcing layer of the adsorption layer 6 by covering the slope 6A of the adsorption layer 6. As the second soil covering layer 7, crushed stone, earth and sand, etc. similar to those of the first soil covering layer 5 can be used. In the present embodiment, the second soil covering layer 7 is preferably formed of teracell (registered trademark), geotextile, geogrid, or the like.

[Water collection structure]
Next, the details of the catchment structure 20 will be described with reference to FIGS. 2 and 3. The water collecting structure 20 monitors the adsorption capacity (arsenic concentration of leachate) of the adsorption layer 6, and corresponds to a plurality of perforated pipe members 21A to C and a plurality of perforated pipe members 21A to C. The drainage ditch 23 connecting the primary water collecting basins 22A to C, the primary water collecting basins 22A to C, and the final water collecting basin 24 for collecting and storing the leachate flowing out from the primary water collecting basins 22A to C. (Shown only in FIG. 3). In FIG. 2, reference numeral 8 indicates a geomembrane sheet that is laid between the ground G and the adsorption layer 6 to prevent the leachate from penetrating into the ground G. The water-impervious sheet 8 is preferably laid from between the layers of the slope reinforcing layers 2B and 3B to the vicinity of the upper edge of the primary water collecting basins 22A to C.

The perforated pipe members 21A to C take out at least a part of the leachate flowing in the adsorption layer 6 to the outside, and the base end side having a plurality of through holes formed on the side surface is embedded in the adsorption layer 6. ing. Further, the perforated pipe members 21A to C have the tip side having no through hole on the side surface protruding outward from the second soil covering layer 7. In the present embodiment, as shown in FIG. 3, the perforated pipe members 21A to C are arranged at appropriate intervals (for example, 50 to 100 m intervals) in the longitudinal direction of the method tail of the second soil covering layer 7. The adsorption layer 6 is divided into a plurality of monitoring sections A to C by the perforated pipe members 21A to C.

The primary catchment basins 22A to C are, for example, substantially housing-shaped concrete catchment basins with an open upper portion, and are embedded in the ground G corresponding to the lower tip portion of each of the perforated pipe members 21A to C. Has been done. The leachate discharged from the corresponding perforated pipe members 21A to C is stored in the primary catchment basins 22A to C.

The drainage ditch 23 is, for example, a U-shaped ditch made of concrete with an open upper portion, and is buried in the ground G adjacent to the buttock of the second soil covering layer 7. Further, the drainage ditch 23 extends substantially parallel to the longitudinal direction of the bottom of the second soil covering layer 7 so as to connect the primary catchment basins 22A to C (see FIG. 3). The groove depth of the drainage ditch 23 is formed shallower than the primary catchment basins 22A to C, and when the water level of the leachate stored in the primary catchment basins 22A to C rises above the bottom surface of the drainage ditch 23, each Leachate flows out from the primary catchment basins 22A to C into the drainage ditch 23. Further, the bottom surface of the drainage groove 23 is formed in an inclined surface shape that descends from one end side (left side in FIG. 3) to the other end side (right side in FIG. 3) in the longitudinal direction, and each primary water collecting basin 22A to 22A. The leachate that has flowed out from C into the drainage ditch 23 is guided to the other end side of the drainage ditch 23.

The final catchment basin 24 shown in FIG. 3 is, for example, a substantially housing-shaped concrete catchment basin with an open upper portion, and is buried in the ground G where the other end (downstream side end) of the drainage ditch 23 is located. Has been done. The final catchment basin 24 is preferably formed to have a volume larger than that of each of the primary catchment basins 22A to C. In the final catchment basin 24, leachate that flows out from each of the primary catchment basins 22A to C and flows in the drainage ditch 23 is stored.

In the catchment structure 20 configured as described above, the adsorption capacity (arsenic concentration of leachate) of the adsorption layer 6 can be monitored according to the following procedures (1) to (3).

