JP7679960B2 - Rainwater storage green soil, rainwater storage green structure, and rainwater storage green space - Google Patents

Description

The present invention relates to urban flood control and urban greening technology.

In recent years, with the progression of global warming, the frequency of torrential rains and super typhoons with hourly rainfall exceeding 100 mm has increased, and this has led to increased damage from flooding due to river overflows and urban flooding. As a result, local governments, including Tokyo, are calling for measures to be taken to reduce the amount of rainwater that flows into rivers, such as installing permeable pavement and rainwater infiltration tanks, in addition to the traditional measures of improving runoff capacity through river maintenance and bank construction.
However, in urban areas, the only places available to build so-called grey infrastructure, such as stormwater infiltration facilities, are underground spaces such as adjustment tunnels, and expanding existing stormwater storage tanks is costly.
Patent Document 1 (JP 2014-177761 A) discloses a base material that is composed of hard granules and an adsorption aid attached to the surface of the granules via a mixing aid, and that is coated on the surface of the granules with a coating layer that has the function of capturing SS substances by the mixture of the mixing aid and the adsorption aid, and that has clogging prevention and rainwater storage and permeability.

Patent Document 2 (JP 2017-94303 A) discloses a storage and purification system 1 that includes a permeation storage tank 2 that stores and purifies water sent to a water collection tank before the water reaches the water collection tank, and the permeation storage tank 2 is a mesh pipe 10 arranged at the bottom of the permeation storage tank 2, and the mesh pipe 10 guides the water that has permeated into the inside of the permeation storage tank 2 through a water guide hole provided in the mesh pipe 10 and sends the water to the outside of the permeation storage tank 2. The storage and purification system 1 has a lower layer 40 composed of particles having a diameter larger than the diameter of the water guide hole, an intermediate layer 50 composed of purified soil that can purify water, and an upper layer 60 composed of particles having a diameter larger than the particles in the intermediate layer 50. The storage and purification system 1 is capable of storing and purifying water before the water reaches the water collection tank, and can easily remove contamination that has entered the inside of the permeation storage tank.
Patent Document 3 (JP 2020-20213 A) proposes a rainwater storage facility that can store water and reduce the impact on plants, and that includes planting soil placed above the existing soil and a side wall surrounding the planting soil, with a portion of the side wall being positioned above the surface of the planting soil so that water can be stored in the space surrounding the planting soil, and the water retention and permeability of the planting soil are set higher than the permeability of the existing soil.

JP2014-177761A JP 2017-94303 A JP 2020-20213 A

In cities, flood control measures are required because during sudden rainfall, drainage systems are unable to keep up with the amount of water, causing small rivers and drainage channels to overflow into residential areas. Meanwhile, paved urban areas suffer from a lack of moisture in the soil, making it difficult for plants to grow.
The object of the present invention is to develop soil that can ensure the amount of moisture available to plants and can regulate the amount of rainwater discharged into sewer pipes, etc., and to develop a method for creating green space with a rainwater storage function.

1. Rainwater storage soil for greening, characterized by a mixture of volcanic gravel with a particle size of 5-10 mm (20-40% by volume), 2.5-5 mm (20-40% by volume), and less than 2.5 mm (10-50% by volume) and black soil (5-20% by volume).
2. The rainwater retaining soil for greening according to claim 1, further comprising 10% by volume or less of organic material.
3. The rainwater retaining ^{soil for greening according to 1. or 2., characterized in that the maximum rainwater infiltration amount (pF0 water amount) is 570 to 620 liters/m 3 ,} the rainwater discharge amount (pF0-pF1.8) is 230 to 300 liters/m ³ , and the plant available water amount (pF1.8-pF3.0) is 70 to 100 liters/m 3 .
4. A rainwater-retaining greening structure characterized in that the rainwater-retaining greening soil according to any one of 1. to 3. is filled into an existing ground recess formed by excavating the existing ground or into a concrete recess formed with concrete.
5. The rainwater storage greening structure according to 4., characterized in that the existing ground depression has a water-permeable sheet provided on the wall surface.
6. It is equipped with drainage facilities that connect to an existing ground depression or concrete depression and a drainage channel.
6. The rainwater retaining greening structure according to claim 4 or 5, wherein the drainage equipment comprises a drainage layer provided at the bottom of the recess, an adjusting basin, and a drainage pipe equipped with a flow rate adjusting device.
7. A rainwater-retaining green space characterized in that it is planted in a rainwater-retaining greening structure according to any one of 4. to 6. in a green space in an urban area, a balancing pond, a residential area, a green belt on a road, a river bank, or the like.

