Performance and prediction of soft clay behavior under vacuum conditions

Transcription

1 University of Wollongong Researc Online Faculty of Engineering - Papers (Arcive) Faculty of Engineering and Information Sciences 26 Performance and prediction of soft clay beavior under vacuum conditions Colacat Rujikiatkamjorn University of Wollongong, colaca@uow.edu.au Buddima Indraratna University of Wollongong, indra@uow.edu.au ttp://ro.uow.edu.au/engpapers/327 Publication Details Tis paper was originally publised as: Rujikiatkamjorn C & Indraratna, B, Performances and prediction of soft clay beavior under vacuum condition, Soft Ground Engineering Proceedings, Australian Geomecanics Society, Sydney Australia, 11 October 26, Researc Online is te open access institutional repository for te University of Wollongong. For furter information contact te UOW Library: researc-pubs@uow.edu.au

2 PERFORMANCE AND PREDICTION OF SOFT CLAY BEHAVIOR UNDER VACUUM CONDITIONS Colacat Rujikiatkamjorn 1 and Buddima Indraratna 2* 1 Researc associate, 2 Professor of Civil Engineering, Department of Civil Engineering University of Wollongong, NSW, 2522 Australia *Corresponding Autor ABSTRACT Tis paper describes te beavior of soft soil foundation under vacuum-assisted preloading at te Second Bangkok International Airport, Tailand. An analytical solution considering te variation of soil permeability and compressibility is proposed. Te associated settlement and excess pore pressure at te embankment centerline are predicted and compared wit te available field measurement. Te field data sow tat te efficiency of tis improvement tecnique depends on te magnitude and distribution of vacuum pressure as well as on te extent of air leak protection. Te eigt of sand surcarge and consolidation time can be significantly reduced in comparison wit te conventional metod of surcarge alone. 1. INTRODUCTION Many coastal regions of Australia and Souteast Asia contain very soft clays, wic possess poor geotecnical properties suc as ig compressibility and very low bearing capacity (Indraratna et al., 1992). Te construction of igway and railway embankments on normally consolidated soft soil deposits as been affected by te excessive differential settlements, lateral displacements in te absence of an appropriate ground improvement prior to construction. To prevent te unfavorable conditions, te application of preloading wit prefbricated vertical drains (PVDs) prior to te construction as been popularly employed in many large scale projects (Hansbo, 1979; QDMR, 1991; Indraratna and Redana, 2; Cu et al., 24). Tis metod accelerates te consolidation by providing a sorter drainge pat (Fig. 1). Te gained sear strngt of te foundation can be acieved due to rapid excess pore pressure dissipation. It is also well-known tat te ig outward lateral movement causing te embankment instability can also be reduced via tis metod. (a) (b) Figure 1 Effect of vertical drain on drainage pat; (a) witout vertical drains and (b) wit PVDs (ttp:// In te case of ydraulic fill used in land reclamation projects were te eigt of surcarge is restricted due to te low sear strengt of soft soil, vacuum-assisted consolidation is an ideal metod for ground improvement (e.g. reclaimed land) (Sang et al., 1998; Indraratna et al., 24; Cu and Yan, 25). Te application of vacuum pressure as apparent surcage load wit PVDs can be used as a replacement of ig embankment fill (Coa, 199). Vacuum preloading metod was first introduced by Kjellman (1952) to improve te strengt of soft soil. Te applied negative vacuum pressure is propagated

3 along te PVDs lengt to deep subsoil layer, resulting in an increase in lateral ydraulic gradients 1 u u vac ( i = + ) and effective stresses in soil were u and u vac are excess pore pressure generated by preloading γ w r r and suction pressure generated by vacuum pump, resprectively. Consolidation can be accelerated witout increasing excess pore pressure (Cognon et al., 1994; Qain et al., 1992). In tis paper, rigorous analytical solutions for radial consolidation under vacuum condition incorporating non-linear soil properties (e.g. compressibility and permeability) are introduced. Subsequenlty, a case istory constructed on soft Bangkok clay is analysed based on te current solution, and compared to field measurements. 2. THEORETICAL BACKGROUND FOR VACUUM PRELOADING 2.1. VACUUM PRELOADING PRINCIPLES In saturated soils, te total stress (σ) at any point witin te soil mass is te combination of te effective stress (σ ) and te pore pressure (u) (Terzagi, 1943). Tus, te total stress at any point witin te soil mass can be written as: σ ' = σ ( +u p ) (1) Under te surcarge load alone, te effective stress is gained by te dissipation of positive excess pore water pressure after te load application. In contrast, te effective stress is increased by te applied negative pore pressure (-u vac ) under te vacuum condition. Equation (1) can be rewritten based on te vacuum and fill preloading as: σ ' σ ( + u ) ( u ) (2) = p It can be seen tat te effective stress increases by negative suction, tereby, reducing te risk of sear failure. Te performance of tis system depends on te vacuum condition under te airtigt membrane (Indraratna et al. 24). Te intensive pore pressure measurement under membrane sould be performed to verify te reliability of te vacuum system. Figure 2 sows te system of vacuum assisted preloading via PVDs. C L vac Vacuum Pump Periperal slurry Trenc Impervious Slurry Wall Membrane Sand Blanket Figure 2 System of PVDs wit sand blanket, airtigt membrane and surcarge preloading (Indraratna et al. 25)

