For the first time in world-wide practice, allowances for minimal cracking in concrete linings of pressure tunnels were included in the Soviet specifications in 1958. Based on experience from performance of hydraulic s~ruc-tures, where some cracks in the concrete occurred, from theoretical and experimental studies in TU 11-58 [1] and subsequently in SN 238-63 [2], directives were issued to permit design of tunnel linings with allowance for formation of cracks in the concrete, provided the following conditions are satisfied: a) size of cracks is limited to a safe value, not to endanger the integrity of the lining as water flows through; b) Ioss of water through the cracks does not result in significant Iosses in power; c) the flow does not produce erosion of material around the tunnel or on the slopes of exit from the tunneL The directives are not too specific and therefore their application presents some difficulties.As early as 1938, V. S. ~ristov [5] proposed the allowance of hairline cracking under internal pressure in tunnel linings. To stop the flow, consideration was given to waterproofing. In 1959, S. K. Shanshiev [6] proposed a type of nonreinforced lining, based on results of studies by TNISG~ I [8,9]. The main advantage of such linings is the lack of restrictions with regard to "resistance to cracking" of concrete and transfer of internal pressure to the surrounding rock.Notwithstanding the economy and simplicity of construction, adaptation of the "cracked lining" design is being held back by lack of proper design methods and specifications.The following recommendations have been worked out at TNISG~I:1. In design of pressure tunnels a type of lining permitting formation of cracks may be considered under the following conditions: a) the rock is isotropic and uniform (either monolithic or with slight blocking), with a specific reaction k0 not less than 100 kg/cmZ; b) rock pressure on the lining is insignificant; c) yearly average composition of the water flowing through the tunnel satisfies the standards for aggressive action of waters on hydraulic structures (SN 249-63).2. Conforming with the basic functions of the lining (to limit water losses and reduce the coefficient of friction),the lining is not counted to resist internal pressure. The size of the cracks is computed by the method explained below and the computed results are within the allowable experimental error.3. The permissible size of cracks is determined by the least value obtained from the following three conditions: a) durability of the lining; b) minimum loss of water, based on economic considerations; c) stability of the surrounding rock and slopes at end of tunnel.4. At the present there are no definite specifications for limit of cracking to ensure durability of linings .of plain or reinforced concrete. The standards of permit cracking only in elements of hydraulic structures which are simultaneously subjected to both positive and negative (tensile and compressive) stresses, whereas linings of pressure tunnels are subjected only to te...

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The new standards specification has been compiled to replace the construction norms 55-59 and to supplement the construction norms and specification in special sections. In connection with the fact that each of these documents has remained in force, unmodified, for at lest 10 years and their contents are to some extent traditional, a critical revision is essential, obsoletestatements being replaced and brought up to date.In the present article we consider certain sections of the projected ~Instructions" and suggest improvements in these important standards. Individual problems have akeady been discussed [1-3].1. Point 1.3 of the projected "Instructions ~ is intended only for structures of class IV strength: for structures of classes I and II strength, however, and, in certain cases class-III structures also, individual technical restrictions must be compiled. We think it expedient to extend the applicability of the ~Instructions" so that they contain only a compilation of the individual technical restrictions for class-I structures; for structures of class-II, and in special eases for class-ilI structures also, asupplement must be provided.2. In point 2.3 the concrete strength for axialtension mustbe given in round figures, at intervals of 5 kg/cm z, i.e., 10,15,20, 25, 30, 35, and 40, The strength of prestressed concrete represents an independent characteristic, and in parts of structures designed on the basis of tensile strength of the concrete, it must be chosen independently of the type of compression concrete.3. The coefficients of the working conditions, which are connected with the level of the production process (Point 3.3b), rcu~t be subdivided according to the conl~olled values of the strength variation coefficients.A great many laboratory data and industrial parameters prove that in an operative accelerated quality control, the modern mean level of concrete-work industrialization enables us to achieve values of the variation coefficient of cv = 0.15. In this case we must take mb= 1 and the coefficient of homogeneity equal to 0.55. When the level of the production process is higher and Cv= 0.10-0.12, which corresponds to a homogeneity coefficient of 0.6-0.65, we can take mb=l.2. Finally, when the production is not well organized and cv=0.18-0.25, which corresponds to a homogeneity coefficient of 0.45-0.50, the coefficient mb of the working conditions must be taken at 0.8.Here it is opportune to repeat the suggestions [1 and 2] on the differentiation of the homogeneity coefficients according to the significance of the complex and class of structures by altering the number of deviation standards.