Applications of FEM in Geotechnical Engineering / State-of-the-Art

APPLICATIONS OF FINITE ELEMENT METHOD
IN
GEOTECHNICAL ENGINEERING
February 2020
Dr Mazin Alhamrany
State-of-the-Art

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Outline of presentation
 Introduction
 Methods of Solutions
 The Finite Element Method (FEM)
 What is FEM
 Basic Concept of FEM
 Usefulness of FEM
 General Comments for applying FEM
 Practical Applications
1) Early Applications of FEM in Geotechnical Engineering
2) Present Applications of FEM in Geotechnical Engineering
 Conclusions

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Geotechnical Engineering
ISSMGE defines Geotechnical Engineering as “a science that explains the mechanics of soil and rocks and it’s
engineering applications to the developments of humankind”.
Geotechnical Engineering is based on using Principles of Soil & Rock Mechanics to (1) investigate subsurface
conditions and (2) to determine the relevant physical/mechanical and chemical properties of these materials and (3) to
analyze / solve / design natural and humankind developments interacted with the ground. The more complex a
geotechnical problem, the greater the care required in each of the steps mentioned above and the more sophisticated
the method of analysis would be used.
Introduction
Practice of Geotechnical engineering:
1. Trial and Error,
2. Observation and Experience,
3. Empirical Experimentations,
4. Scientifically based approach of Soil and Rock
Mechanics, and
5. Soil-Structure Interaction (Numerical Methods).

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Methods of Solutions
 The last decades have witnessed great
expansion in the applications of Numerical
Methods, in particular, the Finite Element
Method (FEM) for tackling rather complex
geotechnical problems.
Empirical
Based on
Experience
Methods of Solution
Analytical
Closed
Form
Numerical
Finite
Difference
Method
Finite
Element
Method
Boundary
Intergral
Method

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Methods of Solutions
1970’s
Main Frame Computers
1980’s
Personal Computer
Current Laptop
 Fortunately, the geotechnical engineer has been one of the first to recognize the usefulness of
numerical methods. The methods are now accepted and employed in practice and considered as one
of the main requirement when dealing with complex soil-structure interaction problems.
 The large, high speed computer has been essential to the phenomenal growth of Numerical Methods.

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The Finite Element Method
What is FEM
• The Finite Element Method (FEM) is a
numerical technique for finding approximate
solutions for partial differential equations
(PDE) for which no exact solution is
available. The technique reduces PDE
system to a system of algebraic equations
that can be solved using linear algebraic
technique, which can be programmed.

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Basic Concept
Step 1: Discretization
Subdivide a continuum into small components or
pieces called “elements” and the elements are
comprised of nodes, which make a grid called
mesh. Fine “mesh discretization” is recommended
in the “zone of interest”.
• Aspect Ration,
• Degree of freedom,
• Compatibility,
• Primary unknowns: Nodal displacements
• Secondary unknowns: stresses and strains

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Basic Concept
Variation of variable fields along nodal line

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Basic Concept
Step 2: Selection of approximation
function
Identifying the pattern of solution (Linear,
quadratic ,cubic, etc.) for the variable field
along the nodal lines of the elements in the
form of polynomials.
Step 3: Derivation of element equation
Defining stress-strain relation and formulation
of element equation that can be expressed in
matrix notation as:
[k]{u}={q}
Step 4: Assembling the element matrix to form
global equation
Combine/Assemble the element equations to
obtain the stiffness relation for the entire system.
[K]{U}={Q}
Where:
[K]: Global (assemblage) Stiffness matrix,
{U}: Global nodal displacement vector, and
{Q}: Global nodal force vector.
Step 5: Computation of the assembled equation
Solving of the global matrix equation. Gaussian
elimination is perhaps the most common procedure
employed for solution of linear equations
generated in numerical techniques.
Other Concepts: Consistency, Stability and Convergence

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Usefulness of FEM
 FEM is useful for its ability for tackling problems involving such complexities as:
 Non-homogeneous media,
 Non-linear material properties and boundary conditions,
 In situe stress condition,
 Spatial and temporal variations in material properties, and
 Arbitrary geometry and boundary conditions.
Why to use FEM
The increase demand on improving infrastructure of modern cities has led to a rise in the number of
challenging projects such as:
 Metro tunnels/stations underneath an existing train station / airport buildings, with minimum
disruption to public services during construction,
 New buildings adjacent to an existing tunnel and buildings,
 shallow tunnels for underpasses crossing highways which would be in service during subway
construction,
The need for using 3D FE Models for tackling rather complex soil-structure interaction problems becomes
inevitable.