1. By combining volcanic gravel of different particle sizes with black soil, we were able to develop soil that has sufficient water retention for plants and reduces drainage.
2. By creating a depression in the existing ground and filling it with this soil to create a rainwater storage green structure, a large amount of the total water stored in the soil can be secured for use by plants, and the amount of rainwater discharged to sewers, etc. can be reduced, so the drainage load on sewers, etc. can be reduced. At the same time, a good growing environment for plants can be maintained. Furthermore, by providing an adjustment basin or gate valve to adjust the amount of water discharged to this rainwater storage green structure, the discharge time can be adjusted to suppress instantaneous flooding.
3. This rainwater storage greening structure can be installed in various green spaces in urban areas, roadside green spaces, river banks, etc. Urban green spaces include parks, gardens of facilities such as schools, exteriors of apartment buildings and various facilities, gardens of individual homes, and other land that can be greened, regardless of size. It can also be applied to greening facilities installed on artificial ground.
4. By using this rainwater storage greening structure, even in existing green spaces with low rainwater permeability, it is possible to easily create green spaces that can temporarily store rainwater by simply backfilling the space excavated from the existing soil with rainwater storage greening soil. Also, even in spaces surrounded on the sides and bottom by impermeable structures such as concrete, such as the exterior of a building, it is possible to create a rainwater storage facility where plants can be planted by simply placing a drainage layer made of crushed stone below and a drainage pipe that connects to the sides, and then spreading rainwater storage greening soil evenly on top of that.
By installing the rainwater storage greening structure of the present invention on roads, rainwater that falls on the paved surface is temporarily stored and the rest, except for water used by plants, is drained away, thereby reducing the burden on the drainage system of the road structure.
5. Because the amount of water available to plants increases, it will become possible to plant plants that are sensitive to dryness (wet plants), allowing for a greater variety of vegetation in green spaces and making irrigation management easier.

A diagram showing the results of a preliminary test of water storage capacity Diagram showing the composition of volcanic gravel Figure showing pF values and adjusted water storage capacity of test soils with different compositions of volcanic gravel Schematic diagram showing the rainwater storage greening structure of Example 1 Schematic diagram showing the rainwater storage greening structure of Example 2 FIG. 13 is a graph showing the water storage capacity using Test Example 2. FIG. 2 is a diagram showing the vegetation condition of a green space where an example 1 of a rainwater storage greening structure was installed.

The present invention has developed soil that is highly permeable and has excellent water storage capacity, and contains a large amount of water that can be used by plants. By constructing a rainwater storage greening structure using this soil on the ground, the invention contributes to a primary water storage function after precipitation, a drainage load reduction function that reduces the amount of water discharged into sewers, etc., and a greening function. This rainwater storage greening structure can be installed in any size of place, such as parks, school planting areas, housing complex planting areas, or private home gardens, making it effective as a measure against urban flooding caused by sudden heavy rains.

The present invention relates to a rainwater-retaining greening soil that is a mixture of volcanic gravel with a particle size of 5-10 mm (20-40% by volume), 2.5-5 mm (20-40% by volume), and less than 2.5 mm (10-50% by volume) and black soil (5-20% by volume).The rainwater-retaining greening structure is formed by filling a depression in the ground or a depression made of concrete with this soil, and the rainwater-retaining greening structure is installed in a rainwater-retaining green space such as an urban park or a street.
A soil improver can be added to the rainwater storage soil for greening in an amount that does not affect the initial range of water storage capacity and plant utilization capacity. The amount of the soil improver is preferably 10% by volume or less, more preferably about 5% by volume.

