ALBERT

Description: Geogrids have been extensively used in subgrade construction for stabilization purposes of unconfined ballast. Based on well-calibrated microparameters, a series of geogrid-reinforced ballast models with different geogrid sizes and particular structures were developed to reproduce the mechanical behavior of the geogrid under pull-out load in this paper. And the rationality of the DEM model is verified by comparing the evolution law pull-out force measured by laboratory tests and numerical simulations under comparable conditions. Moreover, the macro pull-out force and the internal force distribution of the geogrid were analyzed, and the contact force statistical zones of the particle system were divided accurately according to the results. Meanwhile, both the force transfer mechanism in the geogrid-ballast interface and the sectionalized strain of the geogrid were discussed. And results unveil that the pull-out load is transmitted along the longitudinal ribs to the transverse ribs, and nearly 90% of the load is transmitted to the contact network (in statistical zone 1) in front of the first transverse rib, resulting in strong interlocking between the particles occurs in statistical zone 1. And the second transverse rib is the strength dividing line between strong and weak contact forces. Then, additional pull-out tests on the control groups were conducted, and the sectionalized strain of the geogrid and the peak pull-out force, as well as the energy dissipation were systematically analyzed. In addition, the proposed method used in simulation holds much promise for better understanding of the reinforcement mechanism and further optimizing the performance of geogrid-reinforced structures.

Description: As cracks on concrete bridges become severer and more frequent, methods of detecting cracks on concrete bridges have aroused great concern. Conventional methods, e.g., manual detection and equipment-aided detection, suffer from subjectivity and inefficiency, which increases demands for an accurate and efficient method to detect bridge cracks. To this end, we modify the existing percolation method and propose an enhanced percolation method, which detects cracks of concrete bridges automatically. The modification includes three improvements, which are (1) employing photo expansion to eliminate boundary effects, (2) varying shape factors to increase the accuracy of percolating unclear cracks, and (3) decreasing the number of neighbouring pixels to form candidate sets. Combined with the above three improvements, three versions of enhanced percolation methods utilizing three different shape factors are put forward. The numerical experiment on detecting cracks in 200 images of the bridge surface demonstrates the outperformance of the enhanced percolation method in precision, recall, F-1 score, and time compared with traditional detecting methods. The proposed method can be generalized on the application of detecting other types of bridge diseases, which is an advantage for designing, maintaining, and restoring infrastructures.

Description: The growing demand for cement has created a significant impact on the environment. Cement production requires huge energy consumptions; however, Pakistan is currently facing a severe energy crisis. Researchers are therefore engaged with the introduction of agricultural/industrial waste materials with cementitious properties to reduce not only cement production but also energy consumption, as well as helping protect the environment. This research aims to investigate the influence of binary cementitious material (BCM) on fresh and hardened concrete mixes prepared with metakaolin (MK) and ground granulated blast furnace slag (GGBFS) as a partial replacement of cement. The replacement proportions of BCM used were 0%, 5%, 10%, 15%, and 20% by weight of cement. A total of five mixes were prepared with 1 : 1.5 : 3 mix proportion at 0.54 water-cement ratios. A total of 255 concrete specimens were prepared to investigate the compressive, tensile, and flexural strength of concrete after 7, 28, and 56 days, respectively. It was perceived that the workability of concrete mixes decreased with an increasing percentage of MK and GGBFS. Also, the density and permeability of concrete decreased with an increasing quantity of BCM after 28 days. Conversely, the compressive, tensile, and flexural strength of concrete were enhanced by 12.28%, 9.33%, and 9.93%, respectively, at 10% of BCM after 28 days. The carbonation depth reduced with a rise in content of BCM (up to 10%) and then later improved after 28, 90, and 180 days. Moreover, the effect of chloride attack in concrete is reduced with the inclusion of BCM after 28 and 90 days. Similarly, the drying shrinkage of concrete decreased with an increase in the content of BCM after 40 days.

