Rock Mass Classification of Chalk Marl in the UK Channel Tunnels Using Q

Abstract

The Q-system for rockmass, core and tunnel logging was used extensively in the TransManche Link TBM tunnelling, and also in a consortium claim against owner Eurotunnel in the early 1990s. In this connection, the Q-system was also used by the first author for logging precedent conditions in local tunnels in chalk marl, and by the construction consortium on a continuous basis in the TBM service and running tunnels. Although chalk marl is nearly at the weakest end of the strength spectrum for rock (mean UCS ≈ 6 MPa), its distinctly bedded and jointed nature makes it quite amenable to classification by rock mass quality descriptors, such as the Q-system. The steeply dipping jointing and sub-horizontal bedding was Q-logged and photographed in the partly flooded Beaumont (Abbots Cliff) TBM tunnel, and in the Terlingham water tunnel, prior to Q-analysis of core logs and core box photographs from the PB series of eight marine core drillings. Mean Q-values were 3.4, 10.6 and 12.6 respectively. The Grey Chalk seen in the cliff exposures at Shakespeare indicated Q-values in the range 4–33. Jointing appears to have been similar in the significantly weaker underlying chalk marl, where permeabilities of about 1–20 Lugeons in an otherwise very impermeable matrix also indicated the presence of extensive jointing. The jointed and bedded nature of chalk marl, as experienced in the Beaumont, Terlingham and Channel Tunnels, resulted in a lot of distinctly discontinuous as opposed to continuum behaviour. Overbreak was marked where joint sets, bedding joints and an unfavourable tunnel direction combined to give the necessary degrees of freedom for block release. The inevitability of block release problems was increased by the relatively smooth and planar character of the joints and by the destabilising effect of high pore pressures in the case of the sub-sea sections of the Channel Tunnel having low rock cover and higher connectivity.