1. RECENT ADVANCES IN THE APPLICATION OF VERTICAL DRAINS AND VACUUM PRELOADING IN SOFT SOIL STABILISATION
Buddhima Indraratna
Professor of Civil, Mining & Environmental Engineering
Director, Centre for Geomechanics and Railway Engineering
Faculty of Engineering, University of Wollongong
Wollongong City, NSW 2522, Australia

2. Contents Introduction to PVDs and VP application
Role of Smear zone (disturbed soil zone around the mandrel), its assessment and implications
Effect of Vacuum Pressure propagation and variation with time (including vacuum removal &reapplication)
Experimental Investigations
Numerical Modelling and Case History Analysis
Advances in Design and Practice Guides

3. Inward movement due to VP
Potential Benefits of Prefabricated Vertical Drains in Soft Formation Clays
Surcharge Fill
Embankment
Settlement
Surcharge fill only – no vertical drains
Due to PVDs
Depth
Inward movement due to VP
Vertical drains with surcharge
Vertical drains with surcharge and
vacuum preloading
Time
Lateral displacement at toe

4. Drain anchors and Mandrel shapes
Installation of PVDs
Drain anchors and Mandrel shapes 4

5. Principles of Vacuum Consolidation Via PVDs
Membrane system (e.g. Menard)
Membrane less system (e.g. Beaudrain) 5

6. Soft Foundation Stabilisation by Vacuum Consolidation
Surcharge Fill Only
(Anisotropic Load)
Vacuum Preloading with PVD
(Isotropic Loading)
Vertical Stress
Vertical Stress
Slip Surface
No Failure
Risk of Slope failure is minimized by the use of Vacuum Preloading

7. Site preparation for Membrane-type Vacuum Consolidation (Courtesy of Austress-Menard)
Drain Installation
Horizontal drain installation
Peripheral bentonite trench
Connection between horizontal drainage and vacuum pump
Membrane installation

8. Site preparation for Vacuum Consolidation-Membraneless (Courtesy, CeTeau)
Drain Installation
Tube connection

9. Principle of Vacuum Consolidation
Governing Equation
Consolidation: (a) conventional surcharge loading;
(b) idealised vacuum preloading (Indraratna et al. 2005c).

10. Effect of Vacuum Removal and Reloading on Consolidation
After some initial consolidation, putting off the vacuum pump is not going to make the soil swell up again, but the rate of settlement is swiftly retarded. Pumps may have to be switched off from time to time to prevent over-heating.
Suction in the drain (240mm from bottom); b) surface settlement surface settlement
associated with simulated vacuum loading and removal (Indraratna et al. 2004).

11. Experimental Evaluation of PVD + VP system
Large-Scale, Radial Drainage Consolidometer at Uni. of Wollongong
Constant Strain Mandrel Driving
Installation of PVDs by the steel mandrel causes smear around the PVD
Hydraulic Loading
550mm Diameter
1.2m Height 11

12. Assessment of the Extent of Smear Zone
(Indraratna & Redana, 1998, Sathananthan & Indraratna 2006)
Drain
Locations of cored specimens
PVD and smear zone 12

13. Evaluation of Smear Effects
Permeability Approach
Indraratna & Redana, 1998, JGGE, ASCE
Vol. 124(2)
Water Content Approach
Sathananthan & Indraratna 2006, JGGE, ASCE, Vol. 132(7)

14. Vacuum Propagation Model based on Laboratory Data
Soil-drain interface
Soil element
Lateral Propagation of VP
ks < kh
If k1 =1, there is no vaccum loss with depth
Vacuum pressure distribution patterns in the vertical and lateral directions
(after Indraratna et al. 2005).

15. Cyclic Loading and Soil Consolidation via PVDs
Cyclic Loading Actuator
(a) Large-scale triaxial rig; (b) soil specimen (Indraratna et al. 2009a). 15

16. Cyclic Excess Pore Pressure Response of Soft Clay With and Without PVD (Indraratna et al., 2009)
Failure of samples T3 T6
Specimens without PVD fail very quickly as the excess pore pressure rises rapidly! 16

17. Excess Pore Pressure is rapidly created during mandrel intrusion
FEM Simulation of Mandrel-driven PVD – Pore Pressure CREATION due to very high plastic strains
Excess Pore Pressure is rapidly created during mandrel intrusion
Excess PWP dissipates very gradually after mandrel withdrawal in spite of the drain.
Mandrel Driving INCREASES effective vertical stress, hence, the lateral permeability decreases within the smear zone (Indraratna et al. 2009, ASCE J. of Geomechanics).

