Prediction of blast-induced flyrock in Indian limestone mines using neural networks

.Trivedi,T.N.Singh,A.K.incCentrl Institute of Mining nd Fuel eserch,Council of Scienti fi c nd Industril eserch(CSI),Dhnbd,Indi

bDepartment of Earth Sciences,Indian Institute of Technology Bombay,Powai,Mumbai 400076,India

cCentral Institute of Mining and Fuel Research,Council of Scienti fi c and Industrial Research(CSIR),Regional Centre,Nagpur,India

1.Introduction

Due to the explosive force,rock fragments are propelled and thrust high into the air and beyond the safety limit of blast area,thus termed as “ flyrock”.This is mainly due to the flaws presented in the blast design and also due to the mis-interpretation of rock mass behavior.The phenomena of flyrock are always uncontrolled and can never be brought down to zero.But beyond safe permissible limits, flyrock can cause some serious damage to the property and can in flict serious to fatal injury to the personnel,thus making it be one of the main causes of accidents and deaths in opencast mines.Number of accidents as well as number of deaths and injuries due to blasting in Indian mines has been shown in Figs.1 and 2(DGMS,2013,2014).Flyrock is one of the intriguing problems in opencast mining.

By blasting,we obtain a fixed and/or constant size of rock due to fragmentation,which can help in optimizing the further production economics.During explosion,a part of energy is used in breaking and displacing the rock,while the rest of energy(a major portion)is used up in ground vibrations,air blasts,noises,back breaks, flyrocks,dusts,etc.,thus posing a thrust to the nearby life and property(Pal Roy,1995).Flyrock occurrence can be explained with the help of three basic mechanisms,i.e.cratering,ri fling and face burst,as described in Fig.3(Moore and Richards,2005).

Inadequate burden, inadequate stemming length, faulty drilling,back breaks, loose rock on top of the bench due to poor previousblast, very high explosive concentration, inappropriate delay timing,and their sequence, and inaccuracy of delays are the prominent blastdesign parameters responsible for flyrock problems (Workman andCalder, 1994; Siskind and Kopp, 1995; Adhikari, 1999; Rehak etal.,2001).Unfavorable geological conditions, such as open joints,weak seams, and cavities, have been identified as themajor causes offlyrock hazards in opencast mines (Persson et al.,1984; Fletcher andD’Andrea, 1986; Bhandari, 1997; Shea and Clark, 1998).

Blast performance is basically governed by geological and geotechnical data,such as rock quality designation(RQD),unconfined compressive strength(UCS),and joint setting.Joints have an important role to play in any blasting operation as they determine both the safety and performance.Joints are the natural planes of weakness that offer practically no resistance to split.Joints are the zones of discontinuity and weakness and thus during blasting they get affected first,rather than the stable homogenous regions.Therefore,they control the rock breakage process by determining which area gets affected first.Rock fragmentation and over break are influenced by the joint sets.The blast design can be improved by review and analysis of past data of the blasts conducted in the mines(Bhandari,2011;Parihar and Bhandari,2012).

Flyrock due to blasting in opencast mines is complex in nature as it is a random phenomenon.Raina et al.(2007,2011)attempted to devise a criterion for prediction of blast-induced flyrock distances and focused on the factors on which the phenomenon of flyrockdepends.An effective prediction of a blast in opencast mines can addressthe flyrock problem (Ladegaard-Pedersen and Holmberg,1973;Lundborg,1974;Roth,1979;Richards and Moore,2004;Monjezi et al.,2010).

In this paper,an attempt has been made to calculate initial velocity and angle of ejection of flyrock projectile,and maximum horizontal throw of flying fragments.Flyrock distances have been predicted with the help of feed forward back propagation neural network(BPNN),because it is the most versatile and robust technique(Sawmliana et al.,2007;Singh et al.,2008).Predictive problems,such as ground vibrations,air blast, flyrock,fragmentation,and back break,can be solved using back propagation algorithms(Huang and W finstedt,1998;Singh and Singh,2005;Tawadrous and Katsabanis,2005;Remennikov and Mendis,2006;Singh and Verma,2010;Khandelwal and Monjezi,2013;Trivedi et al.,2014).

