Geosynthetics and Reinforced Soil Structures Geosynthetic Reinforced Pile Platforms

Transcription

1 Geosynthetics and Reinforced Soil Structures Geosynthetic Reinforced Pile Platforms Dr. K. Rajagopal Professor of Civil Engineering IIT Madras, Chennai, India

2 Problems Construction on Soft Foundation Soil (a) Slope instability (From Lawson,2012) (b) Unacceptable vertical settlements (From Lawson,2012) 2 (c) Localised differential settlements at embankment surface ( Concept- Lawson,2012) (d) Difficulty to move the construction equipment

3 Methods of Ground Improvement Soil Replacement Preloading Light Weight Fill Preloading with Vertical Drain Vacuum Preloading Stone Column-OSC,ESC Piled Raft Basal Reinforcement Piled Embankment Geosynthetic Reinforced Pile Supported Embankment 3

4 Geosynthetic Reinforced Piled Embankments Rail/Road embankment Soft clay Piles Inclined Piles Firm stratum

5 Advantages of Geosynthetic Reinforced Piled Embankments Faster construction-loading rate not dependent on the rate of consolidation of soil Eliminates differential settlements especially for large height embankments Slope stability Relatively small pile caps and no need for raking piles Low long term maintenance costs 5

6 Embankment Piling CFA (Continuous Flight Auger) piles

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13 13 Load Transfer Platform at Second Severn Crossing (Tensar, UK brochure)

14 Measured data from Second Severn, UK (Tensar, UK brochures) 14

15 Application areas Bridge abutment approach roads (Buchanan,1984) Airport runways (Hossain and Rao, 2005) Subgrade improvement (Han, 1975) Minimize differential settlements under storage tanks (Alzamora et al. 2000) Segmental retaining wall (Alzamora et al. 2000) Widening of the existing roadway embankment (Han and Gabr 2002) To construct confined embankment structures (Lawson 2012) 15

16 Construction Sequence Installing piles with certain grid formation in the soft soil up to a certain depth. Geosynthetic material is laid on top of a thin layer (0.1 m) of granular material. After placing the geosynthetic layer, the embankment fill is constructed to the required height in stages. 16 Finally the construction ti such as railway or road pavement is built on top of the embankment Geosynthetic Reinforced Piled Embankment System

17 Plan Layout of the Piles (a) Layout t( (a) Square and (b) Triangular (b) Geosynthetic Layout (a) (b) 17 Optimal geosynthetic layout (a) direction of placing the layers and (b) direction of load (Lawson,2012)

18 Load Transfer Mechanism (a) Soil Arching (b) Membrane action of geosynthetic (Russell and Pierpoint,1997) (c) Concentration ti of stresses around the pile due to the stiffness difference between the soft foundation soil and the rigid pile 18

19 Design Methods (a) British Standard-BS8006:1995 This is the most widely used method and is very conservative. Based on Marston s aso s (1913) 93) formula for positive projecting pojec conduits, Jones et al.(1990) developed an empirical relationship for the ratio of average vertical stress acting on the pile caps to the average vertical stress acting across the base of the embankment. p c v C ca H where v H 19 p c =Arched vertical stress on top of the pile σ v =Average vertical stress on top of the pile C c = Arching Coefficient (Marston 1913) a = size of pile caps Positive Projecting Conduit (Marston,1913)

20 BS8006 adopted Jones et al.(1990) for the design of piled embankments. BS8006 gives empirical equations for arching coefficient as follows H End bearing piles,cc a H Friction piles,cc a BS8006 considers two cases C c Embankment height is below the critical height of 1.4(s-a): Arching is not fully developed Partial arching Here A= Load acting on the piles due to arching, B= Load taken by the geosynthetic and C= Load acting on the soft subsoil

21 sa H sa For , 2 2 s f fs H fqws p c Load on the geosynthetic, WT s a 2 2 s a v 1 WT s a Geosynthetic Tension, T r 1 2a 6 where is the geosynthetic strain f, f are the partial fact ors used in the design fs q 2. Embankment height isabove the critical height of 1.4(s-a): Full arching is developed 21 Full arching

22 Height of embankment above arching height plays no role in the tension developed on the geosynthetic. Same is the case with surcharge For H>1.4 s a, sf fs s a p c WT s a 2 2 s a v WT s a 1 Geosynthetic Tension, Tr 1 2a 6 22

23 Horizontal force at the slope, Horizontal force at the embankment slope after BS8006 (Satibi,2009) Geosynthetic tensile load needed to resist the horizontal force of fthe embankment tis T rs T 0.5 K ( f H 2 f q) H rs a fs q where K Active lateral earth pressure coefficient f a fs, f = partial factors used in the design q 23

