Explain How the load carrying capacity of group of piles in clay is determined by “Block Failure Method”?

When piles are spaced at closer intervals, the soil contained between the pile move downward with the piles and at failure, pile and soil move together to give a typical block failure. Normally this type of failure occurs when the piles are spaced less than 2 to 3 times the pile diameter. For wide spacing pile in the group fails individually. In the absence of negative skin friction, the group capacity in clay is usually governed by the sum of the single pile capacities with some reduction due to overlapping zones of shear deformation in the surrounding soil.

The following are design recommendations for estimating group capacity in clay:

·         For pile groups driven in clays with undrained shear strengths of less than 2,000 psf and for spacings of three times the average pile diameter, the group efficiency can be taken to be equal to seventy percent (70%). If the spacing is greater than six times the average pile diameter, then a group efficiency equal to one-hundred percent (100%) can be used.

·          For pile groups in clays with undrained shear strength in excess of 2,000 psf, use a group efficiency equal to one-hundred percent (100%).

·          Investigate the possibility of a block failure.

·          Piles should not be installed at spacings less than three times the average pile diameter in clay.

The load carrying capacity of the pile group in clay against a block failure is provided by the following expression:

Qn=(N_c x c_u1 x B x L)+(2 x D x (B+ L) x c_u2 )

Where,

            Qn = Nominal resistance against block failure

            N_c =  9 (bearing capacity factor)

            Cu1 = Undrained shear strength of clay below pile tips

            Cu2 = Average undrained shear strength of clay around the group

            B = Width of group

            L = Length of group

            D = Length of piles