vahab sarfarazi

Regarding the hazard-prone working conditions in underground mines, synchronous monitoring and alarm system is vital to increase the safety. By analyzing the accidents in underground mines in Iran, it can be deduced that most fatalities are related to gas leakage, objects drop off on the head, and not using helmets by the staff. Therefore, a smart helmet with the capability of measuring harmful gasses (regarding the type of the mine), detection of the existence of the helmet on the head, temperature and humidity measurement, and detection of blow on the head is designed and fabricated to eliminate the present dangers and problems. This system displays the evaluated data on a developed software through wireless data transmission hardware. The data transmission hardware is the primary a link between the intelligent safety helmet and the software. To follow the idea, practical experiments have been performed in Parvadeh four and East Parvadeh of Tabas coal mine to confirm the validity of data transmission that culminated in successful results. The results were altered by the complexity of the design of the underground spaces so that in a straight direction, data transmission was held until 430 meters. However, further progress was not possible due to tunnel limitations. Data transmission was reduced to 190 meters in access horizons with curvatures or tilts. According to present standards, some thresholds are defined for each of the mentioned cases such that alarm protocol is activated by exceeding these thresholds in critical circumstances. Then the helmet user and the software’s operator will be informed of the occurred danger and will settle the problem. The system outlined in this study ensures performance reliability through its alarm package. A key innovation is the in-depth examination of the impact of head injuries, transforming it into other factors by analyzing relevant content and setting boundaries for assessment rather than using specific numbers. Furthermore, the most evident aspect of this design is the enhancement of the managerial approach, which includes an attendance evaluation platform and performance reporting within the system.

The mechanical behavior of rock-rock bolt interface considering the effects of indents’ shape and their number was numerically simulated based on discrete element method using the two-dimensional particle flow code. The conventional and standard uniaxial compressive and Brazilian tensile strengths tests were used to calibrate the modelled samples with 100 cm 100 cm in dimension. The numerical models were prepared such that different indent shape and number were inserted in the cable bolts arrangements during the rock reinforcement process. The effects of confining pressure 3.7 MPa and different shear failure loads were modeled for the punch shear test of the concrete specimens. The results of this study showed that the dominant failure mode of the rock-cable bolt interface was of tensile mode and the shape and number of cable indents significantly affected the strength and mechanical behavior of the modelled samples. It has also been showed that the indent dimensions and number affected the shear strength of the interfaces.

Numerical modeling is an essential tool in engineering analysis, particularly within the fields of geoscience and geotechnics. The PFC2D software stands out in this field, using the Distinct Element Method (DEM) to simulate processes related to engineering geology and geotechnical assessment. This study focuses on the analysis and comparison of two common contact models: the Flat Joint Model (FJM) and the Linear Parallel Bond Model (LPBM). The Unconfined Compressive Strength (UCS) test is chosen as a the benchmark for calibrating and validating the PFC models. Sandstone samples for this study are taken from the Aghajari Formation located on the southern limb of the Madar Anticline. The results show that both contact models have a high ability to simulate the UCS in the calibration process. As this test is primarily used to calibrate the failure point (σc) and Young's modulus, the output values for both models are almost identical. However, the post-failure behavior in the stress-strain curves differs between the models, with the FJM demonstrating a more brittle response compared to the LPBM. The ability of the FJM model to simulate rough surfaces and material discontinuities allows for the representation of tensile cracking.