24 (b) Hewlett and Randolph Method(1988) This theory is based on limit state of soil in hemispherical domed region over piles. The stability of arch at the crown and at the pile top of the hemispherical dome formed defines the entire stability. 24 Hemispherical domes (Hewlett & Randolph, 1997)

25 Stress Reduction Ratio ( S 3D ) defined as the ratio of the average vertical stress acting on the reinforcement to the overburden pressure due to the embankment fill was used to check the stability. S S 3D 3D 1 at the crown of the arch 1k p 2 2k p a a a a k 1 p 2 kp 1 s s s s 2 k p 1 a 2sk p sa kp at the pile top 1 1 s 2H2k 3 p 2H 2kp3 - Largest value is the critical S 3D 25

26 (c) The new German Method (EBGEO 2004) In the old German approach the arching model developed by Hewlett and Randolph (1988) was used to calculate the stresses generated due to arching. EBGEO 2004 adopts the mltishell multi-shell arching theory based on the work of Zaeske (2001). 26 Multi shell arching theory adopted in New German Method (Kempfert,2004)

27 3-dimensional soil element is considered and the equilibrium of forces about the radial direction is used to calculate the vertical stress zo, k coming onto the soil 2 p 2 hg 2 2 zo k 1 h1 h 2 h 1 1 h 2, g g 4 g h where 2 a Kcrit s a sa s k ( ), K=tan 45, 1 s a, 21 In the second step the vertical stress acting on the top of the subsoil zo, k is used to calculate the vertical load F k on the geosynthetic. 27

28 Load distribution on the geosynthetic for rectangular pile layout (Kempfert,2004) A 1 s s a s atn A A 1 a s s s atn A 2 y Lx x y, Fx,k Lx zo, k 2 2 s x x Ly x y, Fy,k Ly zo, k 2 2 s y

29 bl J Ers w k The maximum strain k is obtained from the dimensionless design graphs (EBGEO, 2004). Here, J k = tensile stiffness of the geosynthetic (kn/m) L w = (s-a)= pile clear spacing b Ers = width of support 29 (EBGEO,2004)

30 Horizontal force at the slope The additional horizontal force in the reinforcement beneath the embankment slope is given by 1 Ek Eah, k k h k Pk hz k ah 2 where K Active earth pressure coefficient ah 30

31 (d) The Dutch Method (CUR 226) Introduced in Adopts major parts of the German EBGEO Flat terrain-thin embankments are constructed and therefore the EBGEO method was modified to suit the requirements. (Eekelen et al.2010) Main difference from EBGEO-Different set of loadand-resistance factors were adopted in the Dutch Gidli Guideline. 31

32 (e) Guido Method Guido et al. (1987) observed that the inclusion of stiff biaxial geogrid within a granular fill improved the bearing capacity of the foundation soil. Concluded that the angle of load spread through a granular fill reinforced with geogrid would be at an angle of 45 degrees. The approach is mainly for a single layer of geosynthetic at the base of the embankment fill. 32 s a Stress Reduction Ratio= S3 D 3 2H

33 (f) The Swedish Method Carlsson (1987) considered a wedge of soil with an internal angle at the apex of the wedgeequal to 30º. Valid in two-dimensional model. Carlsson adopted a critical height of 1.87(s-a). 33 Miriam and George (2003) presented the expression for S 3D for this model as per Hewlett & Randolph (1997) S 3D 2s asa s a H 6 tan15 Two dimensional model by Carlsson,1987

34 Rogbeck et al. (1998) modified this model into a 3D form which is an inverted truncated pyramid Three dimensional model by Rogbeck et al.,1998 (Lawson,2012) Modified form of this 3D arching model was adopted by Nordic authorities (Svanø et al.2000). 34 In Nordic design thearchinghi anglewaswidened d to include an angle of arching between 68º-75º.

35 Numerical Analyses-Different approaches Axisymmetric i Unit Cell (Russell and Pierpointi 1997, Han and Gabr 2002, Yoo and Kim 2009) 3D Column (Yoo and Kim 2009, Jenck et al. 2009) Full three dimensional analyses (Huang et al.2005,liu et al.2007) 3DColumn Pile Full Embankment 3 5 Axisymmetric unit cell

36 36 Major Numerical Work-3D Column Russell and Pierpoint (1997) carried out a numerical study using FLAC 3D to compare the different analytical methods. -Terzaghi (1943), Hewlett and Randolph (1988) and BS 8006 Two cases were considered-the A13 piled embankment (heavily reinforced) and the Second Severn Crossing embankment (minimal reinforcement). Design methods predicted differently for different embankment geometries Tension force calculated by different design methods Design Methods A13 Embankment (Reinforcement Tension, kn/m) Second Crossing (Reinforcement Tension, kn/m) BS Terzaghi Hewlett & Randolph

37 Major Numerical Work-Axisymmetric unit cell Han and Gabr (2002) investigated the influence of the tensile stiffness of the geosynthetic, the height of the fill, and the elastic modulus of the pile material. One layer of geosynthetic was used and a full bond was assumed between the geosynthetic and the soil. Major findings are given below. Pile Layout and the axisymmetric model considered for the analysis (Han and Gabr,2002) 37

38 (a) (b) Effect of (a) pile modulus and (b) geosynthetic stiffness on the maximum settlements (Han and Gabr,2002) The influence of geosynthetic tensile stiffness becomes less important when the stiffness exceeds 4,000 kn/m. 38 For a pile of elastic modulus of 30,000 MPa, the maximum settlement t for the reinforced if case was reduced d by 20% from that for the unreinforced case.

