ABSTRACT:

The Arenoso Underground Power Plant is part of a 105 MW Hydroelectric project which develops a head of 500 m in the ¥aque del Norte River in the Dominican Republic. The main features of the project are: 00 daJB:iDMmabao(gravity) and- Bejuca1(rockfill), 18,4Ianwater ways, and two power stations in Arenoso (underground) and Aguacate (surface).

The Arenoso power Station is a relatively small plant with two Francis vertical units - with a total design discharge of 17,6 m3/s for an average gross head of 145 m. The main dimensions of the cavern are 33 m (length), 22 m (heigth) and 11 m (width).

The paper describes the geological and geotechnical conditions, and the criteria for the positioning of the cavern, the design of flexible support (shotcrete and bolts), and the anchoring of crane beams.

1.-SCHEME DESCRIPTION

The Manabao-Bejucal-Tavera (MBT) project is a hydroelectric project that develops the upper reaches of the Yaque del Norte River in the Dominican Republic.

This project develops 40 km of the river with an average gradient of 1,5% and a - total fall of 500 m, with two power p1ailts with a global installed capacity of 105 MW.

The upper scheme derives the water from the main river (catching basin of 257 km2) to a regulating reservoir built in a smaller stream (catching basin 72 km2) which forms the head of the lower scheme. The latter has the main plant because it has higher available discharge and regulating reservoir and develops two thirds of the existing head. However, the upper scheme is essential to the whole project because it transfers the water from the main river to the regulating reservoir in the tributary.

This water transfer develops a head of 150 m between the upper derivation dam - and the lower regulating reservoir. These two sites conditionned the whole scheme - and compelled to locate the power plant Underground. This unusual solution for a plant with the small capacity of the scheme of the MBT Project (21 MW) was force~ by the 150 m head developed by the transfer and the lack of ground with an adequate elevation near the down stream regulating reservoir.

Consequently, the upper scheme has the following elements:

  • a 28 m high derivation dam (gravity type)

  • a 4,8 km long headrace tunnel.

  • a 6 m diameter surge shaft and a 2,6 m diameter and 132 m deep power shaft.

  • an underground power plant with an 1,0 km long access tunnel.

  • a 4,5 km long tailrace tunnel discharging in the lower regulating reservoir.

2.- GEOLOGY

Arenoso underground power plant is placed near Arenoso creek, between Yaque del Nor te and Guanajuma valleys, below the nor ~ thern slopes of Limpia-Nariz mountains. - The site is included in the Manabao batho lith, part of the plutonic materials in - the axis of the island Central Range.

The rock can be classed as a tonalite (hi pidiomorphic granitoid) coarsegrained and light-coloured. It includes quartz, pIa - gioclase, and biotitechlorite

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