4 2.2. ANALYTICAL SOLUTIONS FOR VACUUM PRELOADING Te unit cell teory is usually employed in te analysis of radial consolidation of soil around a single drain at te location of te embankment centreline were te lateral displacement is negligible (Barron, 1948; Hansbo, 1981). However, te solutions based on te surcarge load condition were simplified using te constant compressibility and permeability. Indraratna et al. (25) proposed a compreensive analytical solution for a unit cell under vacuum condition considering te constant soil compressibility and soil permeability. Te details of te derivation were explained elsewere in Indraratna et al., 25. Figure 3a sows te unit cell and its dimension. Based on te laboratory observation by Indraratna et al. (24), Figure 3b illustrates te linear variation of vacuum pressure distributed along te drain lengt interface. C L Soil drain interface Smear zone -p k s k d s/2 d e/2 z l Undisturbed zone -k 1 p -p (1-(1-k 1 )z/l) r Soil Boundary of te drain influence zone z l Figure 3 (a) Unit cell and (b) Vacuum pressure distribution along te drain lengt Te dissipation rate of average excess pore pressure ut at any time factor (T ) can be expressed as: ( 1 + k1 ) 8T * ( 1+ k ) u t = p + p exp 1 p 2 µ 2 In te above expression, (3) T * = P T (4) P av av 1 C [ ( ( ) ) c / C k p σ ' i + p 1 + k1 2 ' i ] =.51 σ (5) T = c 2 t / d e n k µ = ln + ln s.75 s k' (6) (7) were, µ = a group of parameters representing te geometry of te vertical drain system and smear effect, n = d e /d w, s = d s /d w, d e = equivalent diameter of cylinder of soil around drain, d s = diameter of smear zone and d w = diameter of drain well, k = average orizontal permeability in te undistrubed zone (m/s), k = average orizontal permeability in te smear zone (m/s). p = preloading pressure, T is te dimensionless time factor for consolidation due to radial drainage, C c = te compressibility indices, C c = te permeability cange index.

5 Te average degree of consolidation ( U ) can now be evaluated conveniently by te equation: (%) = 1 u 1 1 u U, (8) were, u can be calculated by Equations 1 and 6 wen t. Te settlement (strain) based on average degree of consolidation is defined by: ρ = ρ U (9) were, ρ is te ultimate settlement wen t. 3. APPLICATION TO A CASE HISTORY 3.1. EMBANKMENT DETAILS AND SITE CHARACTERISTICS Te Second Bangkok International Airport is located at Samut Prakan province near te city of Bangkok, Tailand. At tis site, soft clays, mainly of marine or estuarine, often ave construction difficulties suc as excessive differential settlement and foundation failure due to te insufficient of soil sear strengt. Terefore, a suitable ground improvement sceme is required prior to te infrastructure construction. Due to te scarcity of good fill material and time limitation, te vacuum preloading combined wit a very low eigt of surcarge fill via PVDs was selected to improve soft soil sear strengt in tis project. Figure 4 sows te sear strengt and compressibility ratio of sub-soil layers. Te minimum undrained sear strengt (C u ) of topmost weatered clay is about 18 kpa at a dept of 1 m. Tis value decreases to 8-15 kpa in te very soft underlying clay layer, wic is igly compressible. Te weatered crust is muc less compressible due to its desiccation and compaction. Te compression ratio of te soft clay layer varies from.3-.5, wereas te weatered crust as a compressibility ratio of about.2. Te soil layer at te crust is igly overconsolidated (OCR ~ ) due to aging, desiccation and oxidation process. sear strengt (kn/m 2 ) Initial void ratio (e o ) compression index C c /(1+e ) Over consolidation ratio (OCR) Dept (m) Figure 4 Average strengt and compressibility indices (After Sangmala, 1997)

