Microstructural changes of an undisturbed, reconstituted and compacted high plasticity clay subjected to wetting and drying

Abstract

a b s t r a c tThe behaviour of soil, and in particular compacted clay fill, can have significant implications on the safe and reliable operation of man-made infrastructure. The mechanical behaviour of soil (e.g. volume change and shear strength) is widely recognised as being associated with the microstructural arrangement (fabric/structure). In the case of high plasticity clays, despite the large amount of research carried out, soil microstructure and its evolution along mechanical and hydraulic paths are still not well understood. This makes incorporation of microstructural analysis difficult in engineering practice and highlights the need for further research. A comprehensive microstructural analysis of Maryland clay, a high plasticity residual soil, based on mercury intrusion porosimetry tests, is presented in this paper. Experimental results obtained from undisturbed, reconstituted and compacted specimens subjected to different hydraulic and mechanical paths are described. As with mechanical investigations, the reconstituted state is proposed to be used routinely as a reference state for comparison of undisturbed and compacted soil. The microstructural evolution of the compacted clay, prepared on the wet side of standard Proctor optimum water content, with an initially high void ratio, is examined along the main drying path. Importantly, a monotonic suction increase from the as-compacted state is shown to have negligible effect on the distribution of macro-pores. However, a new insight is provided based on the evolution of the dominant micro-pore entrance diameter which is shown to reduce with increased suction. This micro-pore entrance diameter is shown to correspond with the theoretical suction back-calculated from a simple capillary tube model, up to a limit. It is observed that, under oedometric conditions, the as-compacted microstructure is erased during saturation (soaking) and resembles the reconstituted microstructure. For this particular material and preparation conditions, it is demonstrated that a bimodal microstructure is not recovered on drying from a saturated state.

One-dimensional compression and swelling of compacted fly ash

“…For material compacted on the wet side of the Proctor optimum water content, the initial bimodal pore size distribution depends heavily on the state of saturation during compaction (Delage et al, 1996). The as-compacted structure is erased on saturation (Burton et al, 2015). Macropore space decreases as the water content or the compaction energy increases, whereas the micropore space remains unaltered by compaction.…”

Matric Suction and Volume Characteristics of Compacted Clay Soil under Drying and Wetting Cycles

“…In contrast, Pires (2008) observed that for compacted soils of mixtures of clay and sand, inter-aggregate pores larger than 500m suffered a major increase after wetting-drying cycles. Burton (2015) demonstrated that a bimodal microstructure is not recovered on drying from a saturated state for compacted high plasticity clay.…”

Impact of Drying and Effective Stresses on the Pore Space and Microstructure of Mudrocks

“…The influence of drying on the microstructure of mudrocks was investigated using MIP and SEM. Systematic comparisons of the pore size distributions reported by MIP for samples prepared by plunge freezing followed by freeze‐drying against oven‐dried or air‐dried samples indicated that only large pores collapse, while small pores remain unaffected (Burton et al, ; Delage & Lefebvre, ; Sasanian & Newson, ; Simms & Yanful, ). Contrary to these findings, SEM analyses reported no difference in the pore space of oven‐dried and plunge‐frozen Boom clay samples, despite a macroscopic shrinkage of approximately 10% (Desbois et al, ).…”

“…The above studies on the influence of effective stresses (Al‐Mukhtar et al, ; Delage & Lefebvre, ; Dewhurst et al, ; Griffiths & Joshi, ; Qi et al, ) and the influence of drying (Burton et al, ; Delage & Lefebvre, ; Sasanian & Newson, ; Simms & Yanful, ) on the pore space assumed that MIP provides accurate estimates of the pore size distribution in mudrocks. However, Diamond () found that, for cement‐based materials, obtain valid pore size distributions using MIP are not met.…”