Small diameter hard rock excavation: comparing the methods

Cost, time, and excavation rates associated with the project can vary widely depending on the selection of the hard rock excavation tools. Some of the tools currently available in the Australian market include small boring units, microtunnelling machines, and Christmas tree head (or similar hard rock attachments).

Small boring units
Small boring units (SBU) are small diameter cutterheads with a bearing housing assembly used in conjunction with an auger boring machine. SBUs are designed to bore through rock with an unconfined compressive strength (UCS) of 25 – 200 MPa. They are welded to the lead section of pipe casing while the ABM supplies torque and forward thrust to the cutterhead. Mounted on the front of the SBU are disc cutters, which enable the machine to excavate rock. Machines are available from The Robbins Company from 600 mm to 1.8 m in diameter.

The SBU is one of the most efficient rock cutting tools for trenchless applications on the market today. The machine’s design is based on that of Robbins’ large diameter tunnel boring machines (TBM). The disc cutters bore through solid rock by fracturing it into chips. The method is unlike other types of cutting tools such as tungsten carbide drill bits, which excavate by ripping away material at the face, creating friction and torque spikes if the rock is fractured. Costs are reduced since disc cutters do not generally need to be replaced during short reaches of around 100 metres.

The SBU can also be customised for a variety of projects and ground conditions. Cutterheads can efficiently bore through unstable rocks such as embedded boulders or rock with clay seams. These cutterheads feature both carbide cutter bits and disc cutters. For highly abrasive rock, such as sandstone or granite with high quartz content, the single disc cutters can be replaced by two-row tungsten carbide disc cutters at any position on the cutterhead.

Due to utility contractor demand, Robbins has designed several types of SBU that are more efficient for longer rock bores, and jobs where line and grade are critical. A motorised SBU (MSBU) is a pipe jacking product that is intermittently manned so it can be steered throughout the entire bore using a pit-mounted pipe laser for guidance. This machine performs well on drives longer than 150 metres, since it is powered by a hydraulic or electric motor with a gear reducer that supplies torque to the cutterhead. With the MSBU, the ABM is still utilised and supplies only forward thrust and muck removal through a small invert auger. For even longer bores, the steerable Rockhead can be utilised. This machine is remotely operated and supplies its own forward thrust, so only a pit mounted pipe jacking station is needed instead of an ABM.

However, while the machine is very efficient in hard rock and mixed ground conditions, the SBU is limited by several factors. Because the ABM supplies forward thrust at the same horizontal plane as the SBU, it has limited ability for excavating below the water table. Small amounts of water can be pumped out, but larger amounts of water will compromise the machinery. The SBU/ABM set up is generally confined to areas in which a large bore pit can be excavated for machine launch, which means a large footprint at the excavation site. This can make launching in tight areas more difficult.

The SBU is only capable of performing straight bores since the steel pipe casing is installed as the machine excavates. As with other excavation methods utilising an ABM, maintaining the accuracy of the bore path can also be difficult on smaller diameter SBUs and on non-motorised models, which can only be steered during the first 6 - 12 metres of the bore. However, the MSBU and Rockhead do not face such issues because they can be continuously steered.

Microtunnelling machines
Microtunnelling machines (MTBM) are small diameter, remotely controlled tunnel boring machines that require the use of hydraulic pipe jacking systems to install pipeline as they bore. This method of excavation is one of the most prevalent small diameter excavation methods in Australia today.

MTBMs can be steered continuously to bore accurate, small diameter tunnels. Like a TBM, they can supply their own forward thrust and like an ABM system, they need a pipe jacking system at the shaft. The pipe jacking system normally is used with short lengths of reinforced concrete pipe behind the machine, creating a smaller footprint at the launch shaft. Because all the material is removed through the slurry system the shaft area can be kept clean more easily.

In terms of excavating rock, MTBMs are more expensive compared to a SBU/ABM combination or the Christmas Tree Head and ABM. Purchasing a typical MTBM will usually cost about five to ten times more than an SBU/ABM combination. Most MTBMs use tungsten carbide drag bits or multi-row tungsten carbide roller cutters, which are good in soft rock, yet highly inefficient for hard rock. These drag bits often break off or chip in hard rock, requiring expensive equipment replacements, slow progress and increased man-hours. The roller cutters pulverise the rock, creating fines and very low production rates as compared to disc cutters.

Time is also a factor in considering MTBMs for a hard rock application. MTBMs usually employ a recirculating slurry system or vacuum extraction for muck removal, which is very inefficient in hard rock bores. Larger chips of rock must first be crushed before they can travel through the slurry system, slowing excavation time. MTBMs also do not allow for cutter changes during the length of the bore, further slowing the boring process.

Christmas tree heads
Christmas tree boring heads are used in conjunction with ABMs in soil to hard rock applications. The Christmas tree head features tungsten carbide teeth that excavate by cutting or ripping away at the material.

Christmas tree heads can be the most cost effective boring method for certain hardness of rock and shorter bores but cost effectiveness is very limited.

The tungsten carbide bits are not efficient in hard rock above 75 to 100 MPa. Since the bullet bits are precision manufactured for low impact loads they tend to break off in hard rock and must be replaced. The drill excavates rock by ripping it away at the face, which creates high torque spikes and unusually high wear on both the ABM and drill head. Thus the final cost of using Christmas tree heads tends to be even higher than SBUs after factoring in replacement and maintenance costs and additional man hours.

SBUs in the field
SBUs, when compared with other methods, are the most time and cost efficient way to excavate hard rock (100 to 250 MPa UCS) in projects above the water table. SBUs have been used on over 500 projects around the world since The Robbins Company began marketing them in 1996.

Big Sky, Montana, USA
A 303 million litre reservoir was excavated to hold treated wastewater for irrigation of a private golf course in Big Sky, Montana. The design called for the installation of a 760 mm diameter casing 97 m in length.

Tunnel Systems of Woodinville, Washington began boring in September 2004 utilising a conventional Christmas tree style head with carbide bullet bits. The crew worked seven shifts averaging about 3 metres per day, but after pulling the Christmas tree head back through the bore several times to replace broken and missing bits, Tunnel Systems decided to change cutter head tooling.

The contractor decided to use the Robbins SBU to complete the remaining 75 metre bore through mudstone and shale, estimated to be in excess of UCS 35 MPa. The decision was made based on economics as well as time constraints.

Once boring resumed with the SBU, Tunnel Systems averaged over 15 metres per day with the 762 mm diameter SBU, finishing the installation in just five shifts. This was their first experience with the technology, and the SBU came out within a few inches of planned line and grade.

Redmond, Oregon, USA
Stadeli Boring & Tunneling of Silverton, Oregon began using Robbins SBUs in January 2006. Their first bore utilised a leased Robbins 762 mm SBU for use on a highway bypass project. The project called for a 37 metre long bore of 762 mm casing beneath railroad tracks in the ‘Deschutes Formation’, which is volcanic basalt.

Stadeli had expected an average of 3 metres a day, based on experience with other rock boring equipment. Instead, the machines bored up to 3 metres per hour - completing the bore well in advance of schedule and allowing overpass construction to proceed ahead of schedule.

This article was published with kind permission of Trenchless Australasia.