Tham khảo tài liệu 'critical state soil mechanics phần 4', khoa học tự nhiên, công nghệ môi trường phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 59 If we accept as a proper approximation that H2 1 2 cvc then we can draw a useful distinction between undrained and drained problems. For any given soil body let us suppose we know a certain time ti in which a proposed load will gradually be brought to bear on the body construction of an embankment might take a time ti of several years whereas filling of an oil tank might involve a time ti of less than a day. Then we can distinguish undrained problems as having t1 2 ti and drained problems as having t tl. Equation can be written 1 2 k in in which form we can see directly the effects of the different parameters mvc k and H. More compressible bodies of virgin compressed soft soil will have values of mvc associated with X and longer half-settlement times than less compressible bodies of overcompressed firm soil having values of associated with k elastic swelling or recompression . Less permeable clays will have much smaller values of k and hence longer half-settlement times than more permeable sands. Large homogeneous bodies of plastically deforming soft clay will have long drainage paths H comparable to the dimension of the body itself whereas thin layers of soft clay within a rubble of firm clay will have lengths H comparable to the thinness of the soft clay layer. This distinction between those soils in which the undrained problem is likely to arise and those in which the drained problem is likely to arise will be of great importance later in chapter 8. The engineer can control consolidation in various ways. The soil body can be pierced with sand-drains that reduce the half-settlement time. The half-settlement time may be left unaltered and construction work may be phased so that loads that are rather insensitive to settlement such as layers of fill in an embankment are placed in an early stage of consolidation and finishing works that are sensitive to settlement are left until a later stage observation of settlement and of gradual dissipation