(1) First, the arsenic concentration of the leachate stored in the final catchment basin 24 is measured, and when a change is confirmed in the measured value (for example, when the arsenic concentration exceeds a predetermined control value). Measures the arsenic concentration of the leachate stored in each of the primary catchment basins 22A to C individually.

(2) Of the primary catchment basins 22A to C, if there is a primary catchment basin 22A to C in which the arsenic concentration of the leachate exceeds the control value, the leachate in the primary catchment basin 22A to C is present. Continue to monitor the arsenic concentration in.

(3) As a result of monitoring, if the arsenic concentration of the leachate exceeds the control value, the monitoring sections A to C of the perforated pipe members 21A to C corresponding to the primary water collecting tubs 22A to C (for example, the primary collecting). If the arsenic concentration of the leachate in the water basin 22B exceeds the control value, it is presumed that the adsorption capacity of the adsorption layer 6 of the monitoring section B) corresponding to the perforated pipe member 21B has deteriorated. In such a case, the adsorption layer 6 and the second soil covering layer 7 of the corresponding monitoring section B are excavated and removed, and the second embankment portion 1B in which the adsorption layer 6 is newly replaced is reconstructed. The detailed procedure for re-construction will be described later.

[Construction procedure, re-construction procedure]
Next, the construction procedure of the embankment 1 of the present embodiment will be described with reference to FIG.

First, as shown in FIG. 4 (A), the mixed soil to be the impermeable layer 2 is spread and compacted on the surface of the ground G, and the slope reinforcing layer 2B is provided on the side surface thereof to provide the impermeable layer. Form 2.

Next, as shown in FIG. 4 (B), a mixed material to be the drainage layer 3 is spread and compacted on the upper surface of the impermeable layer 2 inward of the method head of the impermeable layer 2 and compacted on the side surface thereof. The drainage layer 3 is formed by providing the slope reinforcing layer 3B.

Next, as shown in FIG. 4C, the contaminated soil to be the contaminated soil layer 4 is spread on the upper surface of the drainage layer 3 inside the head of the drainage layer 3 and covers the contaminated soil. Crushed stone or the like to be the first soil covering layer 5 is spread and compacted, and a slope reinforcing layer 5B is provided on the lower side surface thereof. As a result, the first embankment portion 1A in which the impermeable layer 2, the drainage layer 3, the contaminated soil layer 4, and the first soil covering layer 5 are laminated in this order from the ground G is completed.

Finally, as shown in FIG. 4D, the adsorbent layer is compacted by spreading the mixed material to be the adsorption layer 6 on the surface of the ground G and the side surfaces of the slope reinforcing layers 2B and 3B. 6 is formed, and further, crushed stone or the like to be the second soil covering layer 7 is spread and compacted so as to cover the side surface of the adsorption layer 6, and the second embankment portion 1B is completed.

The embankment 1 completed in this way is configured so that the first embankment portion 1A and the second embankment portion 1B can be separated. Therefore, for example, when the leaching of pollutants from the contaminated soil layer 4 is prolonged and the adsorption capacity of the adsorption layer 6 deteriorates due to this, the second embankment portion 1A is not excavated and removed. Only part 1B can be excavated and removed, and the adsorption layer 6 can be replaced with a new one. Hereinafter, the procedure for reconstructing the second embankment portion 1B will be described with reference to FIG.

First, as shown in FIG. 5A, the second embankment portion 1B of the portion where the adsorption capacity has deteriorated is excavated and removed in the order of the second soil covering layer 7 and the adsorption layer 6. At this time, since the slope of the first embankment portion 1A is firmly reinforced by the slope reinforcing layers 2B, 3B, 5B, even if the second embankment portion 1B is removed, it does not collapse and cracks occur. The shape is maintained without any occurrence.

Next, as shown in FIG. 5 (B), a mixed material containing a new adsorbent is spread over the surface of the ground G and the side surfaces of the slope reinforcing layers 2B and 3B and compacted. Adsorption layer 6 is formed. At this time, if the water-impervious sheet 8 or the perforated pipe members 21A to C are deteriorated or damaged, they are also replaced with new ones.

