Abstract

In Italy, during the last decades, most of the in situ stress measurements were carried a shallow depth, in existing geotechnical structures. In these cases, the stress measurements involves concrete or low disturbed rocks material; thus linear, elastic, non-fractured materials. For this reason the stress tests interpretation is straightforward and reliable. In these situations, since the early 90s, the Doorstopper technique became the most used stress measurement technique. It has proven to be effective and cheap. A collection of punctual tests can be interpreted with a suitable numerical model. The numerical model provide the interpolation of the punctual measurements and can give important information about the virgin stress field. After recalling the main features and improvements of the doorstopper method in the last 30 years, this paper illustrates two Italian cases. The first case is the Sparvo Tunnel (Bologna-Firenze highway) where was performed a massive measurement campaign made up of more than six hundred Doorstopper tests in two adjacent tunnels; the results of the two tunnel are compared; the 2d FEM model interpretation is discussed. The second case is a marble quarry in Alta Carnia (Udine): the Doorstopper and stress variation measurements were carried out and interpreted by means of a 2d FEM model, resulting in the definition of the natural stress field.

Introduction

Achieving a satisfactory knowledge of the in-situ virgin stress field may be still today a troublesome and expensive goal. This is particularly noticeable in complex rock formations as pelitic flyshes and highly tectonized clay-stones, where rock stress measurements are often unreliable, even no-feasible.

Also in "easier" rocks, the natural stress field is non-uniform in the domain of the underground excavation. For example: along a tunnel axis the natural stress varies due to overburden and lithological changes; in a cortical marble quarry chamber, the natural stress field varies with the distance from the slope.

In cases like these, a satisfactory knowledge of the virgin stress field can be impossible for economical and timing reasons, because it would require many punctual stress measurements.

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