Rainwater retaining soil for greening of this composition has a maximum rainwater infiltration amount (pF0 water amount) of 570-620 liters/m ³ , a rainwater discharge amount (pF0-pF1.8) of 230-300 liters/m ³ , and a plant available water amount (pF1.8-pF3.0) of 70-100 liters/m ³ .
The pF value of soil indicates the strength with which the moisture in the soil is attracted by the soil's capillary force. The wetter the soil, the lower the pF value, and the drier the soil, the higher the pF value. pF0 indicates a state in which the soil is saturated with moisture, pF1.8 indicates the field water capacity, a state in which gravity and the soil's capillary force are almost equal and all the moisture in the soil is attracted to the soil particles, and pF3.0 indicates a state in which the soil is so dry that there is so little moisture available that plants begin to wither. In this invention, the pF value is measured in accordance with JAS1210.
In the present invention, the water content at pF0 is defined as the maximum rainwater infiltration amount, the rainwater discharge amount Ds = (pF0-pF1.8), the plant available water amount Ps = (pF1.8-pF3.0), and the adjusted water storage amount Rs = (Ds+Ps). In other words, the value obtained by subtracting the amount of water attached to the soil by capillary force (water content at pF1.8) from the maximum amount of water that can infiltrate into the soil (water content at pF0) is the amount of water naturally discharged from the soil = rainwater discharge amount Ds, and the value obtained by adding the amount of water absorbed from the soil by plants for use is the adjusted water storage amount Rs.
The available water content of plants Ps was defined as (pF1.8-pF3.0). In other words, this was calculated as the amount of water that plants can use without dying out of the water that is adsorbed by capillary force after it naturally flows out of the soil. In fact, the adjusted water storage capacity Rs was defined by focusing on the maximum amount of water that the soil can store without dying out the plants during the next rainfall.
In the present invention, we assumed that volcanic gravel, a porous material that does not lose its shape, has a large maximum rainwater infiltration amount pF0, and tested it on the assumption that increasing the plant available water content Ps would reduce the drainage load into sewers, etc. However, we found that even if the maximum rainwater infiltration amount pF0 is large, if the adhesion between water and soil is strong, water will be adsorbed in the soil for a long time, and water storage capacity for the next rainfall will not be obtained. Therefore, in the present invention, we focused on the ability to temporarily store water and delay drainage (rainwater discharge amount Ds) and the amount of water that plants can use (plant available water content Ps) to realize the present invention.

<Soil composition>
Rainwater-retaining soil for greening is based on a mixture of approximately 80-95% volcanic gravel and 5-20% volume of black soil, with supplementary materials being able to be added depending on suitability for the vegetation.

<Volcanic gravel>
Volcanic lapilli are solid materials ejected by volcanic eruptions, with a grain size of 2 to 64 mm. In this invention, volcanic lapilli with a grain size of 10 mm or less are referred to as volcanic gravel.
Volcanic gravel is a porous material that is volcanic sand erupted from a volcano. Volcanic gravel is porous, and in the present invention, gravel with a particle size of 10 mm or less is used.
Volcanic gravel is exposed to high temperatures, and the inside of the hole is free of other soils such as black soil or leaf mold, and microorganisms, so its chemical properties, such as hydrophilicity, are stable. It is also harder, less likely to be crushed, and more stable in shape than other soil materials.
In the present invention, it is important that the soil has a stable shape as used in urban areas such as city parks and streets where foot traffic is constantly applied.
In the present invention, volcanic gravel with a grain size of 10 mm or less is used to suit plant growth, and various grain sizes are used to reduce non-capillary voids in the soil and increase water retention. From the results of the test, a unique composition ratio was found from the viewpoints of water storage and plant utilization.
The composition of volcanic gravel should be 20-40% by volume of 5-10mm grain size, 20-40% by volume of 2.5-5mm grain size, and 10-50% by volume of grain size less than 2.5mm. The ratio of the three grain sizes should be about 1/3 each, and should be adjusted according to the characteristics of the production area, etc.
If the maximum particle size is 10 mm or less, the soil for planting can be easily handled and improved in handling properties.

<Black soil>
Kuroboku soil is a volcanic ash soil. Kuroboku soil is distributed over about 31% of Japan's land area and is said to cover about 47% of the country's farmland, and is commonly found in Hokkaido, Tohoku, Kanto, and Kyushu. Kuroboku soil can contain about 10% humus. Because it contains humus, it does not compact even when it absorbs water, and is a soil that can be properly drained. This humus is organic matter that has become soil, and is different from leaf mold that has not become soil.
Soils with fine particle size composition, such as red soil and araki soil, are easily compacted and retain water, so they are not suitable for this invention.
By adding about 10% black soil to volcanic gravel, the available plant moisture (Ps) can be dramatically increased, the amount of natural runoff can be reduced, and sudden flooding caused by torrential rain can be prevented.
Volcanic gravel alone has a large maximum rainwater infiltration rate pF0, but the available plant moisture Ps is small, and the sewage load does not decrease, making it insufficient for urban flood prevention. By mixing this with about 10% black soil, the available plant moisture can be increased by about three times.

<Supplementary soil materials>
In addition, organic materials such as leaf mold and compost can be added as supplementary soil materials within the effective range of rainwater discharge and available water content for plants. Generally, the amount is 10% or less by volume, or even about 5% by volume, assuming that the rainwater storage greening soil consisting of the basic composition is 100.

The rainwater-retaining soil for greening of the present invention has a rainwater discharge capacity and a plant available water content as part of the soil structure itself, so there is no need to separate it into layers such as a planting soil layer, a permeable layer, etc. However, a drainage layer that performs a water collection function can be provided as an underdrain, and the water can be discharged to a sewer or the like via a drainage pipe system with a temporary storage function.
Rainwater storage soil for greening is easy to lay across a surface, and can be used for greening large parks and balancing ponds. It is also easy to install as a long, linear water storage facility on streets and banks.

