Project Solutions

Case Studies

Double Shield TBM bores through broken rock for Ohio sewer system

Project Overview

The Mill Creek II Sanitary Sewer Storage Tunnel is one of many tunnels undertaken in Cleveland for the management of wastewater. The tunnel prevents sewer overflows in the Cleveland area that were due to increasing population.

The Mill Creek tunnels were divided up into three separate contracts. In 1999, project owner Northeast Ohio Regional Sewer District awarded construction Contract II to a joint venture called KMM&K, made up of Kassouf Co., Mole Construction, Murray Hill Construction, and Kenny Construction. The contractors chose a 7.2 m (23.6 ft) diameter Robbins Double Shield TBM to bore the 4 km (2.5 mi) long tunnel.

Geology

The tunnel passes through fractured gray Chagrin shale, characteristic of the Ohio area. The rock is relatively strong shale with Unconfined Compressive Strength of 41 – 83 MPa (6 – 12 ksi), but is prone to horizontal slabbing.

TBM

Robbins extensively modified and refurbished the TBM, converting it from a Single Shield to a Double Shield for the fractured Ohio rock. The machine separated boring and ring setting into independent operations and allowed both to progress simultaneously for maximum rock support.

In addition, a ring beam support handling and erection system and a probe/grout drill were added for ground stabilization. The machine was also fitted with 17 inch (432 mm) cutters and was driven by 1,900 kW (2,550 hp) of cutterhead power. The TBM was capable of 11,343 kN (2,550,000 lb) of cutterhead thrust and could generate up to 2,486,299 N-m (1,833,800 lb-ft) of cutterhead torque.

Robbins also designed a rolling gantry back-up system and muck transport system for the project. Muck hauling was accomplished by 4,800 m (15,748 ft) of continuously advancing conveyors that wound around a series of eight curves with radii of 300 m (984 ft). The entire conveyor system was provided by Robbins. The conveyors were designed with curve idlers to allow the belt to track through curves with minimum material spillage and belt-edge loading. A vertical belt conveyor and overland belt conveyor also carried the muck from the bottom of the shaft and transported it to a nearby storage area.

Tunnel Excavation

Boring began in April 2001 and the TBM encountered few difficulties. By August of 2001, the machine was over halfway through the drive. The machine achieved advance rates of up to 3 m (10 ft) per hour and accomplished a best shift of 26 m (85 ft) in 8.5 hours.

The TBM finished the tunnel in December 2001, well within its contract schedule requirements.