Small-scale changes of different rock types with different mechanical properties or of different degrees of fracturing are typical especially in sedimentary and metamorphic rock masses. In TBM tunneling, these conditions may pose mixed face conditions with different excavability at the same face. Particularly with large diameter TBMs, these conditions affect the speed of advance and the tool wear. This paper discusses cases from actual projects, offers a new definition for Mixed Face Conditions and suggest their risk management.
Deere (1981) described the ideal rock mass properties for TBM drives: moderate strength rock, homogeneous, little abrasive, unweathered, few discontinuities, no water ingress and no gas. The reality in hard rock TBM tunneling is, however, often very different. By cumbersome manual labor, small tunnels are driven at the roof to rescue a stuck TBM in squeezing rock. Extensive works such as filling of voids, grout—^injections or the installation of umbrella arches to allow for machine advance under adverse conditions are common. Long contractual disputes to overcome problems with advance rates or tool wear dominate the daily work in many projects.
The above-mentioned problems are take place at the interaction between the TBM and the rock mass (Fig. 1). In central Europe the owner's risk (rock mass) and the contractor (choice of equipment for advance) share this interaction. Some rock mass properties may be appropriately described and are mutually understood, e.g. intact rock strength or rock abrasivity (as defined by Cerchar CAI index). Then again there are phenomena such as "mixed face conditions" or "blocky conditions" which are difficult to define and even more difficult to quantify.
Mixed face condition (MFC) has first been described for TBM advances in soils. The presence of cohesive and non-cohesive soil at the face or the occurrence of soil and rock at the face have been titled MFC by Fowel and Johnson (1981). Since at least Büchi (1992) defined mixed face condition being a face with different boreable lithologies. This mixed face condition is not defined and not treated within German standards. The Austrian standard for underground works for TBM tunneling (ÖNORM B2203–2) describes mixed face condition as the simultaneous occurrence of continuous strata with very different excavability.