The initial part of the coursework is a series of small exercises to compare the FEM analysis with theoretical results for problems: Soil-Structure Interaction, UOB, UK

PART 1:

The initial part of the coursework is a series of small exercises to compare the FEM analysis with theoretical results for problems for which the solution is known (see note 1). You should explore the influence of mesh refinement, boundary proximity and boundary fixities as well as loading configuration. If you wish, you may work in a small team and share some results – but please make it clear in your report what is the work of others.

To start with exploring the load-settlement behaviour of a 2-metre wide strip and a 2-metre diameter circular foundation. Each rests on a stratum of stiff clay over bedrock.

(i) Undrained bearing capacity

First, calculate by hand the ultimate bearing capacity of a 2-metre wide strip and circular foundations bearing on the surface of stiff clay with soil parameters (design values) shown below. Look up the values of Nc
for a strip and a circular foundation.

Then set up a plane-strain analysis (with 15-node elements) to model the problem below.

You should model one half of the problem in Plaxis as shown above. The left-hand side of the diagram is an axis of symmetry, and horizontal restraints should be introduced on the two vertical boundaries and vertical and horizontal restraints along the base (default fixities – see note 2). Where the foundation is modelled as a plate explicitly includes a moment restraint on
the centreline (see note 2). Make sure that the mesh is fine enough close to the foundation by refining it several times (refine line or refine point – see note 3) making a note of the number of elements it contains.

Model the soil as an undrained (B) Mohr-Coulomb material, with undrained strength parameters, but using drained stiffness parameters. (Plaxis will convert the stiffness to undrained values).

Model the loading in three ways:

a) apply a vertical displacement of 0.2m (save file e.g. strip1.plx)
b) apply vertical stress of 1000 kPa (save file e.g. strip2.plx) – NB failure should occur
c) include a plate to model a concrete foundation 0.75m thick, again with vertical stress of 1000 kPa (E = 2.4 107 kPa) – (save file e.g. strip3.plx)

(ii) Drained bearing capacity

Repeat the exercise using drained strength parameters. This time place the loading/plate at 0.75m depth to represent a concrete foundation; this is to avoid local bearing capacity failure at the edge of the foundation. Again, look up the values of Nc, Ny and Nq. Compare the results of the FE analysis with your hand calculations. How good is the agreement? See the paper by
Bolton and Lau on Blackboard.

(iii) Settlement analysis

For a flexible strip foundation of width 2m at the surface calculate the settlement at working load (say 150 kPa) using a theoretical solution [e.g. p= (q B /E) x Ip] – see chapter 14 in Lambe and Whitman on Blackboard.

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