Stress path during pressuremeter test and link between shear modulus and Menard pressuremeter modulus in unsaturated fine soils

Vincent Savatier,Maxime Savatier,P Delage,Y.-J Cui,A.-M Tang,J.-M Pereira,S Ghabezloo

doi:10.1051/e3sconf/20160909010

Abstract

This paper presents the stress path before and during a pressumeter test using a Modified Cam-Clay model. This model have been used for unsaturated clay with continuous water phase (D2 domain according to Boutonnier 2007) and for silty to clayey soils with discontinuous water phase (D1 domain – ibid.). After reminding the stress distribution around a pressuremeter cell and the meaning of the Menard pressuremeter modulus which is linked to the soil shear modulus, we explain the stress state during pressuremeter modulus measurement. Then, we show that this stress path diagram can be used to explain classical observations made using Menard pressuremeter tests in continental area and typically: Measurement of high ratio “Menard pressuremeter modulus / pressuremeter limit pressure” in clay subjected to drought. Limit pressure and Menard pressuremeter modulus variation with respect to drought in silty clay situated in water-table fluctuation area. Finally, we will try to show the consequences of these stress paths for performing and interpreting pressuremeter test in unsaturated and collapsible soils.

Highlights

The stress state around a pressuremeter test look like the stress state around an expending cylindrical cavity
The deformation of the radius r follows in elastic state follows (G is the shear modulus): du=dpc. ro u
For limit pressure the stress path will depend on initial saturation degree and suction

Summary

Stress state around pressiometer cell in Cambriges axes

During test while staying in elastic state stress equation are the same that equations (2) replacing pc0 with pc pressure at the hole limit: q= max ( h‐ pc ; h(K0)- pc ). Shear modulus is proportional to average stress It appears around the cell a soil crown in which limits the values of p and q, can be found with: The equation of the limit line for normally consolidated soil with hydric state that allows consolidation: q=( 'rp ‐ ’ p )=M ’m = M.p’. They are given considering two cases: If ’mpo >p= h(1+2K0)/3 deformation will be with effective stress increase and negative interstitial water pressure (U

Pressuremeter behavior of unsaturated soils

D1 soil case

D2 soil case

Plastic stress variation around pressiometer

Stress way of pressuremeter test in unsaturated soil

Unsaturated silts collapsible soil