ABSTRACT
If it is not economical to drive the sheet piles deeply enough to prevent heave, the factor of safety can be increased by placing a weighted filter over the prism aocd as shown by the prism aa/d/d. If the weight of such a filter is Wt' the new factor of safety can be written as
F = _It':_b_+_W_t s U (4.67)
Filter Requirements to Control Piping Filter drains are required on the downstream sides of hydraulic structures and around drainage pipes. A properly graded filter prevents the erosion of soil in contact with it due to seepage forces. To prevent the movement of erodible soils into or through filters, the pore spaces between the filter particles should be small enough to hold some of the protected materials in place. Taylor (1948) shows that if three perfect spheres have diameters greater than 6.5 times the diameter of a small sphere, the small spheres can move through the larger as shown in Fig. 4.25(a). Soils and aggregates are always composed of ranges of particle sizes, and if pore spaces in filters are small enough to hold the 85 per cent size (D85) of the protected soil in place, the finer particles will also be held in place as exhibited schematically in Fig. 4.25(b).
