When choosing the cutting tools for trenchless machinery the type of ground conditions, including the size of cobbles and boulders present, the location of the water table and rock hardness are important mitigating factors.
For mixed ground conditions the choice of the correct cutting tools for trenchless machinery and the installation method becomes more difficult. Possible solutions on this type of project included the auger boring and pipe jacking methods.
At a recent project in Phoenix, Arizona, USA, contractor SSC Boring, Drilling and Vacuum Estimating was looking for a more efficient type of auger cutting head. The company needed to excavate two crossings along a 600 mm diameter pipeline in tough ground conditions consisting of cemented cobbles and boulders. SSC eventually settled on a relatively new type of cutting head, known as a mixed ground Robbins small boring unit (SBU-A). “Before we used the SBU-A, we hand dug these types of jobs,” said Arvid Veidmark III, owner and senior estimator of SSC. “We would push steel casing into the ground and then excavate using jack hammers and buckets. Our typical rates were 1.5 to 2.1 m a day.” By contrast, his average rates using the mixed ground SBU-A were 6 to 9 m per 8-hour shift.
SSC’s auger cutting head was successful in part because of specialised cutting tools, customised for the specific ground conditions. The SBU-A was mounted with widely spaced disc cutters and featured larger muck openings to ingest cobbles and boulders. Whether the method used involves a Small Boring Unit, conventional auger boring, or microtunnelling, the proper cutting tools can make or break the success of a trenchless project
Dry alluvium or clay with cobbles and boulders
One of the more common and most variable types of mixed ground consists of soft ground with cobbles and boulders. Conditions range from dry alluvium to dense glacial till to hard pan clay with a consolidated matrix of rocks. Dry alluvium is soft and generally easy to break; however the hardness of the rocks can vary, necessitating a wide variety of cutting tools.
Conventional ABM Heads
The conventional auger boring machine (ABM) heads attach to the end of a full-face auger utilising either steel ripper teeth of tungsten carbide bullet bits to excavate. The bullet bits operate in mixed ground using a ripping or crushing action. The company said conventional ABM heads are most effective on shorter bores that are less than 30 m with cobbles and boulders less than 100 – 150 mm in diameter.
Larger boulders present problems for conventional heads. Rocky conditions need specialised mixed ground cutting heads or microtunnelling machines.
Mixed SBU-A
The Robbins mixed ground small boring unit (SBU-A), a type of auger attachment, is available in diameters from 600 mm – 1.8 m. The cutterhead consists of a circular cutting face mounted with a combination of single disk cutters, two-row tungsten carbide insert disc cutters, and/or carbide bits, depending on ground conditions.
The operation of the SBU-A involves the welding of lead steel casing to the cutterhead in a launch pit. A standard auger boring machine generates both torque to rotate the SBU-A and forward thrust through the steel casing.
Scrapers scoop the muck from the face into openings on the cutterhead, then to a full-face auger for removal. Typically, SBU-As are used on crossings between 30 – 90 m, though the machines have been used on crossings up to 150 m in length.
In dry ground with cobbles and boulders, cutterheads typically utilise two-row tungsten carbide insert disc cutters with drag bits instead of single disc cutters. This is due to the different methods that single disc cutters and tungsten carbide disc cutters use to excavate rock.
Tungsten carbide cutters successfully excavate cobbles and boulders (rocks) through multiple points of contact at the face. The multiple buttons on each cutter act like traction, allowing the carbide to grip in soft ground and keep the disc rotating.
Single disc cutters are excellent for solid rock conditions with clay or dirt seams. The cutters break the rock into chips, rather than crushing it into fines or ripping it from the face. The rotating disc cutters create crush zones through which fractures propagate. Rock chips are then broken away from the face much more efficiently than the crushing action of carbide cutters.
However, SBU-A technology does have some limitations. Even in dry conditions, the technology must be used with steel casing. As the machine must be welded to the lead casing, other types of casing are not compatible. Motorised, articulated models can be used with RCCP pipe and a pipe jacking unit, most commonly on drives over 150 m in length.
MTBM
Microtunnelling machines (MTBM) are an advantage in certain types of ground because they work on longer drives with curves and with different types of pipe. Microtunnelling systems can employ either an auger or slurry system for spoils removal.
Slurry MTBMs excavate using a pipe jacking unit and a bi-rotational cutterhead. Spoils exit through large openings in the cutterhead and enter a mixing chamber with water to form slurry.
In some dry conditions the slurry pressure balance and spoil removal system can cease functioning, causing the machine to become blocked. The malfunction is caused by a lack of free water surrounding the soil, resulting in absorption of the slurry into the ground. The loss of circulation in the slurry system stops spoil removal at the face.
The loss of slurry on a particular job is determined by the permeability of the ground. In low permeability ground such as clay or rock with fines there will be little water seepage and thus little chance for reabsorption, whereas in highly permeable sands and gravels the system is more likely to fail.
In general, auger-style microtunnelling systems are better suited to dry ground such as alluvium, clay and sands with limited rocks.
Wet alluvium or clay with cobbles and boulders
This type of ground can be excavated utilising auger boring machines if the water level is low, though significant water requires the use of microtunnelling systems.
In dense glacial till or hardpan clay with low porosity, water inflow rates are usually negligible. If sections of the excavation are wet and sticky, shallow de-watering methods such as pumps can be used to temporarily lower inflow rates and dry out the excavation alignment. In these cases, ABM cutting heads can be used for rocks less than 100 – 150 mm in diameter. If larger cobbles and boulders are present, or the job is greater than 30 m in length, a mixed ground SBU-A cutting head is required.
In conditions where water inflow is more significant, a microtunnelling machine must be used. The machines can be remotely controlled from an operator’s station, mitigating any risk of severe water inflows. Launch and exit seals are used in the shafts to prevent groundwater leakage as well as slurry loss from around the outside of the casing. In high water pressures, double seals can be installed.
Rock with clay or dirt seams
The types of cutting tools recommended in rock with clay or dirt seams depends largely upon the expected rock strength.
In soft rock below 35 MPa, drag bits made of tungsten carbide or steel are effective by themselves, though harder rock requires SBU-A cutterheads. Disc cutters are used in mixed ground conditions containing rock strengths greater than 35 to 40 MPa, either in combination with drag bits or by themselves. On cutterheads with both disc cutters and drag bits, the disc cutter stands out a little farther in profile so the drag bits are not in direct contact with the face. The bits clear out clay and dirt that makes it past the disc cutters as the rock is excavated. If very high rock strengths (greater than 110 to 125 MPa UCS) are expected, larger diameter discs are used, or on smaller diameter heads two-row tungsten carbide insert cutters can also be used.
MTBM
Microtunnelling machines have been successful in these conditions with cutterheads utilising disc cutters, though the method is less efficient than an ABM and SBU-A setup. Because the cutting tools tend to wear out prematurely, due to the abrasive nature of the slurry at the face, this can result in downtime and increased costs to the contractor.
In addition, the slurry system is inefficient in rocky conditions. Where SBU-As size the rock chips at the face, microtunnelling machines first chip rock and then send it through the cutterhead to a crush chamber, where the chips must be further resized before being added to the slurry tube for removal. This method results in slower overall excavation rates when compared to SBU-As in rock.
Conclusions
Mixed ground is by nature highly variable, and the information above may not cover all of the possible conditions. Factors such as casing type, location of the water table, curvature of the drive, and project budget will determine the method used. The most important issues to consider when selecting cutting tools include the size of the boulder/cobble, the nature of the soft ground, rock hardness, and length of the drive.