Doug Hood Mining

IT’S A BLAST – Doug Hood Mining Ltd features on Demolition NZ

April 8, 2019

The Broadcast Media NZ team filmed one of our construction blasts during the project to excavate a catchbench above Sumner Road, Christchurch, as part of the road reopening works for Christchurch City Council and New Zealand Transport Agency.

The following video is the Doug Hood Mining Ltd (DHML) segment of Demolition NZ, with thanks to Joe Morgan from Broadcast Media NZ.

On the Sumner Road project DHML Project Manager Rangi Hicks (above) was principal Blast Controller, with holes drilled by Inline Drilling Ltd, and blast design, explosives, and shot-firing supplied by Orica NZ Ltd. The Sumner Road project was managed by McConnell Dowell Constructors as head contractor.

A description of our Sumner Road contract is available here.

The drill and blast process was strictly controlled to ensure project objectives were met including the safety of people and equipment and to minimise:
-mis-alignment of pre-split and blast drill holes,
-poor fragmentation,
-back-break into the final design back-wall of the catchbench,
-fly-rock,
-construction delays to ourselves and other contractors on site,
-air over-blast to keep noise within resource consent limits,
-vibration disturbance of adjacent higher rock formations and keep within resource consent limits.

Blast hole drilling was controlled to ensure that all pre-split and blast holes were as-per-design, with an experienced driller and regular checking of final hole alignment and depth using an inclinometer. The key implications of a misaligned blast hole are reduced burden at the toe of the hole being blasted, under and over-fractured rock and therefore wasted energy, cut-off of adjacent holes during firing and misalignment of the final design highwall.

The first holes drilled in each lineal section of the catchbench were deep pre-split holes which when blasted creates a cut-off curtain to protect the highwall behind it from damage (back-break) from subsequent blasting.

The shot-firer initiates the blast using an electronic blast box linked to surface delay detonators at the holes in a series of rows parallel to the open face with each row separated by millisecond delays, and the holes in each row also separated by millisecond delays. Each surface delay detonator is connected to a down-line which propagates a signal down its core in a split second to initiate a detonator inside a booster charge of high explosive at the base of each hole which sets off the bulk explosive within each hole. The front row of holes initiates first which provides relief for the next row of holes to be thrown into. This allows a high energy blast to occur while minimising the negative factors listed above.

Prior to the first blast a NOTAM or Notice to Airman was established with Airways NZ with our blast location and activities advised to anyone planning to fly over the blast zone.

A 500 metre exclusion zone was also established with the Lyttleton Harbour-master, the Lyttelton Port Company, and Christchurch City Council with regard to the Crater Rim walkway and the Greenwood Park mountain bike track.

At 30 minutes prior to each blast the exclusion zone was manned by blast guards at all entry points and a series of radio checks were communicated between the blast controller and the guards during the countdown and on firing. This zone was held while the McConnell Dowell Project Engineer checked the Tilt Meter data to ensure the blast had not dislodged rock from the bluffs above the catchbench. If this was within the agreed tolerance the shot-firer was authorised to re-enter the blast area to complete the post-blast inspection.

With the all-clear given machinery and people moved back onto the catchbench. Load-out was recommenced with an excavator positioned on top of the shot rock to load articulated dumptrucks which were driven up the Sumner Road to the tiphead at Evans Pass. The material was dumped ready for a loader to push over the edge and into the Gollans Bay Quarry stockpile.

That in a nutshell is the drill and blast process used on the catchbench. To discuss our capability and how we can help with your project please contact Kevin Davies DHML General Manager on 0274 325 869.