Session 2. Deep Excavations and Dewatering in Urban Environment - Midasuk.midasuser.com/web/upload/sample/Deep_Excavations_and... · 2018-05-30 · GTS NX 13 Hardening Soil model

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



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

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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

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Interface Behaviour

• Suggestions for R

• With reference to BS 8002

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Props and Anchor Modelling

Diaphragm wall

Barrettes

Anchors

Modelling of reinforcement






04 Case Study

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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

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Soil Non-linearity

Characteristic stiffness-strain behavior of soil with the ranges for typical geotechnical structures and different tests

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Actual Resulting Strains

Strain contours around an excavation (after Simpson et al., 1979)

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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

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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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Water Drawdown

Excavation dewatering

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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

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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)






04 Case Study

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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.

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Excavation pit

Excavation layout modeled by 3D finite element analysis

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Corner effect

Corner effect for 3D modelling

• Some factors influencing corner effects

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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.






04 Case Study

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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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Introduction

Limit Equilibrium Finite Element Mesh in 3D

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Case Study – Attenuation Tank Construction

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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 – Limit Equilibrium Method

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Methodology – Limit Equilibrium Method

Temporary Conditions Permanent Conditions

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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

)


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)


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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

)


-9

-8

-7

-6

-5

-4

-3

-2

-1

0

-20 80 180 280D

epth

(m

, B

GL

)


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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

)


-9

-8

-7

-6

-5

-4

-3

-2

-1

0

0 50 100 150D

epth

(m

, B

GL

) Wall bending moment (kNm/m)

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Results – Deformation Shapes

3D FEM 2D FEM

Roof slab in compression

Base Slab in tension

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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)


-9

-8

-7

-6

-5

-4

-3

-2

-1

0

-20 30 80 130 180

Dep

th (

m,

BG

L)


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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Conclusions

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Awareness

Disaster Complexity

Incompetent

User

Averaging

material

properties Soil Profile

Uncertainties

Inadequate

validation

FEM PROVIDES A SOLUTION TO MOST PROBLEMS!!!

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Q & A

Documents

Session 2. Deep Excavations and Dewatering in Urban Environment - Midasuk.midasuser.com/web/upload/sample/Deep_Excavations_and... · 2018-05-30 · GTS NX 13 Hardening Soil model