Technical Outline

Flow of Designing Infiltration Facilities

*For the details please refer to “Design Guidelines (Plan)”.

1Calculate the quantity of rainwater that is to be infiltrated

・In case all the rainwater is to be infiltrated
・In case rainwater outflow is to be controlled

2Determine permeability coefficient (or infiltration coefficient)

・Calculating permeability coefficient of the soil in the osmotic layer from soil boring log
(Referring to “Creager chart”, “General values of particle size and permeability Coefficient”)
・On-site percolation test→Permeability coefficient, infiltration coefficient
・Indoor soil test (Permeability test, particle size test)→Permeability coefficient

3Determine standards and cross sections of infiltration facilities

・Consider the standards used in the area
・Consider the size of the site, groundwater level, osmotic layer etc.
・Consider economic efficiency

4Calculate infiltration capacity of the infiltration facilities

5Calculate necessary amount for the quantity of rainwater

Validity of on-site percolation test

The nature of the soil is a key factor in calculating the necessary scale of infiltration facilities. By conducting on-site percolation test and analyzing the results, infiltration coefficient and permeability coefficient can be obtained. Especially infiltration coefficient can be obtained only by conducting the test. When infiltration coefficient is used, infiltration capacity tends to be bigger than permeability coefficient. Hence, by conducting on-site percolation tests, accidents caused by risky designs or overdesigns can be avoided, which results in cost reduction.