Search

One of the main hazards during tunnel boring machine (TBM) excavation in hard rock is related to the occurrence of unforeseen adverse conditions, leading to unfavourable rock mass behaviour during tunnel boring. In this respect, the continuous analyses of TBM excavation parameters may represent an interesting tool for a continuous monitoring of geotechnical conditions at the tunnel face and for early detection of changing ground conditions in the rock mass. This work provides a back‐analysis of TBM data recorded during the excavation of the Frejus Safety Tunnel, bored across the French‐Italian border in the Western Alps. It is a 13 km‐long tunnel excavated by a 9.4 m diameter, single‐shield TBM through strongly anisotropic calcareous schist, with cover up to 1800 m. Main problems faced during TBM excavation were related to the local occurrence of buckling‐related convergence and consequent asymmetrical loading on the precast segment rings. Correlating TBM performance parameters to the encountered geological and geomechanical conditions provides the possibility to identify which parameters are most affected by geomechanical properties of the rock mass at the tunnel face and by rock mass behaviour. Conversely, the analysis indicates which parameters can be adopted as monitoring tool of ground conditions during TBM advance. In addition, this work highlights how buckling phenomena can be distinguished, in terms of TBM operation and performance, from other adverse conditions as, e.g., blocky ground or squeezing. [ABSTRACT FROM AUTHOR]

Boreholes provided with biostratigraphic data indicate major changes in the Miocene sediment accommodation rates in the northern Campine Block, located in the southern North Sea Basin. Low sediment accommodation rates took place during the early to middle Miocene and abruptly increased into the late Miocene. Two processes likely explain these observations. First, the long‐time (±10 Myr) slow sedimentation of fine sands with a high—in part authigenic—glauconite content (of up to almost 50%) suggests that the southern North Sea area was sediment starved during the early to middle Miocene. Sediment starvation ended with the start of the late Miocene when the area became covered by west‐prograding clinoform sets of coarser sands with mainly reworked glauconite as part of the proto‐Rhine system. The latter sands filled the accommodation space that was left after early to middle Miocene sediment starvation. Second, accelerated subsidence during the late Miocene created additional accommodation space to be filled compared with the early to middle Miocene. A mechanism that explains well the regional accelerated subsidence is lithospheric folding or buckling. The timing and geometry of accelerated rates of subsidence in the southern North Sea area are indeed consistent with deformation that was thought to have been related to lithospheric folding in areas further south (in the direction of the Alps) under NW‐Alpine compression. [ABSTRACT FROM AUTHOR]