In the course of the design and construction of the Boβler tunnel it is investigated whether and how it is possible to extend the tunnel sections, which are excavated with a tunnel boring machine, in comparison with the lengths determined in the contract. In this connection, the tunnel section located in the formation Aalenium 2 (al2), for which squeezing conditions are expected, is of major importance. Therefore, a vertical exploration shaft and a horizontal exploration gallery have been excavated starting from an intermediate access, in order to explore the layers of the al2 formation. These works have been accompanied by an intensive monitoring program as well as by field and laboratory testing. The results of geotechnical investigations were evaluated with the aid of three-dimensional finite element analyses. On this basis, the rock mechanical parameters of the layers of al2 were reviewed and upgraded. On the basis of the results of corresponding stability analyses for the segmental lining of the tunnel, it can be expected that the tunnel section crossing Aalenium 2 can be driven with a tunnel boring machine, if the thickness of the segments is increased from 45 cm, as planned in other tunnel sections, to 65 cm.
The 8.800 m long Boβlertunnelis part of the new Railway Line from Stuttgart to Munich which is currently under construction.
Starting from the portal Aichelberg the following strata are crossed: Aalenium 1, Aalenium 2 and Bajocium to Callovium of the Brown Jurassic formation as well as rocks of the Upper Jurassic formation. The maximum overburden of the tunnel amounts to ~280m. In the corresponding area the tunnel is located in the Aalenium 2 and 1 of the Brown Jurassic formation (Fig. 1).
In the course of exploration for this project among other data the uniaxial strength of the different rock types was determined (WBI 2010). The corresponding characteristic values are given in Figure 1. If these values are divided by the unit weight of the rock, the height of overburden can be derived for which the vertical pressure equals the uniaxial strength. These heights are represented with red lines in Figure 1. It can be seen, that in the area in which the tunnel runs through the strata of Aalenium 2 the existing overburden of up to 280 m is much larger than the derived value of 120 m. Thus, squeezing rock conditions were expected in this tunnel section.