HYBRID DESIGN ROCK SUPPORT | GROUND SUPPORT
suggest that Q < 0.1 may behave more isotropically as a result of denser jointing. Therefore, several rock mechanical parameters and
geomechanical systems should be employed when performing a classification of LGBT. The LGBC is similarly built from civil rock tunnelling
experience where in-situ rock stresses were not especially high to create stability problems related to brittle failure of rock. The classification is, therefore, limited to ground conditions subject to low-moderate stresses, i.e., in-situ stresses not more than about 10 MPa. As observed in Table 2, the LGBC is divided into three
main vertical blocks - ‘Ground Conditions’, ‘Ground Behaviour’ (or failure mode), and ‘Support Loading’ (or design considerations). Each of the six different LGBT categories is defined
by a set of characteristics found in the corresponding horizontal rows for each category. The categories are grouped into two subgroups, ‘a’ and ‘b’, which describe two different confinement conditions: ‘Low’ for rock masses subject to low in-situ stresses (maximum horizontal stress typically below 5 MPa) with a horizontal to vertical stress ratio and k0
< 1;
and, ‘Moderate’ representing rock masses where the maximum horizontal stress is in the order of 5 MPa–10 MPa, and a horizontal to vertical stress ratio k0
> 1. For the former, ‘a’, subgroup the rock masses present
challenges to the arch and often lead to a higher propensity for tunnel instabilities and a greater need of tunnel support. On the other hand, subcategory ‘b’ is where the horizontal confinement contributes to rock arching and the rock mass can take a greater share of the ground load. In the absence of stress measurements, best
estimates can be based on overburden, topography, rock mass structure, and the geological/tectonic context of
the site. This can allow application of the LGBC early on a project and interpretations are then update as excavations proceed. The categories I, II and III represent decreasing joint
spacing (relative to tunnel span) and quality of joint surfaces, i.e., declining rock mass quality. Categorisation between I, II and III is primarily based on tunnel span to bed thickness ratio. Figure. 5 shows failure mechanisms that may
arise upon excavation for different categories and subgroups. The cross-joints are truncated at almost every bedding plane. The extent, geometry, and location of the loosening zones over the excavation can vary if persistent cross-joints cut both the bedding and the excavation.
5 GROUND CONDITIONS AT KEY PROFILES Roof failure during the construction of the new Skarvberg tunnel motivated additional site investigations and upgraded tunnel support designs. Site investigations were focused at two locations - Profiles 3+162 and 2+227, respectively.
5.1 Rock Mass Mapping Mapping shows that the rock mass is predominantly layered metasandstones and thin intercalations of metagabbro (Figures. 6 and 7). From face mapping at Profile 3 + 162, the rock mass was characterised by a well-marked and thin lamination, joint surfaces with poor conditions, unstable tunnel abutments with overbreak and flat roofs, and roof delamination. The assessment of RQD was primarily based on measurements in the vertical direction. Cross-joints were also accounted for, through the study of Jv, to adjust and derive a final RQD representative of the overall jointing degree at the face. This resulted in Jv of approx 32 joints/m3
and RQD 10%–20% at profile 3 + 162.
Above, figure 6: Interpretation of the geological conditions at Profile 3 + 162, based on face mapping and a 45° inclined core hole behind the face (3 + 180). Tunnel azimuth ca. 180°
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