A comparative study of the fracture features, strength and deformation properties of pseudo strain-hardening composites based on alkali-activated slag and Portland cement matrices with polypropylene microfiber was carried out. Correlations between their compositions and characteristics of stress–strain diagrams under tension in bending with an additional determination of acoustic emission parameters were determined. An average strength alkali-activated slag matrix with compressive strength of 40 MPa and a high-strength Portland cement matrix with compressive strength of 70 MPa were used. The matrix compositions were selected for high filling the composites with polypropylene microfiber in the amount of 5%-vol. and 3.5%-vol. ensuring the workability at the low water-to-binder ratios of 0.22 and 0.3 for Portland cement and alkali-activated slag matrices, respectively. Deformation diagrams were obtained for all studied compositions. Peaks in the number of acoustic signals in alkali-activated slag composites were observed only in the strain-softening zone. Graphs of dependence of the rate of acoustic events occurrence in samples from the start of the test experimentally prove that this method of non-destructive testing can be used to monitor structures based on strain-hardening composites.

Introduction of new equipment in the production cycle in mines can greatly accelerate the pace of construction and can increase production output. And yet considerable time is spent on preparatory works, transport operations and shunting. To reduce the time consumption, engineers make nonroutine decisions on the arrangement of process equipment in underground excavations, which imposes certain requirements on their geometry and support design. For example, the design size of grinding chambers can exceed 14 m in height and 8 m in width. These dimensions are almost 2 times higher than the size stated in the regulatory documents. The mine support design guidelines are unadapted to the dimensions of such underground openings, and disregard the construction sequence, complicated geological and mining conditions, and the influence of stoping. As a result, geomechanical processes in the vicinity of large cross-section excavations in fractured rock masses can only be estimated approximately. This paper proposes a mathematical modeling algorithm including rock mass fracture intensity, the length/width ratio of a large cross-section excavation and the presence of junctions with other excavations. Such stress–strain analysis of tock mass provides the pattern of geomechanical behavior in the vicinity of the underground excavation, namely, it allows determining the change patterns of damaged zones in rock mass. It is shown that stoping can lead to an increase in the size of the limit state zones up to 3 times, which exceeds the safety factor as per the regulatory documents. The obtained results can widen the field of application of regulatory documents in mining.

Having stated highly complicated ground conditions and hydrogeology of underground extraction of weak iron ore from Yakovlevskoe deposit, Kursk Magnetic Anomaly, using method of top-downward horizontal slicing with backfilling, the authors offer a set of technical and technological actions aimed to enhance safety and efficiency of drivage, support and backfilling under the earlier made backfill. For estimation of stress-strain state of surrounding rock mass and to substantiate actions aimed at stability of underground excavations, 3D finite element model is constructed, including a few stages: generation of in situ stress state of rocks; formation of protective bridge; drivage under the protective bridge with different cross sections of excavations and types of support. It is found that spalling of exposed ore in side walls is induced by the limit state of adjacent ore body, and shearing surface initiates on the ore and bridge contact 1.2 m away of a drive if it has rectangular cross-section and is oriented at an angle of 68°. The modeled limit state zone agrees with in situ data of monitoring in excavations driven under backfill. Shaping the excavation as a trapeze and supporting it with cuttable bolts reduces horizontal distortion of the excavation by 75% as compared with the unsupported rectangular cross-section drive. Based on the finite element modeling of stress-strain state in ore body under mining, the authors substantiate and recommend trapezoidal cross-section excavations (larger base of the trapezoid is the excavation roof), reinforcement of excavation side walls with cuttable bolts and certain sequence of stoping and backfilling using solidifying mixture. The developed set of actions is recommended for application in the framework of construction project stage II in Yakovlevsky iron ore mine with annual capacity of 4.5 Mt.