Innovative mine backfill materials and structures

1.

The mining of natural resources forms three broad classes of mine waste: waste rock, tailings, and coal gangue. These solid mine wastes, which can cause environmental problems, can be employed in recycling, storage and construction forms in making flowable filling slurries to re-fill mined-out voids causing surface subsidence and underground collapses. Capable mining methods with diverse fill types such as cementitious tail/paste backfill (CTB/CPB) in metal mines and cementitious gangue/ash backfill in coal mines can provide local and regional ground supports while ensuring safe ore productions. The stiffness of mine fills in varied underground conditions is getting more and more attentions by operators because it can aid in achieving the long-lasting aim of viable exploring and backfilling processes. Recent developments in filling ingredients and techniques are addressed and it is likely to inspect several advanced products, techniques and constructions to grasp this aim.

Therefore, a variety of papers were collected in this special issue to explore the stability of novel filling products and structures as a main function in most modern mines worldwide. Unique thoughts and novel improvements in mine fill durability were discussed, and strategic methodologies were proposed for the upcoming progresses of this topic. This special issue chooses and gathers fourteen research papers, providing a wide range of data on backfill mining which augments efficient underground mining through diminished stope cycles and augmented ore recoveries. All these papers discuss some instances of characteristics, recipe optimisation, reticulation systems, laboratory and field testing, numerical modelling and monitoring of the different backfill types used in underground mines.

The first paper [1] studies gradient boosting regression tree (GBRT) methods to envisage heavy metal (e.g. Cu, Pb, Zn, Mo, Cr, and Ni) occurrence forms in tailings stored in surface impoundments and/or dams. The authors present tail mineralogical data and heavy metal features as system parameters and the fractions of seven event forms as result parameters. This study noticeably shows that the outcomes obtained by machine learning models could offer a reliable GBRT forecast model for heavy metal occurrence forms. Thus, important information could be gained about the influence of tail mineralogy on creations of heavy metals. Besides, the study could be implemented for pollution analyses and secure utilisation of heavy metals.

The second paper [2] focuses on hydration/hardening characteristics of CPB containing a less expensive, effective, and sustainable cement with 50 wt.% steel slag. It is experimentally shown that the 28-day strength of the improved filings (CPB with a 50 wt.% steel slag, 35 wt.% blast furnace slag and 15 wt.% desulphurisation gypsum) has two times superior than CPB with Portland cement alone. The authors have determined that the outstanding backfill strengths are thanks to considerable gel-like materials acquired by numerous activation practices. Consequently, this study emphasised the importance of the usage of steel slag in CPB for its more valorisation and sustainable implementation in mining sites.

The third paper [3] investigates the effects of backfill ingredients (e.g. water, fibre, binder and tails) and their interfaces on the mechanical/rheological features of CPB through response surface methodology (RSM) model. The authors have indicated that the RSM approach accurately assesses relations between model factors and features of fills reinforced with fibre, assisting the making of ideal fill mixes. The paper has highlighted the efficacy of joint RSM and attraction method in filling mix quantities, offering an advanced engineering method, which significantly diminishes lab tests needed to determine ideal CPB mixture ingredients.

The fourth paper [4] develops a 3D simulation model which predicts the compulsory strength of fill and dislocation outlines, as well as the strength/stress fractions, considering CPB’s strength/failure mechanism. The authors have also validated numerical modelling (FLAC3D) results by employing methodical results and in situ works. They have emphasised that the most delicate parameters for the needed strength of filling are stope length upright to strike/stope inclination. The upper and lower portions of re-filled stope reflect a tensile and shear failure behaviour via FLAC3D. Consequently, one can infer that the developed model could become a beneficial tool to accurately predict fill’s compulsory strength design.

The fifth and sixth papers [5,6] discuss the curing effects on the deformation and mechanical characteristics of CPB specimens exposed to curing humidity values ranging from 60% to 95% to mimic saturate situations. The outcomes specify that curing directly affects both the hydration products and the pore structure. Besides, the dark grey colouration witnessed within backfills exposed to underwater curing is similar to the greening effect observed within cementitious products with blast furnace slag. As a result, these experimental studies have indicated that the curing medium should be taken into account while designing CPB structures in terms of their safety, economy and operational aspects to create an efficient and successful backfill material.

The seventh paper [7] experimentally inspects spatial evolution of mode I/II fracture toughness of CPB specimens subjected to outer sulphate attack conditions through large-scale curing column models. The authors have indicated that seepage-induced external sulphate attack causes spatial variations in mode I and mode II fracture toughness of hardened backfill specimens. Besides, the profounder place within fills offers the upper extent of fracture toughness’s degradation. The early-age fracture toughness is also impervious to CPB’s spatial positions. Finally, mode I fracture toughness could be utilised steadily for guessing mode II fracture toughness under diffusion-induced external sulphate attack, simplifying lab work on CPB’s fracture toughness.

