The Fruta del Norte gold mine in Ecuador’s mountainous province of Zamora-Chinchipe faced challenges of seismic activity, poor ground conditions and heavy rainfall. The previous attempt to provide a new access tunnel to the mine did not utilize a water-tight lining structure, which resulted in flooding and eventual abandonment.
This mine is intended to extract probable reserves of 4.9 million ounces of gold grading at 9.2 grams per tonne, according to mine owner Aurelian Ecuador SA (Lundin Gold), making it a significant prospect and source of local employment.
Lundin commissioned Golder* to find a reliable and safe way to access the ore body. Having undertaken a feasibility study, Golder recommended a construction methodology and technology that has been widely used in some parts of the world, but which is uncommon in the mining sector – and this mine’s location and environment posed some special challenges.
If the access drifts could be formed through the soil and weak surface rock to reach the competent hard rock below, conventional drill-and-blast techniques could then be used to extend the drift. But the big challenge was to reach the ore body through the near-surface weathered saprolite, a very soft, decomposed rock which is highly saturated due to the heavy rainfall in this region. This called for the use of soft ground tunneling techniques utilizing sequential excavation and support.
Golder’s challenge was to design twin access drifts — 7.2 meters (24 feet) inner diameter and 100 meters (328 feet) long — from the surface, through the saprolite and transition into the competent rock. The portal to the mine was developed using cut-and-cover techniques and lined with a sheet steel pipe, with a concrete headwall at the start of the twin access drifts and soil nailing to ensure the temporary slope stability of the commencing face.
To line the drifts through the saprolite in a way that would support the ground above the tunnel and keep out the water, Golder recommended the use of a Sprayed Concrete Lining (SCL). In this construction method, concrete was batched on site and pumped to a semi-automated spraying machine which then deposited layers of concrete onto the floor, ceiling and walls of the excavation to create a self-supporting structural lining.
Steel fiber was utilized within the SCL to improve its structural capacity under small deformations. The SCL was designed utilizing finite element analysis, analytical, and empirical techniques.
The SCL provides a water-resistant design that significantly limits water ingress, providing a robust and durable solution that will require only nominal maintenance during the design life of the mine, minimizing its impact on operation.
Tunnels and other underground structures that are well confined by the surrounding ground mass are proven to perform well under seismic action. The ductility of the SCL material and the longitudinal continuity provided by the tunnel liner mean that the proposed solution is less susceptible to damage during seismic events than other non-continuous structural solutions of varying stiffness.
SCL is tried-and-proven technology with which Golder has extensive experience. It is in common use in parts of the world with ready access to the materials and equipment needed, as well as the skilled personnel able to implement the process. Constructing a SCL in a tropical rainforest, at a mine site located 40 kilometres (25 miles) from the nearest town, and many days by road from the nearest seaport or airport, posed significant logistical and scheduling challenges.
The project team had to factor in significant shipping times, as well as consider potential delays at the port or Customs office. There was also the need to work with local supplies of aggregate, as well as train local staff in working with SCL.
Golder provided continued site support and quality assurance and quality control (QA/QC) throughout the construction phase. By having the designer’s representatives on site, Golder could monitor the construction’s adherence to the design and offer hands-on advice and guidance. Any problems were overcome easily and efficiently, as Golder was able to offer alternative solutions when conditions were not as expected. This allowed for improved productivity during construction and a better-quality product in the long term.
* At the time of this assignment Alan Auld Engineering Ltd was the contract entity. Golder acquired the Alan Auld Group in October of 2017.