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General comments on applications of FEM
• It is essential that engineers have a “reasonable” understanding for
the basic concepts of the theory of Finite Element Method.
• Fully understanding of Physical Problem.
• Geometrical Model; Plane-strain, Plane-stress, Axisymmetric, and 3D
Model.
• Material Model; Mohr-Coulomb, HS Model, HS-Small strain Model,
etc.

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General comments on applications of FEM
• Input data; accuracy & reliability of the input data.
• The heterogeneous and anisotropic nature of soil and rock masses makes the task of evaluating the
parameters very challenging. Thus a careful balance has to be struck between, on the one hand, adoption of
simplified methods of analysis with limited aims as to outcomes but requiring limited number of parameters to
be evaluated and, on the other hand, the use of sophisticated methods allowing far better predictions but
requiring the evaluation of far more data which are generally difficult to obtain even with the commitment of
considerable additional resources. However, as the recent challenges entail using such sophisticated
methods, there is, therefore, developing new techniques (in terms of field and lab tests as well as new
correlations/theories) for evaluating the required input geotechnical data with reasonable accuracy becomes
essential.
• Solid theoretical background and Experience: Whenever possible, prior to start setting the sophisticated 3D
FE Model, it is highly recommended to conduct a sanity check. Despite the complexity of the problem under
consideration, certain simplified assumptions can be adopted in order to obtain a high-level assessment.
Such assessment, which is based on solid theoretical background and intensive experience, should provide
an indication regarding the anticipated results obtained from the proposed complex FE analysis.
• Numerical Analysis has become a standard tool for assessing the serviceability limit state (SLS) and, at least
to some extent, the ultimate limit state (ULS) of the geotechnical engineering problems.

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Vertical
force
Horizontal
force

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Practical Applications of FEM
 Early Application – 4 decades ago!
 Recent Application
 Future Developments in Applications of FEM

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 Early Application
Limited number of elements, simple mesh discretization, Time consuming, Mohr-Coulomb Soil Model,

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 Early Application

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Along with the development of high-speed digital computers, the
application of the finite element method progressed at a very
impressive rate in the last few decades. Currently, the applications of
Numerical Methods, and in particular FEM, for tackling rather
complex problems in the field of geotechnical engineering are
considered as a common standard practice.
Struts
Tunnel Rails
Secant
Pile
Wall
Piles
Waler Beam
Recent developments, such as:
 Window version,
 Graphical features for model display,
 Importing CAD, Excel, field data and other software's data,
 Automatic mesh generation,
 High-Order Elements; 4th order 15-node triangular elements,
 Advanced Soil/rock Models,
 Structural Elements are incorporated to assess stresses for piles,
embedded piles, Retaining walls, Anchors, Struts, Geogrid,
Plates, Beams, Tunnels and other structures,
 Interface Elements,
 User-Friendly,

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Projects
 Impact of New Construction on Existing Adjacent Tunnel - New Construction usually involves certain engineering
activities such as, groundwater lowering, shoring system, excavations and application of load from the proposed foundation and
superstructure. The impact of these activities on existing adjacent tunnel needs to be thoroughly investigated to ensure the safety of the tunnel.
The analysis should provide an insight on the anticipated mechanism of displacements/deformations and the additional stresses on the exiting
tunnel.
 Piled-Raft Foundation Design - Piled-raft foundations have a complex soil-structure interaction mechanism including the pile-
rock interaction, pile-pile interaction, raft-soil/rock interaction, and finally the pile-raft interaction. The 3D Finite Element Model allows examining
the effect of the key parameters (pile spacing, pile length, pile diameter, and raft thickness) governing the performance of this foundation during
loading and, accordingly, the load shared by the piles and the raft.
 Impact of 4 Towers Construction on Existing Adjacent Structures – Impact of two seventy (70) stories towers
and two fifty (50) stories towers on existing adjacent structures (a tunnel and two car parks).

Waterfront
Development
Impact of New Construction (Plots 14a & 16)
on Existing Tunnel
A Soil-Structure Interaction analysis was undertaken to assess the impact
of the proposed construction works in Plots 14a & 16 of Deira Waterfront
Development on the existing Al Shindagha Tunnel.