6 At tis site, Embankment TV2 was raised wit 12 m PVDs@1.m drain spacing and -7 kpa vacuum application (Fig. 5). Total base area of eac embankment was 4 4 m 2 (Asian Institute of Tecnology, 1995). Perforated and corrugated pipes wrapped togeter in non-woven geotextile were placed under membrane liner. Te collection of instrumentation includes piezometers, surface settlement plates, multipoint extensometers, inclinometers, observation wells and bencmarks. Te settlement, excess pore water pressure and lateral movement were measured for about 15 days. Figure 6 sows te fill loading istory of te embankment. 15 m 5 m 1 m 1 m Perforated Pipe Geomembrane (LLDPE) 2.5 m.8 m. m S1 S2 S3 Vacuum Pump S4 LBM -3 m -6 m -9 m -12 m -15 m PVD, S=1. m. Bentonite Legend Surface settlement plate Stand-pipe piezometer Extensometer Electrical piezometer Inclinometer Figure 5 Cross section and location of monitoring system (after Indraratna and Rujikiatkamjorn, 24). Embankment eigt (m) γ t = 18 kn/m 3 TV Time (days) Figure 6 Construction scedule (after Indraratna and Rujikiatkamjorn, 24) SINGLE DRAIN ANALYSIS USING PROPOSED ANALYTICAL MODEL In te field, at te embankment centerline, te condition of 1-D consolidation assumed in te proposed analytical model can be justified. Te soil parameters, te in-situ effective stress and te soil permeability for soft Bangkok clay subsoils are sown in Table 1. Te relevant soil properties were obtained from CK o U triaxial tests (AIT, 1995). Te slope of e-logk (C k ) can be determined by (Tavenas et al., 1983, Indraratna et al. 25): C k =.5e (1)

7 In te analysis, eac subsoil layer was divided into 12 sub-layers (approximately 1m tick) to obtain a more accurate effective stress distribution wit dept. Te value of soil compressibility indices (C c or C r ) are related to te actual stress state, were te current effective stress must be considered in association wit te pre-consolidation pressure of soil at tat particular dept (Indraratna et al., 1994). Te values of k /k s and d s /d w for tis case study were assumed to be 2 and 6, respectively. Te embankment loading was simulated using an instantaneous loading at te upper boundary. Settlement predictions were carried out at te embankment centerline using Equations (3)-(1). At te beginning of te subsequent stage, te initial insitu effective stress and soil permeability were calculated based on te final degree of consolidation of te previous loading stage. As te computation of consolidation settlement at te centerline is uncomplicated and follows te 1-D consolidation teory, te use of an EXCEL spreadseet formulation for tis purpose is sufficient. For te first stage loading, were te effective pre-consolidation pressure (p c ) is not exceeded, te value of recompression index (C r ) may be used. In particular, te surface crust is eavily over-consolidated (up to about 2 m dept). Once p c is exceeded, te value of compression index (C c ) follows te normally consolidated line as indicated by te values in Table 1. Te following 4 models were analysed: Model A: Application of surcarge load alone (i.e., no vacuum application), Model B: Application of surcarge and time dependent vacuum pressure simulating vacuum loss according to te measured vacuum pressure under membrane (Fig. 7). However, te soil compressibility and permability is assumed to be constant (Hansbo, 1981) Model C: Similar to Model B. Te nonlinear variation of soil compressibility and permeability proposed by te Autors is employed Model D: Similar to Model C. Tere is no vacuum loss after 1 mont Pore pressure (kpa) Figure 7 Measured and assumed pore pressure Measured Assumed Time (days) Table 1 Selected soil parameters for single drain analysis Dept (m) C r C c k ( 1-9 m/s) γ (kn/m 3 ) p c (kpa) Figure 8 compares te predicted surface centreline settlement wit te measured data. As expected, te predicted results based on te proposed solutions agree well wit te measured results, wereas te prediction based on te constant k overestimates settlement after 8 days, because, te actual soil permeability decreases considerably at iger stress levels. It was verified tat te combined vacuum application and te PVDs system accelerates consolidation, wile te vacuum pressure performs as an additional surcarge load. As sown in Figure 8, no leakage condition gains more settlements, e

