Prefabricated vertical drains and Preloading by vacuum design and performance University of Shanghai for Science and Technology

Presentation Outline Principal of vacuum consolidation Efficiency and factors affecting vacuum system Unit cell analysis Multi-drain analysis: 2D vs. 3D Case studies Conclusion

Vertical Drains Shorten the length of the drainage path Accelerate the rate of pore water pressure dissipation Accelerate the rate of consolidation / settlement

Conventional Surcharge vs Vacuum Preloading (Chu and Yan, 2005; Mohamed-Elhassan and Shang, 2002) For vacuum application, total stress does not increase. Staged construction can be avoided. Lateral displacement can be controlled. Cost of vacuum pump operation s’=s - (-u)

Inward movement due to VP Potential benefits of Prefabricated Vertical Drains in Soft Formation clays Surcharge With Vertical Drains Ref: Colbond, The Netherlands Embankment Settlement Without 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

Membrane & Membraneless Systems in Vacuum Preloading Advantage Large area Caution Membrane leakage Membrane system (e.g. Menard) Advantage Area can be subdivided Caution Significant cost of individual drain connection Membraneless system (e.g. Beaudrain)

Site preparation for Vacuum Consolidation Drain Installation Horizontal drain installation Peripheral bentonite trench Connection between horizontal drainage and vacuum pump Membrane installation

Reduction of consolidation time through application of vacuum preloading

Efficiency of vacuum preloading depends on Drain spacing and equivalent drain diameter Vacuum pressure distribution Drain core Lateral confining pressure Deformation characteristic (folding, bending, crimping) Installation effects on soil permeability and compressibility Flow characteristics: Darcian vs. Non-Darcian flow

Experimental Evaluation of PVDs Installation of PVDs by a steel mandrel causes smear – reduced lateral permeability Large-Scale, Radial Drainage Consolidometer 550mm Diameter 1.2m Height

Distributions of vacuum pressure along the drain length Laboratory scale 20 kPa vacuum pressure 40 kPa vacuum pressure Field observation Indraratna, B., Rujikiatkamjorn, C., Kelly, R. and Buys, H. (2012). Soft soil foundation improved by vacuum and surcharge loading. Ground Improvement

Effects of vacuum distribution along the drain length Long Drain Short Drain Indraratna, B., Rujikiatkamjorn C., and Sathananthan, I., (2005). Analytical and numerical solutions for a single vertical drain including the effects of vacuum preloading. Canadian Geotechnical Journal, 42: 994-1014. 12

Determination of consolidation & permeability characteristics Permeability Approach Indraratna & Redana, 1998, JGGE, ASCE Vol. 124(2) Water Content Reduction upon Vacuum Application Sathananthan & Indraratna 2006, JGGE, ASCE, Vol. 132(7)

Effect of soil disturbance on soil consolidation parameters Indraratna, B., Perera, D., Rujikiatkamjorn, C., Kelly, R. (2014). Analysis of Soil Disturbance Associated with Mandrel-driven Prefabricated Vertical Drains: Field Experience, Geotechnical Engineering ICE (Accepted)

Single drain vs. Multi-drain installation Indraratna, B., Perera, D., Rujikiatkamjorn, C., Kelly, R. (2014). Analysis of Soil Disturbance Associated with Mandrel-driven Prefabricated Vertical Drains: Field Experience, Geotechnical Engineering ICE (Accepted).

Pore pressure variation during mandrel movement Ghandeharioon, A., Indraratna, B., and Rujikiatkamjorn, C. (2012). Laboratory and Finite-Element Investigation of Soil Disturbance Associated with the Installation of Mandrel-Driven Prefabricated Vertical Drains. J. of Geotechnical & Geoenvironmental Engineering, ASCE. 138(3), 295-308. s1= Locations of pore pressure transducers Mandrel T1 T2 T3 T4 T5 16

Application of Cavity Expansion Theory for Mandrel Driven Vertical Drains Equations of equilibrium (for elliptical expansion): Dsr, Dsq and Dtrq are variations in the radial, tangential, and shear stresses In the plastic region, MCC theory is used with the above equations. S is the component of body force (per unit volume) radially T is the tangential component

UOW Solution: Elliptical Cavity Expansion Theory for Wick Drains R = over-consolidation ratio; Other parameters defined by Modified Cam-clay theory The excess pore water pressure (Du) can be estimated from: ′

Finite element mesh for large strain frictional contact Element type: CAX4P Soil and steel mandrel properties Soil Properties Value Slope of unloading-reloading line,  0.05 Slope of normal compression line,  0.15 Critical state line slope, M 1.1 Critical state void ratio, ecs 1.55 Poisson’s ratio,  (aasumed) 0.25 Permeability (m/s) 5.1 10-10 Lateral stress coefficient (k0) 0.5 Steel Mandrel Properties Young modulus (kN/m2) 2108 Poisson’s ratio,  Interface Properties Friction coefficient, m 0.24

Installation effects by vertical drains

Analytical Solutions for Vacuum-assisted Preloading Assumptions Rujikiatkamjorn, C. and Indraratna, B. (2014). Analytical Solution for Radial Consolidation Considering Soil Structure Characteristics, Canadian Geotechnical Journal. (Accepted November 2014).

