GROUND SUPPORT | HYBRID DESIGN ROCK SUPPORT


Table 3: Rock mass


Q-system RMR


class system


Component values


RQD 15


Rock strength 12


Structure GSI


Blocky, seams. Persistence of bedding planes


Jn 9


RQD 3–5


Jr 1


Spacing 5–8


Ja 6


Conditions of joints 10


surfaces and compacted coatings


Joint surface conditions Poor. With weathered


Jw 1


Ground-water 10


SRF 2.5


Adjustment for joint orientation


-10


Q-value 0.1


RMR


30–35 GSI


25–30


SRF = Strength reduction factor in the Q-system Above, table 3: Assessment of component values for geomechanical classifications Q, RMR and GSI at Profile 3 + 162 in Skarvberg tunnel


A core hole was drilled at Profile 3 + 180 with a


length of 18m and at an angle of 45° pointing towards Profile 3 + 162, reaching an area about 12m–13m above the tunnel roof in the section. Both the lithological sequence and rock mass structure logged were projected to Profile 3 + 162 as observed in the logged layers 1 and 2 (Figure. 6) of the metagabbro. In general, the rock mass was characterised by Q 0.1, RMR 30–35, and a GSI 25–30. A summary of the geomechanical classifications is presented in Table 3. The mapping at the second study location, at Profile


2 + 227 (Figure. 7), shows slightly better rock mass conditions. The resulting Jv was approx 22 joints/m3 and RQD 35–45%. The other data were Q 0.3–0.4, RMR 35–40, and GSI 30–35.


5.2 Joint and Rock Properties At the two study locations, three main joint sets were identified - bedding and two sets of cross-joints. Each joint set was characterised on-site with the assessment of the Joint Roughness Coefficient (JRC), Joint Compressive Strength (JCS), the roughness number in the Q-system (Jr


), and measurements of joint spacing


(S) and persistence (P). As shown in Table 4, joint conditions at Profile 3+162 were significantly worser than those at 2+227. Rock testing was performed in samples retrieved


outside the failure zone (Profile 3 + 162), both from core holes and from samples collected at the face during construction. The strength anisotropy was also studied in intact rock samples. With the relatively low strength


Table 4:


Profile 3+162


2+227


Joint set dipdir/ dip (°) S0


310/10


J1 290/80 J2 020/75 S0 340/10 J1 310/70 J2 160/80


JRC0 a


0.5–1.5 1–1.5 1–1.5 1–2 2–4 2–4


aMeasurement of JRC0 using a 100mm profiler comb and based on Barton and Bandis (1990) b


JCS0


b (MPa) 20 25 25


150 170 170


S (m)


0.05–0.1 0.2–0.4 0.2–0.6 0.1–0.3 0.6–1 0.6–2


Measurement of JCS0 using a Schmidt L-hammer and based on ISRM (1978) and Barton and Bandis (1990) Above, table 4: Joint properties assessed for Profiles 3+162 and 2+227 in Skarvberg tunnel (tunnel azimuth 180°)


24 | September 2025


P (m) 10–20 1–10 1–10


10–20 1–10 1–10


anisotropy and the laboratory scale of the samples, the main source of anisotropy was interpreted to be mechanical weakness introduced by the penetrative and weak bedding planes. Density of the rock material was also tested, finding 27 kN/m3 m3


for metasandstone, 31 kN/ for metagabbro.


5.3 In-situ Rock Stresses Estimation of the in-situ stress state for underground works for a particular site should be based on the broader geological and stress context of the region. Bedrock has traditionally been subdivided into


two main tectonic belts in northern Norway, one of Precambrian age with crystalline rocks like gneiss and granites, and another of Cambrian-Silurian age representing fractured metasediments. While relatively large horizontal stresses have been reported in the former, rocks of the Cambrian-Silurian belt have been generally characterised by lower stresses, which applies to the region of Skarvberg tunnel. Both the magnitude and the orientation of horizontal


stresses can be influenced by the rock mass structure and the degree of jointing. If the general lineament patterns in the region of Finnmark are considered, some degree of rotation of the major horizontal stresses (oriented N-S) should be expected to take place and align with the mentioned structures.


5.4 Deformation Monitoring During construction, convergence measurements were taken some tens of metres behind the face.


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