W. Mudrak, H.A. Mang, R. Beer, S. Fuld:
"Standsicherheit von Hohlraumbauten im Gebirge";
Report for Bundesministerium für Bauten und Technik, Straßenforschung, Heft 252; 1984.
The final report which is presented here consists of an experimental part, a part regarding the technology of construction materials, a numerical part, and a report pertaining to organizational and economical problems.The purpose of the experimental investigations was the development, on the basis of the supporting elements (in particular the rock anchors) which are put into place in the framework of the "New Austrian Tunnel Construction Method", and with the help of the technique of the wire strain gauges, of a measuring system which would be available at a very early date (placement of the anchors). It was aimed at not disturbing the construction work, either by the installation of the measuring elements, or by the measuring operations.
The experimental investigations are subdivided in essentially 5 main tasks, namely:
1. The development of the technological prerequisites for the long-time stability of the wire strain gauge measuring point, with due regard to the conditions prevailing at the construction site of a tunnel. This include the technique of the application (the working order of the measuring point must be maintained during the installation of the anchors) as well as the question of the permanent protection of the measuring point against aggressive water.
2. Investigations of the state of the stresses resulting from the anchor forces in the vicinity of the anchors. It was shown that prestressed anchors with free motion contribute from the very beginning to the creation of the vault, whereas mortar anchors act as a slack reinforcement; this is basically also valid for the prestressed mortar anchors.
3. Field measurements with individual anchors (prototypes) and with a permanent measuring section installed in the northern part of the eastern bore of the Selzthal tunnel. The permanent measuring section is in operation since February 1978 and has enabled the observation of the driving of the tunnel roof section, of the excavation at the face, and of the long-term behaviour.
4. Interpretation of the measured values in view of statements regarding the stability of the total system "surroundings of the excavation and built-in supports". The long-term observations showed that the relief creep can be described well with the creep function of the elastico-viscous three-parameter body. The parameters of this model can be determined with the help of the measurements. In particular, the "coefficient of rheology" is very much dependent on the efficacy of the built-in supports. The efficacy of construction measures can therefore be judged qualitatively and assessed quantitatively with the help of the changes which are brought about by these measures.
5. Air-placed concrete - an essential component of the "New Austrian Tunnel Construction Method" - has technological parameters which are of importance for the tunnel lining. It is not always possible, on the construction site, do delimit and define these parameters already in advance. The work which is presented here gives a short survey of their determination; a report is given of laboratory field tests. The values which are specified are primarily the rebound of the air-placed concrete and, in addition to this, the determination of its mechanical properties, in particular the lexural strength and the resistance to extension by splitting. The problem which is stated is supplemented with a survey of the most important literature.
The main part of the numerical investigations which were pursued with the help of the finite element method was the study of the effect of rock anchors. The description of the numerical computations and the explanation of the results are preceded by a detailed description of those aspects of the realization of the project which are linked to electronic data processing. The numerical investigations are based on the photoelastic examination of a disk model with rectangular cavities which are orientated radially away from the opening of the tunnel, and which represent the holes of the anchors. The decisive characteristic of the "New Austrian Tunnel Construction Method" - the creation of "rock vaults" between adjacent anchors - is given, in the case of free motion anchors, a numerical verification. As against this, it is shown that the mortar anchors essentially only act as slack reinforcement. Finally, the study of the influence of an eventual orthotropy of the rock material shows that were the direction of the "rock vault" differs slightly from the direction of the axis of the "soft" material, no self-supporting "rock vault" can come into being.