Fig.1.Number of fatal and serious accidents due to blasting in Indian mines(DGMS,2013).

Experimental blasts have been conducted in 4 limestone mines in India.Out of datasets of 125 experimental blasts,95 datasets have been used to train the neural network;remaining datasets are used for testing and validation of neural network.Flyrock distances for 30 experimental blasts have been predicted by artificial neural network(ANN),multi-variate regression analysis(MVRA),calculated using motion analysis of videos of flyrock projectiles and compared with the observed data.

Fig.2.Number of deaths and injuries due to blasting in Indian mines(DGMS,2014).

Fig.3.Mechanisms of blast-induced flyrock in opencast mines.

2.Materials and methods

2.1.Field study

Four limestone mines have been selected in this study,i.e.(1)Bamangaon and Mehgaon mines,ACC,Katni,(2)Sheopura-Kesarpura(S-K)mine,shree cement,Beawar,(3)Aditya mine,UltraTech,and(4)Chittorgarhand Kotputli mines,UltraTech,Jaipur.The mining lease of Bamangaon and Mehgaon mines is located at Kymore Village,Vijayraghavgarh Block,Katni District,Madhya Pradesh State of India.It lies between longitude E80°29′and E80°57′E and latitudes N23°48′and N24°8′,survey of India toposheet No.64A/53.The lease area is 51.2022 km2.The true dip of limestone bed varies between 10°and 20°from west to northwest.However,at some places it is gentle in nature.

The S-K mine is located near Beawar City,Ajmer District,Rajasthan.The mining lease of 856.8 ha lies between longitude E74°22′and E74°26′and latitudes N26°1′and N26°5′,on toposheet No.45J/8 of survey of India.The planned capacity of the mine is 2.0 million tons of limestone per year. The major portion of area is rocky and there is no vegetation.There occur two parallel ridges of limestone extending along length of area.Between two ridges,there is alluvium/soil.Structurally the area represents a tight isoclinal synclinorium fold where limestone constitutes two limbs of fold,which are separated by a shallow valley.The general strike direction of limestone beds in the area is N30°E.Generally beds are dipping in WNW direction with dip of 45°to as high as 60°.

The Aditya limestone mine is located in Tehsil-Chittorgarh and Nimbahera,Chittorgarh District(Raj).The site is located 18 km Southwest of Chittorgarh Town.The mining lease area of the Aditya limestone mines forms a part of survey of India toposheet Nos.45L/9 and 45L/10 between latitudes 24°43′and 24°45′north and longitudes 74°35′and 74°37′east.The leasehold area of mine is 760.692 ha with the planned production capacity of 6.6 million tons of limestone annually with a stripping ratio of 1:0.33.Structurally the area represents a syncline fold.In spite of above folds,study of dip and strike readings indicates N-S trend with maximum of 10°deviation on either side.The dip varies between narrow ranges of 0°and 20°.Dip direction changes from east to west due to folding.

The Kotputli limestone mine(Grasim Cement)is located at Mohapura Jodhapura near Kotputli Town,Jaipur District,Rajasthan.It is situated at a distance of 165 km south from Delhi and 106 km north from Jaipur.The lease area of the mine is 5.4878 km2.The mine lies between latitude N27°39′and N27°42′and longitude E76°6′and E76°9′,survey of India toposheet No.54A/2.The production capacity of the captive limestone mine is 6 million tons per year.The limestone formations of this area is light to dark gray in color,low to medium grained crystalline,hard and massive in nature.Color banding was observed at some places.The general strike direction of the rock formation of this area is NE-SW with variable dip ranging from 38°to 80°due east.