18. Pore pressure variation during mandrel installation
Locations of pore pressure transducers 18

19. Maintain geometric equivalence
Analytical and Numerical Simulation Multi-drain Analysis and Plane Strain Conversion
Field condition: Axisymmetric
2D plane strain FEM
Maintain geometric equivalence
Reduce the convergence time and require less computer memory
Must give the same consolidation response

20. Conversion of an Axisymmetric Unit Cell into Plane Strain
Indraratna et al., 2000 & 2005
Conversion must give the SAME time-settlement curve

21. No conversion (some published work)
After conversion

22. Normalized average excess pore pressure in axisymmetric condition with vacuum (Indraratna et al., 2005), CGJ kh ks
-p0
-k1p0 z l
ds/2
de/2
Smear zone
Undisturbed zone
Vacuum pressure distribution C L
= pore pressure at time t (average values)
= time factor
= undisturbed horizontal permeability
= smear zone permeability
= initial pore pressure
= average applied vacuum pressure

23. Vacuum + Surcharge alone
Degree of Consolidation: Pore pressure Based Models (Indraratna et al. 2008)
Surcharge alone
Vacuum alone
Vacuum + Surcharge alone

24. Case Study 1: Port of Brisbane Ground Improvement
Dredging for Reclamation fill
PVD installation
Marine boundary and sandy platform
Vacuum stabilised area

25. Essential Design Aspects
(selected section)
Service 25 kPa,
Max. residual settlement
@ 250 mm over 20 yrs.
Sea wall and future
development area
Soil Properties
Plan view 25

26. Time-Settlement and Pore Pressure Response
More than doubled settlement obtained with VP at the same time scale
Higher k promotes greater PWP dissipation
(a) Settlement and (b) excess pore pressure for non-vacuum site
(a) Settlement and (b) excess pore pressure for a typical vacuum site

27. Effect of OCR and clay thickness on residual settlement
Reduction in lateral movement due to VP
Effect of OCR and clay thickness on residual settlement
Lateral Displacement reduction due to vacuum application 27

28. Case Study (2): Trial Embankment Stabilized with PVD and Vacuum Preloading, Ballina Bypass, Australia
No vacuum
Vacuum
Instrumentation layout
Typical soil properties

29. Test Embankment Cross Section
-70 kPa vacuum and max. 8 m surcharge applied at this site

30. Performance : soil properties before and after vacuum application
The void ratio, compressibility Index and water content decrease significantly in the initial 17m. Beyond that, only a marginal decrease is observed.
PVD+VP system is mainly effective at the upper regions of the clay.

31. Performance: Lateral displacement
No Vacuum
Lateral movement decreases due to vacuum, even at higher fill heights.
Ratio of lateral movement to fill height is a better indicator of the stability provided by VP

32. (Indraratna et al 2005, Int. J. of Geomechanics, ASCE, 114-124)
Case Study (3): Test Embankment Stabilized with PVD and Vacuum Preloading in Soft Bangkok Clay, Thailand
(Indraratna et al 2005, Int. J. of Geomechanics, ASCE, )
-60 kPa design vacuum and max. 2.5 m surcharge 32

33. Vacuum Simulation (selected section) (Indraratna and Redana, 2000)
Model A: Conventional analysis
(no vacuum; only surcharge fill)
Model B: Vacuum pressure is adjusted according to field measurement and reduces linearly to zero at the bottom of the drain (k1= 0)
Model C: Perfect seal (vacuum pressure was kept constant at -60kPa after 40 days); vacuum pressure varies linearly to zero along the drain length (k1= 0)
Model D: No vacuum loss along the
drain length (k1=1)
Field measurements
Model C: Assumed VP 33

34. Surcharge Only
Settlement
Excess pore pressure 34

35. Weathered Crust is much stiffer in reality than the assumed properties
Lateral Movements at Embankment Toe
Weathered Crust is much stiffer in reality than the assumed properties
Key Advantages:
Surcharge fill height reduced from 4.0m to 2.5 m
Time for 95% consolidation reduced from 12 months to 4 months. 35

36. 3D vacuum pressure propagation across the boundaries of treated zone
Effect of vacuum application (negative movements) may extend more than 10 m from the edge of the embankment
Lateral
movement A A A

37. Class A Prediction (Indraratna et al, ASCE, JGGE, 2010)
Case Study 4: Railway Applications: FEM Analysis of Short PVDs at Sandgate
Class A Prediction (Indraratna et al, ASCE, JGGE, 2010)
Very Soft Alluvial Clay
Soft Silty Clay
Rapid dissipation of excess pore pressure
Curtailing lateral displacement 37

