Question

According to Terzaghi, the D15 size material must be more than the size of __________

(a) D40

(b) D85

(c) D20

(d) D10

The question was posed to me in a national level competition.

My question is based upon Protective Filters topic in section Drainage and Dewatering of Geotechnical Engineering I

Answer 1

Correct choice is (b) D85

To explain I would say: The D15 size of filter material must not be more than 4 to 5 times D85 size of the base materials. This prevents the foundation materials from passing through the pores of the filter material.