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Plots 14a & 16 Deira Waterfront Development
As part of the Deira Waterfront Development, Plots 14a & 16 are proposed to be constructed on either side of and along the existing
Al Shindagha Tunnel in Deira, Dubai.
Plot 14a (in brown colour below): Plot 14a will accommodate 3 main buildings with a common single basement. The first building
will comprise 1 No. G+15 to accommodate a 4-star hotel and the second building is of 1 No. G+18 residential building and the third
building is a G+5 car parking building with luxuries amenities & 2 level town houses above.
Plot 16 (in yellow colour below): Plot 16 comprises 4 No. blocks with a common single level basement. Each block is a
combination of a series of 2 No. to 4 No. G+4 storey buildings maximum which are connected at ground level.
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1. Project Description
• Excavation works approximately 8.0m deep in both plots
14a and 16.
• Groundwater level is at +1.5 m DMD which is 2.5m
below the existing ground level;
• Shoring system for Plots 14a & 16 consists of
Diaphragm wall adjacent to Al Shindagha Tunnel and
Secant pile wall on remaining boundaries.
• Toe level of the shoring is at -15.0 m DMD.
Foundation Details Plots 14a & 16
• Pile foundation are used for both plots. The piles are
1000mm diameter with an approximate length of 24.0m
from cut-off level;
• Basement slab is 0.4m thick;
• Building surcharge of 400kPa for Plot 14a and 200 kPa
for Plot 16.

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The existing Al Shindagha Tunnel:
• The Al Shindagha Tunnel runs between both plots with clear distance between the tunnel and Plots 14a & 16 are 30.1m & 22.1m,
respectively.
• Al Shindagha tunnel is 20m wide and comprises tunnel segments, 18.3m (60.0 ft) length each, running in a slope of 3.3% & 5% slope.
• The model used for the analysis comprises a tunnel length of 128m long. This length consists of 6 No. tunnel segments running between
plots 14a and 16.
• A constant internal height of 7.0 m is considered along all the segments.
• The modelled fill height above the tunnel varies between 6.8 and 11.4m approximately.
Segment
D5
Segment
D6
Segment
D7
Segment
D8
Segment
C1
Segment
C2
Plot 16
Plot 14a
Tunnel
• The tunnel segments running between plots 14a and 16 have been assigned as follows; C1, C2, D8, D7, D6 and D5. see the Figure below.
• The top level of segment D5 is -2.75m DMD and the top level of segment C2 is -7.41m DMD.

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Stratum
Top Level
(m DMD)
Bulk Weight
(kN/m3)
Angle of Friction,
φ’ (°)
Cohesion c’
(kPa)
UCS
(MPa)
E50
ref Stiffness*
(MPa)
Backfill material - 18 32 - - 30 (90)*
Fill Sand Layer
(Medium Dense Sand)
+4.0 18 33 - - 30 (90)*
Silty Sand
(Loose Dense Sand)
-0.5 17 30 - - 12 (36)*
Silty Sand
(Medium Dense Sand)
-7.0 18 33 - - 30 (90)*
Silty Sand
(Dense Sand)
-12.0 19 36 - - 70 (210)*
Extremely Weak Calcarenite -14.0 20 31 48 1.0 100 (300)*
Very Weak Calcarenite -20.24 20 34 112 2.0 200 (600)*
*the values between () are for the unload-reload Stiffness to input in Plaxis (Eur = 3xE50).
2. Design Ground Model

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Design Aspects:
• Lowering of groundwater table during excavation works;
• Heave phenomena due to excavation;
• Lateral deflection of adjacent diaphragm walls;
• Settlement due to load application of the superstructures.
Impact on the tunnel:
• Settlement or heave in tunnel;
• Lateral displacement of tunnel;
• Torsion of the tunnel;
• Changes in structural forces in tunnel slabs and walls (axial force, bending moment and shear force).
3. Soil – Structure Interaction Aspects

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Design Performance Criteria
The following design performance criteria have been adopted.
• Tolerable settlement in tunnel in the order of 5.0mm;
• The assessment of the tunnel structural capacity to sustain the additional structural forces, induced due to the proposed
engineering works for Plots 14a & 16, is beyond the scope of works. The current structural condition of the tunnel needs to be
evaluated in order to assess its safety when subjected to the estimated additional stresses resulted from this study.
Design Methodology
• The Design will be based on Eurocode 7, Design Approached 1 (Combinations 1 & 2) and design Approach 2.
• The Soil-Structure Interaction Analysis is carried out considering serviceability limit state (SLS) design approach. Therefore, the
characteristic values of actions, resistances and ground parameters are applied.
• The resulting structural forces in the tunnel will need to be factored in order to obtain the ultimate limit state values.

