Fundamentals of Hydraulic Behaviour of Unsaturated Soils

Transcription

1 Fundamentals of Hydraulic Behaviour of Unsaturated Soils School of Engineering and Computing Sciences, Durham University, UK

2 Hydraulic Behaviour Two aspects we will consider: Soil Water Retention Curves (SWRC) Water in Soils What is Suction? How do we measure suction? Fundamentals of SWRC Comparisons between SWRC measurement techniques Hydraulic Conductivity (Permeability to water) Measuring Hydraulic Conductivity

3 Water in Soils

4 Water in Soils Lourenço, S.D.N., Toll, D.G., Augarde, C.E., Gallipoli, D., Congreve, A., Smart, T. & Evans, F.D. (2008) Observations of unsaturated soils by Environmental Scanning Electron Microscopy in dynamic mode, Unsaturated Soils: Advances in Geo-Engineering, Proc. 1st European Conf. Unsaturated Soils, pp

5 Water in Soils Lourenço, S.D.N., Gallipoli, D., Augarde, C.E., Toll, D.G., Fischer, P.C. and Congreve, A. (2012) Formation and evolution of water menisci in unsaturated granular media, Géotechnique 62(3) pp

6 Volumetric water content, Soil Water Retention Curve s Air entry value Scanning curves Primary wetting curve Primary drying curve Residual suction r Water entry value Suction (log scale)

7 Hysteresis etc. Lourenço, S.D.N., Gallipoli, D., Augarde, C.E., Toll, D.G., Fischer, P.C. and Congreve, A. (2012) Formation and evolution of water menisci in unsaturated granular media, Géotechnique 62(3) pp First wetting Second wetting

8 What is Suction?

9 Suction what is it? Porous ceramic (allows movement of water and salts) Semi-permeable membrane (allows movement of water only) Toll, D.G. (2012) The Behaviour of Unsaturated Soils, Chapter 5 in Handbook of Tropical Residual Soils Engineering (eds. B.B.K. Huat, D.G. Toll, A. Prasad), Leiden: CRC Press

10 Axis Translation u a ( u a - u w ) Air R 0 (Atmospheric pressure) u a ( u a - u w ) u w Water u w Field situation Axis translation

11 The Suction Scale Suction Suction Reference Moisture (kpa) (pf) points Condition 1,000,000 7 Oven dry 100,000 6 Dry pf units represent the logarithm (to base 10) of the suction expressed in centimetres of water (Schofield, 1935) 10,000 5 Suction (kpa) = 9.81 x 10 [pf-2] 1, Wilting point for plants Plastic Limit Moist Liquid Limit Wet Saturated

12 How do we measure suction?

13 Suction Measurement and Control Suction (kpa) Tensiometer (conventional) 2MPa Tensiometer (high capacity) Psychrometer Filter Paper Suction Plate 1.5MPa Pressure Plate Osmotic Control Vapour Control Measurement Control Toll, D.G. (2012) The Behaviour of Unsaturated Soils, Chapter 5 in Handbook of Tropical Residual Soils Engineering (eds. B.B.K. Huat, D.G. Toll, A. Prasad), Leiden: CRC Press

14 Pressure plate

15 High Suction Tensiometers High Capacity tensiometers are capable of direct measurement to -2MPa Relies on a small volume water reservoir Based on the concept first demonstrated by Ridley & Burland (1993) UNINA Tensiometer Rojas et al. (2008) 20 mm Tarantino and Mongiovì (2003) Durham University Tensiometer Lourenço et al. (2006) Toll, D.G., Lourenço, S.D.N. and Mendes, J. (2013) Advances in suction measurements using high suction tensiometers, Engineering Geology, Vol. 165, pp

16 Cavitation -100 kpa kpa

17 Chilled mirror hygrometer Decagon WP4C (Chilled mirror hygrometer) MPa range

18 Filter paper Measurement filter paper (total suction) Separation mesh Measurement filter paper (matric suction) Sealed container Sacrificial filter papers Soil sample

19 Vapour & Osmotic control Soil sample Mesh Aqueous solution of known concentration Sealing lid Beaker PEG solution Soil Sample Semi-permeable membrane enclosing soil sample Magnetic Stirrer

20 Fundamentals of Soil Water Retention Curves

21 Soil Water Retention Curve (gravimetric)

22 Volume Change Behaviour

23 Soil Water Retention Curve (Volumetric) Volume of water Total volume Air Entry Value Residual Water Content

24 Soil Water Retention Curve (Degree of Saturation) Volume of water S r Volume of voids Air Entry Value Residual S r

25 Shrinkage & SWRC e w Volume of water Volume of solid e.s r w.g s w Toll, D.G. (1995) A Conceptual Model for the Drying and Wetting of Soil, in Unsaturated Soils (eds. E.E. Alonzo & P. Delage), Rotterdam: Balkema, Vol. 2, pp

