7.1.2 Limitations
The soil models can be regarded as a qualitative representation of soil behaviour whereas the model parameters are used to quantify the soil characteristics.However，when someone doing numerical analysis，the simulation of reality remains an approximation，which implicitly involves some inevitable numerical and modelling errors.Moreover，the accuracy at which reality is approximated depends highly on the expertise of the user regarding the modelling of the problem，the understanding of the soil models and their limitations，the selection of model parameters，and the ability to judge the reliability of the computational results.Some of the limitations in the currently available models are listed below：
（1）Linear Elastic model
Soil behaviour is highly non⁃linear and irreversible.The linear elastic model is sufficient to capture the essential features of soil.The use of the linear elastic model may，however，be considered to model strong massive structures in the soil or bedrock layers.Stress states in the linear elastic model are not limited in any way，which means that the model shows infinite strength.Be careful using this model for materials that are loaded up to their material strength.
（2）Mohr⁃Coulomb model
The linear elastic perfectly⁃plastic Mohr⁃Coulomb model is a first order model that includes only a limited number of features that soil behaviour shows in reality.Although the increase of stiffness with depth can be taken into account，the Mohr⁃Coulomb model includes neither stress⁃dependency nor stress⁃path dependency nor strain dependency of stiffness or anisotropic stiffness.In general，effective stress states at failure are quite well described by using the Mohr⁃Coulomb failure criterion with effective strength parametersϕ′and c′.For undrained materials，the Mohr⁃Coulomb model may be used with the friction angleϕset to 0° and the cohesion c set to cu（su），to enable adirect control of undrained shear strength.In that case，note that the model does not automatically include the increase of shear strength with consolidation.
（3）Hardening Soil model
Although the Hardening Soil model can be regarded as an advanced soil model，there are a number of features of real soil behaviour the model does not include.It is a hardening model that does not account for softening due to soil dilatancy and de⁃bondingeffects.In fact，it is an isotropic hardening model so that it models neither hysteretic and cyclic loading nor cyclic mobility.Moreover，the model does not distinguish between large stiffness at small strains and reduced stiffness at engineering strain levels.The user has to select the stiffness parameters in accordance with the dominant strain levels in the application.Last but not least，the use of the Hardening Soil model generally results in longer calculation times，since the material stiffness matrix is formed and decomposed in each calculation step.
（4）Hardening Soil model with small⁃strain stiffness
As the Hardening Soil model with small⁃strain stiffness（HSsmall）incorporates the loading history of the soil and a strain⁃dependent stiffness，it can，to some extent，be used to model cyclic loading.However，it does not incorporate a gradual softening during cyclic loading，so is not suitable for cyclic loading problems in which softening plays a role.In fact，just as in the Hardening Soil model，softening due to soil dilatancy and debonding effects are not taken into account.Moreover，the HSsmall does not incorporate the accumulation of irreversible volumetric straining nor liquefaction behaviour with cyclic loading.The use of the HSsmall will generally result in calculation times that are even longer than those of the Hardening Soil model.
（5）Soft Soil model
The same limitations（including the ones for the Soft Soil Creep model）hold in the Soft Soil model.The utilization of the Soft Soil model should be limited to the situations that are dominated by compression.It is not recommended for use in excavation problems，since the model hardly supercedes the Mohr⁃Coulomb model in unloading problems.
（6）Soft Soil Creep model
All above limitations also hold true for the Soft Soil Creep model.In addition this model tends to over⁃predict the range of elastic soil behaviour.This is especially the case for excavation problems，including tunnelling.Care must also be taken with the generation of initial stresses for normally consolidated soils.Although it would seem logical to use OCR＝1.0 for normally consolidated soils，such use would generally lead to an over⁃prediction of deformations in problems where the stress level is dominated by the initial self⁃weight stresses.Therefore，for such problems it is recommended to use a slightly increased OCR⁃value to generate the initial stress state.In fact，in reality most soils tend to show as lightly increased pre⁃consolidation stress in comparison with the initial effective stress.Before starting an analysis with external loading，it is suggested to perform a single calculation phase with a short time interval and without loading to verify the surface settlement rate based on common practice.
（7）Modified Cam⁃Clay model
The same limitations（including those in the Soft Soil Creep model）hold in the Modified Cam⁃Clay model.Moreover，the Modified Cam⁃Clay model may allow for unrealistically high shear stresses.This is particularly the case for overconsolidated stress states where the stress path crosses the critical state line.Furthermore，the Modified Cam⁃Clay model may give softening behaviour for such stress paths.Without special regularization techniques，softening behaviour may lead to mesh dependency and convergence problems of iterative procedures.Moreover，the Modified Cam⁃Clay model cannot be used in combination with Safety analysis by means of c⁃phi reduction.