Pore Pressure Dynamics in Early-Age Cemented Fill Under Multiaxial Stress Conditions

Abstract

In deep underground mines, cemented paste backfill (CPB) is subjected to complex multiaxial stress conditions, including vertical self-loading and horizontal rockwall closure, which can significantly influence the evolution of pore water pressure (PWP). Understanding PWP development under these conditions is essential for ensuring barricade stability and the long-term performance of CPB structures. This study investigates the development of PWP in CPB under realistic deep mine conditions, particularly focusing on multiaxial stress loading and drainage availability. A novel multiaxial stress curing and monitoring apparatus was employed to simulate these coupled conditions and comprehensively capture the evolution of both positive and negative PWP within CPB throughout a 28-day curing period. Although the apparatus can impose field-like temperature histories and drainage boundaries, the present tests fix temperature to isolate mechanical and drainage effects. Groundwater inflow is not explicitly simulated; instead, controlled drainage boundaries are employed to bracket typical deep-mine backfill drainage conditions. Results indicate that horizontal rockwall closure stress significantly amplifies the magnitude and prolongs the duration of positive PWP by actively confining and entrapping pore water, resulting in notably higher peak PWP values and stress-induced pore pressure coefficients (C_n) compared to CPB subjected solely to vertical stress. Practically, this means that even at the same filling rate, CPB material subjected to additional horizontal rockwall closure in deep mines can experience significantly higher positive PWP, thereby increasing the load exerted on barricades at early curing stages and elevating the risk of barricade failure. As curing progresses and positive PWP dissipates, it eventually drops below zero, leading to the development of negative PWP (suction). To interpret this suction development, the study adopts the concept of the air-water interface (meniscus) from unsaturated soil mechanics, illustrating how curing-induced changes in pore structure influence meniscus curvature and consequently affect suction magnitude. Horizontal closure stress promotes consolidation and densification of CPB – supported by mercury intrusion porosimetry (MIP) and thermogravimetric (TG) analyses – which reduces pore size and enhances pore water consumption. This densification leads to smaller pore sizes, generating menisci with smaller radii of curvature, thereby increasing suction (negative PWP). However, rapid and high-magnitude horizontal stresses (rockwall closure 2) at later curing stages can induce a “resaturation effect,” partially reversing this beneficial suction increase, whereas moderate horizontal closure stress (rockwall closure 1) allows stable suction development throughout curing. Furthermore, introducing drainage under multiaxial stress conditions significantly mitigates elevated positive PWP by facilitating rapid dissipation of excess pore water. Drainage also effectively prevents the “resaturation effect” even under high horizontal closure stresses (rockwall closure 2), maintaining higher and more stable suction in CPB throughout curing. These findings underscore the critical influence of multiaxial stress conditions on pore water pressure in CPB, emphasizing the necessity of accounting for these coupled factors in order to manage pore pressure effectively, ensure barricade safety, and enhance the overall stability of CPB in deep mining operations.