Going underground8 October 1998
Juan Carlos Gutiérrez* describes the underground and dam construction work which is taking place at Porce II hydroelectric project in Colombia
Empresas Publicas de Medellin (EEPPM) is one of the main owners of hydroelectric plants in Colombia, South America. EEPPM has several hydroelectric plants in service with an installed capacity totalling 2000MW. One which is currently under construction is the 392MW plant in the Porce II hydroelectric project.
General characteristics of Porce II
The Porce II hydroelectric project is located in the northeast region of Antioquia department, 120km from Medellin. Two large contracts have been awarded for the work on the project. These will cover: dam construction and appurtenant works; and underground powerhouse construction and appurtenant works.
The dam contract includes:
•Construction of a 118m high RCC dam with a volume of 1.3M m3.
•Construction of complementary rockfill and earthfill totalling about 1.6M m3.
•Construction of appurtenant works attached to the dam, such as an intake tower, spillway, and discharge bottom.
The underground powerhouse contract includes:
•Construction of a 4500m long power tunnel with a horseshoe section 60m2 and 8.2m in diameter.
•Construction of a 225m high upstream surge tank comprising a shaft, 7.5m in diameter and 120m high, and a dissipation tank, 16.3m in diameter and 105m high.
•Construction of the 94m long, 21m wide and 43m high powerhouse cavern.
•Construction of the 83m long, 12m wide and 32m high downstream surge tank cavern.
•Construction of ventilation systems for these caverns composed of shafts and galleries is included.
The main hydrological source of the reservoir is the Porce river, which passes through Medellin city and which, for its first 60 km, is called the Medellin river. After passing through the turbines, the water is returned to the Porce river via a 540m long tunnel and a surface channel 120m in length.
The RCC dam will impound a reservoir of 234M m3 and will flood an area of 890ha. The firm annual generation of the project will be 1600Gwh/year.
The basic cost of the dam contract is US$108M and the underground powerhouse contract costs US$54M (May 1994 prices). Both contracts are financed 60% by the InterAmerican Development Bank (IDB) and 40% by Empresas Publicas’ own resources.
Both contracts began simultaneously on 1 November 1994, and were won by an Italian-Colombian consortium consisting of Astaldi SpA, CMC Di Ravenna Soc. Coop. ARL, Federici SpA, Recchi SpA and Topco SA.
Construction of the dam is scheduled to take 1500 days, while the powerhouse contract will take slightly longer to complete at 1590 days.
Participants in the project
The designer of the main contracts of the Porce II hydroelectric project was Integral SA of Medellin. This company is also giving technical advice during construction. Sedic SA of Medellin is supervising the project. Empresas Publicas de Medellin is co-ordinating and managing the technical, contractual and programming aspects of the contracts. The contractor has experienced some economical and financial difficulties during implementation of the contracts. However, it is anticipated that the first generating unit of the plant will begin commercial operation in December 1999.
Project status on 31 July 1998
To date, the contractor has poured 180 RCC layers, out of a total of 406 layers — equivalent to 636,223m3 of RCC, which means 49.9% of the total volume of the dam. During June and July 1998 the contractor placed RCC at an average rate of 230m3/hour. The daily average placing rate was 3225m3 and the average temperature of placing was 16.50C.
Construction has also started on some of the structures attached to the dam, such as the intake tower, the bottom discharge and the spillway.
Some technical problems with the quarry exploitation and with the production of the main crushing plant were experienced and led to several stoppages and delays in placing the RCC.
The total volume of concrete to be poured in the spillway is 77,200m3. The flipbucket slab of the spillway has already been finished, and totals 3900m3 of poured concrete. For the intake tower attached to the dam 5073m3 of concrete has been poured.
Originally the completion date for the dam contract would have meant that all work would be finished on 9 December 1998. However, the contractor will not be able to finish the work by then and the most recent estimated finish date has been set for 24 September 1999. The causes and consequences of delays will be analysed and evaluated by arbitration.
Ninety-three per cent of the tunnel has been excavated. In general, the excavation has been carried out in good rock conditions, almost without groundwater; but the contractor has had several difficulties during excavation in relation to low excavation rates, which have varied from 4m/day to 6m/day, approximately.
Excavation of the power tunnel started in January 1996. It has a section of 60m2 and five excavation sections were established in planning the construction plans. Supporting systems varied from steel arch beams, in fault zones, to bolts alone, in very good rock.
With regard to the power tunnel, for the length excavated to date (4200m) only one 30m-long sector needed steel arch beams. The other sectors have had a variety of supports in different areas, ranging from shotcrete and steel mesh to bolts alone. According to the bid studies, a fault zone 200m long, called the Melguizo fault, was expected. However, during construction the fault zone proved to be better geotechnically than expected. It was supported with two layers of shotcrete (5cm each) with steel mesh between the layers, and bolts. The power tunnel has two adits for construction. Construction of adit No 1 (500m long) began on 16 June 1995 and finished on 8 January 1996. Construction of the power tunnel from this adit began on 9 January 1996. Construction of adit No 2 (290m long, near the dam) began on 25 April 1996 and finished on 16 July 1996. Work on the power tunnel from adit No 2 began on 16 July 1996.
The last, 120m long, sector of the power tunnel, corresponding to the power shaft and the distributor near the powerhouse, is steel lined and the work is almost completed. The final linings of the power tunnel consist of shotcrete in zones of good rock and reinforced concrete in fault zones and in special zones. According to the results of the tunnel excavation, a 500m section will be lined in reinforced concrete in the following different sectors:
•Intake tower zone (abscissa k 0 + 000 to k 0 + 150).
•Fault zone (abscissa K 0 + 365 to K 0 + 395).
•Fractured rock zone (abscissa K 2 + 650 to K 2 + 780).
•Intersection with upstream surge tank (abscissa K 4 + 003 to K 4 + 023).
•Intersection with adit No 1 (abscissa K 4 + 146 to K 4 + 308).
Focusing on construction of the caverns, the powerhouse cavern was excavated from 11 January 1996 to 26 March 1997; the downstream surge tank cavern was excavated from 8 February 1996 to 13 January 1997. The rock volumes involved in these two structures were 80,000m3 for the powerhouse cavern and 31,000m3 for the downstream surge tank cavern. The excavation of the caverns was in good rock, without water, and there were no fault systems that would have stopped the works. The caverns were excavated from the upper part and from the bottom, taking advantage of construction tunnels previously excavated. The supporting systems in the caverns consisted of shotcreting and bolting systematically applied according to the designer’s advice.
The contractor has so far carried out work towards the underground contract at a cost of US$37M. According to the initial programme the contractor had originally planned for the works to cost about US$50.3M and completion was planned for 9 March 1999. However, the contractor will not be able to meet this date and it is expected that work will now finish on 31 July 1999. Again, the causes and consequences of the delay will be analysed and evaluated by arbitration.
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