Newsletter May 2004 Micro-Tunnelling on the Hibiscus Coast Smythe Contractors is undertaking seven challenging drills on the Hibiscus Coast wastewater pipeline project for the Rodney District Council. The job involves laying 15km of large capacity pipeline from Orewa to Army Bay (located at the end of the Whangaparaoa peninsular). The contract also involves pipe design. Drill No3 -Stanmore Bay Road - job details • Pipe Diameter 710mm • Drill Length 310m • Maximum Depth 20m • Pipeline Grade 1.8% • Ground Hard Silt Stone • Drilling fluid recycler with centrifuge • Magnetic North steering tool utilised Difficulties to overcome- • Depth of tunnel through hill • Pipeline passing under many properties • No water available on site • Large diameter pipe • Hard rock with small particle size 710mm diameter pipe Smythe Contractors Ltd PO Box 158 PH: (07) 827-5925 Email: [email protected] CAMBRIDGE Fax: (07) 827 5197 Web site: www.smythecon.com Breaking new ground every day Newsletter May 2004 PILOT BORE Due to the depth of this drill the latest advancement in underground location technology was sourced and imported into NZ by Symthe Contractors last year. A magnetic North steering tool (transmitter) located in a non-magnetic steering head utilizes true magnetic north to obtain direction (Azimuth). A wire inside every drill rode transports the information back to the drill operator and is displayed on a laptop. This system requires no above ground walkover and is ideal for river crossings, deep drills under built up areas and deep drills through hills or bluffs. Special computer software interprets the transmitter information and plots a long section and plan of the pilot drills progression. It also displays the calculated drill head position. Non-metallic drill head Magnetic steering tools are very sensitive to interference and the drill head, transmitter housing and lead drill rode are manufactured from non- magnetic high-grade stainless steel, sourced from overseas. The magnetic steering tool is very accurate. Grade can be achieved in increments of 0.1% and direction down to a tenth of a degree. All information is relayed back to the drill operator via a wire, relayed signal interference is now no longer an issue. The transmitter can also be tracked from above ground like the traditional method. This allowed us to check the location as we launched into the hillside. The pilot drill was completed in six days and within Drill operator in full control tolerances. Bore planning for the job Setting up the magnetic steering tool Breaking new ground every day Newsletter May 2004 HOLE OPENING (reaming) The siltstone was hard. Progress was slow at first and a variety of different rock cutting reamers were build by our workshop engineer. Progress ranged from 10 to 20m a day so it was important to find the most effective reamer for this type of rock. This involved importing components from Australia, USA and India. After 60m of reamed tunnel a tri–cone roller tore away from the main body of the reamer and was lost up the hole. This Barry the workshop engineer makes final presented a major problem as we could adjustments to a reamer before it is sent out not continue with the job until the tri-cone to site. was retrieved. We tried several for several days to scoop it up with an open style reamer but this was unsuccessful. Eventually it was decided to bolt a very powerful magnet inside the open reamer. The magnet worked on the first attempt and the tri- cone was retrieved. Tri-cone roller reamer UNSTABLE GROUND A launch pit was dug, the back cutter attached and the back reaming commenced. The tunnel exit pit was located 15m away from the siltstone / soil interface. When we initially reamed this section it stood open but after a short time we had problems with the ground collapsing at the rock interface. We established that this was probably caused by the ground water tracking along the top of the siltstone saturating the overlying soil at this point and making it unstable. To overcome this we constructed a shaft at the rock interface. This shaft was used to support the unstable interface ground, it was also a safe Rock reamer heading back up the tunnel Breaking new ground every day Newsletter May 2004 place to change reamers and remove drilling fluid for recycling. The reaming in the rock recommenced. In an operation like this huge volumes of drilling fluid are required. In one 10hr day the drill consumes 80,000l and to maintain this supply it was necessary to recycle the drilling fluid. DRILLING FLUID RECYCLING The main function of the fluid recycler is to remove the solids component from the drilling fluid containing the tunnel cuttings. The advantage of this is two fold. 1. Water on the peninsular is an expensive commodity. A fluid recycler enables all of the fluid in the drill operation to be re-used over and over again. The fluid exiting the recycler is trucked back over the hill to the drill for re-use. After the pilot hole was complete no new fluid was introduced to the operation. 2. The solids removed from the drill cuttings could be loaded onto a truck and removed to a clean land fill for a fraction of the cost in comparison to dumping the solids / fluid mix. Tanker truck transporting the cleaned fluid back to the drill. Fluid recycler in the back ground. A fluid recycler has three main stages. 1. Shaker, over a fine mesh screen to remove larger solids 2. Cyclones to remove sand 3. Centrifuge at 1200rpm to remove finer solids Fluid recycling is very environmentally friendly and although Smythe Contractors is the only drilling company in NZ utilising fluid recycling it is very popular in. Europe, Australia and the USA. Drill slurry & cuttings moving across the shaker Breaking new ground every day Newsletter May 2004 ROCK / SOIL INTERFACE A rock / soil interface also exists at the drill end of the tunnel. This was much more difficult to deal with as the face was located well into private property and under a concreted driveway. When the rock reamer reached this interface in was backed out of the tunnel back to the bottom end of the job and replaced with a reamer to handle the clay. This reamer was pulled back to the face and preceded to ream the clay. When the 15m of clay was reamed this reamer was also backed 300m down the tunnel where the waiting polyethylene pipe string was connected. PIPE INSERTION The pipe insertion was undertaken in an 8hr day CONCLUSTIONS Directional Drilling is the most appropriate and cost effective method for undertaking micro tunnelling over this distance with this diameter in this type of ground. A pilot bore (at minimal cost) establishes the correct alignment and the ground conditions along the whole bore before commitment to the entire tunnel is made. Allot was learnt on this project. It was the pioneer for magnetic North location equipment in NZ. Although we had some initial teething problems with the hardware involved with this system, the electronics performed well enabling us to maintain direction and grade at depths not possible with traditional drilling equipment. Silt particles in siltstone are very fine and are well cemented. Allot of energy was required to separate the particles. Once separated and suspended in fluid it is very difficult to remove the silt from the water. Drilling fluid recycling was used for the first time. Learning the capabilities and limitations of the recycler was necessary to obtain the optimum performance from the equipment. The increased difficulty in finding tip sites in the Auckland area to accept the slurry/soup mixture makes fluid recycling economical and invaluable when dealing with large volumes of silt-laden fluid. Rock / soil interface can sometimes present a problem. Above ground access is not always possible. To stabilise this ground slotted PVC pipe can sometimes be drilled to the rock interface to relieve water pressure and reduce saturation. A project of this size will almost always have some problems. A problem is a challenge that always has a solution. This drill was an important step towards the next drill on the Hibiscus Coast pipeline project. The Tindalls Bay tunnel is the same diameter but over 400m long and up to 40m deep. The drill site borders the beach and is confined in a small space. Sound barriers will be required and an extra effort to isolate the public from the works will be required. Lessons learnt on the Stanmore Bay Road project will contribute to the undertaking of the next challenging project on the Hibiscus Coast Wastewater Pipeline Project. Smythe Contractors services include- Directional Drilling, Directional Rock Drilling, Micro tunnelling, thrusting, pipe ramming, slip lining, pipe bursting, Landslip correction, Casing advance, trenching, Drain laying, Sewers, Ducts, Cables, the complete job. Breaking new ground every day .