Surcharge Loading On Reduction of Secondary Compression

Transcription

1 2014 CREBS Workshop at Deltares Surcharge Loading On Reduction of Secondary Compression Yixing Yuan Andrew J. Whittle MIT Jan Page 1

2 Surcharging to Reduce Secondary Compression Post-surcharge OCR OCR p ' vf ' vs ' vf t r t 100 C C c C s (a) (b) (after MIT notes by Prof. Ladd 1989) Amount Of Surcharge Adjusted Amount Of Surcharge AOS ' vs ' vf / ' vf AAOS ' p ' vf / ' vf OCR 1 Page 2

3 Outline Modeling Post-surcharge secondary compression Influence of surcharge on creep behavior Suggestions for more realistic Isotache models Page 3

4 Prior Experimental Study [Ng, SM Thesis 1998] t r t 100 C ' OCR AAOS 1 t Oedometer C NC Page 4

5 Salt Lake City [SLC] clay W P =22.3±3.1% W L =43.6±7.6% G s =2.73±0.04 Test # (Boring) Depth [m] W N [%] e 0 C c p0 / 0 [kpa] B4 (B102A) / 94 B7 (B102A) / 81 B8 (B102A) / 66 B9 (B105A) / 133 B10 (B105A) / oedometer tests, 10 surcharge increments in total Page 5

6 Typical Results Oed. B9 Void ratio e NC Load Increments B9-8 Surcharge Increments B9-9 B9-10 B B Vertical effect stress ' [kpa] (log scale) (from Ng 1998) Page 6

7 Empirical Interpretation [Ladd s Method] (This method was proposed by Prof. Ladd in MIT soil behavior course since 1989) Vertical strain [%] Interpretating Oed. B9-9 with Ladd's method t r =2.09hr Estimated from NC Load incr. t 100 =1.13hr C NC = t s =9.51hr C '= Linear regression Time t [hr] Linear tangential fit C Bilinear intersection t s Easy for practical use Range of linear regression needs subjective judgment Page 7

8 Empirical Correlations 0.80 OCR C '/C NC = log(AAOS%) C ' / C NC B4 B7 B8 B9 B10 R 2 = AAOS(AOS)% (from Ng 1998)

9 Formulation of SSC [Vermeer & Neher 1999] Decomposition e vp Elastic e 0.434RR ' ' Hardening Viscoplastic (flow rule) vp C ' ref p CRRR C vp p p0 exp 0.434(CR RR) - > 0 : always creep Isotaches - Constant isotache distance C Page 9

10 SSC Parameters Parameters Suggested value or data for calibration Compression ratio CR 1D (or Isotropic) compression curve in -log 10 space Recompression ratio RR Reloading curve of 1D (or Isotropic) consolidation Secondary compression coefficient C Secondary compression curve or from C /CR ratio Reference time ref Normally assumed as 24 hrs Initial value of state parameter p0 Preconsolidation pressure from 24hr-IL oedometer test Data fitting CRS compression curve in -log 10 space Page 10

11 SSC Simulation of 1D Surcharge Test Soil k 0 [m/day] C k CR RR C NC ref [hr] pc0 [kpa] e 0 0 [kpa] SLC clay 2x Constant C Page 11

12 Isotache Concept [Suklje 1957] Isotaches 0.434Ĉ ln 1 Integration ' exp 0.434Ĉ d t ' 1 dt 0 0 t 0 0 ' 0.434Ĉ ln 1 t ' Ĉ Page 12

13 Interpreting Post-surcharge Creep with Isotache Concept Post-surcharge secondary compression can be captured by ' 0.434Ĉ ln Ĉ / t ' 1 Fitting two parameters: 1 and Ĉ Page 13

14 0.00 Example I: Fitting to Increment B9-11 (AOS=10%; OCR=1.10) 1 =4.29E-2%/hr B9-11 data Isotach fit R 2 = B9-11 data Isotach fit Strain [%] =4.29E-2%/hr 1 ^ C = fit C = NC ^ C = fit Strain [%] =5.09E-1%/hr 0.434Ĉ ln Time [hr] Strain rate [%/hr] Post-surcharge data is consistent with Isotache concept [Nash, 2001] Page 14

15 Example II: Fitting to Increment B9-9 (AAOS=29.1%; OCR=1.291) 1 =6.79E-3%/hr Vertical strain [%] B9-9 data Isotach fit 1 =6.79E-3%/hr ^ C = fit C NC = R 2 = ^ C = fit B9-9 data Isotach fit Vertical strain [%] =7.52E-1%/hr Time [hr] Strain rate [%/hr] Derived equation represents the entire evolution of post-surcharge creep

16 Existing Models Overestimate Creep Strains 0.00 Vertical strain [%] B9-9 Data Isotache fit Predict. with C NC 1 =6.79E-3%/hr ^ C = fit R 2 = C NC = Time [hr] Existing models overestimate post-surcharge settlement with constant C NC Page 16

17 Summary of Data fitting Surcharge Incr. OCR [%/hr] B B B B B B B B B B4-8* ?? R 2 C NC [%/hr] *Unreasonable > C NC ; Erratic changes in strain rate + short in data points for fitting Page 17

18 Creep Decays with OCR /C NC decreases dramatically with OCR Page 18

19 Initial Strain Rate Decreases with OCR 1 / 100 decreases with OCR following power law Page 19

20 Isotache with Creep Limit after Watabe (2012) Ĉ OCR lim OCR C NC OCR lim 1 OCR lim p / lim after Yin (1999) Ĉ 1 lnocr C NC lnocr lim OCR lim exp lim CR RR 2 Page 20

21 Interpretation of Models with Creep Limit Page 21

22 Summary Simple model of creep dependence with stress history is proposed. This captures development of post-surcharge creep more systematic than existing empirical methods to study effect of surcharge Interpretation of experimental data shows that surcharging indeed reduces 2 compression and post-surcharge creep rate. Ĉ 1 The results highlight the importance of general stress history influence on the creep properties and suggest improvements on isotache model. Page 22

23 Reference Ladd, C. C. (1989), Unpublished Class Notes for 1.322, Soil Behavior, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts. Nash, D. (2001), Precompression design for secondary settlement reduction: Discussion, Geotechnique,51,9, Ng, N.S.Y. (1998). Characterization of consolidation and creep properties of Salt Lake City clays. Master of Science Thesis, Massachusetts Institute of Technology. Suklje, L. (1957). The analysis of the consolidation process by the isotaches method. Proc. 4th Intl. Conf. on Soil Mechanics and Foundation Engineering, Vermeer, P. A., and Neher, H. P. (1999). A soft soil model that accounts for creep. Beyond 2000 in Computational Geotechnics, 249. Watabe, Y., Udaka, K., Nakatani, Y., and Leroueil, S. (2012). Long-term consolidation behavior interpreted with isotache concept for worldwide clays. Soils and Foundations, 52(3), Yin, J. H. (1999). Non-linear creep of soils in oedometer tests. Geotechnique, 49(5), Page 23

24 Thank you! Page 24