Description: Rock masses can be regarded as a blocky rock system. After a disturbance load is applied, the anomalously low friction phenomenon may take place and cause geological disasters. A series of impact experiments on granite blocks were conducted to investigate the anomalously low friction phenomena. Vertical vibration, Fourier frequency spectrum, and horizontal motions were investigated. It can be found that the tensile phases of vertical vibration can reduce the maximum static friction force, namely, the shear strength. The quasi-resonance operating mode of the rock blocks was observed. During the stress wave propagation, the vibration in the loading direction tends to transfer from high frequency to low frequency and the modes of stress wave propagation do not depend on disturbance energies. The observed translational and rotational motions were due to the initial shear force, which is less than the friction force with no disturbance load. Stability of the blocky rock system is very sensitive to the initial stress state. In the subcritical state, friction force reduction can easily break the equilibrium of forces along the contact surface and even a slight disturbance may make the horizontal motions happen, which may lead to geological disasters with great energy release.

Description: A novel method for fracturing coal is presented in this paper. A chemical solution is injected into coal under high pressure, whereby the coal is fractured and subsequently weakened by chemical erosion over time to produce an anti-impact soft structure. In this study, the mechanical properties of coal under chemical erosion were investigated, and the fracturing design parameters were optimized. The uniaxial compression test and the split Hopkinson pressure bar (SHPB) test were used to determine the dynamic and static mechanical properties of coal after 20 days of immersion in different chemical solutions. After chemical solution erosion, the dynamic and static compressive strengths and elastic modulus of the coal decreased according to an exponential power law in the damage variable. The chemical treatment increased the duration of the pore compaction stage and decreased that of the elastic deformation stage, while decreasing the brittleness and increasing the ductility of coal. The acoustic emission (AE) curve of the immersed coal samples consisted of four stages corresponding to those of the stress-strain curve: pore compaction-closure, a slowly rising linear elastic regime, steady-state prepeak crack propagation, and unsteady crack propagation at the peak strain. The increase in the damage variable of the coal sample from chemical erosion led to a lower dissipated energy, a higher fractal dimension, and a more fragmented coal sample. The effect of the investigated chemical solutions on weakening the coal mechanical properties decreased in the following order: alkaline solution 〉 acidic solution 〉 NaCl solution 〉 distilled water. The experimental results provide a reference for weakening fractured coal seams.

Description: The deformation mechanism of the protective coal and rock pillar area outside a stope is an important parameter for setting a reasonable size. In this paper, based on the geological condition of working face 1231(1) in a mine in Huainan, a method that combines the use of a borehole and Brillouin optical time domain reflectometry (BOTDR) was proposed to analyze the stress variation laws of coal and rock pillar areas, and the parameters of the monitoring borehole and installation technique of the sensing optical cables were designed. Based on the monitoring data, the strain distribution characteristics of the sensing optical cables and their relationship with the rock strata were analyzed, the development law of coal and rock strata deformation during the mining process was revealed, and the transverse influence range of the coal and rock pillar affected by mining was reasonably divided. According to the results, the sensing optical cables show an overall trend of tensile strain, with a maximum value of 1800 με, and the main areas of rock strata deformation occur near the interface of rock strata. The range of rock strata disturbance along the borehole direction was approximately 38 m, and the maximum deformation of rock strata after the disturbance, namely, the displacement, was 24.87 mm. A numerical model was constructed to acquire the strain variation characteristic within 100 m in the outer floor of the working face. The transverse range of the floor disturbance was analyzed to be 30–36 m. The field test had good correspondence with the numerical simulation results, which indicates that the optical fiber testing technology can effectively describe the stress variation in the coal and rock strata. The test results can provide technical support for the rational setting of coal and rock pillars and disaster prevention and control. The research direction of deep rock mass testing is discussed, and optical fiber testing in boreholes is considered an effective method for studying deep dynamic disaster control.

