ADVANCED GEOTECHNICAL ENGINEERING

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Session: 2020/2021
Module Title: ADVANCED GEOTECHNICAL ENGINEERING
Module Code: GEOL1028

Coursework: ACADEMIC SESSION 2020/2021

Campus: Medway

Faculty: Engineering & Science

Course code: GEOL1028

Course title: ADVANCED GEOTECHNICAL ENGINEERING

Level: 7

Coursework given: 07/02/2021

Coursework due: 09/04/2021

Module leader/Instructor: Dr Panos Kloukinas

Topics: Computational Geotechnics; Elastic settlement and flexure of piles, loading transfer

on piled rafts, pile group interaction effects

Marking scheme:

– Research & referencing (15%)

– Analysis (25%)

– Computer modelling (25%)

– Results & discussion (25%)

– Presentation (10%)

Detailed description of the assessment criteria will be available on the relevant rubric, in your

Turnitin link.

Weight of coursework towards final grade: 30%

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Session: 2020/2021
Module Title: ADVANCED GEOTECHNICAL ENGINEERING
Module Code: GEOL1028

Coursework brief:

In this open-ended assignment you are asked to perform a preliminary analysis and design of
a deep foundation supporting a wind-turbine, applying all the relevant theoretical and
analytical tools offered in this module, to model the soil behaviour and the Soil-Structure-
Interaction effects. The available data are limited to the design loads and some basic
information regarding the soil material. You must complete the missing information and select
reasonable design parameters, justified through your own research in the literature. Your
understanding, analytical capabilities, engineering judgement, as well as your personal
reflections, need to be evident in your final report.

The problem geometry is depicted in Figure 1: a deep foundation system is needed to support
the wind turbine, taking into account the properties of the 10m silty sand layer and the soft
clay layer underneath. Two foundation options need to be explored: a monopile foundation
and a piled raft foundation with reinforced concrete piles. Split your analysis into the following
steps:

1) Design of the monopile: your design (i.e. selection of diameter and length) needs to satisfy
the ultimate vertical bearing capacity requirements with a factor of safety equal to 2.5.
Monopiles are hollow steel sections – refer to the literature for ranges of parameters
needed. You will also need to calculate the soil strength parameters, for the sand and the
clay material.

2) Then analyse the performance of your monopile under the working loads, i.e. analyse the
settlement and the deflection profiles and the corresponding internal forces (axial and
shear force and bending moment diagrammes). To do that you have to consider two
cases:
a) Apply the relevant elastic solutions, assuming constant (averaged) soil properties – the
approximations are left to your discretion – to get a rough theoretical estimation. In order
to do that, you have to analyse the stiffness parameters of the soil materials.
b) Solve again, for variable soil properties, this time applying a numerical solution with
finite differences. Use Matlab to solve the system of equations (alternatively, you can use
Ms Excel, if you have difficulties in Matlab).

3) Model the problem in Plaxis 2D, using the “Embedded Beam Row” element for your
monopile. Regarding the soil material models, you can use linear Mohr-Coulomb or non-
linear hardening model for the sand and linear Mohr-Coulomb or non-linear soft-soil
model for the clay. You are strongly encouraged to do both and compare the differences.
Compare also with your elastic analysis in question (2).

4) Now consider an alternative foundation, with a 3-by-3 pile group (9 piles overall)
connected to a stiff pile cap. You need to design again the pile group (diameter, length
and spacing between the piles). To estimate the efficiency ratio of the pile group, you can
apply the elastic interaction factors method.

5) Last step, model your pile group in Plaxis 2D, using “Embedded Beam Row” elements for
the piles and a beam element for the pile cap. In this analysis you can use only one of the
soil model combinations (in you have created both a linear and a non-linear set). Apply
the loads in a sequence: first the vertical force, then the horizontal and the bending
moment, and compare the effect on the results. Comment on the pile group interaction
effects.

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Session: 2020/2021
Module Title: ADVANCED GEOTECHNICAL ENGINEERING
Module Code: GEOL1028

Figure 1. Working loads for the wind turbine and soil stratigraphy

Figure 2. Experimental curve from 1-D consolidation oedometer test

Specimen extracted from A:
depth of 15m

Silty sand:

sat = 20 kN/m3

V = 6 MN
30m

soft clay:

sat = 18 kN/m3

10m

H = 8 MN

WT at ground level

A
5m