Limestone mining is being implemented by fully mechanized opencast mining methods in all the mines under the study.The working pit has been excavated by developing working benches of 9-10 m in height.Each bench is connected with a ramp.Necessary haul roads have been developed all around the benches for approaching the working faces.Shovels carry out the excavation and loading operations.Transportation of limestone and waste rock is carried out by dumpers.Before crushing,the limestone from crusher hopper is passed through grizzly screen for screening out intrusive clay.The clay-free limestone is crushed to required size and transported to stock pile located inside plant through belt conveyor.Overburden,mainly clay and soil,is very thin in nature,and it is scrapped by dozer and then lifted with the help of shovel and dumper combination and disposed off to dump yards.

Fig.4.Motion analysis of flyrock projectile using ‘ProAnalyst’in a blast.

Fig.5.Photographs of bench face before and after blasting operation.

2.2.Methodology used in data generation

The problem of blast-induced flyrock is prominent as these mines are surrounded by the villages,and management of blasting events in these mines has been a challenging task.

Table 1Symbols used for blast design and geotechnical data.

Blast design data,such as burden,spacing,stemming,average depth of blast holes,blast hole diameter,charge per hole,linear charge concentration,specific charge(i.e.ratio of explosive consumed to tonnage of rock broken per blast,kg/t),maximum throw of flyrock or flyrock distance from the blasting face,are generated before blasting operation.Geotechnical data,such as volumetric joint count(Jv),joint spacing,dip and strike of major joint set,and joint condition,have been generated at blasting face and at exposed rock in vicinity of blasting face before blasting operation takes place.The RQD has been estimated using volumetric joint count method(Palmstrom,1982).Rock samples from blasting face have been collected from the mines and tested in the laboratory in Central Institute of Mining and Fuel Research,Council of Scientific and Industrial Research(CSIR),Dhanbad,India.The UCS of 1 inch cube samples has been tested and density and UCS of rock samples have been known.To identify the maximum throw of blast-induced flyrock,the distance of fragment of 10 cm or more,in size,from the face blasted has been considered.The portable GPS(global positioning system)was used to measure the distance of flyrock from the face blasted.

Table 2Blast design and geotechnical parameters at various mines.

The experimental blasts were monitored with high-resolution video camera of 24 frames per second.The interval between two consecutive frames is 42 ms.The view of the blasting events was shot from safe distance in view safety of the equipments and personnel.The flying fragment motions were captured in videos of the blasting events.The videos of the blasts have been analyzed to know launching velocity and launching angle of the flyrock projectiles.The AVI files obtained through unloading the images from the camera to the computer for slow motion analysis using‘ProA-nalyst 1.5.6.7’software of XCITEX,USA(Figs.4 and 5).A precalibration technique is used in the calculation.The calibration in the present study was made using three red flags separated horizontally and vertically to known distance.For verification,the other known parameters like bench height,cut length,etc.were used.Launching angle and launching velocity of flyrock projectile for each blast of all the limestone mines under the study with appropriate symbols are shown in Tables 1 and 2.The flyrock distances have been calculated using general trajectory formula(Richards and Moore,2004):

Table 3List of input and output parameters used in ANN.

2.3.ANN approach to predict blast-induced flyrock

The pattern of the result is predicted by ANN on the basis of preceding learning.Once the neural network has been established and trained,any similarities in the new pattern will be detected and the result pattern will change accordingly,thus providing the technique interpolation capability.ANN is trained using back propagation algorithm.The feed forward BPNN comprises 3 layers,i.e.input layer,hidden layer and output layer.Layers are made up of neurons which are the basic processing units.These neurons connect the layers using appropriate weight.The output of the neurons in the input layer serves as input for the neurons in the hidden layer and the same applies to the connection between hidden and output layers.The problem defines the number of hidden layers and the neurons in them.In the present case,the BPNN with‘Levenberg-Marquardt’algorithm and ‘log-sigmoid’transfer function has been undertaken.After trial of a number of different combinations,two hidden layer and ten neurons in each hidden layer have been found as the best predictive model for the case under the study.The input and output parameters used in ANN model are shown in Table 3.Ninety five datasets have been used to train the neural network.Thirty datasets have been used for testing and validations of neural network.Architecture of neural network and its performance during training are shown in Figs.6 and 7.The regression plots of ANN during training,testing and validation indicate the excellence of the selected network as shown in Fig.8.