Finally, as shown in FIG. 5C, the second embankment portion is reinforced by laying out crushed stone or the like to be the second soil covering layer 7 so as to cover the side surface of the new adsorption layer 6. Complete the re-construction of 1B. In this way, when the second embankment portion 1B is reconstructed, only the adsorption layer 6 of the portion where the adsorption capacity has deteriorated and the second soil cover layer 7 covering the adsorption layer 6 are left while leaving the first embankment portion 1A. Since it is sufficient to excavate and remove the embankment and replace the new adsorption layer 6 at the removed part, the construction period can be shortened and the construction cost can be effectively reduced as compared with the conventional structure that requires reconstruction of the entire embankment. can do. Further, each time the adsorption capacity of the adsorption layer 6 deteriorates, only the second embankment portion 1B needs to be reconstructed while leaving the first embankment portion 1A including the contaminated soil layer 4, so that the diffusion of pollutants can be prevented. , Contaminated soil can be continuously stored for a long period of time.

[Adsorption]
Next, the adsorption action of heavy metals by the embankment 1 of the present embodiment will be described.

As shown in FIG. 6, the seepage water that has permeated into the contaminated soil layer 4 due to rainfall or the like flows down the inside of the contaminated soil layer 4 as indicated by the arrow of reference numeral F1. At that time, arsenic contained in the contaminated soil is eluted into the seepage water. Therefore, the arsenic concentration in the permeated water increases toward the bottom of the contaminated soil layer 4. The seepage water seeps into the drainage layer 3 from the lower surface of the contaminated soil layer 4. Here, since the lower surface of the drainage layer 3 and the upper surface of the impermeable layer 2 are in contact with each other, the leachate flowing down to the drainage layer 3 is drained along the upper surface of the impermeable layer 2 as shown by the arrow of reference numeral F2. It flows in the layer 3 toward the outer periphery of the embankment 1. When the drainage layer 3 reaches the slope 3A, the leachate flows downward through the adsorption layer 6 and is discharged from the adsorption layer 6 to the ground G, as indicated by the arrow of reference numeral F3.

Here, the arsenic eluted in the leachate is adsorbed by the adsorbent while flowing inside the adsorption layer 6. In the present embodiment, the adsorption layer 6 is provided on the side surface of the impermeable layer 2 and the drainage layer 3 without being laid on the entire lower surface of the contaminated soil layer 4. That is, in the present embodiment, the adsorption layer 600 is laid over a wide area between the impermeable layer 200 and the contaminated soil layer 400 as shown in FIG. 6 (B). The volume of No. 6 is kept relatively small, and when substantially the same amount of adsorbent is mixed, a large amount of adsorbent is contained per unit volume (adsorbent is concentrated). As described above, when the mixing amount of the adsorbent per unit volume is increased, the contact frequency (contact opportunity) between the arsenic and the adsorbent in the adsorbent layer 6 is surely increased, and the arsenic is discharged from the adsorbent layer 6. It becomes possible to effectively reduce the arsenic concentration in the leachate. Further, in general civil engineering work, in order to uniformly mix the adsorbent with the adsorbent layer 6, it is necessary to secure a ^{predetermined amount or more (for example, 30 to 50 kg / m 3 or more) of the adsorbent per unit volume.} preferable. In the present embodiment, since the amount of the adsorbent mixed per unit volume is large, it becomes easier to mix the adsorbent uniformly as compared with the conventional embankment 100, and the workability can be surely improved.

[Modification example]
7 (A) to 7 (C) show the adsorption layer 6 in which the contact frequency between the arsenic contained in the leachate and the adsorbent is effectively increased by lengthening the leachate flow path in the adsorption layer 6. It is a schematic vertical sectional view which shows various deformation examples.

