Anisotropie des Eigenspannungszustandes der wechsellagernden Locker- und Festgesteinsschichten des Frankfurter Raumes

Summary

In the Frankfurt area several building projects are carried out, of which the subsoil is influenced to a depth. of 40 m and more. The so caused deformations which lead to Settlements at the surface depend on the stresses existing before construction as well as on additional stresses. The same applies to tunnel structures.

As the states of stress depend on depth, the knowledge about the anisotropic State of in-situ stresses is important for exact numeric investigations. The examination of the anisotropic State of in-situ stress was the aim of this research project to which soil mechanical and geological investigations were applied.

Field and laboratory investigations were carried out. In boreholes, reaching a depth of 4 0 m, the horizontal earth pressures at rest σ_h’ were directly measured using a pressuremeter. As the corresponding vertical stresses σ_v’ = ϒ’.t are known, the coefficient of earth pressure at rest MATH TYPE could be calculated in dependence on the depth t (fig. 2). The resulting dependence of K_O = f(t) on depth with values of 1 to 3 near the surface could be confirmed by oedometric tests on specimens from different depths. By these oedometric tests the overconsolidation ratio OCR was determined in dependence on the depth. A well-known empirical relation between OCR and K_O allowed to determine K_O independently once more and to compare it with the KQ-values obtained from pressuremeter tests (fig. 1, 2, 8). The correspondence between these K_O-values was quite satisfactory. Moreover the correspondence between them and the results of geological investigations was satisfactory to the degree which can be expected on such investigations.

The area of Frankfurt is appropriate for the program because of a sequence up to 300 m thickness, slightly faulted and consisting of Tertiary clays, silts, sandstones and limestones is exposed, dipping 3° towards NW (fig. 3, 4). Therefore, a maximum vertical load during the geological past of 50 to 200 m according to the respective location within the city can be derived (fig. 10, 11). The directions of the principal stresses are permanently maintained vertical or horizontal, respectively. Some open joints in clays and open bedding planes encountered during excavation of an underground tunnel indicate locally reduced horizontal stresses. In such areas also increased Settlements caused by tunneling are recorded.

At Frankfurt the KQ-values are distinctly lower than those determined in the London Clay with K_O = 3 − 4 close to the surface. For a comparison clay mineralogical analyses were performed. Characteristic for the Frank furt area is a frequently changing and in the Miocene often high content of swelling clay minerals up to 70 percent related to the grain size 2μm