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oa Civil Engineer in South Africa - Hydraulic roughness change in the Orange-Fish Tunnel : 1975-1990 : technical paper

Volume 33, Issue 8
  • ISSN : 0009-7845

 

Abstract

The hydraulic roughness of the 82,8 km long, 5,33 m diameter, 47,6 m3/sec design capacity Orange-Fish Tunnel was comprehensively tested during commissioning in 1975. The tunnel friction was spot-tested in 1984 and then comprehensively tested again in successive years from 1988 to 1990. These tests showed a hydraulic aging rate for the concrete-lined pressure tunnel that was greater than that expected at design stage and, indeed, outside commonly used ultimate design roughnesses.


The higher-than-expected aging rate (judged to be caused, inter alia, by the build-up of a thin mud/slime layer on the lining) is considered to be a function mainly of the low demand (less than 10 per cent to 20 per cent of capacity) over the first 10 to 15 years of operation. Mechanisms whereby ultimate capacity may be restored are put forward, suggestions made to prevent accelerated aging in the first place and a case made for fixing ultimate design friction co-efficients by probabilistic methods.

Die hidrouliese ruheid van die Oranje-Vistonnel, wat 82,8 km lang is, 5.33 m in deursnee is en 'n ontwerp-kapasiteit van 47,6 m3/sek het, was omvattend getoets gedurende die indiensneming daarvan in 1975. Die tonnel se weerstand was ook eenmalig bepaal in 1984 en is weereens omvattend getoets in opvolgende jare vanaf 1988 tot 1990. Hierdie toetse het 'n hidrouliese verouderingstempo aangedui wat groter was as wat verwag was tydens die ontwerpfase en, weliswaar, hoer as die gewoonlik aanvaarbare ontwerp-ruhede.


Die bo-verwagte verouderingstempo (gereken onder meer veroorsaak te gewees het deur die opbou van 'n modder-slymlagie op die voering) word gereken 'n resultaat te wees van hoofsaaklik die lae aanvraag (minder as 10 persent tot 20 persent van die vermoe) oor die eerste 10 tot 15 jaar sediens. Meganismes waarmee die uiteindelike vermoe herstel mag word, word voorgestel, aanbevelings word gemaak oor hoe versnelde veroudering in die eerste instansie verhoed kan word en 'n pleidooi word gemaak vir die vasstelling van uiteindelike ontwerpweerstandskoeffisiente deur middel van waarskynlikheidsrekeningmetodes.

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/content/civeng1/33/8/EJC25129
1991-08-01
2016-12-09

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