In an example shown in FIG. 7A, a plurality of (two in the illustrated example) water-impervious sheets 9A and B extending in the adsorption layer 6 in a substantially horizontal direction are arranged alternately with each other at intervals above and below. , The leachate flow path is formed in a substantially meandering shape. Specifically, the upper first water-impervious sheet 9A is substantially formed inside the adsorption layer 6 from the slope of the water-impervious layer 2 (the upper end of the side surface of the slope reinforcing layer 2B) toward the inner surface of the second soil-covering layer 7. It extends in the horizontal direction and is buried in the adsorption layer 6 by securing a flow path for the leachate to flow down between the tip end portion and the inner surface of the second soil covering layer 7. The lower second impermeable sheet 9B is located in the adsorption layer 6 below the first impermeable sheet 9A in a substantially horizontal direction from the inner surface of the second soil covering layer 7 toward the slope 2A of the impermeable layer 2. Along with extending, a flow path for allowing leachate to flow down is secured between the tip portion thereof and the slope 2A of the impermeable layer 2 and is embedded in the adsorption layer 6.

That is, the leachate leached from the drainage layer 3 into the adsorption layer 6 is flowed along the upper surface of the first impermeable sheet 9A as shown by the arrow F1 in the figure, and is flown from the tip of the first impermeable sheet 9A. It flows down to the upper surface of the second impermeable sheet 9B. Further, the leachate flowing down to the second impermeable sheet 9B is flowed along the upper surface of the second impermeable sheet 9B as shown by an arrow F2 in the figure, and flows from the tip of the second impermeable sheet 9B to the ground G. By flowing down to the surface, the leachate flows in the adsorption layer 6 in a substantially meandering manner. By forming the leachate flow path in the adsorption layer 6 in a substantially meandering shape in this way, the contact frequency between the arsenic contained in the leachate and the adsorbent can be effectively increased, and the arsenic concentration in the leachate can be increased. It will be possible to reduce it reliably.

An example shown in FIG. 7B is an inclined first and second geomembrane sheets 9A and B in FIG. 7A. Specifically, the upper first impermeable sheet 9A is inclined at a predetermined angle so that the sheet surface has an upward slope from the impermeable layer 2 side toward the second soil covering layer 7 side. The lower second impermeable sheet 9B is inclined at a predetermined angle so that the sheet surface has an upward slope from the second soil covering layer 7 side toward the impermeable layer 2 side.

That is, the leachate leached from the drainage layer 3 into the adsorption layer 6 goes up the slope surface of the first impermeable sheet 9A and is the first from the tip of the first impermeable sheet 9A, as shown by the arrow F1 in the figure. 2 It flows down to the water-impervious sheet 9B. Further, as shown by the arrow F2 in the figure, the leachate flowing down to the second impermeable sheet 9B goes up the slope surface of the second impermeable sheet 9B and from the tip of the second impermeable sheet 9B to the surface of the ground G. The leachate flows diagonally up in the adsorption layer 6 in a substantially meandering manner. When the leachate flow path in the adsorption layer 6 is formed in a meandering shape having an upward gradient in this way, the flow velocity of the leachate is suppressed to a low level, so that the contact frequency between the arsenic contained in the leachate and the adsorbent is effective. It becomes possible to surely reduce the arsenic concentration of the leachate.

In an example shown in FIG. 7C, a water-impervious sheet 9C is embedded in the adsorption layer 6 so that the sheet surface is substantially vertical, thereby temporarily blocking the flow of leachate in the adsorption layer 6. A flow path is formed to stop and stay. Specifically, the water-impervious sheet 9C extends diagonally upward from the surface of the ground G in the adsorption layer 6 substantially parallel to the slope 2A of the water-impervious layer 2, and at the upper end thereof and in the second soil-covering layer 7. A flow path for flowing leachate is secured between the side surface and the surface, and the water flow path is embedded in the adsorption layer 6.

