per cent. Also, the water supply re- quirements will increase which will reduce the primary energy output and correspondingly decrease the load factor, ultimately to about 25 per cent.
Site investigation
an
It was therefore decided that installed capacity of about 210,000 kW was reasonable for both initial and ultimate conditions.
The pos- sibility of future trans-basin diversion upstream was also considered.
As early as 1958, extensive pro- ject site investigations were started by NPC. Geological surface mapping, test pitting and trenching, and dia- mond drilling were undertaken.
The general geology of the site area consists of an alternating series of metamorphised sediments and lavas. The meta-sediments consist of conglomerate beds up to a few hun- dred feet thick and thinner beds of fine-grained elastics.
The meta-volcanics which are mainly andesitic in composition, in- clude a considerable amount of flow breccia (angular fragments formed by fracturing of the solidified
outer
crusts of lava flows) and are locally interbedded with the meta-sediments. The meta-volcanics are closely jointed whereas the meta-sediments are wide- ly jointed.
The rocks have been folded along a general 30 E axis and dips range from 45° to 90°. Shear zones trend- ing mainly along the bedding planes, but also at various angles across these planes, appear to
be fairly common. Bedding plane slips and shears are fairly numerous.
cutting
The flow breccia, shear zones and fractures in the meta-volcanics and meta-sediments appear to have been thoroughly reweldled and recemented by vein minerals. The joints in the meta-volcanics have been generally
healed by vein materials, but those in the meta-sediments are commonly unhealed.
Angat River flows in an alluvial deposit of sand, gravel and boulders overlying a hard base. The depth of significant weathering at and near the dam site varies from a few meters at the toe of the slopes to as much as 20 to 30 meters on the hilltops.
The clay is of medium to low plas- ticity and the moisture content ap- pears to be fairly close to optimum. The water table is generally high. Pressure test data indicated that the rock is impervious. Moderate water losses occur in joints and fissures which are generally filled with clay above the water table but which are clean and groutable below the water table.
Construction contracts
The civil works portion was divid- ed into two major contracts. The first contract, which is of the straight unit-price type, called for the strip- ping of the dam and dyke site areas, drilling and grouting of dam founda- tion from a little above river bed level and construction of the river di- version works. This contract was undertaken by the Philippine Grout- ing & Guniting Company. The same company had previously constructed the 14-kilometer access road to the project site.
The second contract was for the construction of the dam, dyke, the spillway and power facilities. This is being undertaken by Paul Hardeman (Philippines) Inc.
The preparation of the construc- tion schedule was the result of many inter-related decisions. Among others, the diversion works had to be ready before the oncoming rainy season sets in; the dewatering of the dam foundation area had to be done in the dry season. Impervious plac-
ing could only be undertaken effec- tively during the dry season. After the first year, the dam should be high enough against overtopping and finally, there should be time for reservoir build up so that the plant can operate in time for the Decem- ber peak loads.
The main structures for the second contract are a rockfill dam and dyke, spillway, power intake, power tunnel and penstocks, main and auxiliary power stations, surge tunnel and tailrace and low level outlet works. The general layout is shown in Fig. 1. The profile of the power facili- ties are shown in Fig. 2.
On the left abutment of the dam is the power intake. A power tunnel connects the intake to the penstocks. The penstocks branch off to the units of the main and auxiliary power station. East of the dam is the flood spillway and farther east is the dyke.
Diversion works
Two diversion tunels, each 7.6 meters in diameter and horseshoe in cross-section, form the main diver- sion works. Both are concrete lined. Cofferdams upstream and down- stream of the damsite area permit dewatering and preparation of the dam foundation.
It was envisaged that the dam be constructed to El. 144 before the first rainy season, the reservoir stor- age thus available and the two tun- nels forming the diversion works for protection against floods during construction. These works are de- signed for a flood with a peak flow of 3,200 cubic meters per second which is one of the highest on record.
Ultimately, one tunnel will be plugged while the other will contain a 48 in, low level outlet valve for water supply releases in case of failure of the auxiliary plant.
Far East Architect & Builder June, 1965
71