<Rainwater storage greening structure>
A rainwater storage greening structure is constructed by filling an existing ground depression formed by excavating the existing ground or a concrete depression formed with concrete with soil for rainwater storage greening.
The recess is not limited to a groove shape, but also includes planar shapes. Groove-shaped recesses are formed in green spaces along roadways, sidewalks, and embankments. Planar recesses are formed in reservoirs, park green spaces, the exteriors of apartment buildings and facilities, residential lots, etc.
The recess should be dug to a depth of about 300 to 1,000 mm. If tall trees are to be planted, a depth of about 1,000 mm is required.

In existing ground, it is preferable to install a permeable sheet on the wall of the recess. If soil particles such as silt from the existing ground flow into and mix with the rainwater storage soil for greening, the rainwater storage performance and plant available water performance will change and the initial performance will be lost, so by installing a permeable sheet on the side wall surface that prevents soil from flowing in and maintains permeability, performance can be maintained for a long time. It may also be laid on the bottom, but this is not necessary because even if the groundwater level rises from the bottom, there is almost no rise in soil particles associated with it. However, if the excavated recess is filled with muddy water during construction to install a rainwater storage greening structure, it is effective to install a permeable sheet on the bottom as well to prevent mud from mixing with the rainwater storage soil for greening.

The walls and bottom of the recess can be made of concrete. It is also possible to combine concrete with the existing ground, such as using concrete on only one wall and using the existing ground for the bottom. It is also possible to use concrete for part of the wall in the longitudinal direction and the existing ground for the other wall.
For example, in artificial ground, the walls and bottom should be made of concrete. In drainage facilities, both walls and the bottom should be made of concrete. For embankment road shoulders, three sides should be made of concrete.

<Construction of green spaces equipped with rainwater greening structures>
The rainwater greening structure of the present invention can be installed as an urban flood control facility for torrential rains and the like, and as a green space in an urban area. The rainwater greening structure can be installed in a large area such as an urban park, a neighborhood facility such as a children's park, an urban green space, a reservoir, a green space around an apartment building, a private garden, a green belt on a road (urban roads, main roads, expressways, etc.), planted areas around various facilities, and a river bank.

In the case of large-area flood control facilities, a large amount of rainwater storage soil for greening is required, so the following construction work is often carried out.
As mentioned above, the excavated volcanic gravel is classified at the factory and mixed either at the factory or at the construction site before being laid down. When developing a large-scale rainwater storage green space with a rainwater storage green structure, the operations such as mixing at the site are carried out by measuring out the specified amount of classified volcanic gravel into fixed-capacity containers such as backhoe buckets, transporting them to a mixing yard and collecting them in one place, and then mixing them with a backhoe.
Next, add 10% by volume of finely powdered soil such as black soil, and then add about 10% of the total volume of organic material such as compost.Then, use a backhoe bucket to stir and mix the mixture until it is uniform, and prepare soil for rainwater storage and greening.
In large-scale construction projects, it is not possible to mix the entire 1,000 mm thick soil uniformly on-site, so the soil is mixed in advance in a factory or outdoor yard and then packed into flexi-con bags or transported directly by dump truck.
The adjusted rainwater retaining soil for greening is spread using heavy machinery such as backhoes, dump trucks, and bulldozers. Alternatively, the soil for greening with rainwater retaining properties is measured out with a bucket, stored in a flexible container bag with a capacity of about 1 ^m3 , and transported to the installation site. Measuring the weight of the flexible container bag makes it easy to manage the amount of soil put in.

The method for introducing the rainwater storage soil for greening prepared in the factory into the rainwater storage facility is as follows.
The soil that has been mixed and stirred in the manufacturing plant and stored in a FIBC is transported by truck to the rainwater storage facility, and the bottom of the FIBC is broken by lifting it with a crane, and the rainwater storage soil for greening is dropped directly into a recessed area dug into the existing soil, or into a recessed space surrounded by artificial structures such as concrete and where a drainage layer and drainage pipes are placed. Alternatively, the rainwater storage soil for greening stored in the FIBC can be lowered outside the rainwater storage facility, and then transported by hand or with small equipment and dropped into the rainwater storage facility.
The amount of rainwater storage soil for greening to be put into the rainwater storage facility is calculated from the number or weight of FIBC packs according to the volume of the excavated area of the existing ground or the volume of the space surrounded by artificial structures such as concrete, and 1.0 to 1.2 times the volume of the space is put in. At that time, the soil is occasionally compacted by hand and the height is leveled, so that a predetermined amount of rainwater storage soil for greening is put in up to the top of the space. By managing the volume in this way, the performance such as water permeability and water retention of the rainwater storage soil for greening put into the rainwater storage facility is managed.

