Undrained Strength of Deposited Mine Tailings Beds: Effect of Water Content, Effective Stress and Time of Consolidation

Abstract

An understanding of the geotechnical behaviour of mine tailings is imperative when evaluating the stability and erosional resistance of sedimented tailings beds; as well as for the design and long-term management of tailings disposal facilities. Laboratory testing was conducted on mine tailings beds of various ages and thicknesses, deposited from concentrated slurries. Measured index properties allowed classifying the tailings as a coarse grained and non-cohesive material. The results obtained from the performed sedimentation experiments showed that the primary consolidation of the tailings beds was complete in approximately 1 h and negligible volume changes occurred in the beds during secondary compression. The undrained shear strength of the tailings beds was measured using an automated fall cone device at a depth interval of 1 cm and a profile of the shear strength variation with depth was obtained. At each tested surface, moisture content specimens were taken to determine the moisture content profile of the tested tailings beds. The undrained shear strength of the beds varied between 0.008 and 0.975 kPa for effective stresses below 1.19 kPa and increased with depth. Correspondingly, the moisture content decreased with depth and varied between 17 and 27%. The factor controlling the undrained shear strength of the tested beds was the vertical effective stress, with the water content also having some secondary effect. The correlation between the undrained shear strength and the vertical effective stress was expressed with a second order polynomial function. Consolidation time did not appear to influence the observed shear strength.

Role of smectite-rich shales in frequent foundation failures in southeast Nigeria

“…Soil structures, swelling and cementation are the main factors affecting the shear strength of clays (Feda 1995;Cheng et al 2004). But, water content also has some secondary effect (Dimitrova and Yanfu 2011). Therefore, poor cementing material and high swelling potential might have a strong affinity for each other and may be responsible for the average shear strength that eventually could deteriorate at the foundation site, as suggested by the G s and I d values, especially in the presence of water.…”

Improving Tailings Dam Safety via Soil Treatment

“…Effect of Tailings Treatment on Safety Factor Figure 12 demonstrates clearly that the stability of the dam for Case I represented the worst-case scenario among the cases considered. The calculated average S.F for Case I, untreated, is 1.25, which is less than the required long-term safety factor of 1.5 [3,39]. Therefore, a soil treatment scheme is considered to enhance the stability of the dam.…”

“…Duncan (1996) conducted an extensive review of the different constitutive models employed for the analysis of earth dams. He concluded that the elastoplastic and elasto-viscoplastic stress-strain relationships more realistically model the behavior of soils close to failure, at failure, and after failure [39,40]. Similar investigations were conducted more recently to examine different available constitutive models [37,[41][42][43].…”

Effects of Polymers on short-and long-term dewatering of oil sands tailings

“…Consequently, the excess pore pressure dissipates very slowly (Chalaturnyk et al, 2004). The primary consolidation in clay/FFT deposits ends with a complete dissipation of excess pore water pressure (when the excess pore water pressure returns to a hydrostatic equilibrium condition) and the formation of a consolidated bed (Imai, 1981;Leshchinsky, 2004;Dimitrova and Yanful, 2011). However, the creep compression or secondary consolidation, which originates from the plastic deformation of the bed under a constant effective overburden pressure continues long after the primary consolidation ends.…”

“…Although it has been a common practice in geotechnical engineering to use a range of tests including triaxial and direct shear tests to determine the shear strength of soils, such conventional geotechnical techniques have been found unsuitable for the tailings since they have a very low shear strength, often as low as 0.5 kPa (Qiu and Sego, 2001). For these tailings with extremely low shear strength, a fall cone device (Figure 3.16) is considered to be the best tool for measuring the undrained shear strength since the device has the capacity to measure shear strengths as small as 1.5 kPa with reasonable accuracy and repeatability (Dimitrova and Yanful, 2011). Lab vane shear test is another suitable method for measuring relatively low shear strengths in fully saturated samples, however, the sample disturbance caused by the rotation of the vane causes a reduction in the interparticle bonds and a rearrangement of the particles, which makes the assessment of structuration effect difficult.…”