Blasting Overhangs Close to a Vibration Sensitive Structure

R.Farnfield Technical Services Manager, EPC-UK
B.Williams Director of Commercial Operations, EPC-UK

Abstract
This paper describes a situation that many drilling and blasting contractors have found themselves in - a client's fear of the use of explosives.

In the summer of 2007 Blasting Services Ltd was asked to undertake some drilling and blasting work in an abandoned quarry that was being restored for a housing development. Previous quarrying operations had left a series of overhangs that were considered unsafe and required removing. The site contractor considered that the best and safest way to carry out this work was by drilling and blasting. Blasting Services Ltd, with the technical assistance of EPC-UK, designed a drilling and blasting regime to accomplish this work with a series of trim blasts. The design took into account the close proximity of a structure in very poor condition and the imposed vibration limit of 6.0 mm/s. The faces were profiled, holes drilled and explosives ordered. A few days before the first blast was due to be carried out the client decided that the poor condition of the nearby structure meant that the use of explosives could not be permitted. This decision was made despite the blast design indicating that the vibration level would be well below the 6.0 mm/s limit.

Over the next few weeks a series of unsuccessful trials were undertaken with alternative excavation techniques. During this period no work was permitted on site due to the unsafe condition of the faces and the cost to the client and main contractor began to mount rapidly. Throughout this period Blasting Services insisted that the best way to remove the overhangs was by the controlled use of explosives. The client eventually agreed to go ahead with the use of explosives but imposed an even more stringent vibration limit of 4.0 mm/s. The original trim blast was redesigned to take into account the new vibration limit giving a maximum instantaneous charge weight of 2.5 kg and employing a combination of high-energy detonating cord, dynamite and electronic detonators. Two trim blasts were successfully fired with vibrations levels between 2.0 and 3.0 mm/s. The building remained undamaged.

This paper describes the processes followed in undertaking this drilling and blasting contract where the use of explosives was banned.

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Blasting Overhangs Close to a Vibration Sensitive Structure

Abstract This paper describes a situation that many drilling and blasting contractors have found themselves in - a client's fear of the use of explosives.

Abstract
This paper describes a situation that many drilling and blasting contractors have found themselves in - a client's fear of the use of explosives.