Regarding the plants to be planted in the rainwater storage facility, plants from each region can be used depending on the amount of rainwater to be stored and the depth of the rainwater storage facility. The rainwater storage soil for greening of the present invention has a large amount of moisture that can be used by plants, so wetland plants can also be used for planting.
It is possible to plant a combination of plants selected from those whose habitats differ depending on the soil moisture conditions, such as the Japanese maple-zelkova community commonly found in the moist forests around the Kanto region, the Japanese sedge-alder community found in moist woodlands, the Japanese silvergrass-imperata community found in dry grasslands, and the Japanese sedge community found in wetlands.
Furthermore, conventional foundation materials designed primarily to allow rainwater to penetrate have little moisture available to plants, and plants with low drought tolerance cannot grow during periods of continuous sunny weather. As a result, plants that are resistant to drought, such as grasses, are mainly planted, making it impossible to create a diverse green space.

<Vegetation materials>
There is no particular limitation on the plants that can be used in the present invention. Plants that are generally used as roadside trees or for planting in parks can be used.
In addition, because it is possible to maintain a good water-retaining state, it becomes possible to use plants suitable for wetlands, and while green spaces are usually created with plants that grow in dry grasslands, it is possible to develop green spaces equipped with wetland flora even in urban areas. Existing biotopes require power such as pumps to circulate and replenish water, but with this invention, this can be achieved using the soil's natural water-retaining capacity.

Examples of plants that can be used include the following:
Trees that form communities in the dry forests of Honshu include zelkova, Japanese walnut, Chinese hackberry, Zelkova, Japanese cinnamon, dogwood, clover, privet, wild rose, white alder, aucuba, Japanese knotweed, Lilium longiflorum, Centaurea centaurea, water hyacinth, day lily, and Japanese laurel, while trees that form communities in the wet forests include Japanese plum, Japanese willow, Japanese laurel, Japanese knotweed, purple Japanese laurel, alder, boxwood, Japanese einkorn, and Japanese laurel. It is possible to plant plants that grow in dry grasslands, such as water laurel, dwarf fern, and violet, as well as Deutzia japonica, Miscanthus sinensis, Imperata cylindrica, sedge, ominaeshi, loosestrife, Hypericum, bellflower, Japanese anemone, dianthus, and Japanese lawnmower, and plants that grow in wetlands, such as Usagi, Carex gracilis, Japanese sedge, Japanese bean sprout, Chinese laurel, Japanese knotweed, typha gracilis, floatwort, and willow knotweed.

<Soil testing>
In the present invention, the soil material is selected with attention paid not only to the amount of rainwater that can be stored but also to the amount of water that can be used by plants.
(a) Test soil The following four types of soil were tested as preliminary tests.
A1: Volcanic gravel with a diameter of 5 to 10 mm that is irregular, easy to compact, porous and has water retention properties. B: Red soil as a general vegetation soil. C: A commercially available roadbed material that can also be used for vegetation, made mainly of pumice and pumice sand, mixed with 15% compost by volume and auxiliary materials. D: A commercially available mixture of concrete-based crushed stone and humus as a clogging-prevention roadbed material with plant root elongation properties.

(b) Test method The volumetric water content of the soil at pF0, pF1.8, and pF3.0 was measured, and the maximum rainwater infiltration amount (water content at pF0), rainwater discharge amount Ds (pF0-pF1.8), available plant water content Ps (pF1.8-pF3.0), and adjusted water storage volume Rs (Ds+Ps) were calculated.
The pF value was measured according to JAS1210.
To conduct the pF test, 4 liters of each of the four types of soil samples were produced, and the sample soil was packed into a JIS A 1210 mold (1,000 ml) with a 100 ml sample core placed inside. The sample soil was then tamped under conditions for planting soil (a 2.5 kg rammer was dropped 10 times from a height of 10 cm) to produce a test specimen.
The measurement and calculation results are shown in FIG.

Red soil (B), a common planting soil material, has a large pF0, but a small rainwater discharge Ds and a small regulated water storage volume Rs, making it unsuitable as a base material for rainwater storage.
The commercial material (C), a mixture of pumice and compost, has a higher rainwater discharge Ds and plant available water content Ps than the red soil (B), but its adjusted water storage volume Rs is smaller than that of sample (A1). The commercial material (D), a mixture of crushed concrete and humus soil, has the smallest plant available water content Ps, and is therefore unsuitable for vegetation.
Based on this preliminary test, we focused on volcanic gravel that can secure the plant available moisture Ps and has a large adjusted water storage capacity Rs. We then adjusted the particle size and added soil to search for a composition that can secure the plant available moisture Ps while maintaining the adjusted water storage capacity Rs.