That is, the leachate leached from the drainage layer 3 into the adsorption layer 6 is blocked and stays between the slope 2A of the impermeable layer 2 and the impermeable sheet 9C, as shown by the arrow F1 in the figure. When the water level rises above the upper end of the impermeable sheet 9C, the water flows beyond the impermeable sheet 9C as shown by the arrow F2 in the figure. By temporarily retaining the leachate in the adsorption layer 6 in this way, the frequency of contact between the arsenic contained in the leachate and the adsorbent can be effectively increased, and the arsenic concentration of the leachate can be reliably increased. It becomes possible to reduce.

In any of the modified examples shown in FIGS. 7A to 7C, the member forming the leachate flow path has been described as the water-impervious sheets 9A to C, but the flow path can be partitioned in the adsorption layer 6. As long as it is a member, it may be a plate member or the like having a water-impervious property.

[Water flow test]
It was confirmed by a water flow test that the arsenic adsorption efficiency was increased by increasing the contact frequency between the arsenic and the adsorbent. The water flow test will be described below. The water flow test was performed using the test device 50 shown in FIG. The test device 50 includes a water flow column 51, a liquid feeding tube 52, a storage container 53, a liquid feeding pump 54, a three-way valve 55, and a sampling container 56.

The water flow column 51 is a member for passing the simulated contaminated water 57 through the simulated adsorption layer 58, and includes a cylindrical column 61, a first plug member 62, a second plug member 63, and a first filter material 64. , A second filter material 65 is provided. The cylindrical column 61 has an inner diameter of 100 mm and a height (length in the vertical direction) of 150 mm. Then, in a state where the lower end of the cylindrical column 61 is closed by the first plug member 62 and the first filter material 64 is arranged, the simulated adsorption layer 58 is filled until the height becomes 60 mm. After filling, the upper end of the cylindrical column 61 is closed by the second plug member 63 with the second filter material 65 arranged on the upper surface of the simulated adsorption layer 58.

Each of the first plug member 62 and the second plug member 63 is formed with a through hole penetrating in the thickness direction of the plug. Then, the downstream end of the first liquid feeding tube 52A is inserted into the first plug member 62, and the upstream end of the second liquid feeding tube 52B is inserted into the second plug member 63.

The simulated contaminated water 57 is stored in the storage container 53, and the upstream end of the first liquid feeding tube 52A is inserted. The liquid feed pump 54 is provided in the middle of the first liquid feed tube 52A, and supplies the simulated contaminated water 57 stored in the storage container 53 to the water flow column 51 at a predetermined speed through the first liquid feed tube 52A. .. The three-way valve 55 is provided in the middle of the second liquid feeding tube 52B, and the simulated contaminated water 57'discharged from the water flow column 51 can be flowed to the downstream side of the second liquid feeding tube 52B, or a waste liquid container (waste liquid container). Select whether to discharge to (not shown). The downstream end of the second liquid feeding tube 52b is inserted into the collection container 56, and the leachate 57'discharged from the second liquid feeding tube 52B is collected.

As shown in FIG. 9A, in the water flow test, an arsenic solution having a concentration of 0.1 mg / L was used as the simulated contaminated water 57. Further, No. 4 silica sand and No. 6 silica sand were used as the soil of the simulated adsorption layer 58, and the above-mentioned fixall FB was used as the arsenic adsorbent. Then, the arsenic adsorbent was mixed with the soil at a predetermined ratio to prepare a mixed soil as a base of the simulated adsorption layer 58.

In the water flow test, six types of test cases A to F shown in FIG. 9B were targeted. In Test Case A, the arsenic adsorbent was added and mixed with respect to the soil at a ratio of 5 kg / t to prepare a simulated adsorption layer 58. Then, the simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 0.2 mL / min. In Test Case B, the arsenic adsorbent was added and mixed with respect to the soil at a ratio of 5 kg / t to prepare a simulated adsorption layer 58. Then, the simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 2 mL / min. In Test Case C, the arsenic adsorbent was added and mixed with respect to the soil at a ratio of 5 kg / t to prepare a simulated adsorption layer 58. Then, the simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 20 mL / min. In Test Case D, the arsenic adsorbent was added and mixed with respect to the soil at a ratio of 20 kg / t to prepare a simulated adsorption layer 58. Then, the simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 0.2 mL / min. In Test Case E, the arsenic adsorbent was added and mixed with respect to the soil at a ratio of 20 kg / t to prepare a simulated adsorption layer 58. Then, the simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 2 mL / min. In Test Case F, the arsenic adsorbent was added and mixed with respect to the soil at a ratio of 20 kg / t to prepare a simulated adsorption layer 58. Then, the simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 20 mL / min.

