Question

A 20 mm-thick sample in a singly drained laboratory consolidation test reaches 75% consolidation in 5 hours. How long will it take to reach 75% consolidation for a 5 m-thick clay sandwiched between two sand layers in the field?

Answer 1

The expression for time factor with respect to coefficient of consolidation is given by:

                                            

where, coefficient of consolidation

            time taken

            drainage path , for one-way drainage it is equal to clay layer thickness

                                           for two-way drainage , it is equal to half of the clay layer thickness

i) In the lab,

       here, 5 months

              20 mm = 0.02 m      (one-way drainage)

                         

                          ...................(1)

ii) in the field,

       here, time taken in the field ( to be found out)

                  drainage path in the field = 2.5 m   ( since there is two-way drainage as clay is sandwiched       between sand, d = 5/2   )

                             

              .........................................(2)

Since in both the cases 75% consolidation is achieved, remains the same.

and, since it is the same clay in both the cases, remains the same.

hence, dividing equation (1) by equation (2)

hours or 8.91 years

hence it will take 8.91 years to achieve 75% consolidation in the field