Soil Parameters for Drained and Undrained Analysis

Introduction • The aim is to discuss the choice of parameters for the MohrCoulomb model. • More advanced soil models may have some advantages over the MohrCoulomb model (but require the specification of a larger number of parameters) • Typical experimental methods currently used to measure the soil parameters are briefly discussed. • It is also useful, however, to estimate values of soil properties based on previous experience, and on correlations with other soil parameters.

Soil Parameters for Drained and Undrained Analysis Applied Theory Dr Minna Karstunen based on work by Dr H Burd, University of Oxford Introduction • The aim is to discuss the choice of parameters for the Mohr-Coulomb model • More advanced soil models may have some advantages over the Mohr-Coulomb model (but require the specification of a larger number of parameters) • Typical experimental methods currently used to measure the soil parameters are briefly discussed • It is also useful, however, to estimate values of soil properties based on previous experience, and on correlations with other soil parameters Undrained and Drained Loading • In carrying out any analysis in geotechnical engineering it is usually necessary to distinguish between drained and undrained loading • The soil may also be partially drained which means that it lies between these two extremes Undrained and Drained Loading • drained analysis appropriate when – permeability is high – rate of loading is low – short term behavior is not of interest for problem considered • undrained analysis appropriate when – permeability is low and rate of loading is high – short term behavior has to be assessed Undrained and Drained Loading Suggestion by Vermeer & Meier (1998) T < 0.10 (U < 10%)
undrained analysis T > 0.40 (U > 70%)
drained analysis k E oed T= t γw D k = Eoed = γw = D = t = T = U = permeability stiffness in 1-d compression unit weight of water drainage length construction time dimensionless time factor degree of consolidation Drained Analysis Drained analysis may be carried out by using a constitutive model based on effective stresses in which the material model is specified in terms of drained parameters Modelling Undrained Behavior with PLAXIS Method A (analysis in terms of effective stresses): type of material behaviour: undrained effective strength parameters (MC: c', ϕ', ψ‘) effective stiffness parameters (MC: E50', ν‘) Method B (analysis in terms of effective stresses): type of material behaviour: undrained total strength parameters c = cu, ϕ = 0, ψ = effective stiffness parameters E50', ν' Method C (analysis in terms of total stresses): type of material behaviour: drained total strength parameters c = cu, ϕ = 0, ψ = total stiffness parameters Eu, νu = 0.495 Need to be careful in case of stiff OC clays! Mohr Coulomb Model for Drained and Undrained Analysis • For drained loading, a total of parameters are required to specify the Mohr-Coulomb model These are; two strength parameters (c' and φ' ), a dilation angle (ψ) and two elastic parameters • For undrained calculations, a separate failure model based on an undrained shear strength, cu, is used Note that cu is not a fundamental property of the soil; it depends on the stress level and also the stress history Mohr Coulomb Model for Drained and Undrained Analysis Drained or Undrained (Approach A) Undrained (Approach C) Mohr Coulomb Model for Drained and Undrained Analysis • To analyse a problem using the Mohr-Coulomb model, appropriate values of the material parameters must be selected to provide a good match with the soil being modelled • The selection of these parameters is complicated by the fact that real soil behaviour often departs considerably from the fundamental assumptions on which the Mohr-Coulomb model is based Correlations for Friction Angle Determining the relative density of a sand deposit is rather difficult For correlations that relate cone resistance to relative density are described in Lunne et al 1997 Estimation of Stiffness Stiffness of Clay • Option - Use E50 For problems here relatively large strains are expected (e.g for foundation bearing capacity and studies of the deformation of soft soil beneath an embankment) • Option - Use a small strain Young's modulus If the problem involves the calculation of deformations of stiff clay under working conditions (e.g the analysis of the interaction between a tunnel liner and the surrounding ground) • Option - Use the unloading Young's modulus, Eur If the problem is dominated by unloading (as may be the case, for example, in an excavation problem) Measurement of Stiffness in the Triaxial test Not accurate for strains below 1% Measurement of Stiffness in the Triaxial test Correlations for Stiffness Jardine et al (1984) conducted a series of triaxial tests on a range of soils, using local gauges to measure strains Correlations for Stiffness Jardine et al (1984) Correlations for Stiffness Plate loading tests by Duncan & Buchignani (1976) Data correspond to strain values of about 0.1% Correlations