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The 3D Finite Element Analyses involved the following two parts:
• Part 1: Parametric Study, and
• Part 2: Detailed Analysis by simulating the actual construction sequence.
Regarding groundwater table lowering, two different approaches have been adopted:
• Phreatic condition – groundwater table is lowered by the user to a specified level regardless of soil permeability. This approach
is used for the parametric study analyses;
• Steady state flow – more realistic approach, based on lowering the groundwater level within the excavated area only and then
the software assess the groundwater profile in the surrounding area, which mainly depends on the permeability coefficient of
the ground.

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Parametric study on groundwater table lowering, lateral deflection of D-wall and settlements of Plots 14a & 16
• Groundwater Table (phreatic approach): The parametric study is based on lowering the groundwater table at the location
of the tunnel. The groundwater table is lowered to 3 different levels: 0.5, -0.5 and -1.5m DMD, which is 1.0 m, 2.0m & 3.0 m
below the design groundwater level of +1.5m DMD.
• Lateral deflection of the D-walls: Three D-wall lateral deflection scenarios have been considered for the parametric study.
Prescribed lateral displacement of 10mm, 20mm and 30mm have been considered for the analysis.
• Settlement of plots 14a and 16: The parametric study comprises also the impact of settlement of plots 14a and 16 on the
tunnel. A prescribed settlement of 20mm and 40mm have been induced in order to evaluate the impact of such settlement
on Al Shindagha tunnel.
The Figure below shows the locations of the geometry lines used to simulate the prescribed lateral deflection of the shoring walls &
prescribed settlement for Plot 14a & 16. The Prescribed settlement line is taken at 2/3 of pile’s length based on equivalent raft
foundation approach.
4a. 2D Finite Element Modelling – Part 1 Parametric Study
Prescribed Deflection in D-wall Plot 16 Prescribed Deflection in D-wall Plot 14a
Prescribed Settlement in Plot 16 at 2/3 length of piles Prescribed Settlement in Plot 14a at 2/3 length of piles

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30 mm prescribed Deflection
in D-wall Plot 14a
40 mm prescribed
Settlement in Plot 16
Ground water
at -1.5 m DMD
30 mm prescribed
Deflection in D-wall Plot 16
40 mm prescribed
Settlement in Plot 14a
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The Figure below shows clearly that the following aspects:
• lowering groundwater table 3m below the design level,
• Lateral deflection of shoring system for both plots of 30mm,
• Settlement of both plots of 40mm.
Will cause a settlement of less than 4mm to the tunnel. It should be noted that the majority of the settlement is due to
groundwater lowering, which is 2.4mm.

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Summary of the Parametric Study - Plot 14a
1. Groundwater Lowering: Results indicated that
groundwater lowering of 1.0m, 2m and 3m below design
groundwater level at the location of the tunnel will result in
0.8mm, 1.6mm and 2.4mm, respectively.
2. Lateral Deflection of Shoring: The Figure clearly indicates
that lateral deflection of shoring system of 10mm, 20mm
and 30mm have negligible impact on the tunnel.
3. Settlement: It should be noted that applying a prescribed
settlement of 20mm and 40mm has also negligible impact
on the tunnel.

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Summary of the Parametric Study - Plot 16
Following the parametric study for Plot 14a (3.0 m groundwater
lowering), analysis has been carried out considering prescribed
lateral deflection of 30 mm & a settlement of 40mm for Plot 16.
The results indicated that the tunnel will undergo a settlement of
approximately 4.0 mm (1.6 mm more than Plot 14a). It should be
noted that the 1.6mm increase is due to mainly the 40 mm
prescribed settlement. The prescribed 30 mm lateral deflection
has negligible impact.

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19.5 kPa Traffic load
Over the Tunnel
200 kPa Plot 16
Building Surcharge
400 kPa Plot 14a
Building Surcharge
19.5 kPa Traffic load
on the Tunnel Slab
Fill Material
Backfill Material After
Tunnel Construction
Loose Sand
Medium Dense Sand
Dense Sand
Very Weak Calcarenite
Extremely Weak Calcarenite
20 kPa Plot 16
Surface Surcharge
20 kPa Plot 14a
Surface Surcharge
4b. 3D Finite Element Modelling – Part 2 Detailed Analysis

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3D FEM Mesh Detail

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Construction Sequence and Initial Stress Condition of the Tunnel
Tunnel Construction sequence has been considered in the 3D FE Model prior to construction of Plots 14a & 16 in
order to generate initial set of forces in the tunnel segments.
Below is the sequence of construction adopted in the analysis.
• Tunnel Construction
• Plot 14a Construction
• Plot 16 Construction

