Coupled Effects of Tailings Particle Size and Borehole Size on Cement Paste Backfill Fracture Initiation Pressure

Abstract

Cement paste backfill (CPB) is a mixture of mine tailings, water, and binder used to support underground openings. Its mechanical strength is a critical design parameter for safety and the current strength testing methods are highly variable and unrepresentative of in-situ conditions. A recently proposed strength testing method, fracture initiation pressure (FIP), uses a hydraulic fracturing technique for estimating the in-situ tensile strength. Previously, the Point Stress (PS) model was used to interpret the FIP values, which indicate that both the material's internal flaws and borehole size govern hydraulic fracturing behavior. Because the particle size distribution (PSD) controls packing, porosity, and flaw size, this thesis investigates the coupled effects of PSD and borehole size on FIP in CPB. Three ground-silica PSDs (Sil-Co-Sil 52, 90, and 106; with D50 of 16.1, 21.5, 32.9 μm, respectively) were initially tested at 4 and 8 wt.% binder and 7, 14, and 28 days of curing for unconfined compressive strength (UCS), Brazilian tensile strength (BTS), Mode I fracture toughness (KIC), FIP, and isothermal calorimetry. Strength generally increased with binder content, curing duration, and particle fineness with the fineness effects being attributed to the filler effect where increased surface area from fine particles leads to increased hydration product formation. Although the FIP results exceed tensile strength significantly, they correlate linearly with conventional strength measurements, confirming that FIP could be used as a strength measurement. Because the PS model indicates that the FIP will approach the true tensile strength as borehole size increases, the FIP was measured across eight borehole diameters (0.125 inch to 2.375 inch) for the same three PSDs. The results reveal a strong borehole size dependence where FIP decreases with increasing diameter size and plateaus beyond a critical size. The PS model and Lecampion mixed criteria models were used to evaluate the data and a modified mixed model incorporating an additional length factor, taken as the fracture process zone length, best fit the observed results. The plateau pressure at large boreholes is proposed as an effective tensile strength that correlates strongly with conventional strength measures, indicating that FIP measured at a significantly large borehole could provide a reliable relative in-situ CPB strength measurement.