Concretes frequently contain joints and microcrack fractures, and the failure mechanism of these fractures is highly dependent on the pattern of crack coalescence between pre-existing flaws. Determining the non-persistent joints' failure behavior is an engineering challenge that incorporates several factors, including the ratio of the joint surface to the total shear surface, normal stress, and the mechanical characteristics of the concrete. This paper aims to utilize grey wolf optimizer (GWO) and gene expression programming (GEP) algorithms for the prediction of the crack coalescence stress (CCS). For this purpose, 8 input parameters affecting the CCS including jointing coefficient (JC), normal stress (σn), uniaxial compressive strength (σc), tensile strength (σt), Poisson's ratio (υ), modulus of elasticity (E), cohesion strength (C) and internal friction angle (φ) were selected based on the results of 450 direct shear tests conducted on specimens including 2 sets of non-persistent joints made of gypsum, cement, and water. The GWO and GEP techniques were then implemented. Three performance indicators of determination coefficient (R2), root mean square error (RMSE), and mean absolute error (MAE), were employed for the training and testing phases to evaluate the efficiency of the suggested models. The R2 values for GWO and GEP for the training phase were 0.962 and 0.938, respectively, while for the testing phase were 0.996 and 0.981, indicating that the GWO algorithm is more efficient than GEP. Moreover, the findings reveal that the GWO algorithm exhibits lower RMSE and MAE values in both the training and testing phases compared to the GEP method. However, it can be professed that the two methods used have high reliability and accuracy. Also, based on the GEP method, a formula was derived and presented for prediction of CCS. At last, according to the sensitivity analysis, it was found that the normal stress (σn) and jointing coefficient (Cu) have the greatest and least influence on CCS, respectively...

The mechanical behaviour of transversely isotropic elastic rocks can be numerically simulated by the discrete element method. The successive bedding layers in these rocks may have different mechanical properties. The aim of this research work is to investigate numerically the effect of anisotropy on the tensile behaviour of transversely isotropic rocks. Therefore, the numerical simulation procedure should be well-calibrated by using the conventional laboratory tests, i.e. tensile (Brazilian), uniaxial, and triaxial compression tests. In this study, two transversely isotropic layers were considered in 72 circular models. These models were prepared with the diameter of 54 mm to investigate the anisotropic effects of the bedding layers on the mechanical behaviour of brittle geo-materials. All these layers were mutually perpendicular in the simulated models, which contained three pairs of thicknesses 5 mm/10 mm, 10 mm/10 mm, and 20 mm/10 mm. Three different diameters for models were chosen, i.e. 5 cm, 10 cm, and 15 cm. These samples were subjected under two different loading rates, i.e. 0.01 mm/min and 10 mm/min. The results gained from these numerically simulated models showed that in the weak layers, the shear cracks with the inclination angles 0° to 90° were developed (considering 15° increment). Also there was no change in the number of shear cracks as the layer thickness was increased. Some tensile cracks were also induced in the intact material of the models. There was no failure in the interface plane toward the layer of higher strength in this research work. The branching was increased by increasing the loading rate. Also the model strength was decreased by increasing the model scale.

In this paper, the effect of variations in the number and area of the rock bridges on the non-persistent discontinuities is investigated. In this regard, blocks containing rock bridges and joints with dimensions of 15 cm * 15 cm * 15 cm are prepared from plaster. The available rock bridges that have occupied 0.2, 0.4, and 0.6  of the shear surface show latitudinal extension along the shear surface. There are variations in the number and extension of the rock bridges in the fixed area. For each of the samples, tests are performed on three blocks of the same material, by putting it under various direct normal stresses. Normal stresses were 3.33, 5.55, 7.77 kg/cm2. Also the obtained shear strength by laboratory tests was compared with the outputs of Jenning's criterion and Guo and Qi's criterion to determine the accuracy of these criteria for predicting the shear strength of non-persistent joints. The results show that the tensile crack started in the rock bridge under normal stress of 3.33 kg/cm2. Mixed-mode tensile shear cracks were propagated in the rock bridge under a normal stress of 5.55 kg/cm2, while a pure shear crack developed in the rock bridge under a normal stress of 7.77 kg/cm2. With the increase of normal stress, the number of microfractures increased. The variance in the number of rock bridges in the fixed area of the rock bridge does not affect the friction angle along the shear surface. Furthermore, the cohesion along the shear surface shows a small decrease with the increasing number of rock bridges. Also by the increase in the area of rock bridges, the friction angle along the shear surface remains constant, while at the same time, there is an almost linear increase in cohesion. Guo and Qi's criterion predicts the shear strength of the non-persistent joint exactly close to the shear strength of the physical samples.

