Hokkaido Poracon Co. Ltd.

Planning

• What kind of infiltration facilities are available?
  • ・浸透桝
    ● Osmotic box
    ● Osmotic well
    ● Osmotic trench
    ● Osmotic gutter (U-shape, half-round shape) effective in places where the groundwater level is relatively high
    * Unsupported gutter is not included in infiltration facilities.
• What materials are necessary in planning and designing?
  • ・計画平面図
    ● Ground plan
    ● Amount of rainwater runoff
    ● Soil boring log (to grasp soil property and groundwater)
    ● Allowable outflow discharge (If rainwater is drained into public sewers, advanced consultation with relevant public organizations is necessary)
• How is the amount of rainwater runoff calculated?
  • ● The calculation formula is published in Road and Earthwork Outline
    ● Rainfall intensity (determined by materials on heavy rain, Road and Earthwork Outline, effluent standards for sewage of each municipality)
    ● Drainage area
    ● Discharge coefficient
• How is infiltration capacity calculated?
  • ① Calculation method using permeability coefficient
    ● It is calculated with permeability coefficient which is determined by soil properties
    ● It is calculated with permeability coefficient which is gained through laboratory experiments (permeability test, particle size analysis)
    ② Hydrostatic pressure method using hydrostatic pressure
    It is calculated with infiltration coefficient which is gained through on-site percolation test.
• How are soil properties determined in planning?
  • ① Method to estimate permeability coefficient based on boring data
    ② Method to calculate permeability coefficient through particle size analysis of the soil
    ③ Method to calculate infiltration coefficient through on-site percolation test
• I do not understand soil classification.
  • Here is the list of soil type and its permeability in descending order. Detailed information on soil classification can be found in related materials and Design Guidelines for Rainwater Infiltration Facilities (by Hokkaido Poracon, Co. Ltd.)
    ● Gravelly soil​​​ k=10-1cm/sec
    ● Coarse grained sand ​​k=10-1～10-2cm/sec
    ● Medium sand​​ k=10-2cm/sec
    ● Fine grained sand ​​k=10-3cm/sec
    ● Silt​​​​ k=10-4cm/sec
    ● Cohesive soil ​​k=10-5cm/sec
    ● Clay ​​​k=10-6cm/sec
    * Infiltration function cannot be expected under soil conditions below k=10-5cm/sec.
• What kind of place is not suitable for infiltration facilities?
  • ● Fill-up ground
    ● Under groundwater table
    ● Cohesive soil and peat layer of which permeability is below k=10-5cm/sec
• How are infiltration facilities installed on the top and foot of slope?
  • ● As for the foot of slope, it is not a problem because infiltration facilities are not installed on the ground where permeability is low.
    ● As for the top of slope, there is a great risk of slope collapse due to infiltrated water, therefore infiltration facilities are installed in a place where the distance from the top of the slope is twice the height of the slope
• The site area is small and it is difficult to secure the length of osmotic trench in the results of calculations.
  • Osmotic trenches can be installed in parallel at intervals of 1.5m or more, and the calculated length can be secured in this way. If the intervals between infiltration facilities are too small, infiltration capacity declines due to the mutual interference of the infiltration flows. But when the interval is 1.5m or more, there is hardly any decline in infiltration capacity.
• I cannot judge if sand laying is necessary in infiltration facilities.
  • Sand laying is not necessary if the soil property is sand or gravel. If the soil is fine grained sand, sand is laid immediately after excavation because the bottom surface is compacted during work in the bottom, which leads to decline in infiltration capacity.
• Groundwater is high (GL-1.00m) as the result of boring. Is it impossible to build infiltration facilities on this site?
  • Osmotic box and osmotic trench cannot be used. Osmotic gutter can be utilized.

