Search Results (184)

Non-ductile reinforced concrete frames with unconfined joints dominate the collapse hazard of the existing building stock. Their CFRP-retrofit margin at collapse demand is poorly quantified. Two one-third-scale portal sub-frames were tested under Froude similitude. Specimen 1 was bare. Specimen 2 carried a three-ply hoop CFRP jacket on columns, beams, and joints. Both received the Antakya 3141 record from the 2023 Kahramanmaraş $M_w$ 7.7 mainshock at design intensity 0.35 g and collapse intensity 1.0 g. Cyclic response was decomposed into flexural, shear, and slip energy. At design intensity, the retrofit cut peak roof drift by 54%, suppressed residual offset, and lowered the calibrated Park–Ang index from 0.89 to 0.32. Slip share dropped from 47% to 5%. At collapse intensity, the retrofitted frame transitioned to joint-panel debonding-controlled failure at 8% drift with 245 mm residual, and shear share rose to 64%. The dominant-half-cycle ratio $R_1$ ≈ 0.72 emerged as a candidate brittle-damage signature for collapse-level response. A Lam–Teng confinement check confirms that the failure migrates from the column ends to debonding fracture in the wrapped panel rather than being eliminated by the retrofit. Supplementary joint-corner anchorage is recommended for non-ductile joints at collapse demand. Full article

Roof stability plays a crucial role in maintaining the overall stability of surrounding rocks to ensure safety of tunnel construction and operation. In this work, tension cut-off (TC) technique is introduced to modify the Hoek–Brown (HB) criterion to describe the tensile failure of rock strata. Thereafter, stability analysis of anchor bolt-reinforced tunnel roofs in rock strata subjected to a hybrid tensile-shear fracture is performed. The work balance equation is established by equating the external work rates of the falling block and the anchor bolts to the internal energy dissipation rate. Two stability indicators, that is the stability number (N) and the factor of safety (FoS) are proposed to quantitatively analyze the stability of tunnel roofs. Optimization algorithms combining genetic algorithm and particle swarm optimization are programmed to capture the optimal upper bound solutions. The influences of TC, strength criterion parameters, and anchor bolt-reinforcement strength on roof stability are explored in this work. It was found that increasing the anchor tension T improves the FoS of reinforced tunnel roofs, with an increase of up to 68% observed for rectangular tunnel roofs under the selected representative case, while the improvement is relatively less pronounced for circular tunnel roofs. Regarding anchor support, as ξ increases, the N for rectangular tunnels nearly doubles. This work provides a theoretical basis for preliminary designing of tunnels in reinforced rock strata. Full article

Gob-side roadways driven along the floor while retaining top coal in thick soft coal seams are prone to instability under strong mining-induced dynamic loading. To clarify the instability mechanism and develop an effective control method, the 1609 return airway of Jiulishan Mine was investigated using field survey, borehole imaging, FLAC3D numerical simulation, industrial testing, and field monitoring. The results show that, under the combined effects of large mining height, insufficient filling of the gob by the caved immediate roof, weak retained top coal, and low coal strength, shear failure planes tend to develop within the narrow coal pillar and extend from the gob-side roof toward the floor. Once the dominant shear plane cuts through the pillar, the overall bearing structure is destroyed, leading to shear slip, asymmetric rib deformation, roof subsidence toward the coal-pillar side, and rib–roof coupled instability. Based on a multi-index evaluation of pillar load-bearing capacity, plastic zone development, stress concentration, roadway deformation, and coal recovery, a 3 m coal pillar was determined as the rational width. A coordinated “narrow coal pillar + cross-rib anchorage” scheme was proposed, and field verification confirmed its effectiveness in controlling roof separation, roadway surface displacement, and internal surrounding-rock damage. Full article

