Elevation in Meters
250 240
230
220
210
Flow
-Nominal crest El.221,50
Axis of Dam
-8.75
EL 209,0
1.0 type A riprap B.175.0
Max. surcharge H.W EI.219.0
Normal maximum HW. El.217.07
190 Ordinary minimum 1! WL EI, 180.07
Extreme minimum
HW EI. 158.0~
First Eage construction,
El. 198.0
-0.3 facing riprap
-Natural slope
Rockfill, rolled or clumped EL. 140.0
(80
170
IGO
150
140
El 144.0 min:
130
120
Rockfill, rolled or dumped
-Natural slope
110 100
-Impervious fill
Dumped rockfill
Assumed top of fresh rock→
Grout curtain-
Assumed top of slightly weathered rock)
Existing ground surface
Scale O
50 Meters
1:4,000
Figure 4. Typical section of rockfill dam. Height is 125 meters at greatest section.
The dam is approximately 125 meters high, 570 meters long at the crest and 550 meters wide at the base of its greatest section. The height represents nearly the topographic, geologic and structural limit at the dam site.
Dam and dyke
The dam and dyke sections were patterned after the Nantahala type of dam where the impervious core is a
thin inclined membrane sand- wiched between upstream and down- stream rock shells. A typical section is shown in Fig. 4. This type was selected after considering many fac- tors among which is the availability of suitable rock nearby.
Impervious fill is difficult if not impossible to place during the rainy season. By keeping the earth fill to a minimum, a fast build-up is made possible. The upstream and down- stream shells are composed of rolled rockfill and dumped rockfill. The exact line between these two zones was fixed in the field based on the properties of available rock most suitable for the two types.
The earth core which is inclined upstream is constructed of selected overburden soils and severely weathered rock. It is separated from the rock shells by a two-stage filter
and upstream
a three-stage filter downstream. Also, a minimum zone of rolled rockfill is required as transition between the filters and the rockfill shells. The filters are process- ed from materials taken from the Angat River.
a
The dyke will fill the low place in the saddle next to the spillway. It has a cross-section similar to that of the dam.
The dam and dyke constitute the major portion of the civil works. The total volume of the dam is about 7 million cubic meters and the dyke 2 million cubic meters.
Spillway
The spillway structure is of the gated chute type, located in a natural
72
gully. This location offers the most suitable site geologically since sound rock is exposed along the surface of the gully which will provide founda- tion for about 340 meters of chute.
The selected profile of the chute required minimum excavation. It is shown in Fig. 5. Flow is controlled by means of three tainter gates each 12.5 meters wide by 15 meters high.
a
The chute is divided into three equal channels by the use of inter- mediate training walls to allow bet- ter exit conditions for small flows. The spillway was designed for flood with a peak of 7,500 cubic meters per second. This flood will cause a surcharge of 2 meters above normal power pool level and a spill- way outflow of 5,800 cubic meters per second.
The design flood was determined by applying a synthetic unit hydro- graph to a design storm. The design storm was developed from the world record of point rainfall. With proper adjustments for storm efficiency, inflow barrier, elevation and areal effect,
has the resulting storm maximum 24-hour value of 97 centi- meters and a 90-hour rainfall of 202 centimeters.
Model tests
а
Hydraulic model tests were per- formed at St. Anthony Falls Labora- tory under the late Dr. Lorenz Straub to measure discharge capaci- ties, water surface profiles, pressure and erosion patterns. In the course
of the tests, it was found that spill- way operations will eventually make the tailwater higher than would nor- mally occur.
As a consequence, exploratory studies were made of various types of bucket deflectors which will not cause objectionable bar formation in the river channel and backwater effect upstream of the impact pool. It was established that a scheme to direct the jet downstream which would be quite suitable appurtenan- ces at the end of the chute only would be preferable.
A bucket which deflects the jet at right angles to the spillway chute was thus develop- ed. An artist's sketch of the bucket deflec- tors is shown in Fig. 6. The cross- section of the buckets is in the form of а cir- cular arc of 5- meter radius with the upper edge terminated at an an angle of 140° from the plans of the chute floor. The deflector height is progres- sively reduced
until the upper edge terminates at a 90° angle to the chute floor. The buckets are provided with an end sill with a horizontal lip.
Power intake
The power intake consists of a short inlet channel, trashrack struc- ture, tunnel section and gate struc- ture. The scheme chosen was select- ed to obtain a free standing tower of minimum height considered most desirable from the standpoint of economy and vulnerability to earth- quake.
The intake tower houses the gate hoists and service deck. In case of tunnel or penstock failure or in case of valve failure, closure of the power conduit will be against flow.
Tunnel and penstock
An early consideration of a power tunnel scheme incorporated a por- tion of the diversion tunnel in the layout. An economic study showed this scheme to cost as much as one with the power tunnel independent of the diversion tunnels. Because the independent power tunnel scheme allowed more flexibility in the con- struction
it schedule
was finally adopted.
The power tunnel is 8.0 meters in diameter and circular in cross- section.
At the point where the rock cover reduces to 50% of the maximum head (static load plus water ham- mer), the diameter is reduced to 7.0 meters and steel lining is provided. The purpose of the steel lining is to provide a watertight membrane of adequate length to prevent seepage into the power station and to resist internal hydrostatic pressure.
The liners were also designed to resist the unbalanced external pressures due to grouting and ground water.
The penstock then branches into two 4.5 meter diameter penstocks and then 3.0 meter diameter pen- stocks which lead into individual units. A 2.4 meter diameter pen- stock leading to the auxiliary power-
Far East Architect & Builder June, 1965