Cemented Paste Backfill Geomechanics at a Narrow-Vein Underhand Cut-and-Fill Mine

Abstract

Underhand cut-and-fill mining has allowed for the safe extraction of ore in many mines operating in weak rock or highly stressed, rockburst-prone ground conditions. However, the design of safe backfill undercuts is typically based on historical experience at mine operations and on the strength requirements derived from analytical beam equations. In situ measurements in backfill are not commonplace, largely due to challenges associated with instrumenting harsh mining environments. In deep, narrow-vein mines, large deformations and induced stresses fracture the cemented fill, often damaging the instruments and preventing long-term measurements. Hecla Mining Company and the Spokane Mining Research Division of the National Institute for Occupational Safety and Health (NIOSH) have worked collaboratively for several years to better quantify the geomechanics of cemented paste backfill (CPB), thereby improving safety in underhand stopes. A significant focus of this work has been an extensive in situ backfill instrumentation program to monitor long-term stope closure and induced backfill stress. Rugged and durable custom-designed closure meters were developed, allowing measurements to be taken for up to five successive undercuts and measuring closures of more than 50 cm and horizontal fill pressures up to 5.5 MPa. These large stope closures require the stress–strain response of the fill to be considered in design, rather than to rely solely on traditional methods of backfill span design based on intact fill strength. Furthermore, long-term instrument response shows a change in behavior after 13–14% strain, indicating a transition from shear yielding of the intact, cemented material to compaction of the porosity between sand grains, typical of uncemented sand fills. This strain-hardening behavior is important for mine design purposes, particularly for the use of numerical models to simulate regional rock support and stress redistribution. These quantitative measurements help justify long-standing assumptions regarding the role of backfill in ground support and will be useful for other mines operating under similar conditions.

Predicting the Isotropic Volumetric Compression Response of Hydrating Cemented Paste Backfill (CPB)

“…In their study, the induced stresses approached 1 MPa. Other subsequent studies have measured induced stresses in excess of 4 MPa and closure strains up to 15% (Thompson et al 2009;Thompson et al 2011;Thompson et al 2012;Counter 2014;Raffaldi et al 2019). While these induced stresses are smaller than the pre-existing field stresses, they are nonetheless much higher than confining stresses that are typically explored in mine backfill strength studies conducted under controlled laboratory conditions.…”

Determination of the Backfilling Time for the Zinc and Lead Ore Deposits with Application of the BackfillCAD Model

“…Gonet et al [27] pointed out that the backfilling process is sometimes carried out in a mixed manner, alone and with the addition of a cement binder. Raffaldi et al [28] marked that in the underhand cut and fill mining method, the role of backfilling plays a significant role in maintaining the stability of excavations. Dzimunya et al [29] presented several backfilling technologies to optimize pillar recovery.…”

3D Numerical Modelling of the Application of Cemented Paste Backfill on Displacements around Strip Excavations

“…The solid mass of the hardened backfill can absorb significant pressure and prevent deformation of the surrounding rocks, thus enabling the excavation to be secured with a rock bolt support [11], and the subsequent operation of adjacent blocks [12]. Underground mining methods with the use of backfills are often used to reduce the negative impact of mining on the surface [13].…”