A 0.5 m × 0.5 m square concrete pile is to be driven in a homogeneous clayey soil having undrained shear strength, c_u = 50 kPa and unit weight, γ = 18.0 kN/m³. The design capacity of the pile is 500 kN. The adhesion factor α is given as 0.75.
The length of the pile required for the above design load with a factor of safety of 2.0 is _______.