Observations on the revision of the norms for hydraulic tunnels

Abstract

The norms SN-238-68 and TU 11-58, issued in 1959 and 1963, now need revision, based on the experience gathered in the design and construction of large-diameter hydraulic tunnels. The observations and recommendations made here with respect to the norms for hydraulic tunnels are based on experience acquired at the Kazakhstan branch of the Gidroproekt Institute, as a result of the design of pressure and free-flow tunnels for the Kapchagaisk and Atbashinsk hydroelectric plants, as well as of the analysis of tunnels now operating or under construction at the Toktogul'sk, Nureksk, Charvaksk, Ingursk, Aswan, and Chirkeisk hydroelectric plants. I. General suggestions, tn the new norms it would be desirable to incIude recommendations covering nil the components of spillway and supply-tunnel structures: intakes, portals, the linings proper, plugs, and inclined and vertical tunnels and shafts. 2. In the new norms it is extremely important to include a two-stage working design approach for tunnels having a limiting clear area of 60 m z. In the first stage, before the tunnel is driven, the internal cross section, the type of lining, and the thickness of the lining are determined. In the second stage, which should be accomplished after the tunnel (or portions of it) is driven, the actual characteristics of the rock found along the tunnel are taken into consideration, the loads and design schemes are more accurately determined, etc. This recommendation sanctions a generally established approach to the working design of tunnels at construction sites. The adoption of a twostage working design procedure will permit reducing the volume and cost of tunneling work, 3. The new norms should: a) amplify the static analysis methods recommended in a differentiated manner for different types of linings and engineering-geologic conditions and also establish the limits of applicability of the recommendations; b) broaden the recommended methods of technical economic analysis for pressure and freeflow tunnels in spillway and supply conduits; c) amplify the recommended methods of hydraulic and hydrodynamic analysis, including the determination of the loads, or the references to current norms and sources. II. Observations and Suggestions Concerning Chapter 8 SN 238-68 "Analysis of Tunnel Linings," 1. In the section "Loads and Effects, and their Combinations," it would be desirable to indicate the need for the determination of the design loads also in two stages. 2. In the basic and combined loads discussed under point 8.1, the norms do not include the following which should be taken into consideration in the design: the hydrodynamic effects, on the lining, of flows undergoing hydraulic jump; the deviation of the jets at sections where transitions are located; the joining of the jets past the piers of valve chambers, etc.; the pressures on linings of empty tunnels from adjacent pressure tunnels, in the case of small pillars between the tunnels, of thickness (0.7-1.5) dexc, and for relatively weak rock and sufficiently high heads. It is necessary to work out the corresponding recommendations and instructions for these loads. 3. Point 8.2, which reads "The loads and effects should be taken under the most unfavorable combinations separately for the operation and construction cases," should be deciphered and explained. If the loads applied during the construction period cause stresses greater than those resulting during the operation of the structure, then it is necessary to revise the adopted construction procedures, or to give a special justification of this increase in the stresses in the lining. 4. In the consistent analysis of nonclosed dements of the lining, it is necessary to superimpose consistently the forces on the structure, as they arise. The stress state of a lining analyzed in this manner will differ essenrially from the stress state of a closed lining analyzed "directly" for ali the loads, without taking into consideration the stages of their development.