ABSTRACT
When tunnelling through low strength ground the stability of the excavation face is a major concern for the geotechnical engineer. In mechanized tunnelling, closed shields are applied for stabilizing the face. In conventional tunnelling, several techniques could be applied: ground reinforcement of the excavation face by bolts, partial excavation, ground improvement by grouting or artificial freezing. Face reinforcement is a popular solution because of its relatively low cost, its great flexibility and its adaptability to local geological conditions. The bolts are installed on the face with a distribution that depends on the stratigraphy and quality of the encountered ground and are removed as the excavation advances. Fiberglass bolts are often employed because-besides having a high tensile strength-they exhibit brittle behaviour, which is favourable for the excavation process.