Non-persistent joints are geologic occurrences in rocks that weaken pillars because they are present within them. Using practical tests and numerical models, it has been determined how edge notches affect the way pillars break. Gypsum samples that are notched and have dimensions of 70 mm by 70 mm by 50 mm are created. Gypsum's Young modulus, Poisson ratio, compressive strength, and tensile strength are 5.5 GPa, 0.27, 8 MPa, and 1.1 MPa, respectively. 10-, 20-, and 30-degree notch angles are used. The model receives an axial stress at a rate of 0.05 mm/min. On a rock pillar, numerical simulation is carried out concurrently with an experimental test. The findings indicate that the joint angle is mostly responsible for the failure process. The fracture pattern and failure mechanism of the pillars are connected to the compressive strengths of the specimens. At the notch points, two significant splitting tensile fractures spread vertically until coalescing with the top and lower boundaries of the models. On the left and right sides of the pillar, two rock columns are also taken out. The overall number of cracks rises as sample loading increases. The model's deformation at the start of loading reflect a linear elastic behavior, and the number of fractures steadily grows. When the number of cracks increases, the curve becomes non-linear, and the force being applied peaks. When the sample can no longer tolerate the applied force, a dramatic stress decrease occurs. The macro-failure over the whole model is what leads to the greater stress decrease following the peak load. In actuality, the reduced stress reduction is accompanied by more overall fractures. Similar findings are shown in both the experimental testing and numerical modeling.

This work presents the hollow center cracked disc (HCCD) test and the cracked straight through Brazilian disc (CSTBD) test of oil well cement sheath using the experimental test and Particle Flow Code in two-dimensions (PFC2D) in order to determine mode I fracture toughness of cement sheath. The tensile strength of cement sheath is 1.2 MPa. The cement sheath model is calibrated by outputs of the experimental test. Secondly, the numerical HCCD model and CSTBD model with diameter of 100 mm are prepared. The notch lengths are 10 mm, 20 mm, 30 mm, and 40 mm. The tests are performed by the loading rate of 0.018 mm/s. When the notch length in CSTBD is 40 mm, the external work is decreased 48%, related to the maximum external work of model with notch length of 10 mm (0.225 KN*mm decreased to 0.116 KN*mm). When the notch length in HCCD is 30 mm, the external work is decreased 33%, related to the maximum external work of model with notch length of 10 mm (0.06 KN*mm decreased to 0.04 KN*mm). The fracture energy is largely related to the joint length. The fracture energy is decreased by increasing the notch length. In constant to the notch length, the fracture energy of the CSTBD model is more than the HCCD model. Mode I fracture toughness is constant by increasing the notch length. The HCCD test and the CSTBD test yield a similar fracture toughness due to a similar tensile stress distribution on failure surface. The experimental outputs are in accordance to the numerical results.

Experimental and numerical methods (Particle Flow Code) were used to investigate the effect of echelon notches on the shear behavior of the joint’s bridge area in granite. A punch-through shear test was used to model the granite cracks under shear loading. Granite samples with dimension of 20 mm×150 mm×40 mm were prepared in the laboratory. Within the specimen model and near the edges, four edge notches were provided. Nine different configuration systems were prepared for notches. In these configurations, the length of each notch was taken as 3 cm, 4cm and 5 cm. Assuming a plane strain condition, special rectangular models were prepared with dimensions of 100 mm×100 mm using the particle flow code in two dimensions (PFC2D). Similar to those joints’ configuration systems in the experimental tests, i.e. 9 models with different rock bridge lengths and different rock bridge joint angles were prepared. The axial load was applied to the punch through the central portion of the model. This testing showed that the failure process was mostly governed by the rock bridge length and the rock bridge angle.  Shear strengths of the specimens were related to fracture pattern and failure mechanism of the discontinuities. It was shown that the shear behavior of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the rock bridge angle.  The strength of samples decreases with increasing the joint length. The failure pattern and failure strength are similar in both methods, i.e. the experimental testing and the numerical simulation.

