Training in Costa Rica

In 1999 Atlas Copco Craelius delivered three computerised, self-contained grout-mixing stations to the Instituto Costarricense de Electricidad (ICE) in Costa Rica, which were to be used for the construction of the Angostura hydroelectric project. The scheme, the largest hydroelectric project ever to be undertaken in Costa Rica, will substantially increase energy resources and contribute to the development of the country.

Angostura

The 177MW Angostura project is situated some 120km east of the capital, San José, on the Rio Reventazón, in the vicinity of the small town of Turrialba. It will get its power mainly from the Rio Reventazón, while the smaller Tuis and Turrialba rivers are additional sources supplying water to the main reservoir through deviation canals.

The project comprises a rockfill dam with an impervious core with a crest height of 36m and a total crest length of 235m. The reservoir will have an area of 256ha and maximum level oscillation of 7.9m.

The project is financed, designed and executed by the ICE of Costa Rica.

Geology

The grouting equipment acquired by ICE was mainly intended for the conduction tunnel which runs through a very complex set of geological formations. These are a combination of sedimentary and volcanic deposits with transition zones between the two. The complexity is exacerbated by geological faults which form hydro-geological limits. Groundwater pressure had to be monitored very closely.

The tunnel goes though a series of hard soil and soft rock including conglomerates, breccia and lahar (a sort of breccia which sometimes behaves like a fluid), and many of these are susceptible to piping phenomena. Exploration drilling in the area was done with an Atlas Copco Craelius Diamec 262 MP and geologist José F Cervantes followed the geological and geo-technical problems of the tunnel very closely.

Training and commissioning

Start-up and training was an important part of the package requested by and delivered to the ICE. Two Atlas Copco Craelius engineers conducted the training for ICE’s employees’ benefit. This was divided into two areas: theory and practical hands-on work.

The theoretical training was attended by 25 ICE technicians and engineers.

The subjects covered included:

• Drilling techniques, methods and equipment, percussion versus rotary drilling and their effects on hole deviation.

• Mortar and mortar mixing, the effect of the use of sand and additives etc.

• Grouting: considerations of pressure, flow and methods and the (strongly recommended) use of a return grout-line.

The practical training was carried out at the same time as the equipment was being commissioned. The ICE personnel had previous experience of Atlas Copco’s earlier grouting equipment so that training could concentrate on the operation of the weight-batching processor and the recording systems and the benefits they can bring towards a successful grouting operation.

Headrace tunnel

The main grouting job for the ICE at Angostura was undertaken in the headrace tunnel. Water is conveyed over a distance of 6754m from the dam to the penstocks through a 6200m long tunnel. This tunnel has an excavated diameter of 7.7m and is lined with concrete and steel. The final diameters are 7m and 5.8m respectively.

The tunnel is connected to the surface by one large diameter access shaft (18m deep and 340m from the dam) and two 10" holes (one 75m deep hole at 2000m from the dam and the other, 240m deep at 2000m, from the other end of the tunnel). Almost half way along the tunnel there is also a 600m long access tunnel called Ventana which leads into the main tunnel.

The slope of the tunnel is 1% and the total level drop to the power house, including the penstocks, is 141m.

Grouting equipment

In order to have the necessary grouting equipment for this project and for future needs at other projects (such as the Pirris project due to begin in 2002) the ICE, after consultation with Atlas Copco and other suppliers, decided to buy three computerised grout-mixing stations. To meet the final specifications Atlas Copco Craelius built three Unigrouts 400-55 EWB and shipped them in late 1998 to Costa Rica.

The Unigrout 400 is a self-contained, computerised grouting station built on a rigid platform, intended and designed for truck-mounting. The grouting material is held in a 1000 litre silo and fed by a screw into a mixer. The mixer admits the dry material, any additives and water in predetermined proportions as programmed in the weight-batching processor. After mixing, the grout is delivered to one of two identical agitators. Each agitator feeds a high pressure grout pump which can be used for delivering the grout at long distances or for grouting on site. The main components of the Unigrout 400-55 EWB are as follows:

Mixer

The Cemix 402EWB is a high speed, high shear colloidal type mixer which produces a thoroughly homogeneous mix, free from lumps. It consists of two parts: a mixer assembly and a weight-batching stand. The mixer assembly comprises a container, a mixing housing flanged at the bottom and an electro-pneumatic distribution valve system. The mixing housing contains an impeller driven by an electric motor directly flanged on the housing. In the mixing process cement lumps are disintegrated in two steps:

• Firstly, by the vortex in the container, created by the high velocity impeller.

• Secondly, by the shearing action in the specially designed mixer housing. So each cement particle is separated and effectively hydrated.

