Experimental study into the effect of the closed discontinuity dip angle on the particle acceleration resulted from explosion

The most important effect of a discontinuity when encountered with a wave is the division of wave energy. This process is conducted in the form of dividing the input waves to the reflection and transmission waves. Several researchers have studied the effect of physical and mechanical properties of these discontinuities on waves division. In this study, the effect of the closed discontinuity dip angle has been investigated in the form of experimental tests in scale model. Accordingly, the effect of discontinuity dip from 60 to 120 degrees at 10 degree steps was investigated on particle acceleration of the waves. The results indicated that by increasing the discontinuity dip, the reflection of the waves increase and the vibration is enhanced. On dips greater than 90 degrees, the amount of wave reflection is more than the transmission and at dip angles less than 90 degrees, it is opposite. It was also found that on dips greater than 90 degrees, the ratio of reflection was three times more than the 90 degrees dip and the ratio of reflection is approximately 6 to 7 times more than the transmission of waves. By increasing the angle of discontinuity dip, the attenuation rate of particle acceleration increases where this amount at an angle of 100 degrees is about 30 percent more than that of 90 degrees. Some empirical models for the reflection ratio and the attenuation of the particle acceleration were obtained in terms of the dip angle of discontinuity dip with a high determination coefficient.Keywords: Blast-induced particle acceleration, closed discontinuity, reflection and transmission of waves, scale model tes

Minimising Mucking Time by Prediction of Muckpile Top Size in Tunnel Blasting: A Case Study

Drilling and blasting is widely used in underground excavation projects. Timing is considered to be the most important factor in construction projects. In cyclic operations such as drilling and blasting, losing time in each cycle will cause a delay in operation for all cycles and can impose huge amounts of budget loss because of the significance of fixed costs. Therefore, this investigation tries to minimise the mucking time in drilling and blasting operations of the Alborz Tunnel in Iran via controlling the topsize of muckpile in order to eliminate the need for time consuming secondary blasting. Using the Split-Desktop system, the size distribution curve for 25 blasting rounds in Alborz Tunnel were obtained from which the topsize of the muckpile for each round was calculated. 16 datasets were used to develop a multiple linear regression model. The other nine datasets were used to validate the model. Comparing the actual and predicted values of topsizes, R2 and RMSE for the model were obtained as 0.73 and 0.14 respectively, showing that the proposed model can be used for controlling topsize of muckpile. Specific drilling and the ratio of amount of charge to the burden in contour holes are revealed to be the most important parameters in controlling the topsize of the muckpile in this particular case. The proposed model was successfully used and can be used in future excavations as long as the condition of rock mass is not changed

Numerical Modeling Of Rock Blocks With Nonpersistent Rough Joints Subjected To Uniaxial Compressive And Shear Loadings

Characterizing the mechanical behavior of jointed rocks is important to understand the behavior of structures in rock masses. Jointed rocks can be composed of persistent and nonpersistent joints where the impact of nonpersistent joints requires careful consideration for an accurate rock mass mechanical characterization. Most previous investigations into nonpersistent jointed rocks focused on joints with smooth surfaces, and a few experimental studies focused on nonpersistent rough joints and nothing specific has been reported numerically. Therefore, this study investigated several synthetic jointed rocks with nonpersistent rough joints numerically under uniaxial compressive and shear loadings. The PFC2D-based synthetic rock mass (SRM) approach was adopted to assess the impact of bridge angle (γ) and length (L), joint roughness coefficient (JRC), and normal stress (σn) on the shear strength (τn) and cracking in jointed rocks with nonpersistent rough joints. In addition, the impacts of γ, L, JRC, and joint inclination (θ) on the uniaxial compressive strength (UCS or σcm), elastic modulus (Em), and failure pattern in the jointed blocks were examined numerically. First, several numerical models were developed and verified by the laboratory data, followed by an extensive parametric study to assess the effects of the defined parameters further. The effects of JRC and σn on τn were more pronounced than γ and L due to the formation of interlocking cracks, which could cause significant shear resistance during shear loading. In addition, the numerical results under axial loading revealed that an increase in θ could reduce the deformation modulus and the value of the other parameters, in particular the JRC, could lead to an increase in the strength of jointed samples

Physical modeling of soil arching around shallow tunnels in sandy grounds

The distribution of earth pressure surrounding a tunnel is one of the most critical factors in designing tunnel support systems. In this study, a physical modeling setup has been designed and constructed to simulate the excavation procedure of a full-face circular tunnel. Silica sand was used with four different densities and three different cover-to-tunnel diameter ratios. The full-face excavation was simulated with a variation of tunneling-induced volume loss. The variations of earth pressure around the tunnel were measured by means of a series of miniature soil pressure cells. Particle Image Velocimetry (PIV), as a non-destructive image processing technique, was used to monitor the deformation of the soil surrounding the tunnel. The results obtained from both pressure cells and PIV showed that soil arching developed around the tunnel. As tunnel convergence increased, a loosened zone appeared above the tunnel, surrounded by a stress arch. It was discovered that there is a direct relationship between the height of the loosened zone and the depth of the tunnel. A linear equation has been established for the estimation of the height of the loosened zone, which has a direct influence on the design of the support system

Mechanical Behavior of Single-Flawed Cylindrical Specimens Subjected to Axial Loading: A Numerical Investigation

