Integrated Solver Optimized for the next generation 64-bit platform
Finite Element Solutions for Geotechnical Engineering
Session 2.
Deep Excavations and Dewatering in Urban Environment
MIDAS Geotechnical Know-how Sharing Series
JaeSeok Yang Principal Geotechnical Engineer, MIDAS IT
Integrated Solver Optimized for the next generation 64-bit platform
Finite Element Solutions for Geotechnical Engineering
01 Modelling of Excavations
02 Prediction of Ground Movements
03 3D Excavation Modelling
04 Case Study
GTS NX
3
Surcharge Loading
Modelling of deep excavation
ODEON excavation
villas
high school
Ténao street
Point du jour
GTS NX
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Interface Behaviour
• Soil‐structure interaction
Wall friction
Slip and gapping between soil and structure
• Soil material properties
Taken from soil using reduction factor R
Individual material set for interface possible
Integrated Solver Optimized for the next generation 64-bit platform
Finite Element Solutions for Geotechnical Engineering
01 Modelling of Excavations
02 Prediction of Ground Movements
03 3D Excavation Modelling
04 Case Study
GTS NX
8
Material Behaviour in Excavation
• Unloading due to excavation
Vertical unloading at excavation bottom
Horizontal unloading behind wall (may accompanied by shear plasticity)
• Primary loading due to pre‐stressing
• HS-small model is preferred
Non‐linear elastic unloading/reloading behaviour
Shear plasticity due to horizontal unloading
High far‐field stiffness for better settlement trough prediction
GTS NX
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Soil Non-linearity
Characteristic stiffness-strain behavior of soil with the ranges for typical geotechnical structures and different tests
GTS NX
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Actual Resulting Strains
Strain contours around an excavation (after Simpson et al., 1979)
GTS NX
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Determination of Ground Stiffness
Idealized stress paths associated with stress relief due to excavation (after Ng, 1999) (a) Effective stress paths; (b) total stress paths
GTS NX
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Constitutive Models
• Mohr-Coulomb: unrealistic deformations
Use of single E fails to cater for the complex material at various zones
Overestimation over bottom heave
Sometimes heave of soil behind the wall
Soils below excavation behaves with Eur, even soils behind wall behaves between Eur
and E50. Use of E50 is too conservative.
• Hardening Soil model: qualitative realistic deformations
Soil stiffness for Isotropic loading, shearing and unloading-reloading can be catered for
automatically in the model.
More realistic bottom heave
Improved settlement trough behind wall
• HS-small model: qualitative and quantitative realistic deformations
Improved over HS to take care of far field small strain behaviour
More realistic settlement trough behind the wall (narrower and deeper)
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Hardening Soil model
• Possibilities and advantages compared to Mohr-Coulomb
Better non-linear formulation of soil behaviour in general (both soft soils and
harder types of soil)
Distinction between primary loading and unloading/reloading
Memory of pre-consolidation stress
Different stiffnesses for different stress paths based on standard tests
Well suited for unloading situations with simultaneous deviatoric loading
(excavations)
Large stiffness at small stain levels (vibrations) – HSsmall only
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Yield Surface
Excavation (passive side) and construction stress paths in relation to the type of yield surface (after Gens, 1995)
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Dewatering – Undrained Excavation
• For every excavation stage
Excavate soil
Set excavated area dry
Phreatic level outside the excavation remains unchanged
→ Suitable for short‐term excavations in low permeability soils
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Dewatering – Drained Excavation
• For every excavation stage
Excavate soil
Define boundary conditions (heads)
Perform seepage analysis
Phreatic level outside the excavation lowers
→ Suitable for long‐term excavations in high permeability soils
GTS NX
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Non-hydrostatic Water Pressure
Discontinuity in pore water pressure Continuity in pore water pressure using User Defined pressure
Discontinuity
Non-hydrostatic gradient of u (interpolate between layers)
Integrated Solver Optimized for the next generation 64-bit platform
Finite Element Solutions for Geotechnical Engineering
01 Modelling of Excavations
02 Prediction of Ground Movements
03 3D Excavation Modelling
04 Case Study
GTS NX
20
3D FEM numerical modeling
• Advantages of 3D FEM numerical modeling over 2D
Although there are many geotechnical problems that can be approximated to
either plane strain or axi-symmetric conditions, some remain which are very
three dimensional. Such problems will therefore require full three dimensional
numerical analysis.
In reality, most geotechnical problems are three dimensional, and, although
in many, plane strain or axi-symmetric approximations are not unreasonable,
there are some which must be treated as three dimensional.
GTS NX
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Conclusion on 3D modelling
• In comparison with 2D numerical modelling, the most influential factors are the
geometry and mesh set-up.
• The engineering theories applied are the same in 2D and 3D modelling.
• The differences in results are most likely to be caused by the geometry of the
investigating domain, namely the corner effect in 3D modelling. (The rotation and
deformation of corners could not be modelled in 2D models.)
