SSC JE Civil Engineering 30-Oct-2020 Shift 2 Paper

Explanation:
Bertram introduced the ratios ${\mathrm{d}}_{15,\mathrm{F}}∕{\mathrm{d}}_{\mathrm{8}5,\mathrm{B}}\approx 6$ and ${\mathrm{d}}_{15,\mathrm{F}}∕\mathrm{}{\mathrm{d}}_{15,\mathrm{B}}\approx 9$ as empirical filter designs taking into consideration the grain size distribution. The first criterion was given to ensure that the soil particles do not pass through the voids of the filter (retention or stability criterion) and the second to avoid clogging of the filter, i.e. the filter voids should not be filled with too many fine particles (permeability criterion).
Terzaghi & Peck specified more precisely;
●   ${\mathrm{d}}_{15,\mathrm{F}}∕{\mathrm{d}}_{85,\mathrm{B}}\le 4$
● ${\mathrm{d}}_{15,\mathrm{F}}∕{\mathrm{d}}_{15,\mathrm{B}}\ge 4$