The Project – In order to upgrade existing electrical power supplies to larger underground supply in the Elizabeth Bay area, the TT Group used a Grundo Pit with no environmental impact on heritage listed homes and other properties in the area, knowing that all these properties are sitting on tough Sydney sandstone.
The Challenge
Installation of these conduits and cabling, using a non-intrusive trenchless method, so as not to disturb the historic surface features and the buildings themselves. Traditional mudmotors and downhole air hammers were dismissed, due to the size of the drill rigs required and the vibration concerns at such a relatively shallow depth. Also the fact the some of the bores were only six metres long and the longest of only about ten metres, required the need for a very compact machine and system.
The Technique
Utilising the company’s existing Grundopit directional pit launched borer, Peter Decker of TT Asia Pacific suggested to the supervisor that the machine can be converted to operate a new accessory – the Steerable and Trackable Rockhammer developed in Germany by the TT Group. This system would provide information to the operator to control the pitch and direction of the borehead with a very low vibration boring head very suitable for the sandstone formations of Sydney.
The Trial
After much planning, the day arrived when machinery and people assembled in Elizabeth Bay Road, to proceed with the first trial.
As can be expected, old unknown underground services were found, requiring the machine launch pit to be moved and modified to fit around these mains. The traditional white Sydney sandstone was as hard as expected and after a few hours of hydraulic breaker work on the excavator, the machine pit was ready.
The vibration monitoring and recording equipment was positioned and set up to record everything vibration-related including the 30 buses an hour that would trundle past while the work proceeded.
The first bore was about 8 metres in total length, boring into the existing electric meter board position in the house basement with a veritable bamboo jungle between the fence and house, making locating the borehead extremely challenging. The bore was started at about 10 am and was into the basement floor by about 1 pm. After a few core drill holes were put into the floor to pick up the 70 mm bore head position, the borehead was extracted back at the machine pit and the 50 mm orange conduit pushed through later that afternoon to complete the project.
Over such short distances the steering correction was minimal so setup of the machine is still critical to ensure that the borehead comes out on target, which was the case here.
At the time of writing, the vibration analysis report had come back with glowing recommendations that the equipment used was well below the heritage building vibration limits. The company now has only a few hundred more installations to complete in the near future.
Technical Aspect
The Rockhammer only requires 1.2 m3min (20l/sec) of 7 bar compressed air supply to operate and flush the pilot bore, the transmitter is isolated from the vibration and the recommended bore length is up to 20 m, making this an ideal combination of compact directional borer for inner city work in challenging ground conditions. The Grundopit can be powered off an excavator as in this case or off a stand alone hydraulic power pack.