Myths & Facts #13 How particle gradation effects on filter performances?

Particle size uniformity (Cu=D60/D10) can affect soil retention. This soil property plays a role in filter design through the ratio Of/Dl where Of is the largest opening size of the filter and Dl is the largest size of particle retained. Watson and John (1999) studied the effect of Cu on particle arching (or called bridging). They investigated which were the largest opening sizes compatible with stable granular arching structures, for different cases of particle size gradation. They assumed a spherical particle shape and tested their model on the basis of the ratio, O90/D90. They found that the uniformity coefficient (Cu) influences the smallest size of the particles that can form the granular arching structure, and that particles smaller than 0.228 Of are not associated with the bridging formations regardless of the soil grain size uniformity. In general, as Cu increases, the ratio O90/D90 decreases. In practice, this means the filter largest opening size, Of for piping to be prevented should be reduced when the soil is better graded. Giroud (1996) considered the selection of Of*/D85 should take into account the soil uniformity coefficient (Cu) and state of compaction. Three different density states of bridging granular structure were considered: hyperstable (Cu*=3), mesostable (Cu*=6.5) and hypostable (Cu*=13), where Cu* are the coefficients of uniformity, characteristic values related to soil internal stability. The relationship between Of and the finest size of bridging particles was derived for the two case, Cu > Cu* and Cu ≤ Cu*. Both approaches outlined above show similar trends such as relatively high values of Of*/D85 obtained in a dense condition and the relatively low values in a loose condition. However, neither model was based on consideration of actual particle size distributions. Instead, idealized linearly graded soils were presumed.