With each tunnel varying in grade and the route travelling through interbedded layers of shale and limestone, ranging from dry conditions to wet, sticky ground.

Despite the highly changeable conditions, Midwest Mole chose a single machine solution for the pipeline, totaling 2,874 m in length.

The contractor is utilising a 1.8 m diameter Robbins Double Shield Rockhead to excavate the tunnels, ranging from 249 – 614 m long, for the Shayler Run Segment C Sewer Replacement Project in Clermont County, Ohio, US. The $US15 million project will upgrade severely degraded sewer systems and protect the area surrounding environmentally-sensitive Shayler Creek.

Project background

The Shayler Run Segment C project is a priority for the project owner, the Clermont County, Ohio Water Resources Department, because of severe pipeline erosion in the creek bed.

Sanitation Engineer for the Clermont County, Ohio Water Resources Department Lyle Bloom said “The existing sewer pipe was installed in 1978 directly into the creek. Since then, the creek has eroded, exposing the pipe to the environment and putting certain sections at high risk of failure. The exposed pipe has now dumped raw sewage into the creek on several occasions.”

The new pipeline will be installed well below the creek bed, crossing the creek a total of seven times. The route cover ranges from 3 to 30 m, with the majority of the pipeline running some distance from the waterway.

Final carrier pipe will consist of 1 m diameter HOBAS reline, and will be much larger than the current 600 mm sewer line.

“The larger pipe was decided on mainly to allow trenchless construction, though it has the added benefit of providing a larger capacity for future needs in the area,” said Mr Bloom.

Pipeline construction consists of seven tunnels criss-crossing the creek, each connected by a 9.7 m diameter launch and receiving shaft, with a total of eight shafts in all. The shafts will eventually become fiberglass manhole structures to access the new pipeline.

Due to the gravity sewer construction, each tunnel is at a slightly different grade to maintain specified slope.

Vice President of Operations at Midwest Mole Steve Abernathy said “We needed a machine that could hit the specified manhole locations within a foot of line at 1.3 – 2 per cent grade.”

Choosing the machine

The nature of the extensive gravity sewer made the machine selection all the more challenging.

The seven tunnels, with their varying grades, result in a change of 54 vertical metres over the course of the project. As a result of the elevation change, the tunnels travel through different strata consisting of softer shale and limestone for the first 900 m, changing to harder, drier shale and limestone for the later tunnels. To effectively excavate in the conditions, Midwest Mole needed a solution to excavate mixed ground as well as hard rock up to 165 MPa UCS.

“The Rockhead was the best suited to the project conditions based on our previous experience with similar Robbins machines. Two-pass tunnelling with a Double Shield Rockhead was also more cost effective than microtunnelling when we prepared our estimates,” said Mr Abernathy.

The original contract allowed for either microtunnelling or two-pass tunnelling, which allows for installation of a primary liner before placing the carrier pipe.

The Double Shield Rockhead was designed with a mixed ground cutterhead that can be changed out for a hard rock cutterhead later on. The mixed ground cutterhead features 6.5 inch single disc cutters and carbide bits, combined with large openings in the cutterhead to ingest mixed ground and to allow for cutter changes.

The hard rock cutterhead is dressed with 11.5 inch diameter single disc cutters and abrasion-resistant muck scrapers, as well as a cutterhead opening for cutter changes. Crews will switch out the cutterhead between crossings in one of the launch/receiving shafts.

How it works

The Robbins Double Shield Rockhead (SBU-RHDS) is a tunnelling machine for use on longer utility installations, usually over 180 m, in unstable ground. It is also typically used for line and grade-critical installations such as gravity sewers, as the machine can be continuously steered from an in-shield operator’s console. Line and grade are monitored continuously using a laser targeting system.

The machine, available in diameters from 1.3 to 2 m, consists of a circular cutterhead optimised for either mixed ground or hard rock. Hard rock disc cutters are capable of excavating ground ranging from 25 to over 175 MPa UCS.

As the cutterhead rotates, disc cutters penetrate the rock face and create a ‘crush zone’ through which fractures propagate. Material between adjacent crush zones is then chipped from the rock face.

Muck scrapers scoop the muck into openings on the cutterhead called muck buckets, which transfer the material to a machine belt conveyor. Muck removal from the site is by either a belt conveyor or muck cars.

Crossing excavation

Excavation of the first tunnel began on 19 May 2010, from an 11 m deep shaft. The Robbins Rockhead completed its first 474 m long drive in three months, holing through on 18 August into a shaft site.

“The mixed ground cutterhead is ideal for the ground. After 474 m there is almost no perceptible wear of the teeth or disc cutters. The machine also drives very smoothly and has plenty of power for these conditions,” said Mr Abernathy.

Crews reported advance rates averaging 150 mm per minute in low strength shale using the mixed ground cutterhead. Advance rates topped out at 32 m in 24 hours, or 21 1.5 m long ring beam and board sets in two ten-hour shifts. The swift advance comes despite some unforeseen difficulties, including a section directly below the creek bed with significant groundwater, and gripper slippage in the soft, wet ground during machine pushes.

The machine was launched from its receiving shaft to bore a second 575 m long tunnel on 30 August, following some maintenance and modifications. Changes included modifying the hydraulic system for increased gripper and roll correction cylinder pressure. The increases allowed the machine to grip and roll correct in the extremely soft, often wet rock.

As the machine bores, a primary liner of ring beams and lagging is set every 1.5 m. After each ring is built, a muck train consisting of a battery-operated locomotive and three muck cars removes spoils from the tunnel. The muck will be used as back-fill around some of the shaft sites after project completion, and potentially as fill on private property in the area.

Work on the launch/receiving shafts is ongoing – four shafts are now complete using a combination of drill and blast techniques and manual excavation. Some open cut pipe jacking operations are also connecting existing, peripheral lines to the new sewer line.

Midwest Mole recently utilised a 760 mm diameter Robbins SBU-A for a 46 m long crossing of the creek, which will connect a 460 mm diameter PVC sewer to the main line.

The work to date has been successful, in large part due to teamwork between Midwest Mole, Robbins field service, and the project owner.

“If there is one thing we would like to emphasise, it would be how willing the contractor has been to work with us throughout the project. In addition to proposing the tunnelling method, they planned value engineering that resulted in substantial cost savings,” said Mr Bloom.

“When the project was bid, there were originally eleven shafts, but Midwest Mole proposed to eliminate three shafts. They obtained easement rights to alter the project alignment, and shortened the project schedule with their new plan.”

All seven of the tunnels and the eight manhole shafts are expected to be completed by May 2012.