Measurement 168 (2021) 108390 Available online 31 August 2020 0263-2241/© 2020 Elsevier Ltd. All rights reserved. Measurement and control of seismic effects in large scale dragline bench blasts – An approach Nachiket V. Bhagade a, * , V.M.S.R. Murthy a , G. Budi a a Department of Mining Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, India A R T I C L E INFO Keywords: Backbreak mechanism Large scale dragline bench blasting Seismic refraction tomography Near-feld vibration Pre-splitting ABSTRACT Large scale dragline bench blasting is prevalent in many surface mines in India. Blasting in benches as high as 30–50 m to obtain a dragline cut measuring, typically, 80 m in width and 200 m in length requires 250–300 tonnes of explosives. Such a large amount of explosive induces severe seismic effects on the highwall. Backbreak is one such undesirable seismic effect, generally observed in these blasts, which reduces blasting effciency, fragmentation and dragline utilization leading to lower productivity. An attempt has been made in this research to understand the mechanism of backbreak in relevance to dragline bench blasting. Seismic tomography using 24-channels was used to characterize the rockmass. Near-feld vibration measurement was conducted to record ground vibrations using two Triaxial Borehole Geophones placed within a distance of 40 m from the last row of the blast. Vibration levels upto 1026 mm/s and backbreak extending upto 10.6 m were recorded. For modeling ground vibration in near-feld, the Holmberg and Persson (1979) model was examined against the square root scaled distance model and was modifed to incorporate a rockmass parameter. High values of vertical peak particle velocity (PPV) and strain were recorded indicating that long column charges and large diameter holes are responsible for massive cratering and resultant seismic impacts. The plots of PPV against frequency and geometrical attributes of backbreak showed that release of load damage mechanism was most prevalent in high benches. Backbreak was reduced using pre-splitting method of controlled blasting and increasing the effective delay in successive frings. A backbreak mitigation programme is proposed, which can be useful when integrated with dragline bench blasts. 1. Introduction In a blasting event, typically, 7–25% of energy is estimated to be used in fragmentation and throw i.e. useful work (Sanchidri´ an et al., 2007). The rest is wasted in the form of blast-induced ground vibration, air- blast, fyrocks, heat, light and noise as shown in Fig. 1. These wasted energies physically manifest in the form of both backbreak (close to the blast patch) and blast-induced damage in dwellings (far from the blast patch, if not controlled within permissible limits). Backbreak is an important parameter to assess the result of the blast. Backbreak causes highwall damage and induces cracks in the adjacent blast patch (Eades and Perry, 2019). Drilling and charging in cracked patch leads to jamming of drill holes and seepage of bulk emulsion, elevating the specifc charge (kg/m 3 ) and venting useful en- ergy. This leads to poor fragmentation and low dragline productivity. Backbreak also leaves loose rocks hanging from the crest, which are handled by the dragline, damaging both the wall profle and the teeth of dragline. This causes frequent maintenance scheduling and lower pro- ductivity of dragline, the key cost-centre of the project. Therefore, a close-in monitoring of backbreak and its causative factors is necessary. This paper presents the investigations carried out to understand the formation of backbreak using near-feld ground vibration signatures and suggest steps to mitigate backbreak using a control methodology. 1.1. Backbreak: Defnition and mechanisms Backbreak is defned as the extent of blast induced damage beyond the designated periphery of the blast. Backbreak (damage) is an alter- ation in the rockmass matrix that leads to reduction of rockmass strength and modulus. This encompasses creation of new fractures and dilation of pre-existing fractures (Hoek, 2012). There are three mechanisms attributed to the creation of backbreak as presented below 1. Vibration-induced /shock-induced damage: The shock energy component of the explosive is responsible for the high levels of ground * Corresponding author. E-mail address: nachiket.2015dr0160@me.ism.ac.in (N.V. Bhagade). Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement https://doi.org/10.1016/j.measurement.2020.108390 Received 15 June 2020; Received in revised form 14 August 2020; Accepted 21 August 2020