8 wereas te prediction witout any vacuum application yields less settlement. Te efficiency depends entirely on preventing airleaks and te distribution of vacuum pressure along te lengt of te drain. It is noted tat te ultimate settlement can be obtained after 17 days..4 Settlement (m) Field measurement Proposed solution (wit leakage) Constant k and m v No vacuum pressure Proposed solution (no leakage) Time (days) Figure 8 Surface settlement predictions at te centerline 4. CONCLUSIONS A system of prefabricated vertical drains (PVDs) combined wit vacuum preloading is an effective metod for accelerating soil consolidation. In tis study, a revised analytical model for vacuum preloading incorporating te compressibility indices (C c and C r ) was proposed, and te variation of orizontal permeability coefficient (k ) was represented by te e-logk relationsip. Te solution was employed to evaluate te performance of soft clay beneat embankment TV2 using spreadseet software. Te settlement predictions of te soft clay foundation were agreed well wit field observations wen considering te actual field condition suc as te varations of vacuum pressure, soil compressibility and permeabiltiy. It sowed tat te assumption of vacuum pressure distribution along te drain lengt could be applied to tis site as evidenced by te monitored data. Te effectiveness of vacuum system depends on te air leak protection in te field. 5. REFERENCES American Drainage Systems, Inc. (26). Vertical wick drains [Online]. Available: ttp:// [Accessed 26, May 16]. Asian Institute of Tecnology. (1995). Te full-scale field test of prefabricated vertical drains for te Second Bangkok International Airport: Final report, Vol. 1, Asian Institute of Tecnology, Tailand. Barron, R. A. (1948). Te influence of drain wells on te consolidation of fine-grained soils. Diss., Providence, U S Eng. Office. Coa, V. (199). Soil improvement works at Tianjin East Pier project. Proceedings 1t Souteast Asian Geotecnical Conference, Taipei, 1: Cu, J., and Yan, S.W. (25). Application of vacuum preloading metod in soil improvement project. Case Histories Book (Volume 3), Edited by Indraratna, B. and Cu, J., Elsevier, London, Cu, J., Bo, M. W., and Coa, V. (24). Practical considerations for using vertical drains in soil improvement project. Geotextiles and Geomembranes, 22, Cognon, J. M., Juran, I and Tevanayagam, S. (1994) Vacuum consolidation tecnology- principles and field experience, Proc. of conf. on vertical and orizontal deformations of foundations and embankments deformations, College station, Texas. Hansbo, S. (1979). Consolidation of clay by band-saped prefabricated drains. Ground Engineering, 12(5),

9 Hansbo, S. (1981). Consolidation of fine-grained soils by prefabricated drains. In Proceedings of 1t International Conference on Soil Mecanics and Foundation Engineering, Stockolm, Balkema, Rotterdam, 3, pp Indraratna, B., and Redana, I.W. (2) Numerical modeling of vertical drains wit smear and well resistance installed in soft clay. Canadian Geotecnical Journal, 37, Indraratna, B., and Rujikiatkamjorn C., (24) Matematical modeling and field evaluation of embankment stabilized wit vertical drains incorporating vacuum preloading. Te Fift International Conference on Case Histories in Geotecnical Engineering. New York, Vol. 1, Indraratna, B., Balasubramaniam, A. S., and Balacandran, S. (1992). Performance of test embankment constructed to failure on soft marine clay." J. Geotec. Eng., ASCE, 118, Indraratna, B., Bamunawita, C., and Kabbaz, H. (24). Numerical modeling of vacuum preloading and field applications. Canadian Geotecnical Journal, 41: Indraratna, B., Balasubramaniam, A. S., and Ratnayake, P. (1994). Performance of embankment stabilized wit vertical drains on soft clay. J. Geotec. Eng., ASCE, 12(2), Indraratna, B., Rujikiatkamjorn C., Balasubramaniam, A. S. and Wijeyakulasuriya, V. (25). Predictions and observations of soft clay foundations stabilized wit geosyntetic drains and vacuum surcarge. Ground Improvement Case Histories Book (Volume 3), Edited by Indraratna, B. and Cu, J., Elsevier, London, pp Kjellman, W Consolidation of clayey soils by atmosperic pressure. Proceedings of a conference on soil stabilization, Massacusetts Institute of Tecnology, Boston, Qian, J.H., Zao, W.B., Ceung, Y.K. and Lee, P.K.K. (1992). Te teory and practice of vacuum preloading. Computers and Geotecnics, 13: Queensland Department of Transport Registry (1991) Queensland Department of Transport, Sunsine Motorway Stage 2 Area 2 Geotecnical Investigation. Materials and Geotecnical Services Branc Report No. R1765, June Sangmala, S., (1997) Efficiency of drainage systems of vacuum preloading wit surcarge on PVD improved soft Bangkok clay, ME Tesis, Asian Institute of Tecnology, Bangkok, Tailand. Sang, J.Q., Tang, M., and Miao, Z. (1998). Vacuum preloading consolidation of reclaimed land: a case study. Canadian Geotecnical Journal, 35: Terzagi, K. (1943). Teoretical soil mecanics, Jon Wiley & Sons, New York.