<Study of composition based on volcanic gravel>
(a) Adjustment of volcanic gravel The particle size and amount of volcanic gravel were adjusted for the rainwater storage greening soil to be placed in the rainwater storage facility, assuming the use of volcanic gravel that is irregular, easy to compact, porous, and has water retention properties.
The volcanic gravel used in rainwater storage soil for greening is prepared from raw sand mined from layers of volcanic gravel formed by volcanic eruptions. The raw sand is washed and sieved at the manufacturing plant to prepare the material according to particle size.
First, the raw sand is sieved through a sieve with a mesh size of about 10 mm to remove volcanic gravel with a particle size of 10 mm or more. Next, the sieved volcanic gravel is sieved through a sieve with a mesh size of 2.5 mm while being washed with water to remove fine particles with a particle size of 2.5 mm or less. The volcanic gravel remaining on the 2.5 mm sieve is then sieved through a sieve with a mesh size of 5 mm to separate it into volcanic gravel with particle sizes of 5 to 10 mm and 2.5 to 5 mm. In this way, three types of volcanic gravel with different particle size compositions, 5 to 10 mm, 2.5 to 5 mm, and 2.5 mm or less, are obtained from the raw sand.
(b) Add 10% by volume of black soil.
(c) Composition Seven types of compositions were prepared as test samples, as shown in FIG.
(d) Measurement Using the same test method as in the preliminary test, the volumetric water content of the soil at pF0, pF1.8, and pF3.0 was measured, and the maximum rainwater infiltration amount (water content at pF0), rainwater discharge amount Ds (pF0-pF1.8), plant available water content Ps (pF1.8-pF3.0), and adjusted water storage amount Rs (Ds+Ps) were calculated. The seven types of samples and the results are shown in Figure 3.

The test results show the following:
(1) When the amount of particles with a particle size of 5 mm or more was increased (samples A1 to A5), the voids became larger and the maximum amount of rainwater infiltration increased. Among these, the amount of rainwater discharge Ds was higher in soil containing only particles with a particle size of 5 to 10 mm (sample A1) when gravel with a particle size of 2.5 to 5 mm or particles with a particle size of less than 2.5 mm was used (samples A2 and A5).
However, samples A1 to A5 had large non-capillary voids and low water content at pF 1.8 (natural state), and therefore had low available plant water Ps.
(2) In contrast, in samples A6 and A7, which contained less particle size of 5 to 10 mm, the maximum rainwater infiltration amount decreased, but the water content at pF 1.8 increased and the water content at pF 3.0 decreased (good drainage), so the available plant water content Ps increased by about three times compared to the other samples.
(3) From the above results, it was found that mixing 20-40 volume percent of each of the three types of volcanic gravel with different particle sizes results in a significantly higher plant available water content Ps, an equally high adjusted water storage volume Rs, and a lower runoff rainwater discharge Ds than other mixtures. It is preferable to use fine volcanic gravel of less than 2.5 mm to adjust the amount of black soil and soil conditioners mixed.

An example of a rainwater storage greening structure is shown in FIG.
The composition of sample A6, which had high rainwater discharge amount and available water content, was selected as the soil to be used as rainwater storage greening soil for the rainwater storage facility.
When constructing a rainwater storage facility on natural soil, such as in an existing green space, the plants growing in the area where the facility will be constructed are uprooted, and the existing soil is excavated to a specified depth. The depth can be changed as desired depending on the soil thickness suitable for the plants to be planted in the facility. If only ground cover plants are planted to cover the facility, it is desirable to excavate to a depth of about 300 to 500 mm, and if tall trees or other trees are planted, it is desirable to excavate to a depth of about 1,000 mm. Furthermore, regardless of the plants to be planted, the excavation depth can be determined depending on the amount of rainwater to be stored.

Next, in order to prevent the existing soil from entering the rainwater storage facility and clogging it, it is preferable to cover the sides of the space formed by digging the soil with a water-permeable sheet made of nonwoven fabric or the like. The sheet covering the sides can also be a water-impermeable sheet to prevent water from entering. Alternatively, depending on the geology of the existing ground, it is not necessary to cover the sides with a sheet. It is also preferable that the area covered with the sheet is from the surface to the bottom end of the existing soil, but it does not matter if the sheet is raised above the surrounding ground surface. The bottom of the space only needs to be finished almost horizontally, and there is no need to lay a water-permeable sheet or the like between the rainwater storage greening soil to be placed in the space and the soil below it. The rainwater storage facility is established by pouring the rainwater storage greening soil produced by the method described below into the space thus formed. It is preferable to fill the rainwater storage greening soil to a height up to the surface of the surrounding soil, but it may be lower than the surrounding area.