Description: In recent decades, carbon fiber reinforced plastics (CFRP) have been widely used to repair and maintain concrete structures around the world. Since the parameter uncertainties of load and resistance are very important for the reliability assessment of RC bridge strengthened by CFRP, this paper presents a method to estimate the reliability and residual life of RC bridges strengthened by CFRP. In the proposed method, uncertainties of material properties, geometry parameters, load model, and time-dependent resistance model are taken into account. The proposed method combines the inverse reliability method and the calculation method of load and resistance of RC bridge strengthened by CFRP and is illustrated by an example RC bridge strengthened by CFRP during the service stage. The results indicate that the proposed approach can provide valid information regarding parameter uncertainties for the reliability of RC bridge strengthened by CFRP during the service stage. Additionally, the effects of parameter uncertainty of the reliability and residual life of RC bridge strengthened by CFRP during the service stage are analyzed and discussed. The proposed method is more robust and reliable than the traditional method.

Description: During the full-mechanized caving mining, the overburden strata in the double system and extrathick coal seam of the Datong mining area are largely damaged. Water and harmful gas in the old goaf may be discharged when overburden fractures evolve to the upper goaf, which poses a major threat to the normal production of the panel. To study the movement of overburden strata and the evolution of fracture field under the full-mechanized caving mining of the double system and extrathick coal seam, panel 8309 of Tongxin mine was taken as the research object; the evolution rule of fracture field in the full-mechanized caving mining of the double system and extrathick coal seam was obtained through field measurement and physical similar simulation. The results show the following: (1) the far-field and near-field key strata play a decisive role in controlling the fracture evolution of overburden strata. When the far-field key strata break and the development height of fractures reaches 220.9 m, panel 8309 is connected with the overburden goaf. (2) Based on the “O-shape circle” theory of mining fracture, with the continuous advance of the panel, the overburden breaks periodically, and a “fracture surface” with a certain angle of 61°–67° can be formed along with the advancing direction of the panel. (3) When the key strata are broken and the development height of fractures reaches the maximum, the fracture surface is formed as the “main fracture surface,” which is the only downward discharge pathway for goaf water and harmful gas. The overall shape fracture surface is “inverted trapezoid” in the upper part and “positive trapezoid” in the lower part. (4) Based on the field measurement of the water level of borehole and the observation of mine pressure, the correctness of the evolution law of the similar simulated fracture field is verified.

Description: Proper design of the explosion loads is of vital importance in the risk assessment of explosions for offshore oil and gas installations. A quantitative assessment method for gas explosion loads in process modules of offshore platform is proposed in this paper. The proposed approach achieves the following three objectives: (a) defining a suitable number of leak scenarios quantitatively based on the Latin Hypercube Sampling (LHS) technique and statistical analysis; (b) defining the explosion scenarios according to the computational fluid dynamics (CFD) dispersion analysis results in the sampled leak scenarios; (c) designing the explosion loads on interested areas according to the CFD analysis results in different explosion scenarios and exceedance probability methods. The proposed method was applied to a process module of an example offshore platform. The pressure loads on interested areas of the example platform are very close to that suggested in Det Norske Veritas (DNV) codes. The method developed in this paper can benefit the engineers on better assessment of gas explosion risk in process modules for offshore installations.

Description: The rockburst simulation experiments of granite samples with a circular hole under biaxial loading were conducted, and the samples were scanned by computed tomography (CT) at the end of experiment. Through a series of CT images, the failure characteristics of the circular hole wall were analysed to determine the types of rockburst pits, and the detailed formation process of different types of rockburst pits was studied in combination with the crack characteristics around them. The experimental results indicate that there are mainly two types of rockburst pits, namely, pan-shaped and V-shaped, which can occur symmetrically or asymmetrically on the left and right sidewall of the circular hole. The formation of rockburst pits is related to the cracks parallel and perpendicular to the principal stress. Cracks parallel to the principal stress can determine the depth of the rockburst pit and affect the type of the rockburst pit, and cracks perpendicular to the principal stress determine the width of the rockburst pit. There is a correlation between the formation process of pan and V-shaped rockburst pits. During the formation of a V-shaped rockburst pit, several rockbursts occur, and each rockburst forms a pan-shaped rockburst pit. In the process of developing from the tunnel wall to the deep rock, the width of the pan-shaped rockburst pit gradually decreases and a V-shaped rockburst with a stepped upper and lower boundary is formed.