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Re-Evaluation of an Anchored Slope using a Digital Terrain Model

Abstract:

In the eighties the road tunnel Chlus with an overall length of about 800 m was constructed between Landquart and Davos in Switzerland. It consists of two parts excavated by the traditional mining method and a connection part of 70 m length built as a cut-and-cover tunnel. During construction more than 800 permanent anchors were installed, 86 of which in the surrounding slopes in form of prestressed cable anchors.
The aim of the current investigation was twofold: firstly to apply modern design aids to determine potentially unstable rock blocks, and secondly, based on these findings, to re-evaluate the anchor system for ensuring stability. Preliminary work included high resolution photogrammetric recordings which were digitized in the form of a network of triangles and quadrilaterals, exploration of the various discontinuity sets, their spacing and continuity, and laboratory tests to determine the strength along the discontinuities.
Due to the complex geometry of the slope it was decided to carry out the numerical investigations with the computer program AutoBlock, which runs all computations directly on arbitrarily detailed Digital Terrain Models (DTM). First the digitized surface was imported into AutoBlock and automatically extruded to depict a three-dimensional body, a so called Solid Model. This was then intersected by all possible combinations of the discontinuities encountered, which led to all the kinematically admissible potentially unstable blocks. At this point, based on an engineering validation facilitated by AutoBlock, many of these theoretically possible blocks could be excluded, e.g. rather small blocks were not of interest, rather large blocks were highly improbable. For the remaining blocks the factor of safety (FS) against sliding was automatically determined. The results were discussed by considering the value of FS, the anchors installed during construction, and possibly additional anchors required to reach a stable state. These investigations led to an overview about the safety of the present state and recommendations on where additional anchors were necessary.

Authors:

Fritz, Pit and Hauser, Marc

Index Terms:

slope; stability; rock; CAD; analysis; tunnel; AutoBlock; Informatics; TunnelingGroup; Fritz, Pit; Hauser, Marc

Further Information:

Date published: 2007