Effect of the cement–tailings ratio on the shear failure mechanism at the cemented tailing backfill–rock interface: Insights from the morphology of stope surrounding rocks

This paper reports the effects of temperature and fiber architecture on the shear properties and failure mechanism of 3D MWK composites under compressive loading at room and elevated temperatures. The shear experiments of composites with three fiber architectures were performed at five different temperatures. Macro-fracture and SEM micrographs were examined to understand the deformation and shear failure mechanism. The results show that shear stress-strain curves show non-linearity and plastic fracture feature, and exhibit plastic platform at elevated temperatures. The temperature has significant effects on the shear properties which decrease significantly with increasing the temperature due to degradation of matrix properties, especially after 75 oC. The shear properties can be affected greatly by the fiber architecture and these increase significantly with the increase of 45 o direction fiber at different temperatures. The results also show that the damage and failure patterns of composites vary with the fiber architecture and temperature. At room temperature, the interfacial adhesion between fibers and matrix is high, the local delaminating and shear failure feature are prominent. At elevated temperatures, the composites become more softened, cracked and attain plasticity. The damage of matrix plastic and cracking, interface debonding, and fiber layer delaminating change significantly.

In underground engineering, shear failure is a common failure type in coal-rock mass under medium and low strain-rate disturbance loads. Analyzing the shear failure mechanical properties of coal-rock mass under dynamic normal load is significant. In order to reveal the influence of disturbance load on the shear mechanical properties of coal rock, a dynamic and static load coupling electro-hydraulic servo testing machine was used to conduct the shear tests of coal-like rock materials under dynamic and constant normal load. The amplitude of dynamic load is 10 kN and the frequency is 5 Hz. The damage process of the specimens was detected by the acoustic emission (AE) detection system. The results imply that the shear failure process of coal-like rock materials under constant normal load can be divided into four stages. The normal disturbance decreased the shear strength of the specimens and increased the shear modulus of the specimens. With the increase in normal load, the influence of disturbance on the shear strength of the specimen decreased. By analyzing the AE parameters, it was found that the dynamic load made the internal damage of the specimen more severe during the shear failure process. The damage variable was calculated by AE cumulative energy, and the damage evolution was divided into three stages. The shear failure mechanism of the specimen was judged by RA (rise time/amplitude) and AF (average frequency). It was found that from the elastic deformation stage to the unstable development fracture stage, the proportion of shear fracture increased. When the dynamic normal load was 10 kN and 30 kN, the fracture was mainly shear fracture; When the dynamic normal load was 50 kN, the fracture was mainly tensile or mixed fracture. The dynamic normal load affects the shear strength and failure mechanism. Therefore, the influence of disturbance load on coal-rock mass strength cannot be ignored in underground engineering.

Numerical analysis of size effect in RC beams scaled along height or length using elasto-plastic-damage model enhanced by non-local softening

Abstrak Struktur jembatan beton bertulang eksisting yang direncanakan berdasarkan peraturan lama umumnya belum mempertimbangkan konsep perencanaan tahan gempa dan belum mengaplikasikan detailing seismik yang memadai. Hal ini menjadi perhatian bagi jembatan eksisting dengan kolom pendek yang memiliki aspek rasio (a/h) di bawah 2.5 dan berpotensi mengalami mekanisme keruntuhan geser atau geser-lentur. Mekanisme keruntuhan tersebut mengakibatkan performa struktur jembatan akibat gempa memiliki tingkat ketidakpastian yang tinggi. Analisis kerentanan seismik pada struktur jembatan eksisting dapat dilakukan dengan mengembangkan kurva kerentanan menggunakan incremental dynamic non-linear time history analysis yang mampu menghasilkan nilai probabilitas kerusakan pada berbagai intensitas gempa. Penelitian terdahulu umumnya mengembangkan kurva kerentanan berdasarkan idealisasi perilaku sendi plastis pada kolom jembatan yang mengalami mekanisme keruntuhan lentur akibat beban gempa. Studi ini menyampaikan tinjauan terkini (state-of-the-art) yang meliputi penelitian struktur jembatan beton bertulang dengan kolom pendek, khususnya yang perilaku keruntuhannya tidak didominasi oleh mekanisme lentur. Tinjauan ini juga mengusulkan kerangka kerja untuk penilaian risiko seismik dan pengembangan kurva kerentanan yang lebih sesuai untuk struktur jembatan dengan kolom pendek. Hasil tinjauan ini dapat memberikan gambaran yang lebih jelas mengenai risiko seismik yang dihadapi oleh jembatan dengan kolom pendek, serta menunjukkan potensi penelitian lanjutan yang dapat dilakukan untuk pengembangan kurva kerentanan yang lebih akurat dan relevan. Kata-kata Kunci: Analisis kerentanan seismik, jembatan beton bertulang eksisting, kolom pendek, kurva kerentanan, mekanisme keruntuhan geser Abstract Existing reinforced concrete bridge structures designed based on older regulations often do not consider seismic design concepts and lack adequate seismic detailing. This issue is particularly concerning for existing bridges with short columns and aspect ratio (a/h) below 2.5, which has the potential of shear or flexural-shear failure mechanisms. These failure mechanisms result in a high level of uncertainty in the seismic performance of bridge structures.Seismic fragility analysis of existing bridge structures can be performed by developing fragility curves using the incremental dynamic non-linear time history analysis method, which is capable of providing damage probability for various levels of seismic intensity. Previous studies typically developed fragility curves based on idealized plastic hinge behavior in bridge columns subjected to flexural failure mechanisms.This study presents a state-of-the-art review of reinforced concrete bridge structures with short columns, especially those whose failure behavior are not dominated by flexure mechanism. This review also proposes a framework for seismic risk assessment and for the development of more suitable fragility curves for bridge structures with short columns.The findings of this study can provide an understanding of the seismic risk in bridges with short columns, while also highlighting the potential for future research to develop more accurate and relevant fragility curves. Keywords: Seismic fragility analysis, existing reinforced concrete bridges, short columns, fragility curves, shear failure mechanisms

