Rock grouting design: Rheological aspects and radial flow visualizations with ultrasound
Time: Thu 2021-12-02 15.00
Subject area: Civil and Architectural Engineering, Soil and Rock Mechanics
Doctoral student: John Shamu , Jord- och bergmekanik, Division of Soil and Rock Mechanics
Opponent: Associate Professor Chadi El Mohtar, The University of Texas at Austin
Supervisor: Professor Stefan Larsson, Jord- och bergmekanik; Dr. Ulf Håkansson, Skanska Sweden AB; Dr. Liangchao Zou, Resurser, energi och infrastruktur
The rheological properties of cement-based grouts play a crucial role indetermining the final spread in grouted rock formations. In rheological terms,cement grouts are known to be complex time-dependent yield stress fluids,but their steady flow behavior is often described by the simple Binghamconstitutive law. The Bingham parameters obtained from the linear curvefitting to flow curve data are then used in grout propagation calculationsduring the design phase, e.g., for rock fracture grouting in tunnel construction.Since cement grouts are time-dependent and thixotropic suspensions, theinterpretation of their flow curves during conventional rotational rheometryis often complicated by the presence of wall slip, thixotropy, flow localization,and sedimentation, particularly at low shear rates. A systematic approachwas carried out as part of the research work to study these effects within theconstraints of the concentric cylinder geometry (Couette) and for differentcement grout concentrations. Of particular interest were the influence ofgeometry and flow sweep measurement interval on flow curves, includingthe characteristic unstable flow branch that appears at applied shear ratesthat are below the critical shear rate. The unstable flow branch observedbelow the critical shear rate has been described as a characteristic feature inthe flow curves of thixotropic suspensions, e.g., cement grouts and laponite.From a practical standpoint, these crucial shear rate aspects, including wallslip, have not been considered during grouting design calculations whileusing the common Bingham model. Thus, the research also considered theseshear rate aspects as part of grouting design by incorporating them into thedesign approach within the existing framework of the Real Time GroutingControl (RTGC) method.
Another interesting part of the research work presented in this thesis relatesto studies on the radial flow of yield stress fluids. The radial flow betweenparallel plates is an idealized fundamental flow configuration that is oftenused to understand grout spread estimation in rock fractures. In part,the entire radial flow work was motivated by the ongoing discussions inthe literature regarding the different analytical solutions for radial flow.Moreover, compared to other flow configurations, e.g., pipes and channels,only a limited amount of work has presented analytical solutions, numericalmodels, and especially experimental work for radial flow. Thus, during thedoctoral work, a radial flow experimental device was designed, manufactured,and subsequently used to acquire Carbopol YSF radial flow velocity profilesfor the first time. The velocity profile measurements were carried out usingthe pulsed Ultrasound Velocity Profiling (UVP) technique. The velocityprofiles from the initial radial flow study showed that significant wall slip was present. An analytical solution with a Navier slip term was used todescribe the velocity profiles, resulting in a good agreement in the velocityprofile magnitude. However, the plug-flow region extent was smaller in theanalytical solution. Subsequent studies on radial flow sought to addressthe wall slip issue using different wall slip reduction procedures (chemicaltreatment and sandblasting). Both treatments showed substantial wallslip reduction; however, some wall slip effects persisted, especially for thethicker Carbopol gels. In addition, a plug-point estimation algorithm usingTikhonov regularization was developed to calculate the yield points fromthe non-smooth velocity profiles accurately. A final modification was theaddition of frame reinforcement to maintain the required constant aperturebetter. The tests carried out in the final radial flow study followed the testscheme from the previous studies, but with two concentrations of Carbopol.The aim was to compare the measured velocity profiles with two radialflow analytical solutions based on different assumptions that have recentlybeen discussed in the literature. The measurement results showed a goodagreement in the velocity profile shape, but with some velocity magnitudediscrepancies, particularly in the central part of the velocity profiles. Suchdiscrepancies could result from remaining wall slip together with other higher-order flow effects, e.g., nonlinear flow due to inertial effects, that are notaccounted for by the analytical solutions. Nevertheless, within the contextof grouting practice, such magnitudes of differences could be consideredreasonable for scoping calculations during grouting design and execution.Future studies related to radial flow can improve the current understandingby conducting similar tests, but with improved experimental setups, e.g.,better wall slip reduction, larger aspect ratios, and more detailed spatialresolution for smaller flow apertures. Additionally, the understanding of therheological behavior of cement grouts would be improved from a practicalstandpoint, i.e., grouting design and execution, if the wall slip phenomenonis studied in more detail and considered an inseparable feature of yield stressfluid flow.