Construction

• How wide are water permeable sheets overlapped?
  • Generally 10 cm. It is for the purpose of preventing suction of surrounding soil.
• If rolling on the bottom after excavation is not performed, is there no risk of subsidence after completion?
  • Rolling reduces infiltration capacity of the bottom surface, therefore rolling on replacement material and backfill soil in layers performed.
• Osmotic box was going to be used in the design drawing, but after construction started, the groundwater was found after excavation at a high position (about GL-1.00m). Should we continue construction as planned?
  • The groundwater will get into replacement material and infiltration below the groundwater level cannot be expected. It is advisable to discuss with your clients and consider using osmotic gutters.
• Are connections between an osmotic box and an osmotic trench pipe or between pipes fixed with mortar?
  • The gap between osmotic box and trench pipe is filled with mortar. Trench pipes are not fixed with each other with mortar. The connecting part of trench pipes has relief structure (insertion type).
• Is joint mortar necessary for the connection of osmotic gutters?
  • Due to its structure, joint mortar is necessary as is for standard gutters.
• In our plan, a road surface box and an osmotic box will be connected with a conduit pipe. At the side of the osmotic box, is it necessary to connect the conduit pipe with the osmotic box or is it possible to end the conduit pipe in the replacement material and not connect with the osmotic box?
  • The conduit pipe needs to be connected with the osmotic box in order to convey rainwater into the osmotic box quickly. Considering the flow velocity, if the conduit pipe breaks off in the replacement material, it takes a considerable amount of time for rainwater to pass through the replacement material and go into the osmotic box, which would cause overflow on the road surface box.

General

• What kind of preparation needs to be done when applying for drainage plan to the municipality?
  • ● Each municipality has its own drainage plan. Especially when the planned facilities are going to be connected to the public basin, water that exceeds the allowable discharge will be stored on site or controlled at infiltration facilities.
    ● It there is no public basin, the entire amount of rainwater will be processed on site.
• In case there is wastewater treatment facility, do infiltration facilities cost more than general rainwater drainage facilities?
  • It depends on the scale of the facilities.
    ● In case of road construction: if the road section is long and the basin is large, the scale is larger in general drainage facilities (vertical pipes, gutters etc.) going downstream.
    ● In case of site preparation: if the site is large, the scale is larger in general drainage facilities (conduit pipes, gutters etc.) going downstream.
• Large amount of wastewater is discharged from our factory every day and we are concerned that sewage charge will high.
  • Wastewater from factories is different from rainwater. The soil saturation is taken into consideration in planning facilities that are used every day. Infiltration facilities may not be suitable for facilities that are used 24 hours every day all through the year.
• Groundwater will be pumped for a long time during underground construction and we are concerned about ground transformation due to sudden decrease of groundwater.
  • It is desirable to infiltrate (return) the pumped groundwater at a location far from the site.
• There is a drainage facility in the design drawing, but when we checked the site after the construction started, it was found out that there is no wastewater treatment facility.
  • This is a relatively frequent-asked question (several times a year). It is recommended to identify the nature of the soil by machine excavation and consider infiltration facilities.
• We want to install an osmotic box deep underground but we are concerned about its strength.
  • As for a box which is vertical, it has a considerable durability against vertical pressure. Lateral pressure is about a third of the vertical load, which is not a problem. However, osmotic trench pipes receive the considerable vertical road and stability calculation is necessary if it is installed at an extremely shallow or deep place.
• Porous concrete seems to be weak and we are concerned about its strength.
  • As the compressive strength of porous concrete is lower than that of standard concrete pipes and fume pipes, the product strength of porous concrete is maintained by increasing the thickness of the product. It also has reinforcing bars even though they do not count in the intensity calculation.
• We submitted an infiltration facility plan to the relevant public organization but we are told that the infiltration construction method is not accepted.
  • In recent years, increasing torrential rain and localized downpour due to the climate change causes more frequent river floods than ever. The Ministry of Land, Infrastructure, Transport and Tourism has been repeatedly instructing to control rainwater runoff by infiltration and storage of rainwater.
• The combined use of street gully and osmotic trench would reduce cost. Is there a problem about this?
  • That is right about the cost. In the long run, is osmotic trench is used in combination with osmotic box, mud that flows in with rainwater is deposited (functions as mud reservoir) and only clean water is conveyed to osmotic trench (it is difficult to fix clogging inside trenches). In this way, osmotic trench is maintenance free facility when it is used in combination with osmotic box.
• Is there any damage to farms by infiltrating rainwater that contains snow melting agent?
  • There is no damage to farms. Infiltration facilities are installed at a deep position which is deeper than the reproductive range of farm products and do not do any harm on them. Infiltrated water is also purified when it goes through osmotic box and replacement material.
• There is a public basin and a fixed allowable outflow discharge but infiltration facilities are not suitable for this location due to the nature of soil.
  • Storage facilities or an adjustment reservoir can be used instead. BOX culverts were used in conventional underground storage facilities, but recently light-weight resin-based facilities are used, which improved workability and reduced construction time. These facilities are available too, please contact us.