• The advantage of applying 3D FEM modelling would be more significant when the
construction and excavation domain is more complex.
Integrated Solver Optimized for the next generation 64-bit platform
Finite Element Solutions for Geotechnical Engineering
01 Modelling of Excavations
02 Prediction of Ground Movements
03 3D Excavation Modelling
04 Case Study
GTS NX
25
Introduction – Theoretical Background
Complete Theoretical
Solution
Equilibrium
Material Constitutive Behaviour
Boundary Conditions
Compatibility
Conventional Methods
Closed Form
Simple
• Limit equilibrium
• Stress field
• Limit analysis
Numerical Methods
Beam-Spring
Full Numerical
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Case Study – Attenuation Tank Construction
Construction Sequence: • Construct Piles
• Install Steelworks.
• Excavate 4.0m
• Construct Slabs.
• Construct Barrier Wall
Imposed Loadings: • 10kN/m2 surcharge
• 300kN/pile at steel columns
• 30kNm/m moment at steel columns
• 65kN/m barrier line load
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Methodology – Ground Conditions
Material Depth (m) γ
(kN/m3)
γSat
(kN/m3)
E (MPa) φ' C’ K0 Ka Kp ν
Fill 3.5 18 20 16 30˚ 0 0.5 0.29 3.0 0.3
Sandy Gravel 1 19 21 32 35˚ 0 0.45 0.23 3.69 0.3
SAND 1.5 17 20 27 34˚ 0 0.45 0.24 5.5 0.3
SANDSTONE 25+ 23 23 52 38˚ 0 0.4 0.21 7.2 0.3
Concrete - 24 - 27,000 - - - - - 0.2
• Soil Profile Effect
• Sensitivity of E & φ’
• K0 = 1 – Sinø’
• Ka & Kp as per BS 8002
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Methodology – FEM, 2D & 3D Models
Material Properties Ground Piles Slabs Interface
Model Type Mohr – Coulomb Elastic Elastic ---
2D Elements 2D Plane-Strain 1D Beam 1D Beam Interface Elements + Rigid Link
3D Elements 3D Solid 1D Beam 2D Plane-Stress Pile Interface + Rigid Link
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Results – Wall Bending Moment
Limit Equilibrium
FEM
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
-100 0 100 200
Dep
th (
m,
BG
L)
Wall bending moment (kNm/m)
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
-50 0 50 100 150 200D
epth
(m
, B
GL
)
Wall bending moment (kNm/m)
FOS reduced from 3.19 to 2.76
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Results – Wall Deflection
Limit Equilibrium
FEM
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 5 10 15
Dep
th (
m,
BG
L)
Wall deflection (mm)
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 5 10 15
Dep
th (
m,
BG
L)
Wall deflection (mm)
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Results – Sensitivity Study
Soil
Original Scenario Stiffness variation by
±5%
Friction angle variation
by ±5%
Case 1 Case 2 Case 3 Case 4 Case 5
E (kN/m2) φ' E (kN/m
2) E (kN/m
2) φ' φ'
Fill 16000 30 15200 16800 28 32
Sandy
Gravel 32000 35 30400 33600 33 37
Sand 27000 34 25650 28350 32 36
Sandstone 52000 38 49400 54600 36 40
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Results – Sensitivity Study (Limit Equilibrium)
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 5 10 15 20D
epth
(m
, B
GL
)
Wall deflection (mm)
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
-20 80 180 280D
epth
(m
, B
GL
)
Wall bending moment (kNm/m)
GTS NX
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Results – Sensitivity Study (3D FEM)
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 5 10 15D
epth
(m
, B
GL
)
Wall deflection (mm)
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 50 100 150D
epth
(m
, B
GL
) Wall bending moment (kNm/m)
GTS NX
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Results – Method Comparison
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 5 10 15
Dep
th (
m,
BG
L)
Wall deflection (mm)
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
-20 30 80 130 180
Dep
th (
m,
BG
L)
Wall bending moment (kNm/m)
TEMPORARY CONDITIONS (Cantilevered Wall)
PERMANENT CONDITIONS (Slabs act as supports)
Maximum Slab Loads (SLS)
•LE Model 106 kN/m •2D FEA Model 79.2kN/m •3D FEA Model 81.6kN/m
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Results – Modelling Time
• Ability to operate the programs
• Learning and layout of MIDAS GTS NX
• Time of 2D FEM vs 3D FEM
• 3D modelling can be challenging until the program functions are learned in depth.
• Modelling time depends on the complexity of the problem
• 3D modelling and analysis approximately 6 times longer than 2D modelling
• Learning process of LEM
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Awareness
Disaster Complexity
Incompetent
User
Averaging
material
properties Soil Profile
Uncertainties
Inadequate
validation
FEM PROVIDES A SOLUTION TO MOST PROBLEMS!!!