for Stiffness Data from Termaat, Vermeer and Vergeer (1985) may be used to suggest the following correlation for normally consolidated (Dutch) clay: 15000cu E ≈ IP u 50 Case Studies Stiffness profile for various London clay site (Matthews et al, 2000, re-plotted by Simon and Menzies 2000) Case Studies Scott et al (1999) Stiffness Anisotropy • Recent studies on natural clays (normally consolidated and overconsolidated) suggest that their stiffness may be anisotropic Typical data for London clay can be found e.g in Gasparre et al (2007) Stiffness of Sands • Based on data on undrained triaxial testing of sandfs at different densities by Tokheim (1976) and Leahy (1984)Loose sand References: • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Atkinson, J.H (2000) Non-linear soil stiffness in routine design Géotechnique 50(5), 487-508 Atkinson, J.H., Richardson, D and Stallebrass, S.E (1990) Effect of recent stress history on the stiffness of overconsolidated soil Géotechnique 40(4) 531-40 Bolton, M.D (1986) The strength and dilatancy of sands Géotechnique 36(1), 65-78 Bolton, M.D (1987) Discussion on the strength and dilatancy of sands Géotechnique 37(2), 219-226 Burd, H.D (2007) Soil parameters for drained and undrained analysis Numerical Methods in Geotechnical Engineering, 12-14 June, 2007, Manchester Burland, J.B and Hancock, R.J.R (1977) Underground car park at the House of Commons: geotechnical aspects The Structural Engineer, 55(2), 87100 Burland, J.B and Kalra, J.C (1986) Queen Elizabeth II conference centre geotechnical aspects Proc ICE, Part 1,80 Clarke, B.G (1995) Pressuremeters in geotechnical design Blackie Academic Clayton, C.R.I, and Khatrush, S.A (1986) A new device for measuring local axial strains on triaxial specimens Géotechnique 36(4) 593-598 Clayton, C.R.I., Edwards, A and Webb, J (1991) Displacements in London clay during construction Proc 10th Int Conf on Soil Mech and Fdn Engng, Florence, 2, 791-796 Clayton, C.R.I., Matthews, M.C and Simons, N.E (1995) Site Investigation Blackwell Science Cole, K.W and Burland, J.B (1972) Observations of retaining wall movements associated with large excavation Proc 5th European Conf on Soil Mechanics and Foundation Engineering, Madrid, 1,445-453 Duncan and Buchignani (1976) Gasparre, A., Nishimura, S., Minh, N.A., Coop, M.R and Jardine, R.J (2007) The stiffness of natural London Clay Géotechnique 57(1) 33-47 Gordon, M.A (1997) Applications of field seismic geophysics to the measurement of geotechnical stiffness parameters PhD Thesis, University of Surrey, Guildford Hope, V.S (1993) Applications of seismic transmission tomography in civil engineering PhD Thesis, University of Surrey, Guildford Jardine, R.J , Symes, M.J and Burland, J.B (1984) The measurement of soil stiffness in the triaxial apparatus Géotechnique 34(3) 323-340 Leahy, D (1984) Deformation of dense sand, triaxial testing and modelling PhD thesis, NTNU, Trondheim Lunne, T., Robertson, P.K and Powell, J.J.M (1997) Cone Penetration Testing in Geotechnical Practice Blackie Academic Mair, R.J (1993) Developments in geotechnical engineering research: applications to tunnels and deep excavations Unwin memorial Lecture 1992 Proc ICE, 3,27-41 Matthews, M.C., Clayton, C.R.I., and Own, Y (2000) The use of field geophysical techniques to determine geotechnical stiffness parameters Proc ICE (Geotechnical Engineering),143, 31-42 Muir Wood, D.M (1990) Soil Behaviour and Critical State Soil Mechanics Cambridge University Press Scott, P., Talby, R and den Hartog, N (1999) Queensbury House, London: a case study of the prediction and monitoring of settlements during the construction of a deep excavation Proc Int Symp Beyond 2000 in Computational Geomechanics, 163-176 A.A Balkema Simons, N and Menzies, B (2000) A short course in foundation engineering Thomas Telford 2nd Ed Stevens A et al (1977) Barbican Arts Centre The Structural Engineer, 55(11) 473-485 St John, H.D., Potts, D.M., Jardine, R.J and Higgins, K.G (1993) Prediction and performance of ground response due to construction of a deep basement at 60 Victoria Embankment Proceedings of the Wroth Memorial Symposium, Oxford, July 1992, 581-608 Thomas Telford Termaat R.J., Vermeer P.A and Vergeer G.J.H (1985) Failure by large plastic deformation Proc ICSMFE, 4, 2045-2048 Tokheim, O (1976) A model for soil behaviour PhD thesis, NTNU, Trondheim Wroth, C.P (1984) The interpretation of in-situ soil tests 24th Rankine Lecture, Géotechnique, 34(4), 449-89 Wroth, C.P (1988) Penetration testing - a more rigorous approach to interpretation Proc Of International Conf on Penetration testing, ISOPT-1, Orlando, 1, 303-311 Bibliography • Further information on the topics discussed in this lecture can be found in the following books: • Simons, N., Menzies, B and Matthews, M (2002) A short course in geotechnical site investigation Thomas Telford • Potts, D.M and Zdravkovic, L (2001) Finite element analysis in geotechnical engineering Application Thomas Telford • Loo, B (2007) Handbook of Geotechnical Investigations and Design Tables Taylor & Francis