NUMERICAL MODELLING OF PEAT BEHAVIOUR

Transcription

1 NUMERICAL MODELLING OF PEAT BEHAVIOUR Eric R Farrell AGL Consulting and Trinity College, Dublin

2 Objective Look at what the etheoretical/laboratory etca/abo ato ystudies carried out to date add to practical design situations. This is addressed by taking an example of a floating road to take a heavy mobile load It has obvious relevance to the roads used in windfarm developments. 11/09/2013 ET Hanrahan Memorial Symposium 2

3 PRESENTATION Review some special aspects of pseudo fibrous peat bh behaviour Outlinesomeexperience experience of floatingroads onpeat Compare practical experience with numerical model predictions. This comparison will be done with existing soil models, acknowledging that specially adapted soil models are being developed in research. 11/09/2013 ET Hanrahan Memorial Symposium 3

4 ET Hanrahan Ahead of his time in peat Lightweight g fill (bales of peat) Vacuum consolidation trials Effectiveness of surcharge Manyother areas. 11/09/2013 ET Hanrahan Memorial Symposium 4

5 SOME SPECIAL ASPECTS OF PSEUDO FIBROUS PEAT Very low ρ effective stress close to zero in many design situations Drained compression tests are essentially 1 D (very low μ ) (Hebib & Farrell 1998, O Kelly & Zhang, 2013 ) Triaxial compression at low effective stresses, σ 3f ' 0 (Termaat,??, Hebib, bb 1997) High φ in vertical compression,,φ depends on direction of loading. Anisotropicstrength strength andstiffness 11/09/2013 ET Hanrahan Memorial Symposium 5

6 Special aspect (cont) High compressibility Time dependent deformations (Den Haan, 1992) Effect of structure (Zhang and O Kelly, 2013) Permeability dependent on void ratio (Hanrahan, 1954) 11/09/2013 ET Hanrahan Memorial Symposium 6

7 SOME PRACTICAL EXPERIENCE OF FLOATING ROADS ON PEAT 11/09/2013 ET Hanrahan Memorial Symposium 7

8 Floating road test Index test 65mmx 130mm vane versus 130mm x 220mm VST Results Max. height 2m in 8 No. lifts Undrained Shear Strength, c (kn/m ) Peat depth 4 to 5m, ave. 4.65m u Undrained Shear Strength c (kn/m 2 ) Tree trunks 5m long and dia. <0.5m Tensar geogrid and 500mm angular stone Depth (m) Settlement (mm) Geonor-H10 West Geonor-H10 East Geonor H10 Average ESBi Vane West ESBi Vane East ESBi Vane Average Height of crushed rock (m) 11/09/2013 ET Hanrahan Memorial Symposium 8

9 Typical profile through raised bog. Osorio Salas 2012 (m) Depth w (%) 0% 500% 1000% 1500% (m) Depth LOI 0% 50% 100% Transition zone important in some design situations BH 1 BH 2 MBH 3 MBH 4 BH 1 BH 2 MBH 3 MBH 4 11/09/2011 ET Hanrahan Memorial Symposium 9

10 NOTE: CASE STUDY IS ON BLANKET BOG BUT WITH SIMILAR MOISTURE CONTENTS above btm of peat.(m m) Height Moisture content % Edge of slip Edge of slip after test 11/09/2013 ET Hanrahan Memorial Symposium 10

11 Settlementparameters used in the peat modelsgenerally obtained from work of Osorio Salas (2012) on raised bog which found good comparison with field performance with lb laboratory obtained parameters. t 11/09/2013 ET Hanrahan Memorial Symposium 11

12 Selection of effective stress parameters of Peat c' 0 kpa φ = 50 o and 35 o (anisotropic) Actually required c' 3 kpa to 5kPa Due to effect of fibres but perhaps it matches the s u profile from vane. ET Hanrahan Memorial Symposium 12

13 Selection of stiffness parameters Case Histories E=200kPa E=75kPa Landva & Rochelle, /09/2013 ET Hanrahan Memorial Symposium 13

14 Stiffness parameters for soil models Model γ kn/m 3 E' kpa μ' Source Elastic M C To give approx. same strain at 200kPa as lab. Test (ignores stress dependency of E') Model γ λ* κ* μ* e o Source kn/m 3 Soft Soil N/A Osario Salas (2010) Soft Soil Creep Osario Salas (2010) 11/09/2013 ET Hanrahan Memorial Symposium 14

15 Stiffness parameters cont. Model γ m E ref 50 E ref oed E ref ur ur Source kn/m 3 kpa kpa kpa H S See below Hand calculations : e o =12.15; C c = 6.1 ; C s =0.8, σ c = 9kPa 11/09/2013 ET Hanrahan Memorial Symposium 15

16 E kpa O Kelly & Zhang, O'Kelly & Zhang, Eref using 500kPa p' kpa O'K&Z Ref 11/09/2013 ET Hanrahan Memorial Symposium 16

17 Apparent preconsolidation pressure of peat Uncertainty regarding the apparent preconsolidation pressure 9kPa & 9.5kPa from lab. Tests (Osario Salas 2010) Published information 8 to 10 kpa (Farrell, 2012) UPDATED MESH OPTION??? 11/09/2013 ET Hanrahan Memorial Symposium 17

18 Permeability e BALLYDERMOT BOG - BOREHOLE 1 D= m 1.00E E E E E E-06 Log. (D= m) 1.00E-05 k v-log (m/s) D= m Log. (D= m) k o = m/day 2.6x10 8 m/s C k =3.44 k ave = 10 8 m/s 11/09/2013 ET Hanrahan Memorial Symposium 18

