N umerous tunnels constructed in Brazil during the last four decades have used shotcrete for final lining, substantially reducing costs and construction time. Although specifications were less exacting in the past, it is worth noting that this type of lining has performed adequately. There have been no reports of structural deterioration despite the high level of reliability required in most cases; including transportation tunnels and underground hydro-electric facilities. Current specifications require more durability, which is an indirect guarantee of long term performance for tunnels with a shotcrete final lining.
‘Tunnelling in Brazil’[1] contains hundreds of Case histories, and a sample of shotcrete-lined tunnels from this book is examined below. Points made mostly relate to certain design procedures and material technology for these shotcrete final linings.
Shotcrete mechanical properties have been recognised as adequate for structural purposes, but durability concerns have proved more difficult to address. Some index properties have been used for this purpose, but objective assessment is still a way off. In addition to material properties, key factors affecting durability include environmental conditions when spraying and during the lining’s service life. Tests are usually carried out at 28 days, less frequently at 90 or 120 days, and in very few cases at ages of a few years. Durability requirements for most projects range from 50-100 years, so extrapolations to time spans of about 200-400 times are usual[2].
Since there is no record of shotcrete-lined tunnels as old as the time span currently required for durability, a large database of case histories is needed to support decisions in relation to future works.
Improved shotcrete technology
Shotcrete was first used for final lining in underground works in Brazil during the 1960s, and more often with the introduction of NATM for metro tunnels in the early 1980s. Material technology requirements were much less exacting than those of today, but the performance of these tunnels has been satisfactory from all points of view.
Typical requirements from that period were described by Figueiredo[3]. Despite high cement content of about 550kg/m3, strength was low with a typical value of 18MPa at 28-days. Durability requirements were limited to water absorption after immersion and boiling in the range of 8%.
Current requirements are more exacting. Strengths of over 30MPa at 28 days are common. Adding metakaolinite has decreased cement content to less than 400kg/m3, depth of water penetration is limited to 50mm and electrical resistivity of 300kO.cm has been achieved. Robot spraying with wet mix shotcrete is usual, as well as the use of alkali free accelerators and superplasticizers. Examples of such requirements were adopted for Tunnel 3, a pair of 4 lane tunnels with 200m2 cross section for the west section of the São Paulo Ring Road[4], and for the Line 2 Extension of the São Paulo Metro.
Materials have improved, as has technique. For manual application, nozzleman certification introduced in 1996 includes examinations for dry-mix shotcrete[5] and standards are being revised to cover wet-mix shotcrete.
Water infiltration
Shotcrete waterproofing properties have been discussed for decades[6]. While not recognised as watertight, its use under moderate pressure or in cases of low-permeability ground mass is advantageous. In São Paulo, single-shell shotcrete tunnels excavated in stiff, occasionally fissured, Tertiary clays have not presented infiltration problems, despite water pressure of 2 – 3 bars. The same is true for properly grouted rock masses, even with higher pressure.
Road tunnels under Ibirapuera Park excavated in stiff clay with sand lenses, for example, with pressure around 2 bars, raised concerns that the water table might be lowered and trees and lawns in the park affected; but over 10 years have gone by and there are no signs of this happening.
For tunnels in water-bearing sands with pressure in the same range, infiltrations have been inconvenient from the standpoint of electrical systems and equipment. However no signs of mechanical deterioration have been detected. Back-analyses of shotcrete permeability were carried out[7], and most values were consistently below 10-10m/s, with one single case of drained lining surpassing 10-9m/s.
Design considerations
Many reports have pointed to the advantages of single-shell linings, with savings in relation to the conventional double-shell approach estimated at 10-15% for tunnels constructed in Germany under moderate water pressure[8]. Savings would be even more significant if more flexible design philosophy were to be used, according to those authors, who suggested evaluating the effectiveness of the second shotcrete layer using lower strength parameters for ground mass after installing the second layer. This procedure had been successfully adopted 16 years previously for the design of metro tunnels in São Paulo[9], in which total shotcrete thicknesses were much lower than in the single-shell tunnels reported by Pöttler in Germany. An example of this design procedure used for a double-track tunnel is shown in Figure 1. Total shotcrete thickness is 40cm, 25cm for the first layer (figure 1a). Ground mass strength envelopes for both short term (t=0) and long term (t ‘ 8) are indicated in figure 1b.
Moment-thrust diagrams are shown in figure 1c as well as internal forces for both short and long term conditions, including water pressure at the portal and at the central section of the tunnel.
Case histories
A total of 61 cases are shown in Table 1. Cross sections range from 4m2 upwards; shotcrete thickness ranges from 10-55cm; the total tunnel length is over 115km.
A case worth mentioning is the Paulo Afonso IV powerhouse cavern, originally designed with a 1.5m reinforced cast concrete arch at the vault. Excavation of the 31m span in highly fractured rock turned out to be very difficult, and the design was changed to 0.15m shotcrete at the vault. Savings of US$2M were reported. Another case of conventional reinforced concrete being replaced by fibre-reinforced shotcrete final lining are the 3 pairs of 200m2, 4-lane tunnels for the West Section of the São Paulo Ring Road, where construction time was much reduced.
Conclusions
Final lining shotcrete has been used for decades in Brazil in tunnels requiring high levels of reliability, such as transportation tunnels and underground powerhouses. Design procedures have resulted in shotcrete thicknesses lower than similar tunnels elsewhere. Examples include both rock and soft ground tunnel projects.
The outcome in terms of reduced cost and construction time has been very satisfactory.
Shotcrete lining thickness for a double track tunnel (9) in Sao Paulo
