ABSTRACT
The support of modern tunnelling during the excavation is obtained either by sprayed concrete or prefabricated tunnel lining elements. In Switzerland one now requires a planned service life of new constructed tunnels of 100 years. The important technical developments in these two fields will be discussed in this paper.
Sprayed ConcreteSprayed concrete is an important component of modern underground construction and tunnelling activities. In order to fulfil this role properly, it must comply with defined specifications. It is used today as a temporary or permanent rock- and ground-support after drill and blast-, roadheader- or even TBM-excavation. More and more sprayed concrete is used as a final lining. Apart from the increased demands made on quality and durability, economical (output capacity, rebound) and ecological aspects (dust, alkalinity of setting accelerators) have an increasing role to play.
The wet process sprayed concrete has been established on all the major job sites in Switzerland. Regarding the technical and ecological aspects, the introduction of an alkali free non-toxic accelerator has set up a new standard:
In Concrete Technology
Enormous high early strengths in combination with strength gains compared to the reference concrete can be achieved.
Extremely high impermeable sprayed concrete can be obtained in combination with a reduction of the shrinkage behaviour compared to standard sprayed concrete.
In Economy
The high strength impermeable sprayed concrete quality and the much reduced rebound allow optimum economic thickness of the sprayed concrete lining. Due to the high earlystrengths the output capacity can be increased.
In Ecology
No caustic water, mist or dust is generated in the tunnel. The non-toxic accelerator has no hidden health hazards for users during storage, transportation, handling and the spraying application.
290Beside the cement particles in the rebound no additional alkalinity is obtained from the accelerator, so that rebound can be deposited like ordinary rebound from non-accelerated sprayed concrete.
Tunnel Lining ElementsNormally tunnel lining elements are produced in a precast concrete factory on site of the tunnel under construction. The concrete mix shall be designed to allow the continuos production of a consistent concrete quality on site without requiring excessive additional measures. The requirements for the properties of the hardened concrete shall account for the nature of the chemical aggressors identified by trial borings as well as for the degree of importance of the different structural parts in the project. High early strengths (4-4.5h: >15Mpa) shall allow economical production of precast elements. Testing of sulphate resistance shall account for the characteristics of underground water in Switzerland as well as for the high service life expectations for the projects. Immediately after demoulding the concrete lining elements shall be coated with a water-based epoxy resin as a curing agent and at the same time as a barrier to aggressive agents from the ground water. Case studies from major job sites will be discussed.