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RESULTS / Selection of the Cross-Sections at the Tunnel
The Figure indicates the
cross-sections taken at the
location of max deflection in
Plot 14a &16 towards the
tunnel boundary. The distance
between Section-AB &
Section-CD is 32.0 m.
Section-CD at the location of
Maximum Deflection in Plot 16
Section-AB at the location of
Maximum Deflection in Plot 14a

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Tunnel Settlement During Plot 14a Construction
Note: These results are for 3D Steady State, with groundwater table lowering to -4.85m DMD inside excavation

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Lateral Displacement During Plot 14a Construction
Section-AB
Section-CD

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Tunnel Settlement During Plot 16 Construction
Section-AB
Section-CD
Point C
Point A

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Lateral Displacement During Plot 16 Construction
Section-AB
Section-CD

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The results of the Finite Element Analysis indicated the following:
• Lowering of groundwater table - Parametric study results indicated that groundwater lowering of 1.0m, 2m and 3m below design groundwater level at the location of the tunnel will resultin 0.8mm,
1.6mm and 2.4mm, respectively.
Detailed Analysis indicated that lowering of groundwater inside plot 14a will cause the groundwater table at the location of the tunnel to be lowered to approximately 0.7m. This is based on the
adopted value of coefficient of permeability for the ground. The 0.7m groundwater lowering resulted in a tunnel settlement of 0.6mm.
Lowering of groundwater inside Plot 16 will cause the groundwater table at the location of the tunnel to be lowered to approximately 1.0m. This is based on the adopted value of coefficient of
permeability for the ground. The 1.0m groundwater lowering resulted in a tunnel settlement of 0.9mm.
• Lateral deflection of D-wall - Results indicated that lateral deflection of shoring system have negligible impact on the tunnel.
• Settlement of Building Foundation - Parametric study results indicated that settlement of plot 14a has negligible impact on the tunnel while settlement of Plot 16 will impose an additional settlement
in the tunnel of approximately 1.6mm due to its vicinity to the tunnel resulting in a total settlement of 4.0mm. However, the detailed analysis results indicated that both plots settlements will have
negligible impact on the tunnel.
Conclusions
The finite element analysis has proven to be a powerful tool to reasonably evaluate the mechanism of deformation and the induced stresses on the existing
tunnel due to construction of two adjacent buildings in the vicinity of the tunnel. The model considered not only the construction sequence of the new plots
but also the construction sequence of the tunnel in order to assess the initial stresses on tunnel prior to commence the construction of the new plots. The
additional stresses on the tunnel can then be readily be evaluated.

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• Torsion - The Results clearly indicated that the torsion in the tunnel is negligible i.e. in order of less than 1.0 mm settlement in a cross section over a distance of 32.0m (Distance between Section-AB &
Section-CD).
• Structural forces on the Tunnel - The additional bending moment will be less than 5%. The additional axial and shear forces are anticipated to be in the order of 6% & 7% respectively.
The assessment of the tunnel structural capacity is beyond the scope of this study. The provided structural forces are unfactored and need to be assessed by others to ensure that the current tunnel
structural condition can safely sustain the induced additional forces.
• Monitoring Plan - Strict Monitoring System needs to be provided by the Contractor for the tunnel, shoring structures and ground surface settlement behind the retaining walls.
• Risk Assessment - Risk Assessment needs to be provided by the Contractor including the mitigation measures / plan in case settlement of the tunnel, lateral deflection of the D-wall and / or
groundwater table lowering exceed the tolerable limits.
Conclusions

City Tower 1
Redevelopment
Piled-RaftFoundation Design City Tower 1 Redevelopment
The proposedredevelopment of City Tower 1 will involve the demolitions of an existing 14-storey building
(G+14F+R) and the adjacentexisting surface parking, which will be replaced by an eighty-four (84 No.)
storey high-rise building and an eight level car-park building.

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City Tower 1 Redevelopment
45
3D finite element analysis has been carried out to evaluate the mechanism of deformation and stress distribution of the piled-raft foundation for the proposed 84-storey high-rise building. Based on
load combinations used for the analysis, the upper and lower bound values for the spring’s constants of the pile as well as the settlements and differential settlements of the raft have been
evaluated.
Surcharge
(overburden)
Applied Structural
Loads
3.75m thick raft

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The process of the piled-raft foundation design is a complex soil-structure interaction scheme which entails a close coordination between the geotechnical and structural designers in order to come
out with the optimum designin terms of safety and reliability.
Design Philosophy
1. Conventional Approach, Load of superstructure is mainly carried out by piles.
2. Creep Piling, Piles are used as a settlement reducer, and
3. Differential settlement control., piles are located strategically in order to the differential settlement.
DesignAspects
The following aspects have been thoroughly investigated:
1. Piles layout plan / pile spacing,
2. Piles’ diameters and lengths, and
3. Raft’s thickness.
The aim of the design process is to optimize the positions (spacing) and the geometry (diameters and lengths) of the piles in order to achieve a more uniform settlement and thus reducing sectional
forces in the raft and leading to a minimum differential settlement. Using piled-raft with different pile diameters and/or different piles lengths, with unequal applied loads, could have better
operation than piled-raft system with similar piles in terms of reducing differential settlement.

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Design Concepts
Piled-raft foundations have a complex soil-structure interaction mechanism including the pile-rock interaction, raft-soil/rock interaction, and finally the pile-
1. Piles are derived its bearing capacity mainly from the skin friction.
2. Piled diameters of 1800mm and 1500mm are used for the design.
3. To fully mobilise skin friction, a relative pile-soil displacement of approximately 1% of the pile diameter needs to take place. For piles of 1800mm
mobilise the skin friction.
4. The settlement mentioned in point 3 above will contribute in mobilising part of end bearing. A settlement of approximately 10% of the pile diameter
bearing. Therefore, the 18mm settlement mentioned in point 3 above will mobilise 10% of the end bearing.
5. The acceptable settlement of piled-raft foundation is, in general, in the order of 50mm. Therefore, approximately 30% of the end bearing pile capacity is
Based on above, the bearing capacity of piles has been based on fully mobilising the skin friction {fs (kPa) = 0.35x (UCS)0.5} and utilizing only 20% of the end
end bearing has been considered as a reserve capacity.

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Design Objectives
The main objectives from the design analysis are:
• Assessment of raft total and differential settlement,
• Assessment of load distributions on piles. Axial load on each pile will be provided together with the settlement of that piles.
• Assessing the upper and lower bound values of the piles spring constants, based on the load combinations provided by structure team.
• Assessment of bending moment and shear forces on piles.
Design Performance Criteria
The following serviceability criteria are adopted for tolerable settlement and angular distortion in the raft:
1. Tolerable maximum settlement ≤ 50mm;
2. Tolerable angular distortion < 1/750.

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Design Ground Model and Geotechnical Parameters
Ground Conditions
The geological stratification comprised a top layer of dense FILL material
thickness of 3.0 m, overlying about 5.0 m thick layer of loose to medium
followed by medium dense to very dense silty SAND with an approximate
which is underlain by 3.0 m thick loose to medium dense silty SAND.
The rock formation consisted of very weak to weak, reddish brown, slightly
SANDSTONE, overlying weak dark brown to off-white slightly weathered
CALCISILTITE/CONGLOMERATE which in turn is underlain by a weak
SILTSTONE/MUDSTONE

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Design Ground Model and Geotechnical Parameters
StratumNumber Material
ApproximateTop
Level(m DMD)
Approximate
Thickness (m)
1 FILL Material +3.50 3.50
2 Loose to Medium Dense Silty SAND +0.00 5.00
3 Dense to Very Dense Silty SAND -5.00 5.00
4 Loose to Medium Dense Silty SAND -9.00 3.50
5
Very weak to weak, light brown /
medium grained SANDSTONE
-12.50 19.50
6.1
Weak, brown, calcareous
CONGLOMERATE
-32.00 13.00
6.2
Weak, brown, calcareous
CONGLOMERATE
-45.00 30.00
7
Weak, off-white to pale whitish grey
MUDSTONE
-75.00 >10.00
Stratum
Number
Bulk
Weight
(kN/m3)
SPT-N φ (˚) C (kPa)
UCS
(MPa)
Secant
ModulusE50
(MPa)
Unload-Reload
Modulus(MPa)
1 18.0 30 33 0 - 20 60
2 17.0 17 32 0 - 17 51
3 18.0 30 36 0 - 45 135
4 17.0 17 32 0 - 17 51
5 20.0 - 33 65 1.2 180 540
6.1 20.0 - 31 98 1.0 150 450
6.2 20.0 - 33 155 2.0 300 900
7 20.0 - 45 80 7.0 1000 3000

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Pile Layout Plan
The proposed piled-raft solution consists on a 3.75m thick raft and a total of 149 No. piles. The piles are 1.8m (48 No.) and 1.5m (101 No.) diameter and
5.4m DMD. The dimensions of the raft are of approximately 60m x 50m. The larger pile diameters (1.8m diameter) at the center of the raft have been
where higher vertical stresses are applied and thus to minimize the differential settlement and the associate induced stresses in the raft.
The foundation design has been based on
adopting larger pile diameters (1.8m diameter) at
the centre of the raft in order to increase the
stiffness where higher vertical stresses are applied
and thus to minimise the differential settlement
and the associate induced stresses in the raft.
Piles length of 60m have been used under the
central zone where high stresses are applied. The
piles diameter of these piles is 1.8m. Using the
same piles diameter, the piles lengths has been
reduced to 55m. Similar approach has been
adopted for the 1.5m piles diameter. Piles lengths
of 50m, 55m and 60m have been adopted (as
shown in the Figure).

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Pile Layout Plan – Detailed Design
• Piles lengths,
• Piles diameters,
• Minimizing differential settlement in the Raft,
reducing the slab thickness and reinforcement.

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Construction Sequence
i. Initial Phase;
ii. Simulation of the ground overburden pressure
(as a uniformly distributed surcharge);
iii. Excavation to -5.3m DMD (removal of
surcharge in raft footprint);
iv. Pile construction;
v. Raft construction;
vi. Loading of piled-raft foundation according to
the desired load combination.
1.5m diameter pile
1.8m diameter pile
3.75m thick raft
Applied Structural Loads

© Arcadis 2017
54
3D Plaxis Finite Element Model
• The analysis has been carried out considering serviceability limit state (SLS) design approach. This approach is based on applying the characteristic
as for the geotechnical parameters. Therefore, the resulted bending moment and shear forces on piles need to be factored to provide the ultimate
• Ground model adopted for the 3D Finite Element Analysis is based on using Hardening-Soil Model with small-strain stiffness. This is an advanced
behaviour of soil and for unloading-reloading stiffness as well as for considering the soil stiffness as a function of the strain amplitude, which is
assessment.
• The Hardening-Soil constitutive model with small-strain stiffness has been adopted. The Hardening Soil model with small-strain stiffness is based
entirely the same parameters Eref
50 (Elastic Secant Modulus due to primary deviatoric loading), Eref
ur (Elastic Modulus for unloading / reloading), m-
according to a power law) and the Shear Strength parameters. Only two additional parameters are needed to describe the variation of stiffness with
Go and the Shear Strain Level γ0.7 at which the Secant Shear Modulus Gs is reduced to about 70% of Go.

© Arcadis 2017
55
3D Plaxis Finite Element Results
Load Combination
Maximum
Settlement (mm)
Angular distortion
1 46 1:1250
2 25 1:1550
3 70 1:1050
4 65 1:850
5 25 1:1350
The maximum settlement in the raft is estimated to be in
between 25 and 46mm (Excluding load Combinations 3 & 4
– Extreme/Seismic Case).
It is worth mentioning that the settlement for all load
combinations have been assessed. The resulted settlement for
Load combinations 3 & 4 are 70mm and 65mm, respectively.
This is considered acceptable by the structure team.
Load Combination 1Load Combination 2Load Combination 3Load Combination 4Load Combination 5
Settlement

© Arcadis 2017
56
The maximum estimated axial forces on piles and the corresponding
safety factor (with regards to geotechnical single pile capacity).
SLS Load
Combination
1.8m DiameterPiles 1.5m Diameter Piles
Max. Axial Load (kN) SF Max. Axial Load (kN) SF
1 42662 2.6 23341 3.4
2 33208 3.7 18126 5.7
3 51795 2.4 27930 3.7
4 47218 2.6 27324 3.8
5 33529 3.3 19625 4.1
Piled-RaftFoundation DesignResults
Pile Axial Loads
0% 1%
15%
84%
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
<2.0 2.0-2.5 2.5-3.0 >3.0
%Piles
Safety Factor
Histogram

© Arcadis 2017
58
Piled-Raft Foundation Design Results
Pile Vertical Spring Constants
The pile vertical spring constants have been
the axial loads and settlements in each pile.
The overall ranges for pile vertical spring
follows:
 1.8m diameter piles: 500 to 1350 MN/m
 1.5m diameter piles: 350 to 950 MN/m