Discontinuities are inherent components of rock masses and can range from fissures to large faults. Single fissures, the so-called flaws, may affect the mechanical behavior of rock mass, crack initiation, and propagation. In this paper, numerical investigations have been conducted on central-flawed cylindrical specimens subjected to axial loading to investigate the effect of flaw angle (α), length (2a), and aperture (A) on their mechanical behavior and crack development. Particle Flow Code (PFC3D) was adopted to investigate the cracking process of the cylindrical specimens and maximum principal stresses at flaw tips. The numerical models are calibrated and verified using extensive experimental tests. The results show that increasing α, UCS, and E increase while increasing 2a decreases UCS and E, and A does not affect these two parameters. Moreover, numerical simulations reveal that as α rises, the three principal stresses generally fall when 2a = 13 and 26 mm. σ1 and σ3 peak at α = 45°, and σ2 reaches a maximum at α = 30° in models with 2a = 39 mm. The cracking patterns resulting from both methods are highly consistent in that tensile cracks type 1 mainly form at α = 15° to 75°, and tensile cracks type 3 are dominant at other angles. Finally, it is concluded that flaw aperture scarcely affects failure patterns

Interpretation of Rock Mass Behaviour via Multiple Graph Approach: Adit P-C9 of the Alborz Tunnel

The current paper focuses on the application and advantages of the “multiple graph” approach for interpretation of surrounding rock mass behaviour in underground structures. Behaviour of the Argillitic rock mass surrounding Adit P-CP9 of the Alborz Tunnel was interpreted via the “multiple graph” approach resulting in interestingly accurate prediction. The accuracy of the estimation was later observed in the excavation process and afterwards. The observed results are presented which verifies that the “multiple graph” approach can cope satisfactorily with various geological conditions

Modification of rock mass rating system: Interbedding of strong and weak rock layers

Rock mass classification systems are the very important part for underground projects and rock mass rating (RMR) is one of the most commonly applied classification systems in numerous civil and mining projects. The type of rock mass consisting of an interbedding of strong and weak layers poses difficulties and uncertainties for determining the RMR. For this, the present paper uses the concept of rock bolt supporting factor (RSF) for modification of RMR system to be used in such rock mass types. The proposed method also demonstrates the importance of rock bolting practice in such rock masses. The geological parameters of the Shemshak Formation of the Alborz Tunnel in Iran are used as case examples for development of the theoretical approach

Effect of dynamic elastic properties of rock on fragmentation in Choghart Ironore Mine, Central Iran

Blast fragmentation is a measure of efficiency in an open cast blast operation. Specific Charge (SC) plays an influential role on the fragmentation distribution, the quality of product and the production cost. Dynamic properties of rocks can be used for estimation of rock fragmentation and specific charge. Fragmentation analysis by digital image processing is a low cost and quick method. In this paper, the results of the seismic refraction technique are presented for Choghart Iron ore mine in central Iran. The P-wave velocity of the ore body has been measured at the site. The source of vibration generation was by hammering. The fragmentation resulting from blasting was monitored using a digital camera. Split Desktop software was used to quantify fragmentation size distribution. The mean fragmentation size of P50 was obtained as representative of the average fragmentation size. SC of ANFO was calculated. The relationship between SC with P50, Vp and Dynamic Elasticity Modulus (Edyn) were obtained. It was found that P50 and SC are increased with increased Vp and Edyn. P50, increases with increase of SC. These results can be utilised in blasting design in order to optimise fragmentation and SC for improvement in t the blast operation efficiency

Causes of Dynamic Overbreak and Control Measures Taken at the Alborz Tunnel, Iran

Drilling and blasting is widely used in underground excavation projects, where the amount of damage to the surrounding rock mass is crucially important, due to its impact on the safety of working environment and operational costs,. The causes of overbreak in the Alborz Tunnel of Iran are evaluted. In this regard, ten rounds of presplitting and 11 rounds of smooth blasting methods were carried out to determine the dominancy of ground condition over the blasting pattern characteristics. Further study was undertaken to identify the most important parameters of ground condition affecting the overbreak area. These parameters include; joint condition, spacing, orientation, RQD and type of rock mass. As the characteristics of the blasting pattern have very little effect on the amount of overbreak, the smooth blasting technique was chosen for the future operations where the current ground condition is going to be dealt with for about 500 meters of length, based on the data acquired from the Alborz Exploratory Tunnel. Results of this investigation helped to solve disputes between contractors and clients over the issue of permissible overbreak

Causes of Overbreak in Tunneling: A case study of the Alborz Tunnel

Drilling and blasting is widely used in underground excavation projects, where the amount of damage to the surrounding rock mass is crucially important due to its impact on the safety of working environment and operational costs. The causes of overbreak are categorized into three groups namely: geological parametres, drilling accuracy and charging parametres. The present paper focuses on the special case of the Alborz Tunnel of Iran where a discontinuity surface located above the tunnel contour line caused excessive amounts of overbreak in the study area. After introducing the disconformity surface above the tunnel contour line, its impact on the occurrence of excessive amounts of overbreak is discussed. Possible case scenarios for future excavations are pointed outand the problems which may be encountered in each case scenario are predicted. Also, the impact of this special situation on the difficulties faced in working with rock mass classification systems is discussed