Shear failure mechanism and parametric study for seawater sea-sand engineered cementitious composites beams reinforced by FRP bars: Finite element analysis

Impact of weak interlayer characteristics on the mechanical behavior and failure modes of cemented tailings backfill: A study on thickness, strength, and dip angle

The cemented tailings backfill (CTB), which plays a significant role in the stability of mine structure, is made of cement, tailings, and water in a certain proportion. When blasting and excavating an underground mine, the CTB will be disturbed by blasting. The impact load of blasting has an impact on the stability of the CTB, which is directly related to the safety of mine construction. The mechanical behaviour of CTB is generally affected by the cement-tailings ratio (C/T) and average strain rate (ASR). Therefore, a series of impact experiments were carried out on three CTB specimens with different C/T using a SHPB. Combined with the experimental results, this account reports studies on the effects of C/T and ASR on the mechanical properties of CTB, and on the energy transfer laws of CTB during impact compression. The research results show that when the ASR is less than 70 s−1, the peak stress and the peak strain have the same trend, and both of them continue to increase with the increase of ASR.When the ASR exceeds 70 s−1, as the ASR increases, the peak stress continues to increase, but the peak strain decreases gradually. Afterwards, the law of energy transfer of the CTB specimen was analyzed. It was found that as the incident energy increased, the energy reflection ratio of the CTB increased. Both the energy transmitted ratio and the energy dissipation ratio decreased. The volumetric energy showed a sharp increase first and then a trend Because of the slowly increasing trend. Finally, according to the failure morphology of the CTB, it is found that the ASR and the C/T together affect the failure of the CTB. The failure model of the CTB is mainly split failure and crush failure.

The wide application of the fill mining method is necessary for practicing the development concept of "green mountains are golden mountains" and protecting the surface landscape. In the underground quarry, the cemented tailings backfill (CTB) is mainly subjected to the gravity of the overlying rock layer in the mining area, which will cause creep problems in the long term. In order to protect the long-term stability of the mine surface, to study the creep hardening-damage characteristics of the CTB under the uniaxial action, for the maintenance age of 28d, cement-tailings ratios of 1:6 CTB to carry out uniaxial grading loading creep test, combined with the theory of time hardening Explore the creep process of the CTB in the hardening-damage law and the construction of the creep isomorphic model. The results show that the differences in the creep process of 1:6 cement-tailings ratios filler are caused by the joint existence of hardening and damage effects. In the stage of decelerated creep, creep hardening plays a major role; in the stage of stabilized creep, creep hardening and damage play a joint role; in the stage of accelerated creep, the creep damage effect is dominant; the strength of the CTB is strong and then weakened, and ultimately, the cumulative damage is too large to produce a "Y" type damage. The intrinsic model constructed based on the time-hardening theory better characterizes the creep of the 1:6 cement-tailings ratios of CTB under different stress levels. K and r are the material constants of the CTB, and the parameter K affects the time of decelerated creep during the decelerated creep stage, and the parameter r affects the creep rate of isokinetic creep.

The physical and mechanical change processes of rock are closely related to energy transformation, and its deformation and failure is an instability phenomena driven by energy exchange. This study investigated mechanism of shear deformation, failure and energy dissipation of joint using both physical and numerical direct shear tests under constant normal load (CNL) condition. Three kinds of joint surface were artificially prepared. An acoustic emission system was employed to monitor acoustic emission in physical test, and rupture frequency was recorded in numerical test to represent micro-crack development. By research of numerical micro-crack development accompanied with physical acoustic emission results, mechanism of shear deformation and failure of joints were illustrated schematically. By definition of dissipation energy, captured using the particle flow code (PFC2D), energy releasing and dissipation were discussed with microscopic damage evolution of joints. Results showed that joints under shearing present a dissipation trend of four stages including a slow rise stage, a rapid rise stage, a shock rise stage and a rapid decline stage.

