Effect of pre-shearing on the steady and dynamic rheological properties of mud sediments

Mud sediments can exhibit a complex rheological behaviour particularly a thixotropic character or structural recovery after breakup due to the presence of organic matter/biopolymer. Such biopolymers can lead towards the development of flocculated structures having multiple length scales which are sensitive to shearing rate and history. In this study, the extent and rate of structural recovery of mud sediments was studied by measuring the storage modulus as a function of time using small amplitude oscillatory tests after a destructive steady shearing. This linear viscoelastic response of the sediments was further investigated as a function of several parameters including pre-shear rate, pre-shear time, measuring geometry, mud density and organic matter content. The equilibrium storage modulus (G_∞ ^') and the characteristic time (t_r) for the structural recovery of the sediment matrix were estimated by fitting the experimental data to a stretched exponential function. The normalized storage modulus, G^'/G₀ ^' (i.e., structural parameter) was used to relate it with the yield stresses of mud sediments. The results showed that the recovery of structure after shearing was instantaneous (t_r being of the order of seconds), however, the extent of recovery was highly dependent on the studied parameters. The extent of recovery was higher for the samples with lower density and lower organic matter content. The effect of the shearing time on t_r and G_∞ ^' was almost negligible, which implies that the destruction of the structure was achieved within seconds. Using vane geometry, the extent of recovery was higher than using Couette geometry which is linked with the distribution of shear stresses within the cell for each geometry. Yield stresses showed a strong dependency on structural parameter, until it reaches very small values. At low values of structural parameter, the yield stresses were constant as the structural recovery was even faster than the time required to perform the amplitude sweep tests. This study provides an extensive knowledge about the structural recovery in mud sediments under different shearing conditions which can be useful for sediment management.