Small Boring Unit (SBU)

Robbins SBU bores through hard rock in just 8 working days

• Machine Type: Small Boring Unit (SBU)
• Diameter: 66 in (1.67 m)
• Tunnel Type: Water Pipeline
• Tunnel Length: 118 ft (36 m)
• Owner: Charlotte-Mecklenburg Utilities (CMU)
• Contractor: Horizontal Unlimited Inc.
• Location: Pineville, North Carolina, USA

Project Overview

This project, owned by Charlotte-Mecklenburg Utilities (CMU), required the installation of 15 miles (24 km) of pipeline south of Charlotte. The pipeline ranges from 36 inches (914 mm) to 64 inches (1.62 m) in diameter and increases the volume of water to meet higher consumer demands in the area.

Horizontal Unlimited Inc. was contracted specifically for a short 118 ft (36 m) section of pipeline through hard rock. In 2001, Horizontal Unlimited Inc. bought a Robbins 66 in (1.67 m) diameter Small Boring Unit (SBU) for the section of tunnel.

Geology

The portion of pipeline runs through hard gabbro. Gabbro is a plutonic igneous rock that is densely grained and mica rich. Drill and blast was not an option because the section of rock was free of fissures and would not fracture.

SBU Features

Robbins built the 66 inch (1.67 m) diameter SBU, the first manufactured of that diameter, to bore through very hard rock. Robbins SBUs utilize hard rock TBM disc cutter technology adapted for small-bore applications. Each unit includes a rugged cutterhead, high thrust/torque bearing assembly, front shield with stabilizer feet, and standard auger adaptation.

Tunnel Excavation

Workers aimed the SBU at a 5% grade because of close proximity to apartment buildings. Horizontal Unlimited used 5.5 ft (1.67 m) diameter snap-together pipe to minimize welding time. The SBU bored straight and ended up only 1.0 inch (25 mm) high of the target. The entire bore took just eight working days.