The compression of mudrock fills

• Main Author: Nahazanan, Haslinda
• Published: University of Sheffield 2010
• Subjects: 624.15
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.557132

Mudrocks are indurated sediments widely encountered throughout the world, often associated with opencast mining activities where they are used as backfill. Opencast coal mines are frequently reinstated with substantial depths of fill to provide land for development. However, mudstones tend to change from a brittle crushable nature to a ductile clay material when inundated, causing major settlement problems when used as a fill. Three distinguishable settlement stages exist; primary compression due to immediate placement of the fill, relatively short-term but often very large strain collapse due to inundation and finally long-term creep and consolidation (Reed & Singh, 1986; Hills & Denby, 1996 and Goodwin et al., 2003). To provide a tool for engineers to predict primary and collapse settlements for large areas of mudrock fill, Rowe consolidation cell tests were conducted to analyse the influence of initial moisture content, initial particle size distribution and stress on the collapse settlement and weathering of mudrock. Triaxial isotropic tests were also conducted to investigate the stress-strain characteristics of mudrock under three states: dry, short-term inundated, and long-term. The developed predictive model was then compared with a previous model for mudrock collapse developed by Blanchfield (2000). The Rowe consolidation test results show small changes in void ratio for loading under dry conditions but drastic collapse when inundated at the same stress level. The large strains are believed to be due to the wetting of the outer part of the particles acting as a lubricant, causing the wetted particles to slide past one another allowing the soil to collapse. Under all three different soil states, it can be seen that well graded samples show less settlement compared to uniform graded samples. Results of triaxial tests show that well graded specimens always demonstrated the highest maximum deviator stress at failure regardless of the soil states of the specimens. This was followed by uniformly graded specimens with smaller maximum particle sizes and finally the uniformly graded specimens with larger maximum particle sizes. Higher maximum deviator stress at failure means higher load is required in order to fail the soil specimens. This trend is consistent with findings of one-dimensional compression tests· where well graded specimens always exhibited the least settlement in all soil conditions .. It is observed, however, that the values of the maximum deviator stress appeared to decrease as the soil states changed from dry to wetted to degraded conditions. Furthermore, the dry mudrock is seen to exhibit high curved Mohr-Coulomb failure envelopes with the angle of shearing resistance values decreasing with increasing confining stress. The angle of shearing resistance decreasing from 41.20 to 31.30 as the mean effective stress increased from 200 to 700 kPa. This trend is consistent with findings reported by Yasufuku and Hyde (1995) who investigated pile end-bearing capacity in crushable sands. This behaviour which normally exhibited by crushable soil indicated that dry mudrock is crushable and higher effective stress application had caused more crushing to dry mudrock. Results of both Rowe consolidation tests and triaxial isotropic test suggest that the settlement behaviour of mudrock is largely affected by initial moisture content, particle size distribution and soil states.