InterPore2023

The impact of heterogeneity in the pore structure of a laboratory sedimented clay sample when measuring swelling anisotropy

24 May 2023, 16:10

1h 30m

Poster Presentation (MS04) Swelling and shrinking porous media Poster

Speaker

Irene Rocchi (Department of Environmental and Resource Engineering, Technical University of Denmark)

Description

This study sets out to investigate the anisotropy of clay swelling by comparing the free swelling of specimens sampled radially and axially at different location in a large laboratory sedimented clay sample 20 x 6cm (diameter x height). The free swelling was measured on 2 x 2cm (diameter x height) cylindrical specimens sampled with stiff plastic containers having a closed based. The base had a small hole to allow air to escape during the sampling process and a filter paper was position at the base to allow uniform hydraulic boundary conditions during swelling. The sample was then placed in the chamber of a H1 Nuclear Magnetic Resonance setup and porosity was assessed through interpretation of the transverse relaxation time (T2) spectra. In addition, the setup used also provides the water profile along the sample, which allows to study the uniformity of the pore structure before and during swelling. Swelling was induced by filling the chamber with deionized water that entered the specimen from the base of the sampler. The porosity and water profile were then assessed at different time intervals and once marginal changes were observed the specimen was removed and the free swell was additionally measured with a gauge. Specimens taken both in the radial and axial direction were compared, as well at different distances from the center line and the sample faces. The results therefore address two issues related to homogeneity: the extent of initial pore structure homogeneity in a large laboratory sedimented sample and the homogeneity in swelling of the pore structure; besides swelling anisotropy due to preferential clay particles orientation during sedimentation.
The preparation of resedimented artificial soil samples is a common practice in geotechnical laboratory in many instances. Firstly, this may be applied to an intact sample (i.e. either block or core samples) for removing its structure, defined here as the combination of particle bonding and fabric. The aim being to quantifying the effects of such structure by subsequent comparison of the mechanical behaviour of the two samples (e.g. Burland 1990; or Cotecchia & Chandler 2000). Secondly, this technique has been widely used to prepared specimens for mechanical testing of those soils too difficult to sample without significant disturbance. Most commonly mine tailings, soil mixtures and intermediate soils, which tend to liquefy during sampling. In particular, Carraro & Prezzi (2008) devised a method to be employed for the preparation of individual specimens, while others (e.g. Shipton & Coop 2015) opted for batch production by using a large consolidometer to obtain a large “cake” to be then subsampled. Such technique is also typically chosen when artificial mixtures are prepared for ease and speed of production, e.g. when investigating fundamental aspects of soil behaviour. The possible separation of components during the sedimentation process is of particular concern in the case of soil mixture as this might result in heterogeneous samples. However, it has been observed also when resedimenting “pure clay” samples (Stallebrass et al. 2008).

References

Burland, J.B. 1990 On the compressibility and shear strength of natural clays. Geotechnique 30, 329-378.
Carraro, J.A.H. and Prezzi, M. 2008 A New Slurry-Based Method of Preparation of Specimens of Sand Containing Fines. Geotechnical Testing Journal 31(1), 1-11.
Cotecchia, F. and Chandler, R.J. 2000 A general framework for the mechanical behaviour of clays. Geotechnique 50(4), 431-447.
Shipton, B. and Coop, M.R. 2015 Transitional behaviour in sands with plastic and non-plastic fines. Soils and Foundations 55(1), 1-16.
Stallebrass, S.E., Atkinson, J.H. and Masin, D. 2008 Manufacture of samples of overconsolidated clay by laboratory sedimentation. Geotechnique 57(2), 249-253.