The weigh-batching stand is provided with three load cells that sense the weight of the mixer assembly placed on top of it and sends signals, by cable, to a weight-batching processor via a summation-box.

Agitator

The agitator is a CEMAG 701E and has roughly twice the capacity of the mixer. Its function is to keep the solids in suspension and maintain the homogeneity of the mixture as it is being supplied to the grout pump. The cylindrical container has a slanting base and an equally inclined shaft driving a pair of blades which are offset to create the necessary turbulence.

Pump

The ZBE 200 is a single cylinder, double-acting piston pump capable of pumping most grout mixtures. It is designed with easy-to-clean ball valves and driven by an electro-hydraulic system which allows pressure and flow to be regulated independently during the grouting operation.

Weight-batching processor

The complete mixing process is controlled by a programmable weight-batching processor. It receives data from the load cells placed under the mixer and senses the on/off position of the cement feed-screw motor and of all the valves in the grout circuit. It is designed to operate the Unigrout according to any given mixing formula, by weight, programmed in the processor. Up to 25 different formulae, or mixtures, can be pre-programmed, including one for cleaning the equipment, with water and additives, if required. The formula which is being run is also shown on an LCD display.

Recorder

The LOGAC 4288 is a computerised instrument for recording data from the grouting operation. It displays pressure, flow and time and stores pressure, flow, volumes, real time, time and hole number. All data is recorded on a PC card for later use on a computer and for subsequent evaluation. The LOGAC 4288 can read up to eight grout lines simultaneously where each line can be started and stopped individually.

The Unigrout also comprises a small air compressor, an additives pump, electric cabinets and controls.

Grouting operation

For all grouting operations, mixing was done by the Unigrouts 400-55 EW which were placed on surface, and the prepared grout was transferred by a single pipe to the grout pumps placed close to the grout holes in the tunnel. The grout pipes lead from the surface down into the tunnel through the 10" holes. The grout pumps at the front were old ZH 50 and ZBEs which have been in service on previous projects and still are in good working condition.

Several grouting mixes were tested but at the end the most commonly used mixture was 100 litres of water to 150kg of cement. Sand was added for some time but was later abandoned for two main reasons:

• Sedimentation occurred in the pipes when the distance to the grouting front increased.

• A high degree of wear occurred on all the grouting equipment.

Grouting was carried out in two distinct stages: the first one just for filling the space between the crown of the concrete lining and the roof of the excavated tunnel and, once this was achieved, the second for waterproofing the contact between the concrete lining and the surrounding rock.

Grout holes were drilled in a given pattern by an Atlas Copco 353 drill jumbo with COP 1238 drifters. Holes were drilled through the concrete lining and then 50cm into the surrounding rock. Penetration was fast: 30 seconds through the concrete except in reinforced concrete where drilling took up to three minutes.

Three drilling and grouting patterns were used

• Filling the crown

In phase 1A, drill holes 2 (600 left of vertical), 4 (vertical) and 6 (600 right of vertical) are drilled upwards in a fan shape across the section of the tunnel and then grouted. The pattern is repeated every 6m for about 100m. Then in phase 1B, holes 3 and 5 (300 left and right of the vertical respectively) are drilled and grouted to form fans spaced halfway between the fans of phase 1 A.

• Contact grouting

For contact grouting in phase 2A, holes 1, 3, 5 and 7 are drilled, with 600 in between, in the shape of a fan across the section of the tunnel and then grouted. This pattern is repeated every 6m for about 100m along the tunnel. In phase 2B holes 2, 4, and 6 are drilled in a fan shape at angles that bisect the angles of the first phase, and are then grouted. The fans of the second phase are spaced halfway between the fans of the first phase.

• Special Pattern

In some cases, in areas of extremely bad geological conditions and for waterproofing, a special grouting pattern was used. In phase 5A six holes are drilled in the same plane at 60o from one another, forming a circular fan, which was then grouted. The pattern is repeated every 6m along the required stretch. In phase 5B a similar pattern, in the shape of a circular fan, is drilled and grouted and whose holes are offset by 300 in relation to the holes of the first phase. The fans of this second phase are spaced halfway between the fans of the first phase. The distance between grouting coronas was the same along most of the tunnel. Only under very special cases, the distance was decreased to improve waterproofing. Grouting was carried out 24 hours a day by two 12-hour shifts. The amount of cement injected per day was around 40tons. There was a great variation as to the distance covered daily by the grouting teams.

Conclusions

The ICE acquired three mixing/grouting stations to be used for an important grouting operation. The computerisation of these units and the way in which they were used has proven to be an efficient and effective way of maintaining the quality of grouting. The grouting campaign has been relatively short but the equipment and, in particular the technicians, are now extremely well prepared for the next mission, probably on the Pirris hydroelectric project which is due to start in less than two years.