SBUs offer an alternative method of tunnelling in rock at small diameters and at a lower cost to more common trenchless technologies such as microtunnelling.

The Western Corridor Recycled Water (WCRW) pipeline must traverse about 80 roads, railways and rivers—many of them using trenchless methods such as auger boring machines, microtunnelling machines, tunnel boring machines (TBMs) and horizontal directional drills (HDD).

Trenchless techniques

Microtunnelling is the most advanced trenchless construction method and also the highest costing method that has no size constraint in most regions; however, practical limits for pipe jacking means that most microtunnelling is less than 3 m in diameter.

Tunnel construction can be considered a microtunnel if it is operated from a control panel, is guided by a laser beam capable of installing gravity sewers or other types of pipelines to the required tolerance; consecutively pushes pipes and TBM using a jacking system for thrust; and, continuously supports the face of the excavation.

It may be used in most ground conditions but is best used below the water table in unstable ground. Very complex, long drives with curves are also possible with this method.

Auger boring machines (ABMs) produce more than ample thrust and torque for the average geology encountered with most utility installations less than 150 m in length. In soft ground applications, lead pipe casing is hydraulically jacked forward from a drive shaft, cutting the periphery, while the auger removes material from the face and pulls the spoils back through the casing to the starting point for removal to the surface.

For tougher rock, test disc cutter heads, commonly referred to as small boring units (SBUs) have been specifically designed to enable ABM owners to cut efficiently and with attention to cost and production. Figure 1 shows a SBU for use with an ABM.

Installation of the standard SBU, called an SBU-A, is performed by welding onto the front of the lead casing. The torque is transmitted to the head through the auger string. As the casing is jacked forward, it provides thrust to the rock cutting head through the thrust bearing assembly. Rock from the cutting head is fed back into the shield through the SBU-A, and the auger moves the spoils to the bore pit, like any other ABM application.

Excavating rock with small boring units

Using smaller cutters allows the gage to be cut with approximately the same number of cutters that are used on a larger TBM cutterhead, but with a smaller gage-radius. This leaves sufficient space on the cutterhead for spoils openings to remove the cut rock from the face. As utility tunnel diameters increase it is possible to use larger cutters, which is highly beneficial in terms of increased load capacity. SBU cutter diameters range from 165 mm for smaller casing installations to 292 mm for larger bores.

The objective for efficient rock cutting is to break the rock into chips, rather than crushing it into fines or ripping it from the face. Inefficient rock cutting tools increase the cost of equipment operation and maintenance.

For efficient rock cutting to occur, the spacing-to-penetration ratio of the cutter must be optimised. Cutter penetration is governed by several factors including: rock strength, rock mass properties, cutter load, cutter diameter and cutter ring tip-width. Most penetration estimating methods require as minimum inputs unconfined compressive strength (UCS), Brazilian tensile strength (Bt), fracture spacing, fracture dip and strike, cutter diameter, cutter tip-width, cutter load and cutterhead speed (revolutions per minute).

After the rock has been cut it must be drawn from the face and pushed to toward the back side of the rotating cutterhead, to the auger for removal from the casing.

System torque requirements of rock shields

The ABM system provides the torque to overcome friction between the ABM and casing as well as torque to the SBU-A cutterhead. On an ABM, the torque required to turn the auger is a function of the auger diameter, pitch, volume of material in the auger, material size, presence of water, etc. On typical drives under 100 m, the torque required to turn the auger is generally 2 to 3 times the torque required to turn the SBU-A cutterhead.

As the tunnel and auger become very long, the torque requirement to turn the auger eventually exceeds the output of the ABM. For these applications, a motorised SBU (SBU-M) provides a separate cutterhead power unit, either electric or hydraulic, so that all ABM torque can be employed to turn only the small diameter spoil removal auger. Figure 2 shows a side view of a motorised SBU.

The SBU-M is fitted with a hard rock cutterhead like the SBU-A, and is dressed with a set of single disc cutters based on the rock type and characteristics. The disc cutters are identical which allows them to be rotated to increase wear life. A heavy-duty bearing housing assembly supports the cutterhead and is coupled to a high torque in-line planetary gearbox. This assembly is driven by a water-cooled variable-speed electric drive to accommodate changing ground conditions. A torque limiter protects the motor and gearbox from cutterhead jams if fractured or shot-rock conditions are encountered.

Crossing Excavation – WCRW Eastern Pipeline

Winslow Constructors, working for the Eastern Pipeline Alliance, have purchased two 1,500 mm SBU-A systems. The machines, one for hard rock and one for mixed face conditions, average approximately 1 m per hour in clay and medium-strength sandstone. Each crossing, ranging from 60 to 180 m in length, takes approximately one week to complete, with work crews operating in ten hour shifts.

Each crossing must be completed to a contractual 200 mm horizontal tolerance, and a 50 to 75 mm vertical tolerance. Work crews measure the heading of the SBU as it bores using a dutch level. If the SBU is too far from the heading, then the auger is pulled from the bore and reset to keep the machine on track. Manually adjustable stabiliser pads located on the sides of the SBU also allow it to be steered without pulling the auger for the first 6 to 12 m of the bore.

A number of the crossings excavated by Winslow are three pipelines wide for treated and untreated water, as well as for by-product runoff. These crossings are excavated simultaneously in one larger bore pit of 12 m x 8 m. The pipelines are 1.5 m apart centerline to centerline and are installed using 1.5 m diameter steel casing in 6 m lengths. Crews use a double-continuous-feed welding machine to weld pipe lengths together, cutting the welding time from three to four hours per pipe to just one hour. Once the casing is installed a final pipeline of 1 m diameter cement-lined steel water main will be placed inside.

Crossing Excavation – SRWP

On the SRWP pipeline the joint venture has contracted Coe Drilling to carry out many of the crossings. Due to the time constraints Coe Drilling and Robbins Tunnelling and Trenchless Technology have supplied two hybrid SBU-Ms for the project. The machines were jointly designed and engineered with Robbins supplying the cutterheads and cutters with Coe Drilling manufacturing the system in their workshop. The machines were chosen for their greater flexibility in use, articulated steering, and capabilities on drives of up to 250 m.

Coe has completed fifteen 1,400 mm diameter crossings to install 1,350 mm steel pipe. Approximately 1,500 m of crossings had been excavated with drives between 55 and 150 m in length. The drives have involved a mixture of ground types from clay to rock with strengths up to 100 MPa UCS.

SBU-A and SBU-M on target

Many methods of trenchless technology have been used on the WCRW and SRWP projects, though the use of the SBU-A and M systems is a first in Australia. This technology introduces a lower cost alternative to microtunnelling, particularly for use on shorter drives that would not be commercially practical using microtunnelling.

However, it is important to understand that the choice of equipment must be determined by the conditions of each drive. Microtunnelling is best used in soft, unstable saturated soils and other permeable ground types below the water table, especially wet mixed face conditions. The SBU range of equipment is best suited to hard rock and mixed rock ground above the water table or solid rock below the water table with low permeability.

The cost savings to using SBU systems versus microtunnelling may be as much as 50 per cent with an SBU-A or 30 per cent with an SBU-M in the right ground conditions. This includes capital cost of equipment and construction, as well as time savings.