Construction Method
Construction and Maintenance of Infiltration Facilities
Construction Method
The infiltration capacity of infiltration facilities depends on the soil structure of the site. In constructing infiltration facilities, it is necessary to choose the method that does not reduce the infiltration capacity.
Specific construction procedures and points of attention in construction are as follows.
- １
- Excavation
-
-
Bedding and rolling are not conducted after excavation in order to prevent compaction of the bottom.
- Excavation is performed by excavator, and earth retaining work is performed if necessary.
- Only the necessary size of land is excavated without overbreak.
- When the surface of the excavated soil is squashed by bucket claws of excavator etc. in mechanical excavation, the surface is removed with a shovel or a metal brush. The stripped earth is removed.
- When excavation is performed manually using a shovel etc., the side is torn off in a way that the excavated surface is not smooth.
- The excavated bottom surface should not be trodden down in order to keep the infiltration capability.
- ２
- Sand laying
-
-
After excavation, sand is laid immediately to protect the bottom surface.
- After excavation, sand is laid immediately to protect the bottom surface. (This process is not necessary if the ground is gravel or sand.)
- Spreading is performed manually.
- Spread sand is lightly compacted with feet and not rolled with a tamping machine or other machines.
- ３
- Water permeable sheet (bottom and side)
-
-
Replacement crushed stone is fully spread and overlapping should be about 10 cm.
- The sheet is installed on the entire surface where the ground touches the infiltration surface.
- he sheet is overlapped about 10 cm so that earth and sand do not pass through the seam of the sheet.
- ４
- Replacement material (foundation part)
-
-
Rolling is performed at the lower end of the stabilizer (osmotic trench pipe).
- Ideally the replacement material is placed temporarily on a sheet etc. in order to prevent it from being mixed with earth and sand
- When loading, water permeable sheet should not be drawn in.
- Rolling of the replacement material is unavoidable in order to prevent subsidence and collapse, but the number of times and the method should be carefully chosen (as it affects infiltration capacity and storage volume).
- ５
- Box, trench or gutter
-
-
桝
- The bottom plate is not sealed with mortar etc.
- TA temporary cover is put to prevent inflow of earth and sand during backfilling.
- TThe gaps in connecting parts between the osmotic box and the trench pipe is filled with mortar.
Trench
- Pipes are connected with open joint and the connection port is directed upstream.
- By the standard method, pipes are connected horizontally.
- By the standard method, the maximum length of extension is less than 120 times the pipe diameter.
Gutter
- Joints of the gutter connection are treated with mortar.
- For stable installation, a bearer is placed at the connecting part.
- A temporary cover is put to prevent inflow of earth and sand during backfilling.
- ６
- Replacement material (side and top)
-
-
Rolling is performed in layers up to the top of replacement material.
- Box, trench and gutters should not be moved when loading crushed stones.
- Water permeable sheet should not be drawn in
- ７
- Water permeable sheet (top)
-
-
After loading replacement material, water permeable sheet is folded on the top of the replacement material.
- After the process of loading replacement material, the top of the replacement material is covered with water permeable sheet before backfilling.
- ８
- Backfilling
-
-
After loading replacement material, water permeable sheet is folded on the top of the replacement material.
- The backfill soil should be firmly compacted with a machine such as tamping machine.
- Special care must be taken 1 to 2 days after backfilling since there is a risk of initial subsidence due to the engagement of crushed stone etc. ※工事中の排水については原則として浸透施設を使用しない。 ※浸透施設施工後に造園工事等で客土・植栽工事を行う場合、
客土・芝等が浸透施設に流入することが多いため十分注意する。
Maintenance
In order to secure the effectiveness of infiltration facilities, the functions of the facilities must be maintained for a long period of time.
When unit infiltration capacity is calculated, the reduction of infiltration capacity is calculated with the clogging coefficient which is greatly affected by the maintenance status.
If no measure is taken to fix the deteriorated infiltration functions, it may be impossible to recover the functions afterwards. Therefore, maintenance such as inspection and cleaning is essential in order to keep the infiltration functions over a long period of time.
Infiltration facilities are to be inspected and cleaned generally once a year even though the frequency of inspection and cleaning varies depending on the location of the facilities.
In places where refuse accumulates easily due to the geographical features, cleaning may be necessary on occasions such as typhoons or after the snow thaws.
| Facility | Cleaning contents |
|---|---|
| Osmotic box/osmotic gutter | Removal of fallen leaves, refuse, sediment |
| Osmotic Trench | Unnecessary (no maintenance is needed when used with osmotic box) |
As for a cleaning method, manual cleaning, jet injection washing, suction by a vacuum vehicle etc. are available.
Occurrence and removal of clogging
(From “the 1997 Report Collection No. 54 (A) Civil Engineering Society) (Hokkaido University and Hokkaido Poracon Co., Ltd. Joint Research)
Occurrence of clogging
Maintenance method
①Removal of earth and sand
(Suction wheel with a blower of 40㎥/ min or more is used) Because the mud accumulated in the facility is expected to be compacted, the operation is performed more easily when watering is used together.
- ② Suction from the component
- ③ Washing by jet injection etc.
- ④ Suction after washing by jet injection etc.
Rainwater containing sediment and dust etc. on roads constantly flows into rainwater infiltration facilities, which leads to clogging of infiltration parts and gradual decline of infiltration capacity.
In order to keep infiltration capacity as long as possible, appropriate maintenance such as inspection and cleaning is necessary.
Osmotic box: Removal of sediment in the box. In principle once a year.
Osmotic trench (?): Simple maintenance such as removal of refuse.
Standard maintenance contents/p>
| Little sediment inflowOsmotic box | Removal of refuse |
|---|---|
| Little sediment inflowOsmotic trench | None (no maintenance is needed when used with osmotic box) |
| Much sediment inflowOsmotic box | Removal of sediment in the box once a year (Washing by jet injection etc.) |
| Much sediment inflowOsmotic trench | None (no maintenance is needed when used with osmotic box) |
Clogging
| Little sediment inflowOsmotic box | Clogging hardly occurs due to little inflow |
|---|---|
| Little sediment inflowOsmotic trench | Clogging hardly occurs since most sediment is caught in osmotic box |
| Much sediment inflowOsmotic box | Clogging occurs frequently due to much inflow |
| Much sediment inflowOsmotic trench | Clogging hardly occurs since most sediment is caught in osmotic box |
Maintenance such as inspection and cleaning is essential in order to keep the infiltration functions over a long period of time.
Infiltration facilities are to be inspected and cleaned generally once a year even though the frequency of inspection and cleaning varies depending on the location of the facilities.
Inspection
Periodic inspection: To check by a visual observation whether earth and sand, fallen leaves, refuse etc. have accumulated before the typhoon season or after the snow thaws.