This work presents the Semi-Circular Bend (SCB) test and Notched Brazilian Disc (NBD) test of shotcrete using experimental test and Particle Flow Code in two-dimensions (PFC2D) in order to determine a relation between mode I fracture toughness and the tensile strength of shotcrete. Firstly, the micro-parameters of flat joint model are calibrated using the results of shotcrete experimental test (uniaxial compressive strength and splitting tensile test). Secondly, numerical models with edge notch (SCB model) and internal notch (NBD model) with diameter of 150 mm are prepared. Notch lengths are 20 mm, 30 mm, and 40 mm.  The tests are performed by the loading rate of 0.016 mm/s. Tensile strength of shotcrete is 3.25 MPa. The results obtained show that by using the flat joint model, it is possible to determine the crack growth path and crack initiation stress similar to the experimental one. Mode I fracture toughness is constant by increasing the notch length. Mode I fracture toughness and tensile strength of shotcrete can be related to each other by the equation, σt = 6.78 KIC. The SCB test yields the lowest fracture toughness due to pure tensile stress distribution on failure surface.

The tensile strengths of geomaterials such as rocks, ceramics, concretes, gypsum, and mortars are obtained based on the direct and indirect tensile strength tests. In this research work, the Brazilian tensile strength tests are used to study the effects of length and inclination angle of T-shaped non-persistent joints on the mechanical and tensile behaviors of the geomaterial specimens prepared from concrete. These specimens have a thickness of 40 mm and a diameter of 100 mm, and are prepared in the laboratory. Two T-shaped non-persistent joints are made within each Brazilian disc specimen. The Brazilian disc specimens with T-shaped non-persistent joints are tested experimentally in the laboratory under axial compression. Then these tests are simulated in the two-dimensional particle flow code (PFC2D) considering various notch lengths of 6, 4, 3, 2, and 1 cm. However, different notch inclination angles of 0, 30, 60, 90, 120, and 150 degrees are also considered. In this research work, 12 specimens with different configurations are provided for the experimental tests, and 18 PFC2D models are made for the numerical studies of these tests. The loading rate is 0.016 mm/s. The results obtained from these experiments and their simulated models are compared, and it is concluded that the mechanical behavior and failure process of these geomaterial specimens are mainly governed by the inclination angles and lengths of the T-shape non-persistent joints presented in the samples. The fracture mechanism and failure behavior of the specimens are governed by the discontinuities, and the number of induced cracks when the joint inclination angles and joint lengths are increased.  For larger joints when the inclination angle of the T-shaped non-persistent joint is around 60 degrees, the tensile strength is minimum but as it is closed to 90 degrees, the compressive strengths are maximum. However, an increase in the notch length increase the overall tensile strength of the specimens. The strength of samples decreases by increasing the joint length. The strain at the failure point decreases by increasing the joint length. It is also observed that the strength and failure process of the two sets of specimens and the corresponding numerical simulations are consistence.

One of the most important tasks in conducting a laboratory research work is how to make the samples. The purpose of this research work is to create heterogeneous rock-like samples containing non-persistent notches. Regarding that, the molds with dimensions of 250 mm x 200 mm x 50 mm are made. A mixture of plaster and water with different mixing percentages is used to make the heterogeneous samples. Various techniques are also employed to create non-persistent notches on the samples. One of the methods to create a notch is to insert an aluminum blade into the groove of the mold, and finally, remove it after the plaster slurry has hardened. Due to the displacement of the blade and its tilting during slurring, the notches are out of the vertical position. In addition to the mentioned method, other methods such as water jet, cutting by thread, cutting by diamond wire cutting, cutting by rotary saw, and using hand saw are applied. Finally, using a hand saw to create a notch on the samples is chosen as the best method.