Directional long-borehole hydraulic fracturing is an important technique for controlling rockbursts induced by hard roofs. Its effectiveness depends primarily on whether fracturing-induced damage can modify the roof-bearing structure and thereby regulate stress concentration and elastic strain energy accumulation in the coal-rock mass ahead of the working face. However, existing numerical simulations commonly rely on predefined weakened zones or empirical parameter reduction, which makes it difficult to represent the spatial heterogeneity and mechanical evolution of rock damage during field hydraulic fracturing. Taking the 2803 goaf-side working face in Hetaoyu Coal Mine as the engineering background, this study proposes a microseismic-data-driven method for characterizing hydraulic fracturing-induced damage and incorporates it into a FLAC3D finite-difference model. The stress field, elastic strain energy field, and damage distribution ahead of the working face are compared under non-fractured and hydraulically fractured conditions. In the proposed method, the energy of fracturing-induced microseismic events is converted into the Benioff strain of numerical zones according to the attenuation law of microseismic wave propagation, and the corresponding rock damage variable is then calculated using a Weibull damage model. The fracturing-damaged rock mass is further represented by weakening the elastic modulus, cohesion, and friction angle, together with the stochastic generation of strongly damaged zones. The results show that, without hydraulic fracturing, the hard roof maintains a strong, continuous bearing capacity, resulting in a continuous lateral abutment stress concentration zone and a high elastic strain energy accumulation zone ahead of the working face and near the goaf-side boundary. After hydraulic fracturing, a patchy and locally connected high-damage weakening zone forms in the target roof strata. This damaged zone cuts the original continuous load-transfer structure through which the hard roof concentrates load toward the goaf side, reduces the extent of high-stress and high-energy zones in the coal seam, and induces an asymmetric adjustment of the dominant mining-induced energy release zone from the goaf side toward the solid-coal side. These simulation results agree well with the field observation that microseismic activity is mainly concentrated near the roadway on the solid-coal side. The study indicates that the rockburst-control mechanism of directional long-borehole hydraulic fracturing is not limited to simple overall stress dissipation. A key finding is that the fracturing-induced heterogeneous damage zone effectively interrupts the continuous load-transfer and energy-storage paths on the goaf side. This induces an asymmetric spatial redistribution of the mining-induced energy field from the goaf side toward the solid-coal side, thereby mitigating the high static-load and high-energy-storage state ahead of the working face. Full article

Under green mine construction and efficient resource utilization, non-pillar mining has been increasingly applied. However, surrounding rock control remains difficult in traditional gob-side entry retaining under large mining height conditions. To address this problem, a cooperative control method combining roof cutting and pressure relief with a pre-placed backfill strip for coal pillar replacement is proposed. Taking the 15,108 and 15,110 working faces of Wangzhuang Coal Industry as the engineering background, a mechanical model and FLAC3D simulations were used to analyze the effects of roof cutting height and backfill strip width. The results show that roof cutting shortens the goaf-side suspended roof, weakens lateral abutment pressure, and improves the stress state of the strip. When the roof cutting height increases from 11 m to 13 m, the peak vertical stress of the strip decreases from 16.2 MPa to 13.9 MPa, with a reduction of 14.2%. When the strip width increases from 1.0 m to 1.5 m, the peak stress decreases by about 12.0%. Thus, the reasonable roof cutting height and strip width are determined to be 13 m and 1.5 m. Field monitoring shows maximum roof-to-floor and rib-to-rib convergences of 178.5 mm and 143.5 mm, respectively, with no obvious strip instability. Full article

This study addresses asymmetric large surrounding rock deformation induced by narrow coal pillar instability in a gentle-dipping coal seam gob-side coal roadway (GSCR) under water-immersed and high-humidity conditions. The corresponding instability mechanism and control technology are systematically studied via integrated laboratory, theoretical, numerical and field methods. From constant temperature–humidity rock deterioration tests, SEM and XRD analysis, it is revealed that hydration of hydrophilic minerals (kaolinite, chlorite) in immediate roof mudstone intrinsically drives its macro–micro structural disintegration and mechanical degradation, and the catastrophic chain mechanism of water-induced mudstone weakening–force transmission medium failure of coal pillars and overlying strata–sliding instability of key voussoir beam blocks–linked large surrounding rock deformation is clarified. A mechanical model of the overlying voussoir beam structure for the target roadway is established considering both mudstone weakening and excavation-induced load transfer effects. The sliding criterion of key overlying blocks is derived, which quantitatively confirms that higher mudstone weakening and excavation-induced stress concentration elevate the sliding instability risk of the voussoir beam structure. Based on the findings and field conditions, a combined near-field and low-position field support scheme is proposed, including near-field reinforcement (shotcreting sealing, bolt–cable cascade reinforcement, deep grouting modification) and low-position field pressure relief via liquid CO₂ phase transition long–short boreholes roof cutting. Field application verifies that the maximum roadway deformation is controlled within 172 mm, with excellent surrounding rock control performance. Full article

