EXPERIMENTAL RESEARCH FOR THE LINERS OF THE SWISSMETRO TUNNELS
Swissmetro is an innovative concept for a very high-speed underground passenger transportation system. One of its features is that the small diameter tunnels are under permanent partial vacuum. This paper presents the results of an experimental research program conducted on the behavior of concrete liners subjected to service conditions of Swissmetro tunnels. In one series of tests air flow was measured through cracked concrete subjected to a differential of air pressure. In a second series of tests, concrete elements were subjected to a large number of cycles of pressure variations under partial vacuum conditions to investigate possible degradation of the concrete surface properties.
Swissmetro - Tests on Air Permeability of Concrete
Air-tightness of the concrete lining is a critical factor for a railway tunnel to be operated under partial vacuum. The testing described in this article investigates the influence on air permeability of aggregate composition (sand content, maximum grain size), type of cement, addition of steel fibers, degree of saturation of the concrete, and use of a curing compound. Laboratory testing was performed on samples taken from 18 concrete wall elements representing a range of pumped concrete mixtures. Fresh-concrete properties, compressive strength, and porosity were also measured. This report handles the first 12 concrete mixtures. Evaluation of the remaining testing is still in progress.
Air Leakage Characteristics in Reinforced Concrete
Eight reinforced concrete specimens were fabricated and subjected to tensile membrane forces and air pressure to study the air leakage characteristics in cracked reinforced concrete members. A mathematical expression for the rate of pressurized air flowing through an idealized crack is presented. The mathematical expression is refined by using the experimental data to describe the air flow rate through any given crack pattern. Graphical charts are also presented for the calculation of the air leakage rate through concrete cracks. The concept of equivalent crack width for a given crack pattern is introduced. The mathematical expression and graphical charts are modified to include this equivalent crack width concept. The proposed technique is applicable for the prediction of the leakage from concrete containment structures or any similar structures due to high internal pressure sufficient to initiate cracking.