Combined effect of high concentrated loads exerted by TBM hydraulic jacks

During tunnel boring machine (TBM) operations, thrust jacks apply high concentrated forces to the last assembled tunnel ring. These loads generate in the precast segments a disturbed region under the thrust jacks, in which significant tensile stresses (defined as splitting or bursting stresses) occur perpendicular to the load direction.
In addition, due to the compatibility requirement, the combined effect of TBM high concentrated loads may determine high tensile stresses in the area between loads, which lead to cracks defined as spalling cracks. Therefore, rebars or fibre reinforcement are required in these areas. Traditionally, steel fibres were used as fibre reinforcement, although now some types of structural macro-synthetic fibres are used that impart significant toughness and ductility to concrete. In the present study, eight plane specimens were tested under two loads applied on a small area to evaluate the effect of this loading configuration and the contribution of polypropylene (PP) fibre reinforcement in controlling both spalling and splitting phenomena. The results show that failure is mainly governed by splitting phenomena, and that PP fibres could be efficiently used in precast tunnel segments as both equilibrium (splitting) and compatibility (spalling) reinforcement.