39 (a) (b) Effect of geosynthetic (a) Stress Concentration Ratio(b) Tensile force distribution (Han and Gabr,2002) The inclusion of geosynthetic reinforcement enhances the stress transfer from the soil to the piles. Tension is not uniform along the geosynthetic and the maximum tension occurs at the edge of the pile. 39

40 Major Numerical Work-Full three dimensional Geogrid Reinforced Pile supported highway embankment located in Shanghai China-Liu et al. (2007) Case history back analyzed by 3D fully coupled finiteelement analysis. 40 Instrumented cross section of the embankment (Liu et al.,2007)

41 Full three dimensional model developed (Liu et al.,2007) Significant load transfer from the soil to the piles due to soil arching-contact pressure acting on the pile was 14 times higher than that acting on the soil located between the piles. Lateral displacements considerably reduced- stability of the embankment increased significantly. 41

42 Design of Geosynthetic Reinforced Piled Embankment - Example Pulverized fly ash filled embankment Pile caps (1.1 m square) = 14kN/m 3 9 m Soft clay (Without piles settlement = 700 mm) 4 m Embankment Details

43 Reinforcement details Low creep reinforcement Tensile safety factor = 3.0 Peak extension at failure = 12% Geotextiles Longitudinal Transverse Strength (kn/m) Strength (kn/m) A

44 Circular arc Deformation analysis a = = 2.9 m Assuming b = = 0.14 m 2 A R G Geosynthetic From the geometry b a 1 2 tan 2 T T b T T a a 2 R sin G R 758. m G 1 T RG b 2 Weight of the fill, W W kn m T

45 Considering the reaction force as W h18.9 kn m B The tension in the geosynthetic, kn/m T R W W T G T B Consider a single layer of geosynthetic (Optimal), total strength = 1050 kn/m The strain in the geotextile, G %. % From the geometry R 90 a 0. 6% G G

46 As ε G < the predicted Try with b = 0.19 m = 14.93º R G = 5.63 m W T = kn/m T T = 108 kn/m T For this the strain ε G, from the load deformation data = 1.23% From the geometry, ε G = 12% 1.2% As these two are compatible the tension in the geosynthetic T T = 108 kn/m. ε G = 1.2 % G

47 Catenary Deformation analysis From the Equation of the catenary, the tension in the geosynthetic is given by T 1 WT WB a 1 2 a2 a 16b b a 4 b 1 16 b G log 2 8b e a a a Loading coefficient C 169 c. h B c

48 1D Arching: Pressure ratio = C c B c /h 2D Arching: Pressure ratio = (Cc B c /h) 2

49 Loading Coefficient, C 169 c. h B Pressure ratio (1D) = C c B c /h = Pressure ratio (2D) = (1.676) 2 = In any 4 square piles, o Pile area = 1.21 m 2 o Total area = 16 m 2 o Soil area = m 2 Total load c =16149 = 2016 kn Load on the pile = = 428 kn Load on soil = = 1588 kn = kn/m 2

50 WW T = 1074kN/ kn/m W B = 0.15 h = 18.9 kn/m As per the equations shown earlier T T =3098kN/m From load-extension data ε G = (309.8/1050)12 = 3.5 % Using the equation for 1+ε G as shown earlier, ε G = 3.4 % As the two values are in close agreement further iteration is not As the two values are in close agreement further iteration is not necessary.

51 BS Method According to BS8006, the minimum height of embankment required is 0.7 (s-a) and for full arching to develop the height of the embankment should be greater than 1.4 (s-a) In the present case, 0.7(4 1.1) = 2.03 m < 9 m and 1.4(4-1.1)=4.06 m < 9 m - Full arching develops in this case The Arching coefficient (considering end bearing pile). Cc 195. H 018. a = The vertical stress on the pile cap 2 2 Ca c p c v kn/m H 9

52 For H > 1.4(s-a), The distributed load carried by the geosynthetic reinforcement s. s s a pc WT s a 2 2 s a v = 17685kN/ kn/m (Serviceability condition, partial factors in the equations are given a value of 1) Tension in the reinforcement (BS8006-Design strain is 5%) WT s a T 1 1 r kN/m 2a 6 Tension due to lateral thrust, Total tension = kn/m T 05. Ka H kn/m L

