151111 exploring the challenges of underground construction in urban areas_benoit latapie
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Exploring the challenges of underground construction in urban areasUnderground Infrastructure and Deep Foundations UAE 16 & 17 November 2015
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Benoît LatapieTechnical Manager, Ground EngineeringWS Atkins & Partners Overseas
Underground construction in urban areas
1. A good quality ground investigation is essential2. The context of urban areas3. A concise approach to underground design4. Numerical modelling has come a long way5. Recent project example6. Managing geotechnical risk during the project
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Table of contents
1. A good quality ground investigation is essential
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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)
1. A good quality ground investigation is essential
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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 movementsEffects on existing assets misunderstood
Rock modelled as rock
Realistic prediction of ground movementsImpact on existing assets well estimated
Low strength/stiffness High strength/stiffness
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
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2. The context of urban areas in Dubai
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Dubai construction 10 years ago Dubai construction today
2. The context of urban areas in Dubai
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3. A concise approach to underground design
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1. Knowing the location, size and sensitivityof existing assets to ground movementsis key
2. Good understanding of the ground /High quality ground investigation
3. Understanding the interactionmechanisms between existing structures,proposed works and the ground
4. Measuring the impact of the constructionsequence on surrounding assets
Excavation near existing buildings, structures and services means:Topographic surveyAs-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
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 wallDiaphragm wall
Pre-stressed struts/anchors
Inclinometers, strain gauges, observation wells, piezometers,
ground settlement points,building monitoring points,
crackmeters, extensometers
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
4. Numerical modelling has come a long way
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PMTPressuremeter test
DMTDilatometer test
CPTUPiezocone penetration test
SCPTSeismic cone penetration test
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
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A better definition of ground stiffness
Ei
E50
Eur
E
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:
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
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
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
CalcisiltiteLayer
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
5. Recent project example
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FE model in plan:
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.
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
5. Recent project example
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Outputs:Ground settlement contours, sensitive to:
• Wall type andstiffness
• Support stiffnessand applied strutspreload
Maximum settlement 33mm
5. Recent project example
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Outputs:Position of output sections:
• A-A through pier foundation
• B-B through abutmentfoundation
• C-C through wall and quaywall
• D-D through wall, nearexisting road
• E-E through wall, nearbridge abutment
5. Recent project example
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Outputs:
Total ground movement contours at final excavationpier (cross section A, left) and abutment (cross section B, right)
5. Recent project example
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Outputs:
Total ground movement contours at final excavationcross sections C (left), D (middle) and E (right)
5. Recent project example
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Outputs:
5. Recent project example
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What we achieved:
1. Demonstrate that the construction of multiple basement levels is possible in aconstrained environment
2. Temporary works can be designed to have a negligible impact on the bridge and itssupport
3. Evaluate the impact of the proposed works on the adjacent asset with a singleanalysis
4. Instrumentation and monitoring plan tailored to the situation and using the output ofthe analysis to establish AAA values
6. Managing geotechnical risk during the project
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Design1. Employ a competent
ground engineeringspecialist
2. Design the right siteinvestigation
3. Supervise the siteinvestigation
4. Select the right supportstiffness for the localsituation/conditions
5. Do the right analyses
Build1. Employ a competent and
experienced temporaryworks contractor
2. Ensure that designer andcontractor work together
3. Get the designerinvolved in sitesupervision and help thecontractor resolvepotential site issues
Monitor1. Install adequate I&M
apparatus
2. Implement regularmonitoring
3. Keep the designerinvolved in reviewing thedata regularly
4. Back analysis of the insitu condition, as required
+ +
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Thank you
Underground Infrastructure and Deep Foundations UAE 16 & 17 November 2015
Benoît Latapie
Technical Manager, Ground Engineering
WS Atkins & Partners Overseas
[email protected] +971 (0) 553003797