hadi haeri

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.

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.

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.

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.

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 presence of openings in concrete structure has destructive effect on the lifetime. In this paper, the interaction between two openings has been investigated using particle flow code. For this purpose, after calibration of model, one small opening has been situated in various distances and various angularities from big pore opening. The radius of smaller opening is 0.2 of radios of big opening.  This complex is situated in a concrete slab with uniaxial strength of 7 MPa. This slab is under principal stress of 2MPa and 6 MPa. The results show that spacing and angularity of small opening has important effect on the stress distribution and failure pattern around the big opening. The critical position for small opening is when it situated at 0.0 degree. Also the spacing between two openings is less, the failure volume is more.

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, 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.

The rocks with pre-existing cracks are normally under compression. Study of the propagation mechanism of these pre-existing cracks is important in the design of rock structures. In the present paper، a coupled numerical-experimental analysis of crack propagation، cracks coalescence and breakage process of brittle solids such as rocks and rock-like specimens are studied. The pre-cracked cylindrical specimens of rock-like materials (using specially made rock-like specimens from Portland Pozzolana Cement (PPC)، fine sand and water) tested under compressive loading. Several tests accomplished on the rock-like specimens containing two cracks to evaluate the final breakage path in these samples. The effect of the orientations of these two cracks with a constant spacing on the crack propagation paths and cracks coalescence have been experimentally investigate. These results show that the changing in the orientation of the second crack affect the crack growth and coalescence paths. The same specimens simulated numerically by an indirect boundary element method known as displacement discontinuity method. The numerical and experimental results obtained from the tested specimens compared with each other and showing good agreements between the corresponding values. The accuracy، validity and effectiveness of these approaches are quite evident by comparing these experimental and numerical results.