Mechanical response and pore pressure evolution of cemented paste backfill under deep mine-like multiaxial stress and temperature conditions

Abstract

As underground mining advances to greater depths, cemented paste backfill (CPB) is increasingly subjected to complex thermo-mechanical loading conditions, including multiaxial stress states and elevated temperatures. This study investigates the coupled effects of field-representative vertical self-weight and horizontal rockwall closure stresses, along with in-situ temperatures, on the mechanical behavior and pore water pressure (PWP) evolution of CPB. Experiments were conducted using a novel apparatus capable of controlling multiaxial stress and temperature during curing, replicating in-situ stress paths and thermal profiles typical of deep mine environments. Results show that multiaxial stress enhances CPB strength and stiffness by promoting denser particle packing, reducing porosity, and increasing frictional resistance. Elevated temperatures independently accelerate early-age cement hydration, further improving bond strength and stiffness. When combined, multiaxial stress and elevated temperature produce a synergistic enhancement in unconfined compressive strength (UCS) and elastic modulus, as confirmed by two-way ANOVA and synergy index analysis. PWP responses were also highly sensitive to thermo-mechanical conditions. The evolution of positive and negative PWP was governed by the interplay of thermal expansion, hydration-induced desaturation, and mechanical compaction. Multiaxial stress amplified early positive PWP and delayed its dissipation, whereas elevated temperature accelerated hydration and reduced pore pressure, leading to enhanced suction at later ages. A transient ''stress-induced resaturation'' effect was observed under late-stage excessive horizontal stress but was mitigated by elevated temperatures. These findings provide critical insights into the coupled mechanical and hydraulic behavior of CPB under realistic field conditions and offer guidance for optimizing backfill design, binder content, and barricade stability in deep mining applications.

1. Introduction

With the increasing global demand for mineral resources, underground mining operations are extending to greater depths. In many countries, mines have reached depths where the geomechanical and thermal conditions become extreme. Mining depths exceeding 1000 m are now common in countries such as Canada, China and South Africa [1–3]. Currently, coal mines approach depths of around 1500 m, while non-ferrous metal mines are reaching 4500 m [1,3]. In regions such as South Africa, average depths of metal mines is typically around 2000 m, with notable examples like the Wester Deep Level gold mine extending to an extraordinary depth of approximately 4800 m [2]. At such depths, substantial vertical stress from overburden and horizontal stresses caused by rockwall closure impact underground structures, including stopes — large voids created after ore extraction that are typically backfilled to maintain mine stability.

Cemented paste backfill (CPB) is widely adopted in mining operations to address these challenges due to its advantageous combination of flowability, reliable mechanical strength, cost-effectiveness, and sustainability in resource utilization [4–9]. CPB, composed of tailings, water, and cementitious binders (typically 3%–10% by mass), achieves the designed strength through binder hydration during curing. The engineering performance of CPB is highly dependent on several key design considerations, including the type and dosage of binders (e.g., Portland cement, slag, fly ash), the water-to-cement (w/c) ratio, curing temperature, and the overall mix proportion. Binder type and content directly affect the rate and extent of hydration, which in turn influences the mechanical and hydraulic properties of the backfill.