The discontinuities are formed in different levels of concrete as a result of various curing processes. There are only few cases where cause and location of failure of a concrete structure are limited to a single discontinuity. Usually several discontinuities of limited size interact and eventually form a combined shear plane where failure takes place. So, besides the discontinuities themselves, the regions between adjacent discontinuities, which consist of strong concrete and are called concrete bridges, are of utmost importance for the shear strength of the compound failure plane. Also, the coalescence of non-persistent joints is important factors in controlling the mechanical behavior of brittle rocks and can be caused concrete failure in slopes, foundations and tunnels. Therefore, a comprehensive study on the shear behavior of non-persistent joint can provide a good understanding of both local and general concrete instabilities, leading to an improved design for concrete engineering projects. From last decade, direct shear test have been used to study shear behavior of echelon joint. Whereas, only one horizontal shear failure surface can be formed within the shear box therefore, it’s not possible to perform shear test on the echelon non-persistent joint. This paper introduces the modified shear box so the shear test can be performed on the both of the overlapped and non-overlapped echelon joint.

The aim of this study is analyzing the failure modes of sandstones related to Markazi Province using compressive strength changes. For this purpose, the fracture modes of five different types of sandstones with different compressive strength values have been studied experimentally and numerically. Also, The effect of failure modes on uniaxial compressive strength was quantified. block samples were collected from different sandstone formations located in northern part of Markazi province. These sandstones were divided into five types based on the textural and mineralogical characteristics. These samples were subjected to uniaxial compressive strength testing. Examination of the fracture modes showed that simple shear mode occurred at low values of strength and it changes to multiple extension mode with increasing the compressive strength. The dominant failure mode for types 1, 2 and 4 is simple shear and for types 3 and 5 is multiple extension. Investigation of failure modes in different strength ranges showed that the dominant failure modes up to 140 and over 140 MPa are simple shear and multiple extension, respectively. Therefore, strength of 140 MPa was considered as the transisson point. Also, in this study, the failure modes of the experimental samples were numerically simulated using PFC2D software. The results of numerical modeling showed a good agreement with the experimental results.

The aim of this paper is to study the effect of spacing of the joint from ‘u’ shape cutter on the fracture mechanism and to investigate the effect of joint angularity under ‘u’ shape cutter loading on the shear failure mechanism. For this purpose nine sample with dimension of 5 cm *10 cm*10cm consisting non-persistent joints were prepared. The first sets of three specimens have one non-persistent joint with length of 2cm and angularity of 0, 45 and 90. The spacing between joint and top of the sample was 2cm. The second sets of three specimens have one non-persistent joint with length of 2cm and angularity of 0, 45 and 90. The spacing between joint and top of the sample was 4cm. The third sets of three specimens have two non-persistent joint with length of 2cm and angularity of 0, 45 and 90. The spacing between upper joint and top of the sample was 2cm and the spacing between lower joint and upper joint was 2cm. the samples were tested under loading rate of 0.01 mm/s. concurrent with experimental investigation, numerical simulation were performed on the non-persistent joint using FRANC2D. the results shows that the spacing between joint and specimen edge and joint angularity have important effect on the crack growth mechanism. Also, failure mode and failure pattern in experimental test and numerical simulation are similar.