26 Soil Water Retention SURFACES Gallipoli, D., Wheeler, S.J. & Karstunen M. (2003) Géotechnique 53(1), pp Tarantino, A. (2009) Géotechnique 59 (9), pp

27 Comparisons between SWRC measurement techniques

28 Tensiometer vs. Pressure Plate after Lourenço (2008) after Tarantino et al. (2011)

29 Comparison of SWRC techniques High suction tensiometer Durham University tensiometer (2MPa capacity) Pressure plate 1.5 MPa HAEV ceramic Filter paper Whatmans 42, 14 day equalisation, van Genuchten s (1980) calibration Chilled Mirror Dewpoint method Decagon Devices WP4C Toll, D.G., Asquith, J.D., Fraser, A., Hassan, A.A., Liu, G., Lourenço, S.D.N., Mendes, J., Noguchi, T., Osinski, P., Stirling, R.A.. (2015) Tensiometer techniques for determining soil water retention curves. Proc. 6th Asia-Pacific Conference on Unsaturated Soil. Guilin, China pp

30 Soil information The soil was a glacial till (Durham Lower Boulder Clay) Fill material used in the construction of the BIONICS experimental embankment Prepared by sieving through a 2.8mm sieve Sandy clay soil: 30% sand, 35% silt and 35% clay Liquid Limit 43% Plastic Limit 24% Plasticity Index 19% Particle density 2.66 Mg/m 3

31 Soil water retention measurement

32 Methods of SWRC determination Terre School, Napoli, 26 September st

33 SWRC low suction range Terre School, Napoli, 26 September st

34 SWRC Degree of saturation Terre School, Napoli, 26 September st

35 Shrinkage curves Terre School, Napoli, 26 September st

36 Hydraulic Conductivity (Water Permeability)

37 Relative Conductivity Water and Air Permeability k ra k k a d k rw k k w s k d Dry conductivity to air k ra k rw k s Saturated conductivity to water Degree of Saturation, S r (%) Relative air and water permeability as predicted by the Brooks and Corey model (Lu & Likos, 2004)

38 SWRC and Hydraulic Conductivity Data from Brooks and Corey 1964 Fredlund, D.G., Xing, A. & Huang, S. (1994) Predicting the permeability function for unsaturated soils using the soil-water characteristic curve. Canadian Geotechnical Journal, 31(3):

39 SWRC and Hydraulic Conductivity Data from Brooks and Corey 1964 Fredlund, D.G., Xing, A. & Huang, S. (1994) Predicting the permeability function for unsaturated soils using the soil-water characteristic curve. Canadian Geotechnical Journal, 31(3):

40 SWRC and Hydraulic Conductivity Data from Brooks and Corey 1964 Fredlund, D.G., Xing, A. & Huang, S. (1994) Predicting the permeability function for unsaturated soils using the soilwater characteristic curve. 1st Terre School, Napoli, 31(3): September 2016 Canadian Geotechnical Journal,

41 Measuring Hydraulic Conductivity

42 Instantaneous profile method Muñoz, J.J., De Gennaro, V. & Delaure, E. (2008) Experimental determination of unsaturated hydraulic conductivity in compacted silt, 1 st European Conf. on Unsaturated Soils, Durham, UK, pp

43 Infiltration Column Duong, T.V., Cui, Y.J., Tang, A.M., Dupla, J.C., and Calon, N. (2014), Effect of fine particles on the hydraulic behavior of interlayer soil in railway substructure. Canadian Geotechnical Journal, 51, pp

44 Compacted Silt Muñoz, J.J., De Gennaro, V. & Delaure, E. (2008) Experimental determination of unsaturated hydraulic conductivity in compacted silt, 1 st European Conf. on Unsaturated Soils, Durham, UK, pp

45 Interlayer soil in railway substructure Duong, T.V., Cui, Y.J., Tang, A.M., Dupla, J.C., and Calon, N. (2014), Effect of fine particles on the hydraulic behavior of interlayer soil in railway substructure. Canadian Geotechnical Journal, 51, pp

46 Coarse fill Chen, R.P., Qi, S., Cheng, W. and Wang, H. (2016) Effect of Compactness Degree on the Hydraulic Properties for Coarse Soils of High-Speed Railway Embankment, Advances in Transportation Geotechnics 3, Procedia Engineering, Vol. 143, pp

47 Conclusions Soil Water Retention Curves (SWRC) and Hydraulic Conductivity functions are essential to understand and model unsaturated soil behaviour Measurement of suction has greatly improved with high capacity tensiometers and chilled mirror hygrometers Always measure volume change as part of determining SWRC assuming a rigid porous media is not enough Hydraulic conductivity (permeability to water) changes by several orders of magnitude, sometimes with quite small changes in suction Instantaneous profile method provides an opportunity to obtain good experimental measurements of hydraulic conductivity in unsaturated soils

48 Thank you and enjoy Napoli