Backfill of segmental lining – State of the art, redistribution behaviour of pea gravel, possible improvements / Tübbingbettung – Stand der Technik, Umlagerungsverhalten von Perlkies, Verbesserungspotenzial

Abstract

Backfill of segmental lining -State of the art, redistribution behaviour of pea gravel, possible improvements Tübbingbettung -Stand der Technik, Umlagerungs verhalten von Perlkies, VerbesserungspotenzialWith mechanized shield tunnelling the quality of the backfill is of significant importance for the stability of the lining. The used material affects the interaction between support and rock mass. Therefore, the choice of a proper backfilling material is of great importance for the system behaviour. Scaled model tests have been carried out investigating the relocation behaviour of pea gravel within the annular gap. The tests have shown that especially with Double shielded TBMs a distinct relocation mechanism is triggered by the regripping process. This leads to an unfavourable bedding situation immediately behind the TBM shield. Within a numerical study the influence of an incomplete bedding situation on the section forces within the lining segments has been carried out. The simulations have shown that crack propagation is more likely in unfavourable bedding situations. Nevertheless, numerical simulations have proven that cracks within the segments denote a higher utilization level but do not directly imply an acute danger to the load-bearing capacity. As observed in reality, the state of incomplete bedding represents the relevant load case. A design improvement has been introduced overcoming the temporary state of a partially bedded segmental lining. Furthermore, this approach prevents a large-scale relocation process of pea gravel within the annular gap.

From the geological prognosis to the TBM drive: Planning the main construction lots of the Brenner Base Tunnel

“…without active face support), the following basic considerations and experience should be considered: -The unsupported length is at least the shield length and despite the gentle excavation process, it has to be assumed that each potentially unstable block will actually fail in the shield area. -The pea gravel filling of the annular gap normally leads to a slope in the annular gap as a result of the internal angle of friction and the shear forces between pea gravel, rock mass and precast concrete segments [7]. Above all in the case of the frequently expected discontinuity controlled block failure these construction states either have to be appropriately considered in the design of the segments or in the backfilling concept.…”

Detection of voids in the annular gap using ground penetrating radar and determination of the in‐situ deformation properties of pea gravel

“…Model tests [3] on the relocation behaviour of pea gravel have shown that during the re-gripping process of the shield tail a distinct failure development takes place within the annular gap. Hence, the first ring behind the shield tail experiences an unfavourable bedding situation.…”

“…Since neither a single test procedure nor uniform test specimen dimensions are defined, the results of the differ- [3] ent tests are difficult to compare. Therefore, the use of certain parameters in the design phase in this way must be reviewed critically.…”

Bedding improvement of lining segments using geotextile tubes

“…In order to create an immediate contact between the segmental lining and the rock mass, a concept has been developed that can guarantee an immediate bedding behind the shield tail [4]. In this case, folded geotextile tubes (Bullflex) are attached to the exterior surface of the segments in a slot, in order to allow a damage free passage at the shield tail seal.…”

“…A disadvantage of backfilling with pea gravel is the uneven distribution immediately behind the shield tail. Model tests have shown that a cone forms slightly below the shield tail to the crown ( Figure 1) [4]. This effect is further triggered at double shield TBMs due to the rapid movement of the gripper shield.…”