Investigation of behavior of non-persistent joint is important in rock structure stability. This leads to improvement in rock engineering project design. Rock bridges in non-persistent joint increase shear strength of failure surface. For investigation of shear behavior of rock bridge, 24 gypsum samples with dimension of 10 cm × 10 cm × 5 cm were prepared. The joint lengths in various samples are different but in the one sample the joint length are similar. Joint lengths change from 1 cm to 4 cm. in each joint length, joint angularity was 0, 15, 30, 45, 60, 75 degrees. These samples were tested under uniaxial compression test. The results show that failure pattern was affected by joint length, joint angularity and rock bridge length while failure load was controlled by failure pattern. Concurrent with experimental test, numerical simulation was performed using PFC2D software. The joint lengths in numerical model change from 1cm to 4 cm with increment of 1cm. In each joint length, the joint angularity is 0° and 45°. Failure pattern in numerical model was similar to experimental sample while failure load in numerical model was more than experimental outputs.

In this paper, a compressive to tensile load convertor (CTC) device has been introduced which can be used for induction of tensile failure in specimen. This devise was consisted of 7 different parts. Parts number 1 and 2 which have U shape and П shape section have been made from stainless steel. Parts number 3 and 4 were made from two semi-cylindrical stainless steels with dimension of 10mm × 75mm × 60mm. Parts number 5 and 6 were made from two stainless steels with dimension of 190mm × 10mm × 20 mm. The concrete specimens used in this test have rectangle shape with internal pore. This geometry was gained from FRANC2D simulation outputs. The concrete samples has been prepared by mixing water, fine sand and cement by the ratio of 40%, 30% and 30%. The CTC device and sample were inserted in uniaxial test machine. The tensile test was performed by conversion of compression load to tensile load using CTC test. The tensile failure pattern occurred in the sample. For validation of experimental results, numerical simulations have been done using FRANC2D and high order displacement discontinuity method. The good accordance between failure pattern in numerical simulations and experimental test shows the validation of introduced device in induction of tensile failure in specimens.

In this paper, a simultaneous experimental and numerical analysis of opening mode toughness in The Pre-joined Brazilian disc using Brazilian tests are carried out. These numerical results are compared with the existing experimental results. For this purpose, two concrete disc specimens of 54 mm diameter and 27 mm thick were prepared. These specimens have a central joint of 20 mm in length and an opening in mm 1. Specimens are made from a mixture of water, fine sand, and cement with a ratio of 1-5 / 0-1. The same specimens are numerically simulated by a two-dimensional particle flow code (PFC).The results indicate that the crack formed from the tip of the joint and grows parallel to the load and connects to the edge of the specimen. The Fracture toughness obtained from the numerical method is in good agreement with experimental results.

In this paper, the effect of the ratio of tensile strength to confining pressure on the penetration of U Shape disc has been investigated using Discrete Element Method. For this purpose, three numerical models with different tensile strength of 5 MPa, 15 MPa and 25 MPa were built. From each model two similar samples were prepared and compressed by two different confining pressures of 5 MPa and 25 MPa, respectively. The U shape cutter penetrates in the model by the rate of 0.02 m/s till 4 mm of disc is penetrated. In total, 6 simulations have been done. The rock materials, below the cutters, show three different mechanical behavior including failure, plastic and elastic behavior. The failure zone is fully fractured while the plastic zone is consisted of partially micro crack with several major fractures. The elastic zone has remained undamaged. The effect of the ratio of tensile strength to confining pressure has substantial effect on the extension of three introduced zones. When tensile strength is 5 MPa, the initiation force and failure stress is increased by increasing the tensile strength but the extension of failure zone and chipping thickness is decreased. When tensile strength is 25 MPa, the initiation force, failure stress, extension of failure zone and thickness of chipping is constant by increasing the tensile strength.

In this paper, a compression-to-tensile load converter device is developed to determine the anisotropic tensile strength of brittle material. A cubic sample with an internal pore was used as the test specimen, and a series of finite element analysis and DDM simulations were performed thereafter to analyse the effect of pore dimensions on the stress concentration, as well as to render a suitable criterion for determining the anisotropic tensile strength of concrete. The results obtained by this device show that the tensile strength of concrete is similar in different directions because of the homogeneity of bonding between the materials.