Maturing of contacts and ageing of silica sand

Abstract

A typical surface texture of silica sand grains is rich in asperities and mineral debris. Consequently, a nominal inter-granular contact is composed of many ‘contact points’. Associated with the contact loading process is micro-fracturing of the textural features, causing the contact to evolve. Even under a constant load, the process of micro-fracturing continues, although with a decreasing frequency. This static fatigue at contacts (or contact maturing) under a constant load is considered to be a major contributor to ageing of silica sand. The rich surface micro-morphology of silica sand grains is demonstrated using scanning electron microscopy and atomic force microscopy. Grain-scale tests indicate time-dependent deflection under constant load, attributed to delayed fracturing of textural features at grain contacts. The process is greatly dependent on the initial roughness of the surfaces. The rate of the deflection for a grain with large initial roughness (root mean square of grain surface elevation (RMS) = 621 nm), loaded with 2·4 N, was found to be 17·6 nm/h at the end of the first day after loading, whereas for a grain with approximately half the initial roughness (RMS = 321 nm) it was only about 2 nm/h. A consequence of the contact maturing process in grain assemblies is a time-dependent increase in macroscopic stiffness, and a resulting alteration of the stress state in a confined sand. This was confirmed in soft-ring oedometer tests on specimens subjected to constant load. The temporal scale of the radial stress increase in the oedometric samples was about the same as that in convergence tests on single grains. Contact maturing is accelerated by the presence of moisture and pressure dissolution, and this was manifested in tests on samples saturated with pore fluid. The results of both grain-scale and sand specimen tests are consistent with the hypothesis indicating contact maturing as the major contributor to time-dependent effects in silica sand.