INTRODUCTION

Within the scope of the engineering geological evaluation of the area for the water- supply gallery line for the city of Brno geophysical methods were employed to verify weak zones.

GEOLOGY

The line of the water-supply gallery is 15 km long and is designed to be situated at the depth of approximately 100 m in the metamorphic rocks that belong to the Moravian- Silesian crystalline part of the Bohemian Massif in the central part of Moravia - Fig. 1.

The tectonic pattern of the whole area is very complicated. Although for some mineral accumulations of Ag-Pb-Zn and Cu ores it has attracted attention since as early as the Middle Ages, the problems of its origin and structure have not yet been reliably solved.

For this reason only general geological maps at the scales of 1: 50 000 and 1: 25 000 were available. These maps do not contain complete data concerning the tectonic structure of the region or of the weak zonesand offered a survey of the fundamental rock types only (Frejvald 1965, MÍsaŕ 1969).

Therefore, the engineering geological mapping was carried out once again over the entire line of 15 km in a 500 m wide strip. The principle of the area convenience classification from the point of view of driving the gallery, i.e. the excavation

  1. Variscian syenite

  2. pre-Cambrian granodiorite and diorite

  3. orthogneiss

  4. paragneiss and m'gmatite

  5. two-mica schists

  6. green schists

  7. the border between metamorphic zones

  8. the border between regional units

  9. faults

of rocks were observed. The tectonic analysis was performed as a part of the geological mapping. The information obtained from the confrontation of ore veins in abandoned old mines has also been used as well as the river course morphology.

(Figure in full paper)

(Figure in full paper)

Two-mica orthogneiss is present in the largest part of the area of our interest. Its texture is granolepidoblastic, the structure is parallel. It consists of quartz, plagioclase albite, orthoclase or microcline, biotite and muscovite.

Two-mica schist is the second most frequent rock type. It exhibits the lepidoblastic texture. Its schistosity is very intensive intensive. It consists of quartz, muscovite, biotite, orthoclase, plagioclase, chloritized garnet, apatite and zircon being accessory minerals.

Gneissose granite with garnet occurs at the contact of orthogneiss with mica schists. They are of the granolepidoblastic texture or even of the porphyroblastic one.

Amphibolites represent another rock type. They are greenish-black, of the granonematoblastic texture and parallel structure. They consist of common amphibole, which sometimes encompasses andesine. Biotite and garnet are less frequent. Ore minerals are represented by magnetite and pyrrhotine.

The regional metamorphosis of these rock formations is pre-Devonian and it may correspond to the so called paleo- Moravian phase. The Variscian metamor phosis is periplutonic, retrograde, responsible for the origin of weak zones. The tectonic faulting has manifested itself as a multiple repetition of the lenticular structure of crystalline limestones along strike faults.

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