With the increased extraction thickness in fully mechanized top-coal caving faces in extra-thick coal seams, the caved gangue in the goaf is unable to effectively support the roof, resulting in aggravated deformation of the pre-driven withdrawal channel. Taking the No. 221304 working face in the No. 13 coal seam of Xiaojiawa Coal Mine as the engineering background, this study combined theoretical analysis, numerical simulation, and field measurement to investigate the effects of the top-coal caving stopping position, suspended roof length, and presplitting roof cutting on the stress and deformation of the rock surrounding the withdrawal channel. The results indicate that the convergence of the roof and floor and that of the two ribs of the withdrawal channel decrease in a staged manner with the increase in the top-coal caving stopping distance, but increase nonlinearly with the increase in the suspended roof length. With the increase in the presplitting roof-cutting height, the surrounding rock deformation first decreases significantly and then tends to level off. When the roof-cutting height is 30.5 m, the reductions in roof displacement and rib convergence reach 33.46% and 37.76%, respectively. When the roof-cutting height is further increased to 35.0 m, the improvement becomes insignificant. Therefore, the reasonable roof-cutting height for the No. 13 coal seam is determined to be 30.5 m. Field monitoring results show that the convergence of the roof and floor and that of the two ribs of the withdrawal channel are reduced by 41.2% and 36.8%, respectively, and the distance between the stopping line and the terminal mining line is shortened by 15 m. The research results provide a useful reference for determining the top-coal caving stopping position and roof-cutting height, and for improving the stability of the surrounding rock of the support withdrawal channel during the final mining stage of fully mechanized top-coal caving faces with thick and hard roofs in extra-thick coal seams. Full article

In view of the complex stress environment of the underlying roadway and the difficulty in ensuring the stability of entry retaining under the combined action of the goaf and the residual coal pillar, this paper takes the auxiliary intake roadway of the 8307 working face in Xinjing Mine as the engineering background and carries out research focusing on two aspects: the determination of the pressure-relief environment beneath the goaf and residual coal pillar, and surrounding rock control through roof cutting and pressure relief. Two conditions, namely the structural basis and the stress basis, are proposed for determining whether a roadway beneath the goaf is located in a pressure-relief environment. For the identification of the stress basis, a floor stress calculation model is established, and the stress calculation method is corrected, thereby clarifying the necessity of entry retaining by roof cutting under a pressure-relief environment. Meanwhile, a FLAC3D numerical model is established to comparatively analyze the stress and displacement characteristics of the surrounding rock of the roadway under roof-cutting and non-roof-cutting conditions, and the results are verified by an in situ industrial test. The theoretical calculation results indicate that the 8307 auxiliary intake roadway satisfies the two conditions for being located in a stress reduction zone. The numerical simulation results show that, after roof cutting, the vertical stress on the solid coal side decreases by 19.7%, and the maximum vertical displacement decreases by 20.6%. The field statistical results show that the average convergence between the roof and floor after entry retaining is 399 mm, the average convergence of the two ribs is 160 mm, and the average deformation rate of the retained-entry section is 17.44%, which satisfies the reuse requirement. The research results can provide a reference for the design of entry retaining by roof cutting under similar goaf conditions. Full article

Mining coal seams with shallow, thick, and hard roofs often results in extensive roof suspension. This issue poses significant challenges regarding stratum control and mitigation of strong mining pressure, especially within the confined working space of a mining face. This study focuses on the 13101 working face of Shengfu Coal Mine. Through field observations, theoretical analysis, and numerical simulations, the characteristics of support resistance and microseismic activity were investigated. This research elucidates the mechanism behind the strong mining pressure driven by the structural coupling and synergistic breakage of two key strata, highlighting how their interaction dictates weighting intensity. A small-aperture hydraulic fracturing technology, specifically designed for inter-support spaces, was developed. The results indicate that the working face exhibits alternating “minor weighting” and “major weighting” events. Minor weighting occurs at an average interval of 12.38 m with a dynamic load factor of 1.14, while major weighting occurs at 41.07 m with a factor of 1.56. The roof structure was found to form a combination of an “inclined stepped rock beam” and a “voussoir beam.” Field applications demonstrate that the proposed technology reduces the major weighting interval by 41.46% and total microseismic energy release by 35.01%. This study provides a theoretical and technical basis for preventing roof disasters under similar geological conditions. Full article

