The design and construction of a headrace tunnel in Costa Rica was carried out according to Norwegian codes of practice

THE Brasil hydro power project near San José in Costa Rica is an augmentation of an older project. Completed in 1912, the original 2.4MW scheme consisted of a dam, canal, tunnel, pipeline and power station. In 1996 Norwegian companies NCC Eeg-Henriksen Anlegg, Kværner Energy and ABB Kraft – collectively known as the Consorcio Noruego (CN) – signed a turnkey contract with Compañía Nacional de Fuerza y Luz (CNFL) to upgrade the Brasil project to 28MW. Completion of the US$43M contract was scheduled for within 18 months of signing.

The new project was constructed in the same place as the original and consists of a 35m high gravity concrete dam (approximately 55,000m3 of concrete),with a concrete culvert leading to the enlarged 750m long tunnel. Furthermore, a 3.2m diameter pipeline leads to the power station, where a 28MW Francis turbine was installed. The power plant is a peak period plant and during the dry season the regulated volume allows for a total of seven hours production per day.

The cross-section of the old tunnel was increased to 23m2 with a gradient of approximately 1.5%. The old tunnel was used as a ventilation tube and dewatering channel, and the major part of the new tunnel was driven from one side. This was mainly due to ground conditions: the northern part consisted of tertiary sedimentary rocks of medium to low quality, and the southern 60m consisted of quaternary sediments of low stability and quality.

The final tunnel lining was designed and constructed according to a modified Q-system, where five different classes of lining were proposed. After several months of negotiations with the client, a final lining consisting of steel fibre reinforced sprayed concrete, with a compressive strength of 40MPa and a minimum thickness of 25cm, was chosen. The invert of the tunnel was a reinforced cast concrete slab, 10-20cm thick and with a compressive strength of 25MPa. The temporary lining was performed with 5-10cm of steel fibre reinforced sprayed concrete and rock bolting.

In Costa Rica few tunnels have been driven and even less have been lined with sprayed concrete. So the first challenge with the tunnel was to teach the supplier the specialities of modern sprayed concrete mixes. The European contractors also had difficulties mixing SI units and US units which are used in Costa Rican industry. Silica was also brought in from Norway – it is rarely used in Costa Rica due to its extremely high price.

Work started on preparing a mix suitable for robot application with the available aggregates on site. Soon it turned out that these aggregates were suitable and a grand tour of the country’s quarries began. A quarry was located some 200km from the construction site with good quality sand for the desired purposes.

The above mix is rather unusual, especially in cement, silica and water content. This is mainly due to large variations in the quality of cement and aggregates. Costa Rica is, geologically speaking, a very young country and the aggregates are therefore made from young and weak raw materials.

The design compressive strength was 40MPa, but the design mix strength had to be increased to as much as 50MPa to ensure that variations in cement and aggregate quality did not impair the final result. Testing showed that typical variations in compressive strength ranged from 38MPa to 63MPa on samples taken within the tunnel, with a mean value of approximately 54MPa.

During the final phase of the tunnel construction, approximately 1100m3 of sprayed concrete was delivered over 24 days.

At the site there were several locations where concrete was used and all kinds of concrete qualities were produced, ranging from blinding to a high quality C45, in addition to the sprayed concrete. In the beginning, the supplier had only one industrial mixer on site with a mean monthly production of 2000m3 of concrete.

Tunnelling techniques

The new tunnel was excavated by traditional drill and blast techniques, except for the southern part with sediments. A two-arm Atlas Copco 352 boomer was purchased for the job, together with a Atlas Copco Wagner ST 1000 and a Byggs Robomat 2000 robot shotcrete rig. The tunnel was started with only one 12-hour shift but due to delays, machine problems and an increase in tunnel sediments, this had to be increased to three excavation shifts and one shift working with sprayed concrete and rock bolting.

The rock conditions in the tunnel varied a lot and the temporary support ranged from only scaling to cast concrete lining at the face. The design of the permanent support was not decided until after break through due to differences in understanding of the behaviour and quality of the sprayed concrete.

In the southern part it was decided to perform the excavation with a lining of reinforced ribs of sprayed concrete and spilling. The excavation rate was 1m per round and a double layer of reinforced ribs was installed. The excavation method proved successful as long as the procedures for excavation and support intervals was followed. Deformations were measured and maximum deflection in the roof was less than 1mm and less than 2mm in the walls.

Unfortunately at the end of the alluvial part, wall supports were not installed and the right wall caved in over a length of 11m, leading to total collapse. As a consequence, the access road to the power station was cut off and had to be re-established before the new excavation could begin. The collapse resulted in a excavation and support with steel ribs and mainly manual work. Of course this slowed down the progress, but one month after the collapse the affected area was excavated and lined again. The invert in the alluvial part was lined with a 30cm double reinforced concrete slab.

At the beginning of March 1998 the tunnel works were finished, ten months after the start up and one and a half months ahead of schedule.

Sprayed concrete mix…