Soil structure change with particle loss

  Soil particle loss can occur through biodegradation, erosion and dissolution. Yet there is little understanding of the mechanical changes that accompany these phenomena, especially where the size of particle removed is concerned. This study investigated the influence of particle loss on the volumetric, strength and smallstrain
sti�ness of analogue soils. These consisted of uniform Leighton Buzzard sand with selected salt particle sizes. Particle sizes chosen for experimental tests are representative of the �fines that might be lost through an erosion process called suffusion in embankment dams or the various sizes that might be degraded and/or decomposed in landfill and mining wastes. A triaxial apparatus was modified to allow the in-situ dissolution of samples under triaxial stress states. This was achieved through the circulation of water through the pore-water pressure line with ow controlled by differential pressure using a peristaltic pump. Bender elements were installed to
monitor changes in shear wave velocity before, during and after dissolution. Test results showed increases in void ratio in all dissolution tests. The influence of salt size and the stress under which tests were performed was found to have a limited impact on the magnitude of void ratio increase. Salt particle size did, however, affect the initial packing density of the sand-salt mixtures with �fine salt sizes resulting in lower void ratios. Therefore, these tests showed lower post-dissolution void ratios. Coarse salt sizes initially densely prepared resulted in high post-dissolution void ratios close to the maximum void ratio for the Leighton Buzzard sand. Ultimately, post-dissolution void ratios determined the large-strain shearing behaviour.
Therefore the fine salt tests, in which the post-dissolution void ratios were lowest, were the only tests to show minor peak strengths prior to the critical state with a shear behaviour described as strain-softening dilative. The comparatively high void ratios obtained in coarse salt tests showed no peak strength but a strain-hardening
contractive behaviour. The structural role of salt particles within sand mixtures was continually assessed with evidence suggesting that salt particles maintained
their structural integrity under the stresses applied through loading and subsequent shearing in this study. The influence of particle loss on the critical state was also
probed. Post-dissolution samples consistently showed higher critical void ratios than sand-only samples not subjected to particle loss. Most of the findings might be explained in the context of strong force chains and their stability which is in turn influenced by the amount and size of soluble particles. Shear wave velocities were shown to decrease significantly with dissolution of 15% of weight of salt irrespective of size. Associated small-strain stiffness moduli were found to decrease even more substantially. The reported changes illustrate the significant influence that particle removal has on the mechanical properties of soil and are discussed and analysed
within this thesis.

  • Dates:

    2009 to 2015

  • Qualification:

    Doctorate (PhD)

Project Team