7th INTERNATIONAL SYMPOSIUM ON SPRAYED CONCRETE – Modern Use of Wet
Mix Sprayed Concrete for Underground Support
– Sandefjord, Norway, 16. – 19. June 2014
Q-SYSTEM APPLICATION IN NMT AND NATM AND THE CONSEQUENCES OF OVERBREAK
Nick Barton and Eystein Grimstad
NB&A, Høvik, Norway and Geolog Eystein Grimstad, Oslo, Norway
nickrbarton@hotmail.com and eystein.grimstad@vikenfiber.no
SUMMARY
The Q-system of rock mass classification for assisting in support and reinforcement selection for rock tunnels and caverns has now been in use for 40 years. During the last 20 years it has been used in order to assist in the choice of permanent single-shell fiber-reinforced S(fr) support and systematic corrosion protected rock bolt reinforcement. Twenty years ago the original S(mr) mesh reinforced recommendations were updated to fiber-reinforced shotcrete, in order to reflect the by then more than ten years of experience of wet process, robotically-applied S(fr) in Norway. This revolutionary product now has a 35 years track record. The Q-system is also used to select rib-reinforced shotcrete arches (RRS) which are superior to steel arches and lattice girders, because intimate contact with the tunnel arch and wall, and systematic bolting of these arches are integral and essential components of the method. The bolted RRS arches therefore help to prevent further deformation instead of allowing it as in NATM, which is a labour- intensive method which does not address these two problems adequately. In this paper some of the other differences between single-shell and double-shell tunnelling will be emphasised, including the frequent use of Q to select only the temporary support and reinforcement in double-shell tunnelling, using the 5Q and 1.5 ESR rule-of-thumb. Hong Kong road and metro authorities have applied this method in the last 25 years in hundreds of kilometres of tunnels and in station caverns. The B+S(fr) applied in such cases as the first stage of double-shell NATM, is considered as temporary support and reinforcement, prior to casting the final concrete lining with its drainage fleece and membrane. The temporary support is ignored in the final lining design. This of course is wasteful and adds to the cost. This paper also briefly addresses some of the useful Q-correlations to rock engineering parameters such as P-wave velocity, deformation modulus, and tunnel and cavern deformation. All are depth or stress-dependent.
INTRODUCTION
The first wet-process fiber-reinforced shotcrete applied in Norway was in a hydropower cavern in 1979 and in a main road tunnel in 1981. Mesh-reinforced shotcrete S(mr) ceased to be used by about 1983. The Q-system development in 1974 [1], which was first based on B+S(mr), was updated ‘late’ in Norway [2] by Grimstad and Barton, but obviously ‘early’ for many other countries. In Austria, B+S(mr)+lattice girders are seemingly still favoured as temporary support for transport tunnels. The writers have been surprised to see Austrian consultants continued recommendation of S(mr) in good quality but over-breaking rock in Asia. However the very strange and diametrically-incorrect instructions are given to accept the use of S(fr) when there is no significant overbreak, but to use S(mr) where there is overbreak. As we shall see in a moment, overbreak is of course a fundamental geometric factor that increases the volume of S(fr) in both single-shell and double-shell tunnelling, and can have quite adverse influence in the latter concerning concrete volumes and the need to construct a more time-consuming three- dimensional membrane and drainage fleece. In many Scandinavian tunnels and in many of the world’s much larger hydropower caverns, single-shell and Q-system based methods are preferred due to their economy. However, thorough jet-cleaning of the rock surface prior to shotcreting, and use of good quality and corrosion protected B+S(fr) are obviously essential.
THE ADVANTAGE OF A LOGARITHMIC QUALITY SCALE
Unlike RMR or GSI (= RMR-5) and the Austrian F1 to F7 rock mass quality scale, the Q-value resembles a logarithmic scale of quality with its six orders of magnitude from approximately 10-3 to 103. With the normalization Qc = Q x UCS/100 described in [3], the Qc scale can reach almost eight orders of magnitude, and then approaches the actual variability found in nature. One only needs to consider the range of deformation moduli and shear strengths depicted in the deliberately contrasted photographs in Figure 1 to realise that not only these parameters, but also the need for support and the loading of the support (i.e. Sfr) can cover an extremely big range: from zero up to 100 t/m2. As will be seen later, there is a clear inverse proportionality between support pressure/capacity needs, and the simply estimated deformation modulus, which itself can vary by a factor of 100, or even 1000, between the extremes of saprolite/soil and hard unjointed rock. The non-linearity of nature does not link in a simple way to linear (RMR or GSI) qualities.
7th INTERNATIONAL SYMPOSIUM ON SPRAYED CONCRETE, Norway, 16. – 19. June 2014
