Research On The Mechanical Behavior And Function Of Backfilling With Underhand Cut-and-fill Method In Jinchuan No.2Mine

Jinchuan No.2Mine was one of underground mines with the hardest mining conditions all over the world. Its rockmass stability was extremely poor, and it was subjected to higher tectonic strata stress. In order to achieve the goal of efficiently extracting mine deposit under safe mining operation, different mining methods including top slicing and caving, cut-and-fill mining and underhand cut-and-fill mining, had been carried out in decades, and the mechanized underhand cut-and-fill mining with drift (MUCFMD) was finally selected as the mine's primary mining method due to the method's merit in mining productivity and operation safety.With the mining levels extending downwards, the strata stress increased gradually with the increase in the mining depth. For MUCFMD, the backfilling was the artificial roof of mining drift which was located in the next mining slice, and its stability directly influenced the safety of miners and operation devices. Whereas, there still was lacking in systematic insight into the stability mechanism of backfilling under the mining conditions of higher tectonic strata stress. It was positive that the investigation on the mechanical behaviour and function of backfilling was carried out.Mainly based on the in-site mining engineering situation in Jinchuan No.2mine with MUCFMD in the extremely poor mining conditions, the study on the mechanical behaviour and function of backfilling was systematically carried out in the work by means of comprehensive ways, mainly including field surveying, theoretical analyse, material model test, and3D numerical modelling with FLAC3D and FLAC3D secondary development programmed by the author. The main research contents and conclusions were as follows.(1) Under change in the layout of mining drifts in conjunctive mining slices, the internal forces(M, FQ) state in the artificial roofs, which were supposed as continuous beam structure, was studied according to the 'three-moment equation'of the structural theory. The equations of internal forces were derived. Further, the quantitative distribution of internal forces for three types of artificial roofs, which respectively corresponded to the three drifts layout (the drifts in upper slice perpendicular to those in next slice, the drifts in upper slice parallel to those in next slice, and the drifts in upper slice oblique with those in next slice), were given. The tension or shear failure in artificial roof adjacent to intermediate supports, which actually were temporary ore ribs or artificial ribs made of backfilling, might be controlled or eliminated by optimizing the span among the supports.(2) As most simplified backfilling models did not consider construction of steel in thin reinforced backfilling, it was difficult for the simplified models to resolve and reflect the change in both stress state and loading paths of steel parts in backfilling resulting from mining operation in extreme geological conditions. For Jinchuan No.2mine, its basic mechanical structure of backfilling was abstracted as a dualistic structural model by author. That is, the backfilling consisted of continuous-homogeneous cemented motar and steel rod, and both of them together undertook the loading. The "cable" element of FLAC3D Platform was acted as steel parts in the mine's backfilling, and the change in both stress state and loading paths of the steel parts was numerically simulated. The numerical modelling result for steel parts in backfilling was basically proved by the steel deformation or failure found from the in-site survey.(3) Using Google Earth(GE), the three-dimensional coordinates of topography had been obtained by extracting the elevation of surface around the Jinchuan No.2mine in term of a certain degree of flat networknet on line. Further, topographic map and DTM model of surface around the mine were established. These works addressed the problem that the range of data collected were insufficient to cover the surface of model, and provided the preconditions for global modeling of Jinchuan No.2mine.Utilizing mining software Surpac established rich ore model, lean ore model of Jinchuan No.2mine, and other parts of the solid model surface, the ore-bearing block unit database, and various constraints. Using step modeling approach, by coordinate transformation, round of data and other measures to ensure the various parts of the whole block some accurate docking model of convergence, then transforming Surpac ore-bearing block model, surface block model data into Flac3D, adding some of the resulting divergent transition, final the Flac3D mechanical model was established. These methods overcame the difficulties using Flac3D to establish complex geological models directly, and make the foundation to apply Flac3D to simulate the production process of mine.(4) Through the secondary development of Flac3D, the Generating Code System for Flac3D, GCS, had been developed. The programmable components formed by dividing the mining&filling process of Jinchuan No.2mine according space position, order, structural function and attribution of engineering geological body were researched. The problem of the model unit centroid localization, unit capture, restructure, application variables, Flac3D variables, function, sentence, the mapping between data structure etc were resolved.Developed by VC++, This application program used the database of cell model as a data resource, through a series of steps, the process and parameters of excavation process necessary can been selected and confirmed, which can been transferred to the simulation code generation system, and then the Flac3D codes would be generated by the application program. By the system, the three-dimensional mechanical simulation for the main production process of Jinchuan No.2mine, such as production order, mining process, layout of mining drifts, layout of reinforcement, filling etc., can be realized. The system can greatly improve the efficiency and reduce the error rate of programming the3D mechanical simulation code.(5) Through the more accurate simulation by Flac3D for the production process of Jinchuan No.2mine, the mechanical behaviours associated with the backfilling of No. mine, such as the features of the stress redistribution and the law of the displacement movement in the backfilling, pillar and near-ore rock, the law of the rock movement on the surface, the destructive state of backfilling, the stress state and changes of force in the reinforcement, were fully described and induced, and the backfill stable mechanism was revealed.(6) Two sorts of comparative investigation into mechanical performance of physical models made of different backfill motars, including beam models (BM) and slab models (SM) formed with joining three pieces of beam together, was carried out by indoor model test. In BM sort investigation, the flexural strength of beam models, including reinforced beam models (RBM) and plain beam models (PBM), was comparatively tested and analysed. The results of BM sort showed that the flexural strength of RBM was obviously bigger than that of PBM the failure characteristics of RBM was closer to that of elastic-plastic materials, and the failure of PBM was characterized in brittle fracture. In SM sort investigation, the shear strength of weak interfaces existing in slab models, including the slab models with reinforced interfaces by shear connecting element (RSM) and other slab models with plain interfaces existing naturally (PSM), was comparatively tested and analysed as well. The results of SM sort showed that, under like conditions, the shear strength of RSM was bigger than that of PSM. It was good for backfilling stability to be improved by suitably reinforcing drift backfilling and by suitably installing shear connecting elements along interfaces between drift backfill.This paper closely combines the important technical problems of the engineering practice, stands on subject frontier, comprehensively uses the means of material model tests,3D mining software, secondary development of Flac3D and3D numerical simulation of mechanics, establishes a set of3D mechanical analysis method for the complex production process of mine, obtains systematic beneficial results, has important theoretical significance and engineering value.