The eighth paper [8] focuses on the damage/energy behaviour of fibre-reinforced CTB specimens subjected to different strain rates through the SHPB tool armed by confining stresses. It is experimentally detected that the strain rate strengthening effect has a key role on failure/energy behaviour when considering lower confining stresses. Nevertheless, as confining stresses rise, the strengthening influence lessens slowly. Energy density exhibits a growing tendency with increasing confining stress, while energy dissipation ratio displays a quadratic drop with increasing strain level. The present investigation subsidises the endowment of detailed information and theoretic basics for fibre reinforced CTB’s design. It also assists mine firms in improving their practices to accomplish better productivity and security.

The ninth paper [9] deals with ideal Poisson’s ratio of grainy products like CPB by replicating volumetric strain against axial strain link by numerical models with Mohr–Coulomb model. Poisson’s ratio μ_0.02 can be used in numerical modelling as long as the dilation phase of grainy backfill products is absent. Poisson’s ratio μ_0.36 appears to be suitable in numerical model with the Mohr–Coulomb model as long as the dilation phase of grainy backfill products is present. Finally, one can infer that, under small strain, the mechanical behaviour of granular material can be simulated using the Mohr–Coulomb model, but it is not apt under large strain conditions.

The tenth paper [10] experimentally evaluates the CPB’s stress features when re-filled into a semi-confined narrow wall witnessed at temperatures shifting from 30°C to 60°C. The authors have indicated that arching influences limit CPB’s vertical expansion, leading to temperature-induced thermal stress which exceeds the total stress assessment. The stress gain’s quantity is reliant on the degree of heating, the total temperature alteration from its initial temperature of fill pouring, and fill’s material state where hardened fill makes thermal stress with increasing temperature. As a result, outcomes of this study advise the need to consider thermal features in the design stage for improving safety in mines.

The eleventh paper [11] proposes an inclinometer state analysis method for field observation of virtual slip between CPB and nearby rock formations, based on the values of elastic beam and elastic foundation beam theory. The authors have maintained that it is practical to assess suitable installation distance, measurement’s extent, and inclinometer tube’s active state. The inclinometer tube’s strength response is also scrutinised by novel/finite element methods. In inclinometer tube’s failure analysis, the novel technique offers more conservative results than the finite element method. Finally, a novel technique could be implemented for governing inclinometer tube’s maximum measuring range and ultimate bending strain.

The twelfth paper [12] proposes the final backfill ratio to describe the filling effect characterisation model (including multiple parameters) of CPB specimens in coal mines, establishing a relationship between strength and compressibility of filling by mechanical tests and theoretical analysis. The influence mechanism of geological, technical and material factors on the final backfill ratio is experimentally revealed. The link between the final backfill ratio and geological parameters including mine depth and height can be described by a logarithmic and power function. The authors have also obtained the control principle and method of the final backfill ratio through sensitivity analysis. The numerical simulation verifies the good reliability of the control method.

The thirteenth paper [13] explores mixing features of waste rocks cast in CPB through lab experiments. The authors have proposed descriptions of solid concentration by the weight of waste rock and mixing degree of mixes to quantitatively describe blend between waste rocks and CPB. It is experimentally indicated that diffusion of waste rock into CPB and mixing degree could be improved by using CPB with low solids, bulky grain sizes of waste rock and/or by the rise of dropping height of waste rocks cast. As a result, one can conclude that the anticipated meanings can be adopted for assessing natural mixture’s mix class.

The fourteenth and final paper of this special issue [14] investigates the use of microgrid fibres as a novel polypropylene fibre additive for cementitious rock fills in comparison with orthodox polypropylene fibres. It is experimentally shown that microgrid fibres have much better strength and adherence to cemented rock fills than orthodox polypropylene fibres. Finally, one can settle that microgrid fibres could offer a good reinforcement impact in cemented mine backfill applications by increasing their compressive and tensile strength values. Microgrid fibres are a candidate to become popular in mines.

Generally, the papers collected in this issue take critical subjects linked to advanced mine fill materials and structures. I sincerely hope that this special issue escalates the consciousness of applicants and community on progress of advanced fill products and methods that can be applied to unmanned/smart mining for reducing mining’s adverse impact on ecosystem/community.

To conclude, I state my heartfelt appreciation to all authors/referees who are specialists in their fields for their important recommendations, assessments, and explanations. I also wish to most sincerely acknowledge Professor Erkan Topal, the Editor-in-Chief of IJMRE, and reproducing editors who have assisted the execution of extended and perilous arbitrating practices, offering precious procedural/technical aid. Lastly, there is no doubt that this special issue would not have been so effective without the non-stop care and aids of these substantial individuals.

Disclosure statement

No potential conflict of interest was reported by the author(s).