Fig.6.Training of back propagation neural network.

2.4.Prediction of flyrock by MVRA

The relation among the variables can be calculated using MVRA and the least squares fit method to solve the dataset.Regression matrix solves the simultaneous equations thus created.MVRA has been conducted by the same datasets and the same input parameters used in ANN.The equation for prediction of flyrock by MVRA is as follows:

Fig.7.Performance of neural network during training.

3.Results and discussion

3.1.Performance of neural network model

Flyrock distance has been predicted by neural network and multiple regressions.Relationships between flyrock distance predicted by ANN and MVRA have been compared with the observed ones as shown in Figs.9 and 10,respectively.Also,a line of 1:1 slope intersecting origin has been drawn in Figs.9 and 10.It is evident that the predicted values are closer to observed values in ANN when compared with MVRA.Mean absolute error(MAE)is 2.5 m in case of MVRA,whereas it is 0.92 m in case of ANN.Similarly,root mean square error(RMSE)is 3.1 m in case of MVRA and 0.99 m in case of ANN.Coefficient of determination(R2)is 0.815 in case of MVRA whereas it is 0.983 in case of ANN.Maximum flyrock distances have been calculated by projectile motion(Eq.(1))as explained earlier and compared with the observed values as well as values predicted by ANN and MVRA,as shown in Fig 11.MAE and RMSE in terms of flyrock distances calculated by projectile motion theory are as high as 5.2 m and 6.1 m,respectively.Higher MAE may be due to the fact that projectile motion theory does not take into account the air resistance offered to flying fragments during its travel.These indices of model performance indicate an excellent correlation between flyrock distances predicted by ANN and observed values.Thus,the proposed ANN model has exceptionally high capability in predicting complicated blasting problems.

Fig.8.Regression plots of ANN during training,testing and validation.

Fig.9.Relation between flyrock distances predicted by ANN and observed values.

Fig.10.Relation between flyrock distance predicted by MVRA and observed values(Solid line indicates best fit line and dotted line is 1:1 slope).

Fig.11.Comparison between predicted,calculated and observed flyrock distances.

Fig.12.Effects of the six independent variables on maximum throw of flyrock.

3.2.Sensitivity analysis

In order to assess the impact of an independent variable on maximum throw flyrock,sensitivity analysis has been performed in‘Matlab 7.11.0(R2010b)’using ‘simulation tool’.While performing sensitivity analysis of an independent variable,all other independent variables are set at their mean values as specified in Table 3.Sensitivity analyses for all the six independent variables have been performed,as shown in Fig.12 and the results have been described in Table 4.The linear charge concentration and specific charge have a positive correlation with the flyrock distance,whereas burden,stemming,UCS and RQD have a negative correlation with the flyrock distance.Impact of rate of change in burden and stemming are more pronounced at blast hole of d=165 mm,whereas impact of rate of change in UCS and RQD are more pronounced in case of blast hole of d=115 mm,and impact of rate of change in linear charge concentration and specific charge are more or less same in both cases.

Table 4Results of sensitivity analysis.

4.Conclusions

The ANN with two hidden layers and log-sigmoid transfer function has been found as the best predictive combination in back propagation algorithm.While solving the blast-induced flyrock predictive problem,six independent variables have been selected,i.e.linear charge concentration,burden,stemming length,specific charge,UCS,and RQD.Among the various flyrock models namely ANN,multiple regression and ‘projectile motion model’,ANN has been proved to be an excellent model due to its highest coefficient of determination and lowest MAE as well as RMSE.Sensitivity analysis has been performed and we found a positive correlation of linear charge concentration and specific charge with the flyrock distance,whereas burden,stemming length,UCS and RQD bear a negative correlation with the flyrock distance.Impact of burden and stemming length are more pronounced at blast hole of d=165 mm whereas impact of UCS and RQD are more pronounced in case of blast hole of d=115 mm.

Conflict of interest

The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

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