TBM overcomes challenges in the Andes 26 Jan 2017

A Robbins 4.56m (15ft) double shield TBM has completed boring the 900m (2,950ft) long access tunnel for the Los Condores hydro-electric power plant in Chile in January 2017 and is soon to start boring the first section of the headrace. The machine started its journey in May 2016, and has excavated more than 1,300m (4,270ft) of tunnel so far. It is designed to be suited to the high cover, the hard rock challenge and features efficient segment unloaders to minimise downtime during tunnelling.

The launch of the machine overcame logistical challenges including the remote jobsite which is located 2,500m (8,200ft) above sea level and the high overburden of some 500m (1,640ft) above the alignment. “The location of the project is a major constraint due to the rugged terrain and the geology high in the Andes,” said Pello Idigoras, Tunnel Production Manager for contractor Ferrovial Agroman, “but we are anxious to perform work in an efficient manner.”

Efficient segment unloaders minimise downtime

The Los Condores project is a new 150MW hydro power plant with a 12km (7.5 mile) intake tunnel being bored in the mountainous Maule region of Chile through sedimentary and volcanic rocks. As the TBM advances the double shield is erecting rings of 250mm (10in) thick x 1.2m (3.9ft) long concrete segments in a four segment plus key arrangement. The project will increase energy production and benefit surrounding areas of Chile. The Robbins TBM is excavating at rates of up to 25 rings/day, working two 10-hour shifts per day with one 4-hour shift for maintenance. This is despite challenging conditions, including water inflows of up to 3,500litre/min (925 gal/min). Cementitious and chemical grouting with polyurethane and foam is being used for water control in two significant fault zones. Such ground conditioning techniques were anticipated and the Robbins double shield was designed to apply effectively.

After boring the two sections of the intake tunnel, at 6km (3.7 miles) and 4.4km (2.7 miles) each, a section between the two tunnels will be excavated by drill to complete the intake tunnel of about 12km (7.5 miles) long.