The Springtime of Nations in 1990 was a great victory over the Evil Empire. We will examine some of its background in Croatia, concentrating on cultural phenomena and fine arts in particular. Even under Socialist Realism, there were some flights into “decadence”, such as the EXAT 51 exhibition in Zagreb, as Yugoslavia, threatened by the Russians, wanted to show its “pro-western” leanings. We will examine two outstanding Croatian artists who spanned the periods of Communism and its aftermath. The sculptor, Ljubo Dekarina (b. Rijeka, 1948), has mostly worked in the small Istrian township of Brseč. The painter, Ivan Rabuzin, from the Croatian Zagorje village of Ključ (b. 1921–d. 2008), maintained considerable ties with the Croatian capital of Zagreb. Thanks to them, we have discovered a world of an artistically contrived beauty and truth of eternal presence. Rabuzin’s is one of hills and groves, of rounded trees, and scattered hay-roofed homes, a short cut to paradise. Dekarina’s penetrations are entranceways into secret spaces of the beyond. Rabuzin and Dekarina just see better and know how to better communicate their experiences. It is the humanities that makes us human. Aristotle, as opposed to Plato, saw the arts as an improved and not a debased version of reality. Thus, the art experience comes close to religion, making us better, happier, more complete human beings. Art and religion are two aspects of that same pool. Full article

To address the stability challenges of surrounding rock in large-span open-off cuts within weakly cemented strata of western China, this study investigated the 1219 open-off cut at the Shila Wusu Coal Mine. An analytical elastic model for rectangular roadway stress was developed using complex variable function theory to examine the influence of the lateral pressure coefficient on stress distribution. Furthermore, numerical simulations were employed to characterize plastic zone evolution and evaluate support effectiveness. The results demonstrate that the lateral pressure coefficient significantly dictates the stress field: circumferential stress at the ribs intensifies with the increasing lateral pressure coefficient, while stress in the roof and floor decreases accordingly. Notably, tensile stresses develop in the roof and floor when the lateral pressure coefficient is less than 1. Stress extremes are concentrated at the roadway shoulders, exhibiting a distribution pattern where the ribs experience higher concentration than the roof and floor. The circumferential stress concentration coefficient exhibits a marked positive correlation with the lateral pressure coefficient. Numerical results indicate that post-support compressive stress at the shoulders reaches 39.24 MPa, with plastic zone widths of 1.64~2.06 m at the ribs, 2.70 m at the roof, and a significant 5.33 m at the floor, highlighting a pronounced risk of floor heave. Field loosening zone measurements of 1.08 m in the roof and 2.49 m in the rib align closely with numerical findings, confirming that the implemented support effectively constrains plastic zone development. By integrating theoretical derivation, numerical modeling, and in situ observations, this study establishes a robust theoretical and technical framework for the support design of large-span roadways in similar geological settings. Full article

The stability of retained roadways in closely spaced coal seams beneath a goaf is strongly affected by complex stress redistribution and the deterioration of roof structures under downward mining conditions. To address this issue, a combined approach involving theoretical analysis, numerical simulation, and field monitoring was adopted to investigate the deformation characteristics and stability control of gob-side retained roadways in short-distance coal seam groups. The movement characteristics of the roof and the deformation law of surrounding rock of the retained roadway under downward mining were revealed. An embedded short-arm beam structural model for a roof cutting retained roadway was established, and a calculation method for determining the required support resistance of the retained roadway was proposed. Based on this model, design criteria for the passive support system of the retained roadway were developed. A surrounding rock control technology with hollow grouting anchor cable support and low-disturbance directional roof cutting as the core was proposed, and the support resistance of a one-beam–four-column support system was determined to effectively limit roof subsidence. Field application results show that the surrounding rock displacement was controlled within 350 mm, and the roadway section shrinkage rate was maintained at 16.4%, indicating good stability of the retained roadway and satisfying the requirements of ventilation and transportation. This study provides a mechanical basis and practical guidance for stability control and support design of roof cutting retained roadways in closely spaced coal seams beneath goaf. Full article