Analytical Solutions for Vacuum-assisted Preloading Effects of soil permeability and compressibility variation Indraratna, B., Rujikiatkamjorn C., and Sathananthan, I., (2005). “Radial consolidation of clay using compressibility indices and varying horizontal permeability.” Canadian Geotechinical Journal, 42: 1330-1341.

Installation effects on consolidation responses CASE A (Single Drain), CASE B (Multi-drain), and CASE C (Ideal Case: No smear). 23

Analytical Solutions for Vacuum-assisted Preloading: Application to Case histories Muar Clay Embankment (without vacuum) Embankment Centre line only Second Bangkok International Airport (with vacuum) Indraratna, B., Rujikiatkamjorn C., and Sathananthan, I., (2005). “Analytical modeling and field assessment of embankment stabilized with vertical drains and vacuum preloading.” The Proceedings of the 16th ICSMGE, Osaka, Japan, Edited by the 16th ICSMGE committee, Millpress, Rotterdam, the Natherlands, (1049-1052).

Effects of flow behaviour: Darcian and Non Dracian Flow relationship Kianfar, K., Indraratna, B. and Rujikiatkamjorn, C. (2013), Radial consolidation model incorporating the effects of vacuum preloading and non-Darcian flow. Geotechnique. 63: p.1060-1073. 25

2D FEM Multi-drain Analysis and Plane Strain Permeability 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!!

The equivalent plane strain permeability is: After conversion Before conversion Indraratna, B., Rujikiatkamjorn C., and Sathananthan, I., (2005). “Analytical and numerical solutions for a single vertical drain including the effects of vacuum preloading.” Canadian Geotechinical Journal, 42: 994-1014.

Case History: Second Bangkok International Airport, Thailand Vertical cross section -60 kPa vacuum and 2.5 m surcharge applied at this site FEM mesh discretization (ABAQUS) Indraratna, B., Sathananthan, I., Rujikiatkamjorn C. and Balasubramaniam, A. S. (2005). Analytical and numerical modelling of soft soil stabilized by PVD incorporating vacuum preloading. International Journal of Geomechanics, Vol. 5 No. 2, 114-124.

Case History: Soil Parameters Case History: Construction Schedule

Case History: Vacuum Simulation Measured vacuum pressure Indraratna, B., Rujikiatkamjorn C., Balasubramaniam, A. S. and Wijeyakulasuriya, V. (2005). “Predictions and observations of soft clay foundations stabilized with geosynthetic drains and vacuum surcharge.” Ground Improvement: Case Histories, Elsevier, 199-230.

Case History: Vacuum Simulation Model A: Conventional analysis (i.e., no vacuum application) 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 (i.e. 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)

Case History: Results and Discussions Settlements Excess pore pressures

Case History: Results and Discussions Lateral Movements at Embankment Toe Advantages Embankment height reduction from 4.0m to 2.5 m Time reduction from 12 months to 4 months

Case History at Tianjin Port, China 2D and 3D analysis Soil Profiles Embankment plan view Rujikiatkamjorn C., Indraratna, B. and Chu, J. (2008). 2D and 3D Numerical Modeling of Combined Surcharge and Vacuum Preloading with Vertical Drains. International Journal of Geomechanics, ASCE, 8(2), 144-156.

2D and 3D FEM mesh discretisation

Excess pore water pressure Ground Settlement

Lateral Displacement Lateral Displacement

Effect of vacuum pressure at the border of embankment Effect of vacuum application (negative movements) may extend more than 10 m from the edge of the embankment Lateral movement A A A

Case Study: Ballina Bypass DESIGN APPROACH FOR SOFT CLAY IMPROVED BY VERTICAL DRAINS WITH VACUUM PRESSURE – UoW Method Rujikiatkamjorn and Indraratna (2007, 2008), CGJ. AS8700 Sand layer Case Study: Ballina Bypass One way drainage l =24m Impermeable layer qu=80kPa, Ut = 90%, l = 24m, dw = 0.06 m, ch = 1.8m2/year, uo= -70 kPa, cv = 0.9m2/year, kh/ks = 5, ds/dw = 3, t = 1.5 year ds/dw kh/ks

Now determine (a, b) to get spacing parameter, n g = f (Ut, Tv, u*) de =n*dw Drain spacing, S =de/1.128 for square pattern S =de/1.05 for triangular pattern de = 1.0-1.2 m @ POB was justified. Latest Version: Rujikiatkamjorn, C. and Indraratna, B. (2007 and 2008). Canadian Geotechnical Journal

Conclusions The effects of permeability and compressibility variation are included in the proposed analytical model Plane strain with appropriate conversion procedure is useful for multi-drain analysis in terms of calculating time Vacuum preloading through PVDs applies a direct increase in lateral hydraulic gradient rapidly decreasing the excess pore pressure, thereby effecting rapid consolidation PVD system subjected to vacuum preloading will only be effective as long as the potential air leaks can be minimized in the field Factual data from well–monitored site such as measured applied vacuum is important to obtain the accurate predictions