Ansys Geotechnical Geology - Finite Element Modeling 562q2u

STABILITY ANALYSIS OF SPREAD FOOTINGS ON WEAK MASSIVE & LAYERED FLOOR STRATA USING FINITE ELEMENT MODELING(FEM)

VISHNU.CS n.No:613 222

Selection of the Topic Powerful

technique for finding “approximate” solution for “real-world” problems. Widely accepted by scientific and industrial community. Less time - maximum observation Ability to handle complex problems of unusual shapes,sizes & loading condition.

Need of understanding the behavior of foundation strata… Very

important for the safe as well as economic design and implementation of mega to mini engineering structures like dams etc.,

Objectives of this investigation: Settlement

analysis of weak floor strata(massive&layered) by Finite Element Modeling. Thereby

finding the stability of spread footings on it and comparing it with field/ model plate load test data.

Pros and Cons of FEA of present topic. unusual

shapes and/or unusual loading conditions as well as in situations where the foundation rock is highly variable.

 The

primary advantage is that the method is able to handle complex conditions and also provides direct solutions for deformation/ settlement.  The primary disadvantage of the finite element method is that the method does not provide a direct solution for the ultimate bearing capacity.

What is FEM & Why FEM?  FEM

is a computational technique used to obtain approximate solutions of boundary value problems in engineering.  The method utilizes an integral formulation to generate a system of algebraic equations.  Can be applied to irregular shaped objects composed of several different materials and having mixed boundary conditions.  Applicable to steady-state and time dependent problems as well as for problems involving nonlinear material properties.

How FEA gets done? (1)Preprocessing:Define

the geometric domain of the problem. Define the element type(s) to be used. Define the material properties of the elements. Define the geometric properties of the elements (length, area, and the like). Define the element connectivities (mesh the model). Define the physical constraints (boundary conditions). Define the loadings. (Conti…)

(Conti…) (2)Solution:-

During the solution phase, finite element software assembles the governing algebraic equations in matrix form and computes the unknown values of the primary field variable(s). (3) Postprocessing : Sort element stresses in order of magnitude.  Check equilibrium.  Plot deformed structural shape.  Animate dynamic model behavior.  Produce color-coded plots. 

Stability analysis of spread footings:  FE

modeling(ANSYS 11) for floor strata of various thickness(T) and for various sizes(B) and shapes of footing keeping B/T ratio constant(0.625 ) to varying ratios.

 The

input parameters obtained from existing literature for laboratory simulated weak floor strata and footing.

What to find?  Settlement

of foundation(simulated floor) with regards to the size and shape effects (i.e, effect of size and shape of footing on stability floor strata).

 Settlement

of foundation containing weak layers of varying thickness.

Parameters used for FE Analysis

Parameters used for FE Analysis

Parameters used for FE Analysis  For

FEA ,the bearing strength of simulated floor strata was taken as loading pressure which has to be applied for the observation of settlement of foundation.  The Non-linear analysis (stress-strain relation is non –linear) with “Drucker-Prager” criteria for non metal plasticity was followed as failure criteria.

FE Models

Floor strata: Massive Thickness of strata(T):4 cm Footing plate size(B):2.5 cm

B/T ratio= 0.625 Footing plate shape:circular

visualize what is happening -the biggest advantage of FEA

FE Models Floor strata: Massive Thickness of strata(T):12 cm Footing plate size(B):7.5 cm

B/T ratio= 0.625 Footing plate shape:circular

More models

FE Models of strata containing weak layer.

Results of FEA

FEM result of the footing settlement (central circular footing).

FEA result of the footing settlement (central square footing).

FEA result of the footing settlement (central square footing in the presence of weak layer).

Why the variation ??  In

FEM analysis it is always assumed that the model material is fully isotropic. But the material properties of physically simulated floor strata might not have been fully isotropic as they were prepared in the laboratory using the mixture of cement, sand and water.

 In

FEM the model floor strata were fully confined using boundary constraints. Whereas in laboratory the physical model may face problems in confinement.

Conclusions  The minimum and maximum deviation of results

from lab data are about 0.01% and 40% respectively.  Tensile cracks are initiated at the rim of footing forcing a depression beneath the footing leading to a stable crack growth as load increases.  The maximum stress concentration extends to a distance 2 to 3 times the footing plate width in all direction.  It can be concluded that the result obtained by FEM analysis is closely matches with experimental results.

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