To address the difficulty of directionally cutting thick, hard key strata in gob-side entry retaining using conventional blasting or hydraulic fracturing, this paper proposes a high-pressure water-jet slotting-induced pre-cracked weakening belt (PCWB) roof-cutting technology. Several finite-length PCWBs are arranged within the key stratum and designed to coalesce into a plane, inducing through-going roof failure along a pre-determined path. A fixed–fixed key strata beam model combined with linear elastic fracture mechanics shows that the double-belt configuration forces the bending moment and shear force to concentrate in a thin rock bridge, where bending and shear stresses are amplified by about 1.5–2.8 times and 1.2–1.7 times, respectively, for 2–4 m thick key strata, providing a mechanical basis for preferential tensile–shear failure. Two-dimensional RFPA2D simulations reveal “width-dominated, length-assisted” control of cutting performance and identify an optimal weakening belt geometry of about 400 mm in width and 200 mm in length. Three-dimensional numerical modeling of parallel slot pairs indicates that intra-pair spacing of about 40 mm produces a continuous, directional weakening belt, whereas smaller or larger spacing causes, respectively, destructive interference or loss of connectivity. High-pressure water-jet tests (320 MPa, 0.33 mm nozzle, 1.30 mm/s traverse speed) on limestone blocks confirm that single slots can penetrate the full thickness and that cracks from adjacent slots coalesce through the rock bridge, forming a wide, straight fracture band. Field application in the Dongjiang Mine (3.5 m limestone key stratum, ~400 m depth) shows that the first weighting is advanced from the 7th to the 3rd day, peak support resistance is reduced from 8.8 to 7.4 MPa, and periodic weighting becomes more frequent and smoother. The PCWB technology is therefore suitable for panels with 2–4 m thick hard key strata at similar depths, offering precise key stratum severance, active stress relief, and safe, controllable construction. Full article

Large-scale surface subsidence induced by extra-thick seam longwall top-coal caving (LTCC) is strongly amplified by thick unconsolidated overburden, posing serious serviceability risks to overlying linear infrastructure. Taking the S103 Provincial Highway above Panel 6118 in Inner Mongolia, China, as the engineering background, this study integrates theoretical analysis, numerical simulation, and in situ monitoring to investigate the subsidence-control mechanism of directional presplitting roof cutting. The results show that roof cutting mitigates surface subsidence by reconstructing the overburden structural system and weakening the stress-transfer chain, thereby transforming key-stratum deformation from integral bending to segmented block movement and narrowing the subsidence-affected zone. An equivalent mining-depth model for subsidence-boundary convergence is proposed to characterize the inward migration of the subsidence-basin boundary under thick unconsolidated cover, and a segmented probability-integral model is developed to explain the kink-like high-gradient feature in the post-cut subsidence profile. Parametric simulations of roof-cutting positions (p = 0, 2, 4, …, 32 m) show that the most effective mitigation occurs in the range p = 4–12 m; using minimum–maximum highway subsidence together with profile flattening as the optimization criteria, the representative optimum is identified at p ≈ 10 m, for which the maximum highway subsidence is approximately 57 mm, about 76% lower than that in the non-cutting case. The results further indicate that, although roof cutting significantly reduces subsidence and deformation gradients, fissure localization and possible discontinuous deformation near the pre-split weak plane still require careful field monitoring. Full article

During coal mining, parallel voids ahead of an advancing working face often trigger intense dynamic loading and structural instability, posing significant risks to operational safety. Using the 43,201 working face of the Shiyangou Coal Mine as a case study, this research investigates the mechanisms of surrounding rock instability and proposes an integrated synergistic control strategy. Based on voussoir beam theory, a mechanical model of the roof structure—incorporating the nonlinear coupling between the gangue and immediate roof—was developed to establish the critical thresholds for the rotational instability of key blocks. Analytical results indicate that the limit breaking distance for “Key Block B” in the main roof is 24.49 m, which defines the primary zone for advanced reinforcement and hazard prevention. Furthermore, applying short-arm beam theory, this study clarifies how pre-split roof cutting disrupts the transmission of advance abutment pressure, identifying 8° as the optimal cutting angle. Building on these insights, a multi-faceted control system was implemented, combining hydraulic fracturing for pressure relief, pumpable backfill pillars, and an artificial false roof (utilizing a suspended I-beam structure 1.2 m above the floor). Field monitoring confirms that this collaborative approach effectively stabilizes the surrounding rock, ensuring the safe and continuous passage of the working face through parallel void areas. Full article