In the construction of Example 1, a depression 1000 mm deep and 4 m square was created in the existing ground of the Kanto loam layer, a water-permeable sheet was laid on the side walls of the excavated depression, and the soil for rainwater storage and greening with the composition of Sample A6 was filled in and plants were planted. This construction assumes a fully sunken adjusting pond, and there is no discharge facility for sewage or the like.
After observing the situation for about a year, it was confirmed that in addition to the planted trees, local grasses were also thriving, creating a good green space.

An example of a rainwater storage greening structure is shown in Figure 5.
This embodiment is a rainwater storage greening structure 12 installed in an urban area. This is an example of a rainwater storage facility installed in a space surrounded on the sides and bottom by an impermeable structure such as concrete on the exterior of a building.
A concrete recess 4 is excavated in a corner surrounded by a building 7 and a pavement 71, and the sides and bottom are made of concrete 41 to provide the recess. A drainage layer 61 made of crushed stone or the like is laid on the bottom of the concrete recess 4, and rainwater-retaining soil 1 for greening is filled on top of that, and desired plants 2 are planted.
A drainage facility 6 is provided to connect the concrete recess 4 to a drainage channel 7. The drainage facility 6 is composed of a drainage layer 61, a drainage pipe 63 that connects to the drainage channel 7 from the side or bottom, an adjustment basin 65 installed midway along the drainage pipe, a valve 64, etc. A sewer pipe or a small or medium-sized river is used as the drainage channel 7.

In the formed recessed space, first, a drainage pipe that connects to a drainage channel installed outside the rainwater storage green structure is placed from the side, and then crushed stone is laid evenly to surround the drainage pipe. It is desirable to be able to adjust the drainage capacity of the drainage equipment that connects to the drainage channel installed outside the rainwater storage green structure as desired by changing the diameter of the drainage pipe or the valve attached to the drainage pipe depending on the amount of rainwater to be stored and the amount of water to be discharged to the outside per hour.
The crushed stone used for the drainage layer should be larger in grain size than the rainwater storage soil for greening that will be placed above it, and should be about 100 mm thick. The drainage layer can also be formed around the drainage pipe or in stripes on the bottom surface toward the drainage pipe.
It is desirable for the bottom surface to have a slope of about 0.5 to 1.0% toward the drainage equipment to be placed on the outside. It is desirable for the walls surrounding the sides to rise up to the surface of the existing soil or pavement, but it is acceptable for them to be higher than that surface. In the space thus laid with the drainage layer, rainwater-retaining greening soil is poured and plants are planted to create a rainwater-retaining greening structure. The height at which the rainwater-retaining greening soil is poured may be at or below the top of the structures surrounding the sides, such as the pavement.

The space of the recess is designed depending on the type of plant to be planted, the amount of rainwater to be stored, or the size of structures surrounding the rainwater storage greening structure 12. The installation location can be underground, in an artificial ground, on a pavement, inside an artificial structure, etc.
In urban areas, drainage channels for sewage and the like are installed, but many of the surfaces are paved, and their capacity is insufficient for drainage from sudden torrential rains. Even if drainage channels are installed, they are too large for normal use, and there is also a lack of land, making installation itself difficult. In urban centers, green spaces around facilities, roadside tree belts, sidewalks, squares, and parks are being developed, so by installing this rainwater storage greening structure in these spaces, it is possible to flexibly respond to momentary floods and provide rich green spaces. Even in residential areas around urban centers, by installing rainwater storage greening structures 12 in the yards of private homes, it is possible to handle sewer pipes with relatively poor drainage capacity. In private homes, concrete can be used only for some of the side walls.

A test specimen was created by filling a concrete box with a depression in the natural ground with green soil that retains rainwater.
1. Test specimen configuration (natural ground excavation recess)
Volcanic ash soil was excavated and two rectangular recesses, each 3600 mm on a side and 1000 mm deep, were installed for testing. Test piece 1 was designed to be permeable, while test piece 2 was designed to be impermeable. A border was placed around the perimeter to prevent overflow from the neighboring land.
The rainwater retaining soil for greening was the mixed soil of sample A6.
The test site was divided into four sections with different elevations.
The test period was four months during the summer.
A rain gauge was installed at the test site.
Test specimen 1: Plywood was set up on the sides, a water-permeable sheet was placed, and sample A6 rainwater-retaining soil for greening was filled in.
Test piece 2: Concrete panels were erected on the sides, concrete panels were laid on the bottom, and waterproof sheets were installed on the sides and bottom. A drainage pipe was placed through the waterproof sheets on the sides, crushed stones were poured in to create a 10 cm drainage layer, and sample A6 rainwater storage soil for greening was filled on top of that. A weighing meter was installed on the outlet side of the drainage pipe to measure the amount of drainage.

