FINITE ELEMENT ANALYSIS IN SOIL/ROCK
FGS Engineers & Innovators
A mutli-facet civil engineering consulting company
Finite element analysis (FEA) involves complicated geometric and mathematical models of real-world problems. FEA will provide us with more reliable results if only the modelling and analysis has been carried out with proper care. An attempt has been made to explain how to utilize the FEA to the maximum extent for obtaining reliable results while explaining the importance of conventional methods too. This article is written by Er. Gowtham B, Geotechnical Design Engineer at FGS Engineers & Innovators.
Well, talking mathematically, conventional methods or analytical methods develop the governing?equations for a considered element which are in the form of partial differential equations. These equations can solve real life problems but not some of the complex problems keeping in view the complex geometry and boundary conditions. There comes the need of finite element analysis (FEA) which converts those partial differential equations into simple algebraic equations which are easy to solve to obtain approximate solutions.
?In geotechnical field, soil/rock itself is a complex and heterogenous structure which displays non-linear response under loads. Using FEA, this complexity can be solved using different constitutive models. Notwithstanding the hurdles like complexity in modelling and time consumption, FEA will provide us with the reliable results and simulate the realistic ground conditions compared to the conventional methods which are being mostly used by designers.
?ARE CONVENTIONAL METHODS ALWAYS THE SOLUTION?
>> Complex ground behaviour like non-linear stiffness, anisotropy etc.
>> Ground improvement problems
>> Complex hydraulic conditions
>> Unusual geometry
>> Soil–structure interaction
>> Complex loadings
>> Back-analysis of field trials
>> The effect of construction stages
>> Time effects like creep & consolidation
So, conventional methods are useful only upto a certain level of complexity, but for simulating better ground/field conditions and to get the better conservativeness for more complex problems, finite element analysis has to be applied. The below figure is an example for slope stability analysis using RS2.
HAS FINITE ELEMENT ANALYSIS BEEN UTILISED WELL?
The finite element analysis (FEA) will always provide us with good results if only proper care is taken in modelling with optimum boundary conditions and suitable constitutive models. Below are the few points in which care must be taken in carrying out finite element analysis.
UNDERSTANDING THE PROBLEM AND PROGRAM
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2D vs 3D
?HOW FAR THE BOUNDARIES HAVE TO BE CONSIDERED?
?FIXITY CONDITIONS FOR THE BOUNDARIES
?DISCRETIZATION TO BE USED
CONSTITUTIVE MODELS TO BE SELECTED
Constitutive models for the respective soil/rock have to be selected so that the soil/rock should behave with sufficient accuracy for all the applied loading conditions. Nowadays there are number of constitutive models which will simulate all the soil/rock behaviour, but one should not complicate the model by giving complex model to a simple problem. The selection of constitutive models mainly depends upon the following criteria:
Some examples for the constitutive models for the conditions of soil/rocks ?
Apart from all these, there are certain other factors which will affect the finite element analysis to a certain extent, such as giving sensible input parameters, assessing the output values etc. So, the user should be aware of the program and problem which has to be solved thoroughly so that the model and the results are sensible.
APPLICATIONS OF FEA IN GEOTECHNICAL FIELD
Tunnels, anchored walls used to stabilize landslides, building foundations, cellular cofferdams, embankment dams, excavation bracing systems, long-span flexible culverts, offshore structures, plastic concrete seepage cut-off walls, reinforced embankments, reinforced slopes, retaining walls, seepage through earth masses, slurry trench seepage barriers, u-frame locks, unbraced excavations etc., are the main areas where FEA will be useful in the geotechnical field.
FEW BOOKS ON FEA TO REFER:
?There are numerous good books available for reference on FEA in Geotechnical analysis. Below are a few good ones: