1. Exploring the challenges of underground
construction in urban areas
Underground Infrastructure and Deep Foundations
UAE 16 & 17 November 2015
16 November 2015 1
Benoît Latapie
Technical Manager, Ground Engineering
WS Atkins & Partners Overseas

2. Underground construction in urban areas
1. A good quality ground investigation is essential
2. The context of urban areas
3. A concise approach to underground design
4. Numerical modelling has come a long way
5. Recent project example
6. Managing geotechnical risk during the project
16 November 2015 2
Table of contents

3. 1. A good quality ground investigation is essential
16 November 2015 3
Weak to medium strong, off-white to pale
yellow Limestone with inclusions of silt/clay
Non intact core recovered as medium to
coarse gravel of weak dolomitic limestone
Conventional single tube core barrel Rotary coring with double tube core barrel
Two sites a few kilometres away
from each other, at similar depth
(with plastic lining)

4. 1. A good quality ground investigation is essential
16 November 2015 4
Weak to medium strong, off-white to pale
yellow Limestone with inclusions of silt/clay
Non intact core recovered as medium to
coarse gravel of weak dolomitic limestone
Conventional single tube core barrel Rotary coring with double tube core barrel
(with plastic lining)
Rock modelled as soil
Erroneous prediction of ground movements
Effects on existing assets misunderstood
Rock modelled as rock
Realistic prediction of ground movements
Impact on existing assets well estimated
Low strength/stiffness High strength/stiffness

5. 2. The context of urban areas in Dubai
• Prime locations/sites are mostly developed
• Remaining sites are constrained:
– Existing buildings
– Existing infrastructures (roads, services, quay walls…)
– Irregular plot shapes
– Building height race: deep basements and deep foundations
• Additional constraints often means that additional authorities/parties
approval are required
16 November 2015 5

6. 2. The context of urban areas in Dubai
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Dubai construction 10 years ago Dubai construction today

7. 2. The context of urban areas in Dubai
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8. 3. A concise approach to underground design
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1. Knowing the location, size and sensitivity
of existing assets to ground movements
is key
2. Good understanding of the ground /
High quality ground investigation
3. Understanding the interaction
mechanisms between existing structures,
proposed works and the ground
4. Measuring the impact of the construction
sequence on surrounding assets
Excavation near existing buildings, structures and services means:
Topographic survey
As-built drawings
Pre-construction condition surveys
Experienced SI contractor
+
Design and supervision by
specialist consultant
Finite element modelling including:
• Soil/rock behaviour
• Existing structures
• Temporary supports
• Dewatering
• Construction sequence

9. 3. A concise approach to underground design
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1. Design the appropriate support stiffness
for temporary works
2. Instrumentation and monitoring of
temporary works
3. Instrumentation and monitoring of
existing assets
Excavation near existing buildings, structures and services means:
Contiguous bored pile wall
Diaphragm wall
Pre-stressed struts/anchors
Inclinometers, strain gauges,
observation wells, piezometers,
ground settlement points,
building monitoring points,
crackmeters, extensometers

10. 4. Numerical modelling has come a long way
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1. Commercial codes have become much quicker to run
2. 3D analyses no longer require top spec computers nor long calculation times and are
used more routinely on projects
3. Input procedures have become more user-friendly which enables very fast model
generation
4. It is possible to generate 3D ground models with several boreholes and import
information from drawing files
5. Improved constitutive models closer to predicting true soil/rock behaviour
6. Experience gained in confidently deriving more sophisticated parameters from ground
investigation results
7. 2D and 3D groundwater flow analyses are possible, in conjunction with the modelling
of a multi-stage construction sequence

11. 4. Numerical modelling has come a long way
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PMT
Pressuremeter test
DMT
Dilatometer test
CPTU
Piezocone penetration test
SCPT
Seismic cone penetration test

12. Mohr-Coulomb:
A single stiffness parameter, the
Young’s Modulus E = E50.
Hardening Soil:
Three different stiffnesses
E50: secant stiffness
Ei: initial stiffness
Ei ≈ 1.8 x E50
Eur: unloading/reloading stiffness
Eur ≈ 3.0 x E50
4. Numerical modelling has come a long way
16 November 2015 12
A better definition of ground stiffness
Ei
E50
Eur
E

13. 5. Recent project example
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1. Small, complex plot geometry
2. Plot constrained on all sides by existing structures and services:
– Existing road bridge on one side
– Existing quay wall on one side
– Existing roads and services on two sides
3. The temporary works proposal required approval from 3 different authorities
4. Shallow groundwater, near the sea
5. Stratigraphy comprising 13m of various cemented sand densities overlying rock
Project context:

14. 5. Recent project example
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Atkins approach:
1. Obtain as-built information for the existing bridge:
– Foundations’ positions and dimensions
– Loads and moments transferred through the bridge bearings
– Structural tolerance to movement
2. Obtain as-built information for roads and services and for the quay wall
3. Produce a 3D finite element model of the temporary works for the entire plot and
including the surrounding assets. This enables:
– Consideration of 3D effects and asymmetrical loading due to the plot geometry
and different surcharge intensities around the site

15. 5m
15m
5. Recent project example
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Site Plan:
Quay wall
canal
Bridge pier
Bridge abutment
Gravity
retaining walls
Edge of road
Temporary retaining wall
Temporary support
Plot boundary
Bridge
6m

16. 5. Recent project example
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Ground conditions: +4.5mDMD
-8.6mDMD
-21.0mDMD
13.10m
12.40m
Final Excavation level for 3
basements at -10.5mDMD
In situ groundwater
level at +1.3mDMD
Cemented
Sand
Calcarenite
Calcisiltite
Layer
γ UCS Erm ϕ’ c’
[kN/m3] [MPa] [MPa] [°] [kPa]
Sand 19 – 50 32 5
Calcarenite 21 1.0 200 40 125
Calcisiltite 21 3.4 750 30 300

17. 5. Recent project example
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FE model in plan:

18. 5. Recent project example
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FE model in 3D:
NB: the top soil layer was ‘turned off’ so that the bridge’s elements are visible.

19. 5. Recent project example
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Construction sequence – following the site history:
1. Construct the quay wall
2. Construct the bridge components, including the approach embankment
3. Apply bridge service loads to the pier, abutment and embankment
4. Build perimeter retaining wall
5. Excavate and apply supports in sequence until reaching formation level

20. 5. Recent project example
16 November 2015 20
Outputs:
Ground settlement
contours, sensitive to:
• Wall type and
stiffness
• Support stiffness
and applied struts
preload
Maximum settlement 33mm

21. 5. Recent project example
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Outputs:
Position of output sections:
• A-A through pier foundation
• B-B through abutment
foundation
• C-C through wall and quay
wall
• D-D through wall, near
existing road
• E-E through wall, near
bridge abutment

22. 5. Recent project example
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Outputs:
Total ground movement contours at final excavation
pier (cross section A, left) and abutment (cross section B, right)

23. 5. Recent project example
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Outputs:
Total ground movement contours at final excavation
cross sections C (left), D (middle) and E (right)

24. 5. Recent project example
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Outputs:

25. 5. Recent project example
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What we achieved:
1. Demonstrate that the construction of multiple basement levels is possible in a
constrained environment
2. Temporary works can be designed to have a negligible impact on the bridge and its
support
3. Evaluate the impact of the proposed works on the adjacent asset with a single
analysis
4. Instrumentation and monitoring plan tailored to the situation and using the output of
the analysis to establish AAA values

26. 6. Managing geotechnical risk during the project
16 November 2015 28
Design
1. Employ a competent
ground engineering
specialist
2. Design the right site
investigation
3. Supervise the site
investigation
4. Select the right support
stiffness for the local
situation/conditions
5. Do the right analyses
Build
1. Employ a competent and
experienced temporary
works contractor
2. Ensure that designer and
contractor work together
3. Get the designer
involved in site
supervision and help the
contractor resolve
potential site issues
Monitor
1. Install adequate I&M
apparatus
2. Implement regular
monitoring
3. Keep the designer
involved in reviewing the
data regularly
4. Back analysis of the in
situ condition, as required
+ +

27. 15 November 2015 29
Thank you
Underground Infrastructure and Deep Foundations
UAE 16 & 17 November 2015
Benoît Latapie
Technical Manager, Ground Engineering
WS Atkins & Partners Overseas
benoit.latapie@atkinsglobal.com +971 (0) 553003797