Since in situ soils seldom behave like remolded laboratory soils or disturbed field samples, it is important to 'identify' or 'calibrate' the engineering properties of field soil by means of in situ tests. A response surface methodology based on an orthogonal regression in the parameter space has been developed to 'identify' engineering properties of any material based on in-situ tests. The proposed methodology was verified for the case of a two parameter hypo-elastic model for soil as well as a complex five parameter model for soil which includes nonlinear material behaviour in elastic range, yield based on Drucker-Prager yield criteria and associated plastic flow upon yield.