Following consideration is not included in design of sand drains:
1) 1) Secondary consolidation is not taken into account in the design of sand drains. In fact, the sand drains are ineffective in controlling the secondary consolidation for highly plastic and organic soils.
2) 2) In case of deriving equation for effectiveness of sand drains, it is not considered that the excess pore water pressure developed, actually in soil where sand drains are exist, is generally less than that of the case having no sand drains. Sand drains tend to act as weak piles and reduce the stresses in the clay.
3) The typical design parameter for sand drain may vary as below :
a) Spacing of sand drains, S = ( 2 ~ 5) m
c) Radius of sand drains well, rw = (0.2 ~ 0.3) m