August_1971 — Page 28

dam. Hydraulic turbines are of the Francis type, vertical shaft, 176.5 rpm, rated output 15 MW at 32 metres head. The runner is a single casting of 13% chromium stainless steel. The spiral cases are made of welded steel plate, assembled with flanged sections.

Progress

In the past, access to the site re- quired crossing the Nam Ngum river just downstream of the confluence of the smaller Nam Lik river. Here a high- level prestressed concrete bridge was built in early 1968 by Christ & Nielsen (Thai) Ltd.

The general contract for the civil works was awarded to a joint venture of Hazama-Gumi and Mitsui. Two separate contracts were awarded for the permanent equipment. The Gates and Penstocks Contract went to Mitsui (as representatives for Sakai Iron Works) and the Generating Equipment Contract was awarded to Hitachi.

A concrete-lined diversion tunnel was holed through the north bank of the river around the dam and power- house site. Coffer dams upstream and downstream from the dam and power house site were also constructed and closed in December 1969, thus causing the river to divert through the tunnel.

The Nam Ngum has a low flow during the dry season which generally

extends from October to May and it is for this capacity that the diversion tunnel was designed. From May to October the river swells with flood flow to low flow ratios of 10 to 1. During the flood period in 1970, the coffer dams were intentionally over- topped, inundating the

inundating the dam and powerhouse works in the riverbed. During this period work in the river- bed was suspended. Later when water receded, the coffer dams were reinstat ed and the main construction work continued.

There was a critical time of deci- sion during the dry season of 1971, with the scheduled closure of the diversion tunnel in April when the dam wall was still incomplete. It was essential that the upper pours of the essential that the upper pours of the dam should advance ahead of the im- pounding water in the reservoir so as to preclude the possibility of over- topping. The decision to proceed with closure and impounding or to resort to emergency diversion of the river dur- ing the flood season was based on the height of dam at the time and a very conservative estimate of the concreting rate as well as some security factors. Now the concreting is safely ahead of the upstream water flows and if all work progresses as scheduled the Nam Ngum station should go into operation by the end of this year.

Principal features of the project:- Catchment area (above dam site):

8,460 km2

Reservoir:

-

Proposed high water level - EL 202 m Proposed low water level EL 196 m Drawdown - 16 m

Gross storage capacity - 7,030 million

m3

3

Active storage capacity - 4,700 mil-

lion m3

Dam:

3

Volume of concrete - 350,000 m3 Height above foundation - 75 m Crest length - 470 m Width at crest — 5.5 m Width at base (average) – 50 m Slope (upstream) – 1 to 0.20, up to EL

175.0 (vertical above EL 175.0) Slope (downstream) 1 to 0.77 up to

—

EL 165.0 (1 to 0.67 above EL 165.0)

Foundation condition:

The dam site foundation consists of alternate sandstone and shale layers. The main body of the dam is placed on a massive sandstone layer. The average excavation is about 10 metres.

Spillway:

-

Type Overflow chute spillway with

flip bucket

Dimensions 9.5 m high, 57.5 m wide

including piers

Intake:

Type - Dam opening, gate guides and

trash racks

Inlet dimension For 15 MW unit 3.5

-

m by 3.5 m approx.

Penstock:

Type Welded steel pipe encased in

concrete

Number of lines - 5

First development

diameter

—

2 lines of 3.4 m

Future development - 3 lines of 6.0 m

diameter

Power station:

Type - Above ground, reinforced con-

crete with steel roof trusses

Dimensions (first development) 50

m long, 27.5 m wide

-

Dimensions (future development)

Possibly 120 m long, 27.5 m wide Turbines 2 units of 15 MW each,

-

vertical Francis type

Generator - 2 units of 17,500 KVA

Far East BUILDER, August 1971

23