Design of concrete linings of pressure tunnels with allowance for minimal cracking

Abstract

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...

Consideration of groundwater pressure on linings of pressure tunnels and shafts

“…b) If we take into account the seepage resistance of the rock surrounding the tunnel then, following the course of calculation analogous to that used in the literature [11][12][13], we can write the following equation for determining the seepage discharge q through the lining per cm of tunnel length:…”

“…According to Verbetskii's formula with a crack in the concrete 6 _< 0.02 cm the coefficient of impermeability kf (equal to the discharge of water through 1 cm of the width of the crack over the flow front with a linear law of resistance and pressure gradient equal to unity) is determined according to [12,13] kf = (10~) 3, cm3( see "cm);…”

“…Grotmdwaters Present. The seepage from the rock into a unit length (1 cm) of the tunnel is determined from equations in the literature [11][12][13] as…”

Discussion of construction standards

“…The most advanced and simplest of all known calculation procedures for crack opening is the Verbetskii method [2]. The maximum crack opening at.…”