GROUND SUPPORT | HYBRID DESIGN ROCK SUPPORT
Table 2: LGBT
confinement Ia IIa
LOW, k0
<1
LOW, k0
<1 IIIa
LOW, k0
<1 Ib IIb
MODERATE, k0
1>=1
MODERATE, k0
1>=1 IIIb
MODERATE, k0
1>=1
CONDITIONS Horizontal
GROUND
Tunnel span/ bed thickness
<25 25-50 Rock mass conditions RQD>40%; GSI>35; RMR>40; Q>0.4
Fair to good, slightly rough, unweathered joint surfaces
RQD 20%-40%; GSI 30-35; RMR 30-40; Q 0.1-0.4
Fair, smooth-slightly rough, partially unweathered joint surfaces
RQD 10%-20%; GSI<30; RMR<35; Q 0.1 >50
Poor, smooth, unweathered joint surfaces
<25 25-50
RQD>40%; GSI>35; RMR>40; Q>0.4 Fair to good, slightly rough, unweathered joint surfaces
RQD 20%-40%; GSI 30-35; RMR 30-40; Q 0.1-0.4
Fair, smooth-slightly rough, partially unweathered joint surfaces
>50
RQD 10%-20%; GSI<30; RMR<35; Q 0.1 Poor, smooth, partially unweathered joint surfaces
Failure of the immediate roof slab; Detachment at bedding plans; Shear along vertical joints at the abutments
Larger done of loosened rock; Gravity failure of the roof slabs through shear along vertical joints
Widened dome of loosened rock can reach verticality ca. ½ x tunnel span; Combination of both shear failure
along vertical joints, and buckling or crushing of roof beds. Block overbreak
Overbreak produced by fall of rock blocks intersecting the excavation contour
Tensile rupture or failure of roof beds as a result of excessive surcharge loads; Block overbreak
along vertical joints, and buckling or crushing of roof beds. Block overbreak
Peaked dome of loosened rock can reach vertically ca. ½ tunnel span; Combination of both shear failure
overlying rock slabs; Immediate bolt support including the abutments to avoid further relaxation
Moderate roof loading from
from overlying rock slabs; Immediate bolt support and lean load bearing support (i.e., lean arches)
Moderate and uneven roof loading
Significant, uneven roof loading from overlying, loose and broken rock slabs; Immediate support with bolts and load bearing support with full arches
Minor/moderate loads from unstable rock blocks with rock mass
considered self-bearing; Bolt to secure unstable blocks
Moderate and uneven roof loading from overlying and broken rock slabs; Immediate bolt and shotcrete support at roof and abutments
Significant, uneven roof loading from overlying, loose and broken rock slabs; Immediate support with bolts and load bearing support of full arches
Above, table 2: Chart for layered ground behaviour classification (LGBC) of layered and hard rock masses into layered ground behaviour type (LGBT). Based on D-shaped tunnels with arched roof and rounded walls. Bedding is assumed horizontal
Buckling (Figure. 4a) and crushing (Figure. 4b) in
multi-layered rock masses would be expected for a high ratio of span to bed thickness, typically greater than 10. If the rock is hard, buckling failure is produced when moment loading exceeds moment capacity of a bed; where rock material is relatively weak, failure by crushing can be initiated at the upper midspan and at the lower support (hinges) by an excess of compressive stress. Both buckling and crushing failures would be accompanied by interbed slip and delamination, causing bed cracking and roof deflection. For low span to bed thickness ratios, however, the
beds are considered thick and shear failure (Figure. 4c), can take place for shear loads in excess of shear resistance at the abutments. A fourth failure type is diagonal tensile rupture (Figure. 4d), which also can occur under low ratios of span to bed thickness.
4.2 Further Parameters Controlling Layered Ground Other parameters like the joint spacing and joint persistence have also a direct influence on the extent of the dome of loosened rock formed above the tunnel, hence on the ground loads. In-situ rock stress also controls ground behaviour since horizontal confinement usually contributes to roof arching and stability. However, such stresses are seldom accounted for in the Voussoir solutions. Similarly, rock mass structure and changes in rock stiffness across the geological sequence can contribute
22 | September 2025
to stress reorientation and fluctuation in magnitude, which can lead to misinterpretation of the stress state. Other important parameters for roof stability analysis are geometry and reinforcement.
4.3 Proposed Classification A classification of ground behaviour for layered ground has been elaborated in Table 2. This proposed Layered Ground Behaviour Classification (LGBC) combines the basic failure modes of jointed rock beams (Figure. 4) and the relevant engineering geological parameters discussed. It has categories of Layered Ground Behaviour Type (Layered GBT, or LGBT). The LGBC aims to supplement a hybrid procedure for
tunnel rock support design in layered and hard rock masses, as proposed by Terron-Almenara et al. (2023). The main purpose of such classification is to better anticipate ground loading conditions (approximate distribution and size). Final, hybrid designs of permanent rock support
should, however, be performed on the basis of an elaborated analysis process. According to Terron- Almenara et al. (2023), such a hybrid procedure should principally be used when rock mass quality is Q < 1 in anisotropic and hard rock masses. Defining a lower boundary may be challenging and use of LGBC not recommended, as rock mass anisotropy is scale- and stress-dependent. As such, the relative size of span to bed thickness along with in-situ stress conditions should be evaluated in each case when Q is very poor. Some authors like Barton (1998) and Brady and Brown (2006)
GROUND BEHAVIOUR & FAILURE MODES
& DESIGN CONSIDERATIONS
SUPPORT LOADING
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