19 Comparison with lab. results Stress Stress train % S train % S m to 3.2m Oed 4 Spreadsheet 2.4m to 3.2m Oed 4 Spreadsheet Soft Soil SSCreep Soft Soil SSCreep HS Lin E M C HS Lin E M C 11/09/2013 ET Hanrahan Memorial Symposium 19

20 UNDRAINED TRIAXIAL q/p' plots q kpa M C 10kPa M C 40kPa SS 10kPa SS40 SSC 10kPa SSC 40kPa HS 10Kpa HS 40kPa p' kpa 11/09/2013 ET Hanrahan Memorial Symposium 20

21 s u /σ 3c ' s u kpa 20 M C 15 Soft Soil SSC 10 HS σ 3c ' 11/09/2013 ET Hanrahan Memorial Symposium 21

22 Undrained stress vs strain Triaxial compression σ 1 σ M C HS SS SSC Axial strain % 11/09/2013 ET Hanrahan Memorial Symposium 22

23 Modelling Floating Road 11/09/2013 ET Hanrahan Memorial Symposium 23

24 Parameters used to model floating road Model Length (m) E GPa Trunk dia. EA kn/m EI knm 2 /m W kn/m/m Timber Plate m Model E' μ' c' φ' γ kpa kn/m 2 Gravel Elastic M C /09/2013 ET Hanrahan Memorial Symposium 24

25 Construction process modelled. Allowed to consolidate after construction of road. Two loading scenarios considered: Loading completely undrained Loading applied over 0.5 days with consolidation. 11/09/2013 ET Hanrahan Memorial Symposium 25

26 FEA Modelling of load test 30kPa Loading stage undrained d 15/08/2013 ET Hanrahan Memorial Symposium 26

27 Observations at centreline Vertical settlement are uniform Effective stresses are low, as expected 5 σ yy σ'kpa yy 'kpa Deformations sensitive to pre consolidation pressure. c' = 3 to 5 kpa required for stability with φ = 35 o φ = 50 o lesser benefit. Depth (m m) Depth (m) Depth (m) Floating Rd only Floating 30kPa Rd only Cons No 0.5day cons 30kPa 30kPa No cons No 0.5d cons cons 30kPa After 0.5d cons Floating 30d Rd Full cons σ yy ' 11/09/2013 ET Hanrahan Memorial Symposium 27

28 35 ertical Stress (kpa0 Total V No cons Soft Soil Model Distance along Floating Road (m) 250 ) Vertical Displacement (mm No cons Distance along Floating Road (m) 11/09/2013 ET Hanrahan Memorial Symposium 28

29 Settlement (m mm) SS No cons SS wt cons 300 SSC No cons SSC wt Cons Measured 200 M C No cons M C Cons Applied Stress on road surface kpa Settlement after 24hrs was about 0.9m 11/09/2013 ET Hanrahan Memorial Symposium 29

30 Comments NEED TO IDENTIFY SOURCES OF THE DIFFERENCE BETWEEN FIELD PERFORMANCE AND NUMERICAL PREDICTIONS SS No cons Settlemen nt (mm) SS wt cons SSC No cons SSC wt Cons Measured M C No cons Soft Soil model with σ c = 7kPa (with cons) Applied Stress on road surface kpa M C Cons SS 7kPa wt cons Large plastic strains with σ c = 6kPa. 11/09/2013 ET Hanrahan Memorial Symposium 30

31 Time/Settlement under 30kPa 0.00 Time (days) , Sett tlement (m mm) SS Full cons SSC m c /09/2013 ET Hanrahan Memorial Symposium 31

32 Some points made by O Loughlin(2001) based on lab. and field studies. Summary of test Field ildload test applied 7.9kPa over a 5.2m by 4.175m area using large capacity water tanks (lh (although hsubmergence of the tanks with ihtime reduced the effective stress). The tanks settled about 860mm in two years. 11/09/2013 ET Hanrahan Memorial Symposium 32

33 O Loughlin, /09/2013 ET Hanrahan Memorial Symposium 33

34 CONCLUSIONS Virgin pseudo fibrous peat has many unique features which make it difficult to model. Numerical modelling can assist in interpreting the behaviour of peat but further advances are required. More long term field monitoring is required to feed into the numerical modelling developments. Given the prevalence of floating roads for windfarms, data on their performance, preferably with comparison with predictions, should be collected for research. 11/09/2013 ET Hanrahan Memorial Symposium 34

35 References Farrell & Hebib, 1998, The determination of the geotechnical properties of organic soils, Problematic soils conf. Zang and O Kelly, 2013, The principle of effective stress and triaxial compression testing of peat. O Kelly and Zang, 2013, Consolidated drained triaxial compression testing of peat, Geotechnical Testing Jnl. Hanrahan, 1954, An investigation into some physical properties of peat. Geotechnique Den Haan, 1992, The formulation of Virgin Compression of Soils, Geotechnique. 11/09/2013 ET Hanrahan Memorial Symposium 35

36 References contd. Hebib, 1997, Study of the shear strength of peat under different stress paths, MScthesis thesis, TCD. Osorio Salas, 2012, Vacuum consolidation field test on a pseudo fibrous peat, PhD Thesis, TCD. Hebib, 2001, Experimental investigation on the stabilisation of Irish peat. PhD Thesis, TCD. O Loughlin, 2001, The one dimensional compression of fibrous peat and other organic soils., PhD Thesis, TCD. O Loughlin & Lehane, 2001, Modelling one dimensional compression of a fibrous peat, 15 th ISSMGE. 11/09/2013 ET Hanrahan Memorial Symposium 36