HYBRID DESIGN ROCK SUPPORT | GROUND SUPPORT
It should be noted that the main goal of this hybrid
methodology has been to provide a complementary tool for ground characterisation of layered rocks and the assessment of permanent rock support design. Practitioners should, therefore, be aware that further elaborated design work may be needed to derive final, detailed designs. As observed, a hybrid approach still contains
an empirical essence since part of the rock mass characterisation and support assessments are based on the application of several rock mass classifications. Although a detailed study with such an analytical approach would require more specific information on, among other parameters, the joint spacing, rock stiffness, and the level of horizontal confinement, simple analyses using the Voussoir beam model can still help to predict roof stability and the needs of support on the basis of roof -deflection and -loading. In light of this discussion, it is obvious that the type
and amount of rock mechanical information needed for more elaborated analyses with such a hybrid procedure may theoretically place the use of such hybrid procedure more towards detailed design and/or the construction stages of a tunnel project. However, the structure of this procedure is conceived for use at any stage of tunnel construction as more rock information is gained. In looking to design optimisation, the results obtained
for models A2 and B2 with the application of a hybrid methodology, combined with UDEC calculations, show that such procedure can contribute to reduced amounts of support without compromising tunnel stability. Use of a hybrid approach demands more knowledge of the rock mass conditions and ground response which may appear, a priori, more expensive and time consuming, but it would be quickly compensated, as extra costs, possible delays, and safety concerns related to tunnel instabilities can be significantly reduced. There was an overall optimisation in the use of bolts
of different lengths, with combinations of 4m and 3m bolts in the hybrid designs providing a notable reduction in rock support consumption when compared to empirical recommendation with 5m and 4m bolts. Put in an economic perspective, in terms of 2022 prices for tunnel construction in Norway, a saving of about Euro 300,000 may have been achievable by the approach in only 175m length of tunnel. Savings of time and carbon were not considered in the analysis. Although such hybrid methodology has been proven
rather effective to predict ground behaviour and support design in layered rocks, some additional remarks should be given to practitioners intending to use this hybrid methodology approach in other projects. Note that the methodology should be only used in underground rock conditions comparable to those studied in this article.
10 CONCLUSIONS The current empirical approaches in Norway for design of rock support has been investigated in layered rock masses of poor rock quality (Q < 1) and subject to low horizontal confinement of the new Skarvberg tunnel.
Above, figure 8: UDEC model A1 with empirical support at Profile 3+162. a Ground behaviour of the supported and failed tunnel roof, with plot of total vertical displacements, bending moments on support and bolt axial loading, and b Plot of stress contours for major principal stresses. Horizontal and vertical model scale in (m×10)
Above, figure 9: New Crown (C)-Si-RRS 30/6 arches in a D-shaped tunnel, proposed for hard and horizontally layered rock masses of quality Q 0.3–0.4 and subject to low-moderate horizontal confinement
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