Assumption:
    Equal load on all piles, corresponding to loading over a flexible pile cap
    Equal settlement of all piles, corresponding to a perfectly rigid cap on the piles.

The following factors are incoporated in the analysis:
    Factor of finite layer: N_h, normally, set h > 5L, N_h = 1.0
    Factor of enlarged pile base: N_db
    Factor of poisson's ratio: N_v
    Factor of linearly increasing modulus of elasticity: N_es
    Factor of finite compressibility of bearing stratum: N_FE, for end-bearing piles only

The shaft friction is dominant of the pile capacity for floating piles.

The pile end bearing is dominant of the pile capacity for the end-bearing piles.

E_b is modulus of elasticity of soil under the pile tip, E_s is average modulus of elasticity of soil along the pile shaft. factor of E_b/E_sis only applicable to end-bearing piles with soils between the pile tip and rock.

Pile stiffness factor: K = E_p.R_A / E_s
  E_p - modulus of elasticity of the piles
  E_s - average modulus of elasticity of soil along the pile shaft
  R_A = A_p / (0.785 d²), A_p - the area of pile cross section, for solid pile R_A = 1

The basic equation for the analysis:
    r_k = S(r_1j .P_j.a_kj) + r_1k.P_k

    r_1j = displacement of single pile j under unit load
    r_k = final displacement of single pile k
    a_kj = interaction factor for spacing between piles k and j, and for the geometrical parameters of pile j
    P_j = the loading on pile j

N_x < 31, N_y < 31

Reference:
Poulos H. G., and Davis E. H., Pile Foundation Analysis and Design. John Wiley and Sons, new York.