Cone Penetration Testing in unsaturated soils at two instrumented test sites

SUMMARYCavity expansion theory assists in the interpretation of in situ tests including the cone penetration test and pressuremeter test. In this paper, a cavity expansion analysis is presented for unsaturated silty sand exhibiting hydraulic hysteresis. The similarity technique is used in the analysis. The soil stress-strain behaviour is described by a bounding surface plasticity model. Results of oedometric compression tests, isotropic compression tests and triaxial shear tests for both saturated and unsaturated states are used to calibrate the model. The void ratio, suction, degree of saturation and effective stress are fully coupled in the analysis. The influence of where the initial hydraulic state is located on the soil-water characteristic curve on the cavity wall pressure is investigated and found to be significant. Also, the effects of three different drainage conditions (constant suction, constant moisture content and constant contribution of suction to the effective stress) on cavity wall pressure are studied. It is found that the drainage condition in which the contribution of suction to the effective stress is constant offers a good approximation to the other two. This may simplify interpretation of in situ tests. When testing occurs quickly, meaning a constant moisture content condition prevails, a constant contribution of suction condition can be assumed without loss of significant accuracy. The contribution of suction assumed in the interpretation can be taken as being equal to the in situ value, although this discovery may not be applicable to all soil types, constitutive models and soil-water characteristic curves.

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“…These observations are consistent with CPT results (e.g. [8,[17][18][19][20][21]), which show that suction has a pronounced effect on q c .…”

Section: Introductionsupporting

confidence: 93%

Cavity expansion in unsaturated soils exhibiting hydraulic hysteresis considering three drainage conditions

Yang

Russell

2015

Int. J. Numer. Anal. Meth. Geomech.

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“…So far these have been limited to a clean quartz sand and a silty sand (decomposed granite). These and other studies (Lehane et al 2004;Collins & Miller 2014) show that suction significantly increases qc.…”

Section: Introductionsupporting

confidence: 80%

Pitfalls in interpretation of cone penetration test data recovered from unsaturated geomaterials

Russell¹,

Reid²

2018

Proceedings of the 21st International Seminar on Paste and Thickened Tailings

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Laboratory-controlled and field cone penetration test (CPT) results for unsaturated geomaterials (sands, silty sands, tailings) show that suction has a pronounced influence. The penetration resistances increase significantly due to the presence of suction when compared to those for saturated or dry states, for a given relative density and net confining stress. For unsaturated sands, when suction is included in the effective stress, the same semi-empirical expressions used for saturated conditions are found to link penetration resistance to the relative density and effective confining stress. However, the same is not true for unsaturated silty sands, as saturated and unsaturated silty sands behave very differently around a penetrating cone. Suction also has implications when using classification charts. Cone penetration test results from a range of soils and tailings for both saturated and unsaturated conditions are plotted in one type of chart. Suction-induced increases in penetration resistances, and changes of soil behaviours from partially drained/undrained to drained cause incorrect classifications and assessments that geomaterials are dilative. Failure to account for suction influences, and the altered material response, may also lead to overestimation of cyclic resistance ratio.

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