A square pile group similar to the one shown in Fig. 15.27 passes through a recently constructed fill. The depth of fill L_{n}=3 m. The diameter of the pile is 30 cm and the piles are spaced 90 cm center to center. If the soil is cohesive with q_{u}=60 kN / m ^{2}, \text { and } \gamma=15 kN / m ^{3}, compute the negative frictional load on the pile group.