TECHNICAL | DRILL & BLAST, CONVENTIONAL


thickening SCL layer integrates the drifts and pillar into a continuous structural ring, three rounds behind the active face. This staged approach ensures progressive stabilisation of the rock mass, limits stress concentrations and effectively mitigates the risk of crown collapse, face instability and deep shear failure. For the subsequent section that was more than 70m


Above: Figure 6. Site photographs and numerical model highlighting the progressive development of plastic zone around the excavation with each construction stage.


discontinuities, together with minor water ingress, presented significant swelling potential further exacerbating ground instability risk during excavation. To assess these risks and inform the excavation strategy,


finite element analyses (FEA) were undertaken. Initial incremental deconfinement analyses were performed using the Generalised Hoek-Brown (GHB) and Ubiquitous Joint Model (UJM) constitutive formulations to evaluate the onset of loosening and shear failure during excavation. The analyses indicated early development of plasticity and deep shear failure within the shale, confirming that a full-face top heading excavation would be unstable, necessitating subdivision of the heading. Convergence-confinement analyses were subsequently carried out to calibrate relaxation factors for 2D staged construction models. Further FEAs were used to assess the progressive


development and extent of plastic zones using an isotropic Hoek-Brown formulation. Support elements were checked for axial force and bending moment capacity at early-age SCL strength – 5 MPa at 24 hours – intermediate strength (15 MPa at 10m behind the face) and final top heading completion strength (30 MPa prior to benching). All support elements were confirmed to remain within their design capacity. The top heading breakout for the first 10m of the portal


was executed using a double sidewall drift sequence with a central pillar (see Figure 6). Excavating in staged increments reduces the excavation area, providing better control over deep shear failure and face instability. Drift 2 is then excavated under the same support measures. The central pillar is sequentially removed while maintaining sidewall support. As excavation progresses, a 150mm jointless


in length, where GSI mapped ≤ 15, a single sidewall drift excavation strategy was adopted. The overburden of more than 31m enabled three-dimensional ground arching, providing inherent stability and reducing the likelihood of large-scale face or crown failures. The support system followed the same principles as the initial portal breakout, with modifications: the top heading was excavated and supported in two stages rather than three; and, the pipe roof pre-support was substituted with an R38 Self-Drilling Anchor (SDA) canopy. In the subsequent section, where the GSI mapped ≥20,


a single sidewall drift approach with parallel drive was adopted to enhance production rates. The excavation and support sequence followed the same principles as the preceding section; however, in this case the sidewall was not retained during the rear excavation. Instead, the sidewall was removed as the rear drift excavation progressed and a jointless thickening SCL layer was sprayed to integrate the drifts into continuous structural ring, allowing both Drift 1 and Drift 2 to be excavated and supported simultaneously.


4.0 GFRP PERMANENT ROCK BOLT AND THIN SECONDARY LINING The DVEXT-15 tunnels incorporate permanent-grade GFRP rock bolts, representing a pioneering application in India. Their adoption enabled a reduction in secondary lining thickness from 300mm to 150mm, in competent ground conditions, optimising material usage while maintaining long-term structural performance. The GFRP bolts, designed for a 100-year service life, carry loads from rock blocks and wedges, while the cast-in-place (CIP) secondary lining resists long-term loads including permanent internal fixings and accidental cases. The bolts were specified with stringent performance


criteria, including 32mm solid bars manufactured with E-CR glass fibres (≥ 80%) and vinyl-ester resin, an Ultimate Tensile Strength (UTS) ≥ 960 MPa, ultimate


Table 3 - Summary of ground condition encountered and support measures adopted in Zone III Zone GSI UCS


Encountered ground conditions


Zone A 20–30 25–50 MPa 15–20 5–25 MPa


Zones B and C


10–15 5–15MPa


Siltstone in the tunnel face


Mudstone Shale by Enlargement


Pilot heading followed


<10 1–5 MPa


Disintegrated weathered rock


● ISHB 150 ribs in enlarged profile in both pilot and enlargement


Excavation sequence


Single sidewall drift


Support measures adopted


● 100mm + 150 mm SCL ● Rock bolt ● Lattice girders in each advance ● 100mm Face sealing ● Face bolts (if required)


● 100mm + 150 mm SCL ● Rock bolt ● Lattice girders in each advance in Mudstone unit


● 100mm Face sealing


Pre support measures


● 12m-long R38 SDA spiles at 400mm c/c


● Pre-excavation grouting with Ordinary Portland Cement


● 15m-long x 114mm long R38 SDA spiles at 400mm c/c


● Pre-excavation grouting with ultra-fine cement in mudstone and colloidal silica in shale and disintegrated rock


14 | August 2026


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