The WA Water Corporation has commenced construction of a new wastewater treatment plant in Perth’s northern corridor as part of a plan to cater for an expected population explosion in the region.Treated wastewater, using reverse osmosis, can be reused in the domestic water supply, however, there is still a degree of wastewater from the process to be disposed of. A 3.7km ocean outfall pipeline is to be buried up to 4m below the beach and will sit in a trench on the seabed 8-12m below sea level. Towards the end of the pipeline the depth of water is 25-30m.Water Corp has entrusted the underwater trenching job to Coastal Dredging, a New Zealand-based company with extensive experience in precision subaquatic excavation and installation.Coastal Dredging is using a super long-reach Komatsu PC 1100 excavator mounted on a jack-up barge fitted with the Trimble GCS900 machine control system.Coastal Dredging’s Dirk Wheeler said that after some initial, minor teething problems, The Trimble GCS900 had made the dredging job easy for the operators of the 20-tonne excavator. “We had some challenges with water-proofing the sensors on the bucket initially,” he said. “The depth of digging gave us some water pressure and some salt water corrosion issues, but [WA Trimble dealer] HL Solutions have come up with an answer by moving the sensor further up the arm and connecting it back to the bucket hydraulics via sealed cable – a simple solution really.“HL Solutions have been at pains to make sure the system works for us and are implementing refinements as they are needed. We wanted to have a split screen in the cab so the operator could see plan and cross-section views of the digging zone at the same time, and they have been quick to devise software for that.”The required design is loaded into the GCS900, which then receives information from the GPS receivers and sensors mounted on the body, boom, stick and bucket, and uses that information to give the operator his bucket teeth position.The Trimble Virtual Reference Station, a satellite network in the Perth metropolitan area operated by a local company, is used in place of a traditional base station to send corrections to the excavator for centimetre accuracy. The excavator digs about 10m at a time, as this is the maximum reach of the PC 1100, before moving the barge to the next position. Digging is accurate to less than 100mm, although the hardness of rock on the sea floor can challenge that. Piero Liscia from HL Solutions is very pleased with the way the system is performing. “It’s just amazing how adaptable and versatile the GCS900 system is proving to be,” he said. “We’ve got this system fitted to conventional earthmoving vehicles such as graders, scrapers, excavators and dozers, then to more specialised equipment such as salt harvesters, surface miners and customised tow-along scoops. “Utilising satellite navigation technology to accurately and economically control the grades for underwater excavation just shows how versatile and adaptable Trimble machine control systems have become.”
