A group of nine piles, each 10 m long and 350 mm in diameter, is arranged in a square 3 × 3 pattern in a clay soil of cohesion c = 60 kN/m2. Find the centre-to-centre spacing of the piles for a group efficiency of 1. Neglect end bearing resistance. The adhesion factor α = 1 for both individual pile and group action.

After a certain time lapse, clay A achieved 50% average degree of consolidation (U) and clay B achieved 20% average degree of consolidation. Find the ratio of the coefficient of consolidation of clay A to that of clay B (CvA : CvB). Given time factor Tv = 0.20 for U = 50% and Tv = 0.03 for U = 20%, and both clays have the same drainage path lengths.

The plastic limit of a soil is 20% and its liquid limit is 50%. If its flow index is 30%, find the value of the toughness index of the soil.