38. CONCLUSIONS
Vacuum preloading rapidly decreases excess pore pressure, and directly increases effective stress with time. Conventional surcharge models simulate the increase in total stress, and associated increase in the excess pore pressure.
Smear effects adversely affect PWP dissipation, and the application of VP and corresponding increase in the hydraulic gradient partially compensates for this.
PVDs in combination with vacuum and surcharge fill curtail lateral movements and provide stability for the superstructure. Excessive VP generates high inward movement causing tensile zones.
Sophisticated 3-D numerical modelling is not required if appropriate conversion to 2D plane strain can be made for most sites for multi-drain analysis. Exceptions would be marine boundaries and corners.
Field monitoring for VP sites is essential to ensure performance, and to establish any time-dependent variation of VP distribution.

39. CONCLUSIONS
Vacuum preloading rapidly decreases excess pore pressure, and directly increases effective stress with time. Conventional surcharge models simulate the increase in total stress, and associated increase in the excess pore pressure.
Smear effects adversely affect PWP dissipation, and the application of VP and corresponding increase in the hydraulic gradient partially compensates for this.
PVDs in combination with vacuum and surcharge fill curtail lateral movements and provide stability for the superstructure. Excessive VP generates high inward movement causing tensile zones.
Sophisticated 3-D numerical modelling is not required if appropriate conversion to 2D plane strain can be made for most sites for multi-drain analysis. Exceptions would be marine boundaries and corners.
Field monitoring for VP sites is essential to ensure performance, and to establish any time-dependent variation of VP distribution.

40. Acknowledgement Australian Geomechanics Society (AGS)
Dr Geng Xueyu and Dr Cholachat Rujikiatkamjorn during compiling and editing of a vast amount of data from the past 15 years of research in vertical drains and vacuum preloading conducted at University of Wollongong (UOW).
More than a dozen past and present research students who have contributed to the contents of this lecture directly and indirectly.
Australian Research Council – Linkage and Discovery project funding
Research Collaborations with many Industry Partners and Institutions over the years: Queensland Department of Main Roads, Port of Brisbane Corporation, Roads & Traffic Authority, Coffey Geotechnics, Asian Institute of Technology, Thailand; Polyfabrics, Geofabrics, ARUP, Douglas Partners, Snowy Mountains Engineering Corporation, RailCorp, ARTC, Chemstab, Queensland Rail and Austress-Menard
Technical staff, University of Wollongong

41. Thank You

42. Settlement close to the embankment centreline
Practicing Engineers’ Dilemma - Disparity between Excess Pore Pressure and Settlement
Indraratna, Balasubramaniam & Ratnayake,
Journal of Geotechnical Engineering, ASCE, Vol. 120, No. 2, pp ,
Rate of excess pore pressure dissipation influenced by:
high visco-plastic strains
clogging of drains
malfunctioning piezometer tips
Sudden high rainfall
Settlements may continue to occur, when excess pore pressure is still not dissipated.
Solution: Increase hydraulic Gradient towards drains by applying vacuum pressure
Settlement close to the embankment centreline

43. PWP/initial total stress
Extent of the smear zone based on permeability measurement and finite element prediction
Predicted and measured pore pressure during vertical drain installation

44. Applications (4): 3D FEM Application: Land Reclamation Stabilized with PVD and Vacuum Preloading Tianjin port , China
(Rujikiatkamjorn, Indraratna and Chu 2008, Int. J. of Geomechanics, ASCE)
Soil Profile 44

45. Soil Profile, Embankment Cross Section & Instrumentation
Embankment Plan View & Instrumentation 45

46. Settlement Predictions
Depth
(m) l k n e0 gs
kN/m3 kv
10-10 m/s
kh,ax
k’h,ax
kh,ps
k’h,ps
OCR
0.12
0.03
0.3
1.1
18.3
6.67 20
5.91
1.46
1-1.1
0.14
0.25
1.0
18.8
13.3 40
11.8
2.92
0.20
0.04
1.35
17.5
0.10
0.02
0.27
0.9
18.5
1.67 5
1.48
0.365
Soil parameters
Settlement Predictions 46

47. 3D FEM mesh
2D FEM mesh (converted)

48. Finite element analysis: Vertical settlement
Soil parameters
Surface Settlement Predictions
2009 E H DAVIS LECTURE Buddhima Indraratna