The test results of the water flow test are shown in the graphs of FIGS. 10 (A) and 10 (B). The vertical axis of each graph shows the arsenic concentration contained in the leachate 57'collected in the sampling container 56, that is, the arsenic concentration in the leachate 57' after passing through the simulated adsorption layer 58. The horizontal axis of each graph shows the amount of water flowing through the simulated adsorption layer 58.

In the water flow test, the arsenic concentration of the leachate 57'was measured every time a predetermined amount of water was passed through the simulated adsorption layer 58 in each of the test cases A to F, and the relationship between the water flow amount and the arsenic concentration was obtained. Regarding the arsenic concentration, the Environmental Agency Notification No. 46 sets the environmental standard value in soil to 0.01 mg / L. Therefore, in the water flow test, the amount of water flow until the arsenic concentration reached 0.01 mg / L was compared in each test case.

As shown in FIG. 10 (A), in the test cases A to C in which the arsenic adsorbent was added at a ratio of 5 kg / t, the water flow rate was about 80 L and the arsenic concentration exceeded 0.01 mg / L.

As shown in FIG. 10B, in the test cases D to F in which the arsenic adsorbent was added at a ratio of 20 kg / t, the arsenic concentration was substantially abbreviated over the period until the water flow amount reached about 200 L. It generally changed around 0 mg / L. Then, when the water flow amount exceeded about 300 L, the arsenic concentration increased and reached 0.01 mg / L at about 400 L.

In test cases A to C in which the arsenic adsorbent was added at a ratio of 5 kg / t, the amount of water flowing until the arsenic concentration reached 0.01 mg / L was about 80 L, and the arsenic adsorbent was added at a ratio of 20 kg / t. In the test cases D to F, the amount of water flowing until the arsenic concentration reached 0.01 mg / L was about 400 L. Therefore, the larger the mixed amount of the arsenic adsorbent per unit volume, the more arsenic and the arsenic adsorbent. It was confirmed that the arsenic processing capacity was improved by increasing the contact frequency of arsenic. By keeping the volume of the adsorption layer 6 small and increasing the mixing amount of the arsenic adsorbent per unit volume as in the embankment 1 of the present embodiment, the contact frequency between the arsenic and the arsenic adsorbent in the adsorption layer 6 can be increased. be able to. Therefore, in the embankment 1, arsenic can be effectively adsorbed on the adsorption layer 6.

[others]
The present invention is not limited to the above-described embodiment, and can be appropriately modified and implemented without departing from the spirit of the present invention.

In the above embodiment, the adsorption layer 6 has been described as being provided so as to be in contact with the slopes 2A and 3A of the impermeable layer 2 and the drainage layer 3, but for example, as shown in FIG. 11, the adsorption layer 6 is provided. The height may be set to be about the same as the height of the impermeable layer 2, and the drainage layer 3 may be extended to the upper surface of the adsorption layer 6. Also in this case, the volume of the adsorbent layer 6 is suppressed to a small size, and a large amount of the adsorbent per unit constant volume is secured, so that the contact frequency between the arsenic and the adsorbent in the adsorbent layer 6 can be surely increased. ..

Further, in the above embodiment, the drainage layer 3 is provided between the impermeable layer 2 and the contaminated soil layer 4, but the drainage layer 3 is omitted and the upper surface of the impermeable layer 2 is contaminated. The soil layer 4 may be directly laminated. In this case, it is preferable to provide a horizontal trough between the impermeable layer 2 and the contaminated soil layer 4 to guide the leachate in the width direction of the embankment 1.