In this study, a one-part alkali-activated slag (AAS) composed of ground-granulated blast furnace slag, desulfurized gypsum, and hydrated lime is proposed as alternative to cement for the production of cemented fine tailings backfill (CFTB), which is an environmentally friendly binder consisting of 93.72 wt.% industrial solid waste. Results show that AAS with 67.83 wt.% slag, 25.92 wt.% desulfurized gypsum, and 6.25 wt.% hydrated lime yields the highest strength, which is 1.7-3.2 times that of ordinary Portland cement (OPC). Aside from calcium silicate hydrate gel, appreciable quantity of ettringite characterized by interlocking needles structure and high bound water is also produced during the AAS hydration process. In addition, the hydration heat of the AAS binder is 48% less than that of OPC. Moreover, CFTB made of AAS provides better workability than that of CFTB with OPC up to 20h. The findings of this study will contribute to the production of more cost-effective, durable, and environmental-friendly cemented fine tailings backfill.

Cemented tailings backfill (CTB) is the most cost-effective and environmentally friendly method to recycle tailings for filling mining. It is of great significance to study the fracture mechanism of CTB for safe mining. In this study, three cylindrical CTB samples with a cement-tailings ratio of 1:4 and a mass fraction of 72% were prepared. An acoustic emission (AE) test under uniaxial compression (UC) with WAW-300 microcomputer electro-hydraulic servo universal testing machine and DS2 series full information AE signal analyzer was carried out to discuss the AE characteristics of CTB, such as hits, energy, peak frequency, and AF-RA. Combined with particle flow and moment tensor theory, a meso AE model of CTB was constructed to reveal the fracture mechanism of CTB. The results show that (1) the AE law of CTB under UC has periodic characteristics, which can be divided into the rising stage, stable stage, booming stage, and active stage. (2) The peak frequency of the AE signal is mainly focused on three frequency bands. The ultra-high frequency AE signal may be the precursor information for CTB failure. (3) The low frequency band AE signals represent shear crack, while the medium and high frequency band AE signals represent tension crack. The shear crack initially decreases and then increases, and the tension crack is the opposite. (4) The fracture types of the AE source are divided into tension crack, mixed crack, and shear crack. The tension crack is dominant, while a larger magnitude AE source is frequently shear crack. The results can provide a basis for the stability monitoring and fracture prediction of CTB.

Study of internal pressure strength of the titanium-steel composite tube based on yield and shear failure mechanisms

Study on the nonlinear deformation characteristics and constitutive model of cemented tailings backfill considering compaction hardening and strain softening

Mineral resources are increasingly being developed in cold and permafrost regions. However, the mechanical and physical properties of cemented tailings backfill (CTB) cured at normal temperature are no longer applicable. To clarify the reasons for this variability, a series of tests were performed. The mechanical properties of CTB with different cement–tailings ratios (CTR, 1:4, 1:8, 1:12, 1:16, and 1:20) were tested at different curing ages (3, 7 and 28 days) and curing temperatures (20 °C, 5 °C, −5 °C, and −20 °C). The differences of CTB in mechanical and physical properties under positive- and negative-temperature curing conditions were analyzed, and the microscopic failure process of CTB under negative-temperature curing conditions was discussed. The results revealed that the mechanical properties and deformation behavior of CTB under positive- and negative-temperature curing conditions were different. The frozen CTB had higher early strength than the standard-temperature curing condition (20 °C), and the lower the temperature, the higher the early strength. The low-temperature curing condition, on the other hand, was not beneficial to CTB’s long-term strength. The low-temperature curing condition was not conducive to the long-term strength of CTB. After yielding, strain hardening and strain softening appeared in the deformation behavior of frozen backfill, indicating ductility. In contrast to the typical-temperature curing condition, the frozen CTB showed a new failure pattern that has little relation to curing time or CTR. Furthermore, the failure process of frozen backfill was reviewed and studied, which was separated into four stages, and altered as the curing time increased. The results of this study can act as a guide for filling mines in permafrost and cold climates.

Understanding the shear behaviour of the interface between cemented tailings backfill and retaining wall structures (barricades, bulkheads) is important for the optimal design of barricades or bulkheads, and for mine operators to balance strength and safety against cost. However, our understanding of the shear behaviour of the aforementioned interface is limited. This paper is aimed at investigating the interfacial behaviour and properties of cemented tailings backfill-retaining wall structures, including stress–strain behaviour, cohesion, friction angle and shear stiffness through direct shear tests. Two different types of barricade or bulkhead materials (brick and concrete) are used in this study. Interface shear tests are performed at various curing times of the cemented backfill. Valuable results are obtained with regards to the interface shear behaviour of backfill-retaining structures. Based on these results, interfacial properties between cemented tailings backfill and barricades or bulkheads show a significant time-dependent variation.