Evaluating the coefficient of consolidation using penetration tests

Abstract

This paper discusses the potential for using variable rate penetration tests as a means of assessing the consolidation coefficient for fine-grained soils. Previous testing has shown that the penetration resistance increases as the rate of penetration decreases, owing to partial consolidation in the soil ahead of the advancing probe. This led to the idea of reducing the penetration rate over a short interval during a standard (rapid) test, in order to identify the velocity at which consolidation effects become significant. By matching that point against a normalised backbone curve of penetration resistance versus non-dimensionalised velocity, the consolidation coefficient, cv, of the soil may be deduced. To explore this idea, a series of penetration tests in kaolin clay were undertaken on a drum centrifuge at the University of Western Australia (UWA), using a cylindrical T-bar penetrometer. A backbone curve was first established using constant penetration rate tests, but with velocities covering nearly 3 orders of magnitude. Then, a series of ‘twitch’ tests were undertaken, where the velocity was successively halved over 8 steps, from a high value (corresponding to undrained conditions) down to 0.4% of the initial value, with the penetrometer being advanced by either 1 or 2 diameters in each step. Comparison of the normalised penetration resistance with the backbone curve provided an estimate of the coefficient of consolidation, which was compared with independent estimates from oedometer tests. The tests gave very encouraging results, with the potential for the value of cv to be estimated within an error band of ±20%.

A simplified mechanically based model for predicting partially drained behaviour of penetrometers and shallow foundations

“…As reported by House et al (2001), the tests were conducted in normally consolidated Kaolin clay in a drum centrifuge. The T-bar was 5 mm in diameter and 20 mm long.…”

“…With the centrifuge at 100g, the scaling factor for diffusion time is 1/100 2 and for distance 1/100 (Garnier et al, 2007). As water was used in the tests of House et al (2001), the prototype velocity is therefore 1/100 that used in the model. A c v value was estimated at 3?3 m 2 /year (House et al, 2001), resulting in normalised velocities vD=c v of between 143 and 0?28.…”

“…As water was used in the tests of House et al (2001), the prototype velocity is therefore 1/100 that used in the model. A c v value was estimated at 3?3 m 2 /year (House et al, 2001), resulting in normalised velocities vD=c v of between 143 and 0?28.…”

“…Unfortunately, these tests did not extend into fully drained conditions. However, based on the previous experiences of Watson and Suemasa as reported by House et al (2001) and Chung et al (2006), a backbone curve assuming a ratio of drained to undrained capacity of 3?77 was fitted to the data and is represented in Fig. 5.…”

“…Under purely vertical load and in a single layer, recent tests at different penetration rates have provided evidence of this change in capacity. Backbone curves that normalise resistance against a normalised velocity have been established: initially for circular footings by Finnie (1993) and, more recently, for T-bar, ball and piezocone penetrometer site characterisation tests in unpublished data of Watson and Suemasa (quoted by House et al (2001) and Chung et al (2006)) and Randolph & Hope (2004), Schneider et al (2007) and Yafrate & DeJong (2007). An illustration of the vertical capacity with changing penetration rates is provided in Fig.…”