2. Planting Plants were planted according to the height. In the highest section, plants that make up the moist Zelkova-Acer palmatum community were planted, such as Zelkova, Acer palmatum, White alder, Aucuba japonica, Oriental holly, and Red fern. In the area adjacent to the sunken section, plants that make up the Alder-Acer sedge community, such as Alder, Japanese laurel, Boxwood, and Sedge, which are species that take precedence over moist forests, were planted. In the bright section, herbaceous plants that make up the Miscanthus-Imperata cylindrica community, such as Japanese silver grass, Japanese yew, Rosa japonica, Scutellaria baicalensis, and Loosestrife, which are often found in dry grasslands, were planted. In the sunken section where water accumulates during rainfall, plants that make up the Ustilago candida community, such as Ustilago candida, Sedge sedge, Japanese laurel, Lythrum salicaria, and Cattail serrata, which are species that take precedence over waterside ustilts, were planted. The planting conditions are shown in Figure 7.

3. Evaluation Regarding the performance of the constructed facility, in order to quantify the percentage of rainfall that was discharged as a rainwater storage effect, a rain gauge and a data logger were used to continuously measure the amount of rainfall near the facility and the amount of drainage from test body 2, the bottom of which was covered with a waterproof sheet. In addition, the growth of the planted plants was evaluated by conducting vegetation surveys (visual measurement of coverage and population density) once a month.

4. Test results (1) An example of the measurement results of the rainwater storage effect is shown in Figure 6.
The measurements were taken over five days in the summer near the facility. Rainfall was 102.6 mm/ ^m2 in the first two days. The amount of drainage from test piece 2 was 42.0 mm. In other words, 41% of the rain that fell was drained and 59% was stored within the facility, confirming a reduction in drainage load and the assurance of effective water utilization for plants. Note that the amount of rainfall and drainage volume in Figure 6 are per 12.96 ^m2 .
It was also observed that discharge continued after the third day, indicating that the water-retaining effect suppressed a sudden increase in discharge volume.
(2) Plant growth The growth state of the plants in test specimen 1 is shown in Figure 7. The plants were grown for about four summer months without irrigation, receiving only rainfall. All the plants planted in the four plots survived, and it became clear that all plant species, whose habitats differ depending on the moisture conditions, survived and grew in the rainwater-retaining green space equipped with the rainwater-retaining green structure filled with the rainwater-retaining green soil developed this time.

1 Rainwater storage greening soil 2 Plant 3 Existing ground depression 4 Concrete depression 41 Concrete 6 Drainage equipment 61 Drainage layer 62 Drainage amount adjustment means 63 Drainage pipe 64 Valve 65 Adjustment basin 7 Building 71 Pavement, etc. 8 Park 10 Rainwater storage greening structure

Claims (7)

This rainwater retaining soil for greening is characterized by being mixed with volcanic gravel having a particle size of 20 to 40% but less than 10 mm , 20 to 40% but less than 2.5 mm , and 10 to 50% but less than 2.5 mm in a ratio of 5 to 20% by volume of black soil. The rainwater retaining soil for greening according to claim 1, further comprising 10% by volume or less of organic material. The rainwater retaining ^{soil for greening according to claim 1 or 2, characterized in that the maximum rainwater infiltration amount (pF0 water amount) is 570 to 620 liters/m 3 ,} the rainwater discharge amount (pF0-pF1.8) is 230 to 300 liters/m ³ , and the available water content for plants (pF1.8-pF3.0) is 70 to 100 liters/m 3 . A rainwater-retaining greening structure characterized in that a rainwater-retaining greening soil according to any one of claims 1 to 3 is filled into an existing ground recess formed by excavating the existing ground or a concrete recess formed with concrete. The rainwater storage greening structure described in claim 4, characterized in that the existing ground depression has a water-permeable sheet provided on the wall surface. It is equipped with drainage facilities that connect to an existing ground depression or concrete depression and a drainage channel.
6. The rainwater storage greening structure according to claim 4 or 5, wherein the drainage equipment comprises a drainage layer provided at the bottom of the recess, an adjusting basin, and a drainage pipe equipped with a flow rate adjusting device. A rainwater-retaining green space, characterized in that it is planted in a rainwater-retaining greening structure according to any one of claims 4 to 6 in green spaces such as urban green spaces, balancing ponds, residential areas, road green spaces, and river banks.