The design of deep excavation pits in rock mass is dependent as well on the mechanical properties of the rock mass as for example the orientation and the type of discontinuities like bedding planes, joints and fissures, the length of the discontinuities (persistence), the grade of weathering, the strength of rock mass, and the shear strength in the discontinuities, etc., and also on the respective boundary conditions specific for the project, and here especially the geometrical boundary conditions.

The basic mutual influences and interactions between these boundary conditions are explained and exemplary presented at an excavation pit with a depth of 18,5 m, situated at a slope with a surface near groundwater table.

Resume

La planification des excavations profondes depend aussi bien de la qualite mecanique du roc, par exemple la location de la texture de la surface de moindre resistance, le degre de rupture, le degre de decomposition, la durete du roc et la resistance limitee au cisaillement de la texture de la surface de moindre resistance que des conditions limitees specifiques pour le projet, ici en particulier des conditions limitees geometriques.

Les influences et interactions essentielles et mutuelles entre les conditions limitees isolees seront expliquees et presentees à l'aide d'une excavation 18,5 m de profondeur, situee sur une pente où le niveau des eaux souterraines est près de la surface.

Kurzfassung

Die Planung tiefer Baugruben im Fels hangt sowohl von den mechanischen Eigenschaften des Fels wie z.B. der raumlichen Lage des Trennflachengefueges, dem Durchtrennungsgrad, dem Verwitterungsgrad, der Gesteinsfestigkeit sowie der Scherfestigkeit des Trennflachengefueges als auch von den projektspezifischen Randbedingungen hier insbesondere von den geometrischen Randbedingungen ab.

Die wesentlichen gegenseitigen Einfluesse und Interaktionen zwischen den einzelnen Randbedingungen werden aufgezeigt und an einer 18,5 m tiefen Baugrube in Hanglage mit oberflachennahem Grundwasser vorgestellt.

Introduction

In Kronberg / Taunus in the north western of Frankfurt am Main in Germany, an office building with an area of approx. 4.000 m2 and three subfloor levels used as a parking garage has been constructed. For the construction of the subfloor levels a deep excavation pit had to be built.

Within the soil investigation program 6 rotary borings have been executed. The positions of the drilling locations are shown in figure 1.

(Figure in full paper)

The subsoil on the site can be described as the following. The Top soilconsists of silty and clayey sand with organic components and partly stones and has a thickness between 0,1 m and 0,3 m. Partly Fillwith a thickness between 0,3 m and 1,3 m was found which consists of silty and clayey gravel with rests of brick stones. Under the top soil there is heterogeneous Talus materialwhich is between sandy clay with a few stones and clayey gravel with many stones and has a thickness between 0,25 m and 1,60 m. After a zone of Weak rockwith a thickness between 0,70 m and 3,4 m there is Sericite gneiss and partly Green schistin different grades.

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