1206
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Figure 7. 3D picture of model.
Eq. 3 depends upon two fundamental parameters: the initial
suction,
Ψ
0
, and the constant
C
.
The initial suction may be obtained by any of the techniques
for measuring suction in soils compatible with the range of
suctions of the sample: tensiometer, filter paper, psicrometer,
etc. To obtain the constant
C
, suction controlled oedometer test
must be performed subjecting the sample at least to two
different vertical pressures.
This way, with the determination of
Ψ
0
and the
C
coefficient, it is possible to establish a simple method that will
represent with sufficient approximation the collapsing behavior
of a collapsing soil from the initial suction until saturation.
5 CONCLUSIONS
Recent models to predict the collapsing behaviour in low
density, partially saturated soils, obey to very complex
formulations with a high number of parameters. These
parameters can only be carried out using advanced laboratory
tests, not commonly available even in advanced laboratories.
For many problems of foundations on collapsing soils it may
be assumed that displacement are one dimensional and also that
wetting of the soil and collapse occurs after the soil is loaded.
For this cases, a simple model has been presented in this paper
that describes, with sufficient precision, the behaviour of a
collapsing soil (a mixture of sand, silt and clay) when subject,
under oedometric conditions, to a wetting stress-path under
constant vertical stress.
The model proposed is based on a linear relationship
between volumetric deformation, the log of relative suction and
the log of the vertical pressure. The equation depends upon a
coefficient called the Instability Index, which in turn is
proportional to the log vertical pressure.
The model is valid until the samples have reached the field
capacity; then the volumetric strain becomes constant.
This model needs only two parameters to be defined: the
initial suction and the coefficient
C
that relates the Instability
Index with the log of the vertical pressure. This parameter is
obtained in suction controlled oedometer tests, for two different
constant vertical pressures.
The simplicity of the proposed model makes it interesting for
a quick estimate of the collapse vertical strains in partially
saturated soils.
6 ACKNOWLEDGEMENTS
This work has been financed by the Spanish Ministry of Science
and Innovation (Project BIA 201020377).
7 REFERENCES
Alonso E.E., Gens, A. and Hight D.W. 1987. Special problem soils.
General report. In Proceedings of the 9th European Conference on
Soil Mechanics and Foundation Engineering, Dublin, Vol. 3: 1087-
1146.
Aitchison G.D., Peter, P. and Martin R. 1973. The Instability Indices
Ipm and Ips in expansive soils. Third International Conference on
Expansive Soils, Haifa, .
Balmaceda A.R. 1991. Suelos compactados. Un estudio teórico y
experimental. Ph.D. thesis. Polytechnical University of Catalonia,
Spain.
Booth, A.R. 1975. The factors influencing collapse settlement in
compacted soils. Proc. 6th Reg. Conf. For Africa on S.M.F.E.,
Durban, Vol. 1: 57-63.
Cox, D.W. 1978. Volume change of compacted clay fill. Clay fills,
London, ICE: 79-86.
Cui Y.J. Delage P. and Sultan N. 1995. An elasto-plastic model for
compacted soils. Proceedings of the 1st International Conference on
Unsaturated Soils. Volume 2, 703-709. París.
Dudley A. 1970. Review of collapsing soils. Journal of the Soil
Mechanics and Foundations Division. ASCE, Vol. 96: 925-947.
Escario V. 1969. Swelling of soils in contact with water at a negative
pressure. Proc. of the 2nd International Research and Engineering
Conference on Expansive Clay Soils, Texas: 207-217.
Escario V. and Sáez J. 1973. Measurement of the properties of swelling
and collapsing soils under controlled suction. Proc. 3rd Int. Conf.
Expansive Soils, Haifa: 196-200.
Fredlud D.G. y Morgenstern N.R. 1976. Constitutive relations for
volume change in unsaturated soils. Canadian Geotechnical Journal
13, No 3: 261-276.
Habibagahi G. and Mokhberi M. 1998. A hyperbolic model for volume
change behavior of collapsible soils. Canadian Geotechnical
Journal. 35, 264-272.
Jiménez Salas J.A., Justo J.L., Romana M. and Faraco C. 1973. The
collapse of gypseous silts and clays of low plasticity in arid and
semiarid climates. Proc. 8th I.C.S.M.F.E., Moscú : 161-190.
Josa A, Balmaceda A, Gens A and Alonso EE. 1992. An elastoplastic
model for partially saturated soils exhibiting a maximum of
collapse. Proceedings of the 3rd International Conference on
Computational Plasticity. Volume 1, 815-826, Barcelona.
Justo J.L. and Saettersdal R. 1979. Design parameters for special soil
conditions. General Report. In Proceedings of the 7th European
Conference on Soil Mechanics and Foundation Engineering,
Southampton, Vol. 5: 127-158.
Li X.S. and Fang X.W. 2011. Consistent modelling of expansive and
collapsive response of unsaturated soils. Geotechnical and
Geological Engineering, 29, 203-216.
Maswoswe J. 1985. Stress path for compacted soil during collapse due
to wetting. Ph.D. Thesis, Imperial College, London.
Romero E. 1999. Thermo-hydro-mechanical behaviour of unsaturated
Boom clay: an experimental study. Ph.D. Thesis, Polytechnical
University of Catalonia, Barcelona.
Sheng D, Sloan S.W. and Gens A. 2004. A constitutive model for
unsaturated soils: thermodynamical and computational aspects.
Computational Mechanics. 33(6), 453-465.
Terzaghi K. and Peck R. 1948. Soil Mechanics in Engineering Practice.
Wiley, N.Y.
Vilar O.M. 1995. Suction controlled oedometer tests on a compacted
clay. Proc. 1st Int. Conf. on Unsaturated Soils, Paris. Editors E.E.
Alonso and P. Delage, Balkema/Presses des Ponts et Chaussées, 1:
201-206.
Yudhbir 1982. Collapsing behaviour of residual soils. Proc. 7th
Southeast Asian Geot. Conf., Hong-Kong, Vol.1: 915-930.
Yuk Gehling, W.Y. 1994. Suelos expansivos: estudio experimental y
aplicación de un modelo teórico. Ph.D. Thesis, Polytechnical
University of Catalonia, Barcelona.
Wheeler S.J. and Sivakumar V. 1995. An elasto-plastic critical state
framework for unsaturated soil. Géotechnique. 45(1), 35-53.
