The quality of the backfill applied at a shield driven tunnel is of significant importance failure of the lining. The used material also affects the interplay between support and excavation behaviour.
In this research the state of the actual bedding situation is investigated by an analytical approach, laboratory tests and numerical simulations. The numerical simulation using continuum methods have shown shortcomings. These shortcomings regarding the instability of large strain simulations restrict the possibility to reproduce the relocation of the backfilling material. The performed scaled model tests have shown that the regripping process of a double shielded tunnel boring machine affects the soil failure within the gap. With the analytical results the influence of the wall friction can be predicted. Therefore the angle of the failure surface can be determined. The results have shown that the wall friction significantly increases the failure angle.
For tunnel boring machines (TBM) with single (SM) or double (DSM) shield, precast lining segments are used as rock support. These segmental linings are installed within the protection of the TBM shield. In order to reduce the friction while advancing, the shield diameter is smaller than the diameter of the cutter head and therefore also smaller than the excavation boundary. The difference depends on the expected radial convergences. Since the lining segments are mounted within the shield the steering gap between the shield and the excavation boundary increases and is hereinafter called annular gap.
In the process of tunneling in hard rock, the annular gap is usually filled with fine grained and closely graded gravel, termed as pea gravel. The backfilling process starts as soon as possible after the shield tail passes the lining. Due to the operational procedure within the working area of a TBM, a fully backfilled annular gap might not be established after every ring closure. This leads to an unfavorable distribution of the pea gravel leaving the segmental lining partially without bedding. Segmental linings can develop sufficient resistance only when properly bedded.
The rapid advance of the shield tail of a DSM after a boring stroke has been completed increases the non-bedded area by removing the abutment of the backfilled material in the annular gap. This leads to shear failure of the pea gravel and to its relocation within the annular gap. Therefore, the position of the backfill within the annular gap is unknown and can be estimated only. The shape of the failure mechanism is approximated by the dumping angle of gravel.