Further, the heavy metal to be adsorbed has been described by taking arsenic as an example in the above embodiment, but the heavy metal is not limited to arsenic. In addition to arsenic, heavy metals specified by soil environmental standards, such as cadmium, lead, hexavalent chromium, mercury, selenium, fluorine, and boron, are heavy metals to be adsorbed.

1 ... embankment, 1A ... 1st embankment part (filling body part), 1B ... 2nd embankment part, 2 ... impermeable layer (impermeable layer), 2A ... impermeable layer slope, 2B ... impermeable layer method Surface reinforcement layer, 3 ... Drainage layer, 3A ... Drainage layer slope, 3B ... Drainage layer slope reinforcement layer, 4 ... Contaminated soil layer, 4A ... Contaminated soil layer lower slope, 4B ... Contaminated soil layer Upper slope, 4C ... upper surface of contaminated soil layer, 5 ... first soil covering layer, 5B ... slope reinforcing layer of first soil covering layer, 6 ... adsorption layer, 6A ... slope of adsorption layer, 7 ... second soil covering layer , 9A-C ... Impermeable sheet (impermeable member), 20 ... Water collecting structure, 21A-C ... Perforated pipe member, 22A-C ... Primary water collecting basin, 23 ... Drainage ditch, 24 ... Final water collecting basin, 50 ... Water flow test test device, 51 ... Water flow column, 52 ... Liquid feed tube, 52A ... First liquid feed tube, 52B ... Second liquid feed tube, 53 ... Storage container, 54 ... Liquid feed pump, 55 ... Three-way valve, 56 ... sampling container, 57 ... simulated contaminated water, 57'... leachate, 58 ... simulated adsorption layer, 61 ... cylindrical column, 62 ... first plug member, 63 ... second plug member, 64 ... first filter Material, 65 ... 2nd filter material, G ... Ground

Claims (7)

Turn water shield layer from the ground side and the polluted soil layer containing heavy metals is laminated leachate flowing down the upper surface of the water shield layer from the polluted soil layer is guided toward the slope of the water shield layer and 1 Sheng soil part,
Outside the slope of the water shield layer provided in contact with該法surface, with circulating the leachate leaching from the first-up soil portion, the adsorbent which adsorbs the heavy metal contained in the leachate A storage structure for heavy metal contaminated soil, which comprises a second embankment portion having an adsorption layer mixed with the above. Wherein the barrier layers of water layer and the polluted soil layer of the first Sheng soil portion, a high drainage layer of permeability is provided than the water shield layer Rutotomoni, the adsorption layer, the water-impervious layer and, provided in contact with each slope of the drainage layer, heavy metal polluted soil according to claim 1, leachate flowing down from the polluted soil layer is the proposal to the adsorption layer flows the drainage layer Storage structure. The heavy metal according to claim 1 or 2, further comprising a water-impervious member embedded in the adsorption layer and partially blocking the flow of leachate in the adsorption layer to lengthen the leachate flow path in the adsorption layer. Storage structure of contaminated soil. The storage structure for heavy metal-contaminated soil according to claim 3, wherein the water-impervious member is a plurality of water-impervious sheets that are alternately arranged in the adsorption layer at intervals above and below to meander the leachate flow path. .. The heavy metal contaminated soil according to claim 4, wherein the water-impervious sheet is embedded in the adsorption layer by inclining the sheet surface so as to have an upward slope from the upstream side to the downstream side in the flow direction of leachate. Storage structure. The water-impervious member is a water-impervious sheet embedded in the adsorption layer in the vertical direction, and leachate is temporarily blocked between the sheet surface of the water-impervious sheet and the side surface of the water-impervious layer. The storage structure for heavy metal-contaminated soil according to claim 3, wherein the soil is retained. Wherein the portion contacting with at least the adsorption layer of the first-up soil portion is Homen reinforcing layer is formed, said second fill portion from claim 1 is configured to be separated from said first fill portion 6 Storage structure for heavy metal contaminated soil according to any one item.

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