Characteristic Load Merthod (CLM) uses dimensional analysis to characterize the nonlinear behavior of laterally loaded piles by means of relationships among dimensionless variables

The larger the value of P_c, the greater is capacity of the pile to carry lateral loads, the larger the value of M_c, the greater is capacity of the pile to carry moments.

for cracked sections, the moment of inertia of concrete is reduced to 40-50% of the value for the full uncracked concrete section.
In fixed head piles, the maximum moment occurs at the top of piles, in free head piles, the maximum moment occurs at some depth below ground.

Assumption: the top of pile is at the grade elevation.

Short piles in uniform soil deflect more than calculated using the CLM, and the bending moment in them are smaller than calculated using CLM. for Short Piles in Uniform soil, Check Winkler Method
for soil information, use the average properties of the soil within the upper eight diameters.

Limitations of Group Amplification Procedure:

it was developed for uniformly spaced piles, can be used for rectangular groups with non-uniform spacing.
It doesn't apply to battered piles.

References:
Duncan J. M., Evans L. T., Ooi P. S. K., Lateral Load Analysis of single Piles and Drilled Shafts, Journal of Geotechnical Engnieering, Vol 120, No. 5
Ooi P. S. K., Duncan J. M., Lateral Load Analysis of Groups of Piles and Drilled Shafts, Journal of Geotechnical Engnieering, Vol 120, No. 6
Mokwa R. L., Duncan J. M.(2001) Lateral Loaded Pile Group Effects and P-y Multipliers ASCE Geotechnical Special Publication, N0 113, pp.728-742