Filter rule
The Terzaghi filter rule is a criterion for the filter stability of soils. It specifies the grain distributions that two adjacent soils can have without fine components of the denser soil being washed out when water seeps or flows through the soil. The grain distributions of soils are determined by sieving the soils with standardized sieves of different sizes. The more permeable soil with the larger grains (the filter material) must not have such large gaps that the fine grains of the denser soil can fit through. In order to guarantee this, the so-called mechanical filter resistance must be met. The ratio of the grain size D15 of the filter material to d85 of the soil to be filtered must be 4 or less. D15 and d85 are the sieve sizes in which 15 and 85% of the soil grains pass through the sieve.
- D15 = diameter at 15% sieve passage (of the filter material)
- d85 = diameter at 85% sieve passage (of the soil to be filtered)
Another rule when selecting the filter material is the permeability rule or the so-called hydraulic effectiveness. It is intended to ensure that the pore spaces of the filter base are not clogged by basic grain particles loosened by the flow, as otherwise increased pore water pressure will result. The hydraulic effectiveness is calculated with:
as an additional condition
- be fulfilled.
Filters from soil layers, for example, as a drainage zone behind the clay - sealing layer of dams used. If the filter rule is not met, the sealing clay can be eroded and washed away, which can lead to a dam break .
There are other filter rules, including those that provide information on whether the filter is hydraulically effective at all; however, the Terzaghi filter rule is the most important. The filter rules are important to prevent erosion , suffusion and colmation .