12.2.3 Experimental and observed data

Note that the forward analysis is not only restricted to FEM，but also any framework for solving geotechnical problems，such as limit equilibrium method and simplified methods involving semi⁃empirical models.When the excavation is carried out in a heterogeneous or layered soil deposit，back analysis of soil parameters can be quite complex because parameters of each layer have to be treated as separate variables.In theory，it is not a problem to back figure the parameters in FEM.In theory，this is not a problem if the analysis model，which is a required component in any updating scheme based on field observations，allows for consideration of soil parameters separately in each of multiple layers，as in the case of finite⁃element modeling of an excavation in a layered soil deposit.However，back⁃analysis using FEM that involves multiple sets of soil parameters is computationally intensive，especially under updating framework with uncertainty.

As for the experimental data，displacements are the most accessible observation objects in excavation issues.Thus it is the most commonly practice to back figure parameter based on displacement data.In theory，the two concerns in deep excavation monitoring system，data of retaining wall displacements and surface settlements，could be used in back analysis.However，it is a much more complicated task to predict ground settlement because of various factors（such as small strain behaviors of soils），which makes wall displacements based back analysis more common.