Rock layering influence on rock mass displacements in tunnelling

Rock layering features, such as foliation or bedding, strongly affect the displacement characteristics and magnitude during tunnel construction. This has been observed in a number of construction sites, where absolute displacement monitoring has been executed. The mechanisms include shearing along the foliation, as well as dilation perpendicular to the foliation. The longitudinal displacement development also depends on the orientation of the tunnel axis in relation to the foliation. It was observed that for perpendicularly striking foliation to the axis, with steeply dipping angle, the displacements mainly occur close to the face and in the case of tunnelling parallel to the foliation; displacements have a larger influence on the tunnel length. Additionally the magnitude and orientation of the displacements significantly change because of the changes in the relative orientation between foliation and tunnel axis. Tunnelling parallel to the strike yields the highest displacement magnitudes. The displacement distribution ahead and behind the face is effected by the excavation direction relative to the dip. A systematic study was conducted, using site data and 2D (UDEC) and 3D (FLAC3D) numerical modelling. The 3D numerical analyses were mainly performed using the continuum ubiquitous joint model and 3DEC was used to evaluate the discrete approach. The simulations generally confirmed the in situ observations, although the ubiquitous joint model displays some shortcomings. The findings allow a better prediction of the behaviour during the design stage. Moreover, understanding the effect of foliation allows the reliable interpretation of monitoring data.