TECHNICAL | DRILL & BLAST, CONVENTIONAL
Right: Figure 7. Illustration of the QA/QC procedures developed for permanent GFRP rock bolts.
Below: Figure 8. Figure showing stripping failure of rock bolts and the plots showing settlement trends.
load ≥ 560 kN, elastic modulus ≥ 50 GPa, creep-rupture resistance ≥ 54% of UTS and alkali resistance ≥ 83% of UTS after 5,000 hours. To support this first-of-kind implementation, comprehensive pre-construction trials were undertaken prior to site deployment. These included short encapsulation pull tests, conducted in different rock units to validate design assumptions and to quantify the development of bond strength with time, which was explicitly incorporated into the final design. A bespoke material, installation and QA/QC specification
was developed to ensure long-term performance (see Figure 7). This required pull-out testing of 5% of installed bolts to a load of 350 kN at 24 hours age, equating to
approximately ten bolts tested per day. In addition, visual inspection of grout continuity was carried out using borehole viewers to verify full-column grouting. All testing outcomes and inspection records were reviewed and formally reported through the DRM process, providing continuous assurance that the permanent GFRP bolt system was installed and performing in accordance with design intent. Implementation of the GFRP bolts presented significant
challenges during the suitability testing stage. Initial tests indicated premature failure, with pull-out loads reaching only 50 kN within 24 hours. To address this, the rock mass conditions were reassessed and a special grout mix was developed. Laboratory trials were conducted to optimise both the grout formulation and the installation methodology, ensuring compatibility with the GFRP bolts and the surrounding rock. Through rigorous field validation, the installation method was refined, consistently achieving pull-out loads exceeding 400 kN within 24 hours, meeting the design performance targets. To evaluate the structural performance and load
transfer to the secondary lining, FEAs were conducted considering temporary, permanent and accidental load cases, including: ● striking of shutters after CIP secondary lining construction – temporary support elements remain active with CIP in place at 12 MPa strength
● degradation of temporary supports – temporary SCL removed; permanent GFRP bolts and secondary CIP active with a slip membrane to simulate sheet waterproofing
● accidental water pressure – blocked drainage causing water loading at the tunnel axis
● seismic loading – horizontal acceleration applied ● accidental wedge loading due to bolt failure – conservatively modelled as plane-strain ‘free-falling’ wedges along discontinuities, assuming GFRP bolts fail and the wedges directly load the 150mm-thick CIP secondary lining.
16 | August 2026
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45
