An experimental investigation on collapsible behavior of dry compacted phosphate mine waste rock in road embankment

Abstract

Prescribing compaction at optimum water content in arid and semi-arid climate regions generally leads to unjustified ecological practices. Indeed, to achieve optimum densities for road embankment compaction, significant volumes of water are required, which is perceived as a very scarce resource in drought-prone areas. Dry compaction with the promotion of the use of materials at their natural moisture content can be a promising alternative that could save water resources. Nevertheless, dry compaction can affect the infrastructures’ stability under flooding conditions.This paper presents field and laboratory tests carried out to investigate the effect of dry compaction on Phosphate Mine Waste Rocks (PMWR) behavior. In situ tests were conducted and the PMWR dry compaction protocol has been defined. Furthermore, the collapse deformation and the shear strength properties were evaluated through double oedometer and triaxial tests, respectively. The microstructural evolution on wetting was evaluated using mercury intrusion porosimetry (MIP) and 3D X-Ray computed tomography techniques. The water retention curve and mineralogical quantification tools were used to explain the influence of hydraulic hysteresis and clay minerals properties on the hydro-geotechnical behavior of PMWR. The results show a significant effect of applied stress level and dry density in addition to the type and amount of clayey expansive minerals on the structural deformation of PMWR on wetting. PMWR showed a maximum collapse settlement of 5.33% under a pressure of 1500 kPa, while no collapse is expected for stress less than 100 kPa. Low content of clays was identified as well as the swelling shrinkage potential within PMWR. The ability of using PMWR as embankment materials at dry moisture content under total overburden stress below 200 kPa ensuring a dry density of at least 95%.σd max is confirmed. The results of this work can also be applied to standard materials with similar hydro-geotechnical characteristics.