© Arcadis 2017
Conclusions
59
The results of the Finite Element Analysis indicated the following:
• The foundation design has been based on adopting larger pile diameters (1.8m diameter) at the centre of the raft in order to increase the stiffness where higher vertical stresses are applied
and thus to minimise the differential settlement and the associate induced stresses in the raft.
• Piles length of 60m have been used under the central zone where high stresses are applied. The pile diameter of these piles is 1.8m. The piles lengths have then been reduced to 55m to support
the zonesof lessapplied stresses. Similarly, depending on zonesstresses, for 1.5m piles diameter, piles lengths of 60, 55 and 50m have been adopted.
The Finite Element Analysis has enabled to come out with the optimum design for the piled-raft foundation in term of:
1. Identifying the most effective piles layout plan,
2. Providing the most effective piles lengths and piles diameters underneath the raft foundation,
3. Providing the minimum thickness of the raft,
4. The model allows obtaining a better insight for the interaction between the raft-piles-ground,
5. The Finite Element Design Analysis has led to minimise the total and differential settlement of the raft and ensured
providing the most economic solution for the piled raft foundation,

Qatar Lusail Plaza
Towers
Impact of Plots Construction on Existing Adjacent Car Park & LRT Tunnel-
Qatar Lusail Towers
Lusail Towers are part of largest Lusail City Development which involves transforming and developing north of
Doha into a residential and commercial district. Lusail Plazatowers are divided into 4 plots. Each Plot contains a
tower, podium building & basement. The development comprisestwo seventy (70) stories towers and two fifty
(50) stories tower includingbasements and fourteen (14) low rise buildings in 4 plots.

© Arcadis 2017
Plot 12A Plot 12C
Plot 12DPlot 12B
Cark Park Cark Park
LRTTunnel
Qatar Lusail Plaza Towers
Qatar Lusail Towers
Plot 12A
50 Storey
Buildings
Plot 12B Plot 12C Plot 12D
70 Storey
Buildings
Car Park LRT Tunnel

© Arcadis 2017
Conclusions
Ground Investigation / Factual Data
Thorough site ground investigation needs to be conducted, in order to reasonably identify the ground stratigraphy and the factual data necessary
for determining the Geotechnical design Parameters.
Question: Do the current methods/procedures satisfy the requirements for providing “reliable “Factual Data”?
Geotechnical Interpretive Report
Applying the basics of soil mechanics, experience and empirical formulas, to determine the geotechnical design parameters from the factual data.
Question: Do the current approaches/correlations, for determining the Geotechnical design Parameters, satisfy the needs for providing
representative Geotechnical design parameters?
The use of most sophisticated 3D finite element software can note be justified without adopting reliable input data. There is a real need to develop
field and lab tests as well as new correlations/methods that will ensure obtaining reliable input geotechnical design parameters. Currently,
sensitivity analysis, that should cover any potential variation of geotechnical design parameters, is considered essential.
Methods of Solution
Solid theoretical background in Soil/Rock mechanics, experience and engineering judgement in addition to understanding the basic concepts of
the numerical methods are considered essential for providing reliable Finite Element Design Analysis for complex geotechnical problems.
Sanity check, which based on simplified assumptions, adopting analytical analysis, experience and engineering judgement is an essential exercise
for the feasibility analysis of complex projects and for providing an initial indication for the order of magnitude of variable field(s) under
consideration.

© Arcadis 2017
Conclusions
Numerical Methods / Finite Element Method
Numerical Methods, in particular, Finite Element Method seem to be the most promising technique, for the near future, considering its potential power in
dealing with rather complex geotechnical (soil-structure interaction) problems.
Digital Computers: “High” speed digital computers are an essential requirement in order to enable geotechnical engineers to deal with even more complex
problems.

© Arcadis 2017
Conclusions
Ground Numerical Models
Question: Do the current “advanced” ground models, which are based on considering the ground as an Elastic Media” accurately simulate the
actual ground behaviour? Do Mohr-Coulomb, Hardening Soil and Small Strain Models reasonably simulate the behavior of natural soil/rock?
Codes and the Standards for the design Analysis
Fully understanding of the Codes and Standards. Recently, Limit State Design and Eurocode 7 becomes as one of the main requirement.
Geotechnical engineers, therefore, need to realise the main different between the proposed partial factors and the ‘Global” safety factor commonly
used in the conventional methods.
The main philosophy behind the Factor of Safety for design analysis is UNCERTANITY. The more certain geotechnical engineers are, the lower the
factor of safety can be justified.
Risk Assessment, Contingency plan and Monitoring system
Geotechnical Design Engineers are obliged to provide the risk associated with the provided design. Risk Register is an essential design document,
which should include the mitigation measures should the behaviour of the ground approaches the Alert and/or Alarm levels.

© Arcadis 2017
THANK YOU
81
Today’s “State-of-the-Art
is
Tomorrow’s “Out-of-the-Ark”!
QUESTIONS?
Dr Mazin Alhamrany: mazin.alhamrany@arcadis.com
?

Applications of FEM in Geotechnical Engineering / State-of-the-Art

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