Material

• How about using concrete bottomless box?
  • Since suspended substances and mud in rainwater accumulate at the bottom of the box and turn into sludge causing clogging, bottomless box usually last only two to three years. It is advisable to have a box that can infiltrate water both at the bottom and the sides. Also, when the concrete bottomless box is used in combination with trench pipes, mud flows into trenches and causes clogging.
• How is replacement material selected?
  • It has to be single particle sized. In standard designs, crushed stone size 3 (30-40mm) or size 4 (20-30mm) is used, but it is available only in limited areas in Hokkaido. Most types can be used as a substitute except for 0 (0-40, 0-80mm). For details, please refer to Design Guidelines for Rainwater Infiltration Facilities (by Hokkaido Poracon Co. Ltd.).
• Are there any points to pay attention in terms of the quality of water permeable sheet?
  • Other than tensile strength, water permeability is important. Sheets of which permeability is lower than permeability coefficient used in the calculation cannot are not suitable. Extra attention should be paid when the soil is course sand or gravel or when on-site percolation test by “PRWI method” is conducted because the coefficient value is large in those occasions. Also, some sheets used for horizontal drainage have low permeability in vertical direction and high permeability in horizontal direction, and attention needs to be paid because infiltrated water travels in the vertical direction.
• How is the height of the middle box in the osmotic box determined?
  • ● Soil structure (Thickness of impermeable stratum etc.)
    ● Scale of gutters, vertical and horizontal pipes
    ● Thickness of the roadbed
    ● Other factors

Maintenance

• How often are the facilities inspected and cleaned?
  • Infiltration facilities are inspected and cleaned generally once a year even though it depends on the location of the facilities (the environment of surroundings etc.)
    ① Places where there is a little accumulation of refuse and earth and sand (In the following cases, the bottom place of the osmotic box can be seen even after 3 years)
    ● In case the surrounding area is asphalt such as a parking lot
    ● In case facilities are installed in roadways with low traffic volume such as residential areas
    ② Places where there is much accumulation of refuse and earth and sand
    ● Road gutters into which rainwater from farms and fields etc. flows
    (Inspection is necessary especially before the typhoon season and after heavy rainfalls)
    ● In case facilities are installed in roadways with high traffic volume or a lot of traffic of large vehicles.
    (In these conditions, some places have about 10 cm of accumulated earth and dust on the bottom after 3 years.)
• How are facilities cleaned?
  • Other than manual cleaning, jet injection washing and suction by a vacuum vehicle etc. are available.
    When osmotic box is used in combination with osmotic trench, the osmotic box is a pretreatment facility and the osmotic trench does not require maintenance. If mud accumulates at the bottom for a long time and becomes hard, suction by a vacuum care while washing with jet spray is effective. Our business includes cleaning of infiltration facilities. Please contact us.
• With how much accumulation of earth and sand does an osmotic box need to be cleaned?
  • In order to prevent sediment from getting into the infiltration trench, the osmotic box needs to be cleaned when the accumulation of earth and sand reaches 50% of the length between the bottom of the box and the bottom of the osmotic trench pipe.
• There was a heavy rainfall immediately after construction and earth and sand flowed into the osmotic box up to the top of it. Is it necessary to replace the infiltration facilities with new ones?
  • It is no problem if the earth and sand in the box is removed by vacuuming and the box is cleaned. It is also no problem in case the osmotic box is used in combination with trenches. The earth and sand stops at the edge of the trench and does not go further into the trench.