53 Results of Design By Circular arc method T T = 108 kn/m; ε G = 1.2 %; W T = kn/m T ; G ; T By Catenary deformation method T T = 310 kn/m; ε G = 3.4 %; W T = kn/m By BS method T T = kn/m; ε G = 5 %; W T = kn/m T N/ ; ε G 5%;W T N/

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55 References 1. Alzamora, D., M. H. Wayne and J. Han (2000) Performance of SRW supported by geogrids and jet grout columns Proc., ASCE Specialty Conf. on Performance Confirmation of Constructed Geotechnical Facilities, Geotechnical Special Publication, 94, British Standards BS8006: 1995 Code of practice for strengthened/reinforced soilsand other fills. Section British Standard Institution. 3. Carlsson, B. Reinforced soil, principles for calculation, Terratema AB, Linköping (in Swedish), CUR (2010) ( ) Dutch CUR design guideline for piled embankments. ISBN EBGEO (2004): Bewehrte ErdkÖrper auf punkt - und linienförmigen Traggliedern, Entwurf Kapitel 6.9, 05/16/2004 version. 6. Guido, V.A., J.D. Knueppel, and M.A.Sweeny (1987) Plate loading tests on geogrid - reinforced earth slabs Proceedings of Geosynthetics 87 Conference, New Orleans, Han, R. (1975) Piled Embankment Supported by Single Pile Caps. Proceedings of the Conference on Soil Mechanics and Foundation Engineering, Istanbul, 1, Han, J. and MA M.A. Gabr(2002) Numerical analysis of geosynthetic-reinforced and pile-supported earth platforms over soft soil. Journal of Geotechnical and Geoenvironmental Engineering, ASCE,128(1), Hewlett, W.J. and M.F. Randolph (1988) Analysis of piled embankments. Ground Engineering, 21(3), Hossain, S. and K.N. Rao (2006) Performance Evaluation and Numerical Modeling of Embankment over Soft Clayey Soil Improved with Chemico-Pile. Transportation research record, USA, Issue Number: 1952, , ,

56 References 11. Huang, J., J.G. Collin, and J. Han (2005) 3D Numerical Modelling of a Geosynthetic Reinforced Pile-Supported Embankment- Stress and Displacement Analysis 16 th International Conference ence on Soil Mechanics and Geotechnical Engineering, ing, Osaka, a, Japan, Kempfert, H.G., C.Gobel,D.Alexiew and C. Heitz (2004) German Recommendations for Reinforced Embankments on Pile-Similar Elements. Proceedings of the EuroGeo3,Munich DGGT, Jones, C.J.F.P., C.R. Lawson, and D.J. Ayres Geotextile reinforced piled embankments., pp In Den Hoed (eds.) International Conf. Geotextiles, Geomembranes and related products, Balkema, Rotterdam, Lawson, C.R. (2012) Role of Modelling in the Development of Design Methods for Basal Rif Reinforced dpilde Piled Embankments, to be published blihdin the Proceedings of feurofuge 2012, Dlf Delft, the Netherland. 15. Liu, H.L., W. W. Charles, and K. Fei (2007) Performance of a geogrid-reinforced and pilesupported highway embankment over soft clays-case study. Journal of Geotechnical and Geoenvironmental Engineering, i ASCE, 133(12), Marston, A. and A.O. Anderson (1913) The theory of loads on pipes in ditches and tests of cement and clay drain tile and sewer pipe. Engineering experiment station, Bulletin No Miriam, E.S., and M.F. George (2003) Influence of Clay Compressibility on Geosynthetic Loads in Bridging Layers for Column-Supported Embankments.Geotechnical Special Publication, no ,

57 References 18. Reid, W. M. and N. W. Buchanan(1984)Bridge approach support piling. Piling and Ground Treatment, Thomas Telford Ltd., London 19. Rogbeck, Y., S. Gustavsson, I. Sodergren and D. Lindquist(1998) Reinforced Piled Embankments in Sweden-Design Aspects. Proceedings of the Sixth International Conference on Geosynthetics, 2, Russell, D. and N. Pierpoint (1997) An assessment of design methods for piled embankments. Ground Engineering, 30(11), Satibi, S. (2009) Numerical analysis and design criteria of embankments on floating piles. A PhD thesis submitted to the Universität of fstuttgart, Stuttgart, Germany. 22. Yoo, C. and S.B. Kim (2009) Numerical modeling of geosynthetic-encased stone columnreinforced ground. Geosynthetics International, 16(3), Zaeske, D. (2001). ZurWirkungsweise i von unbewehrten und bewehrtenmineralischentragschichtenu berpfahlartigengru ndungsetementen. SchriftenreiheGeotechnik,University of Kassel, Germany, Heft 10, February. 57

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