Mrazovka tunnel was excavated under one building whose subsurface space was not possible to reinforce against additional settlements induced by digging. In the presented contribution the risk analysis, warning levels and results of deformation measurements are described.

Ľespace souterrain d'un des batiments sous lequel le tunnel Mrazovka a ete creuse n'a pu être renforcee contre l'affaissement supplementaire provoque par l'emplacement du tunnel. Le materiel presente decrit l'analyse des risques, le niveau des limites et les resultats des mesures des deformations.

Der Untergrund eines der Gebaude, unter dem der Tunnel "Mrazovka" ausgeschachtet wurde, war es nicht möglich, gegen nachtragliches Setzen zu versteifen, hervorgerufen durch die Untertunnelung. Im vorliegenden Beitrag sind Gefahrenanalyse, Grenzwerte und Ergebnisse von Deformationsmessungen beschrieben.

Introduction

The Western tube of Mrazovka tunnel passes a reinforced concrete building. This object used to be the chapel of an evangelic. Considering a low thickness of tunnel overlay, the designer was trying to minimize the impact of tunnel excavation to this structure by reinforcement of its sub-basement similarly as in other cases. However, the owner has not permitted the access to his property.

The tunnel face has been already close to the subsoil of the building in question and the construction company refused to accept any risk for logical reasons. The client has approached therefore the company in charge with a suggestion for geotechnical supervision and has asked for performing a detailed geotechnical analysis of existing hazards. He has asked also for a proposal of procedures of passing through the dangerous zone.

The ground plan position of the assessed building with respect to the position of tunnel route is obvious from Figure 1. The tunnel axis passes in the distance of approximately 14 meters from the northeastern corner diagonally to the axis of the tunnel.

(Figure in full paper)

Figure 1: Position of the Chapel with respect to the axis of the tunnel also indicating the orientation of supporting elements of the ceiling structure, main crack direction in the ceiling structure, and inspection monitoring elements

The thickness of the tunnel overburden over the tunnel calotte up to the level of the assumed building foundation base was approximately 15 meters. No significant faults have been found during the site survey performed in March, 1997. The only exception was a manifestation of a single transversal crack in the ceiling in the level of the ground floor. The crack was passing all over the width of the building. The building was found to be in good technical conditions corresponding to its age.

Description of the tunnel and the tunneling procedures

The western tube of the Mrazovka tunnel has a transverse section area of 165 m2. A vertical heading division has been used. The tunneling order was following, at first the right side tunnel, then the left side tunnel and finally the calotte and bottom excavations. The total length of the work queue under standard conditions was 75 m.

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