In the heart of the Swiss Alps, a major base tunnel expansion faced unexpected geological hazards: squeezing ground, high water ingress, and fractured rock. The 35 km Gotthard Base Tunnel expansion, part of the AlpTransit initiative, encountered a 200-meter stretch of kakiritic rock and faulted dolomite at depths exceeding 2,000 meters. With water pressures reaching 15 bar and rock mass rating below 20, conventional steel rock bolts failed to provide adequate long-term stability. Corrosive groundwater threatened steel anchors, and the need for rapid installation to maintain TBM progress was critical. Falls of ground and water inflows had already caused several stoppages, threatening both the completion deadline and worker safety. The project's geotechnical team needed a support system that could be installed quickly and provide immediate load-bearing capacity while resisting the aggressive chemical environment. After a thorough evaluation, the project consortium selected SupAnchor's GFRP hollow injection bolt system—a self drilling anchor bolt that integrates drilling, grouting, and permanent support in a single operation.
SupAnchor's SupFRP® GFRP Hollow Injection Bolt is a hollow bar anchor made of glass fiber reinforced polymer. It serves as both a drill rod and a permanent rock bolt, eliminating the need for separate casing or post-grouting. Its design features a hollow core for grout injection and a waved thread on the exterior for superior mechanical interlock. The waved thread structure, unique to SupAnchor's hollow bar anchor, increases the surface area and mechanical bond with the grout by up to 40% compared to smooth bars, ensuring load transfer from the rock to the bolt without relying on end-plate friction alone. This geotechnical reinforcement system provides immediate active support and long-term durability in the most demanding environments.
Weighing up to 75% less than steel, the GFRP bolt significantly reduces manual handling fatigue and installation time, especially in confined TBM face support or forepoling setups. Its high corrosion resistance ensures no degradation in acidic, alkaline, or wet environments, a critical advantage over traditional ground anchor bolts. Additionally, the bolt is cuttable with standard tools, making it safe for sequential excavation and eliminating sparking risks in flammable atmospheres. The material's low thermal conductivity also prevents freezing issues in permafrost or cold regions, a benefit rarely offered by metal alternatives.
The installation process begins with the bolt being coupled to a sacrificial drill bit. Using high-torque drilling rigs, the hollow bar anchor is drilled into the borehole while simultaneously flushing with air or water. The self-drilling nature of the bolt eliminates the need for pre-drilling and casing, reducing equipment requirements to just a standard rotary-percussion drill and a grout pump. Once the required depth is reached, a cementitious grout is pumped through the hollow core at pressures up to 30 MPa, filling the borehole and penetrating surrounding fissures. The entire cycle—drill, inject, tension—takes less than 15 minutes per bolt in favorable conditions. A plate and nut are then tensioned against the excavation face, providing immediate active support. This drill-and-grout bolt method ensures a monolithic bond between the bolt and the ground, eliminating the voids that often plague conventional anchor systems and creating a watertight, fully encapsulated reinforcement element.
SupAnchor offers six standard models to match varying load requirements and borehole diameters. Key parameters include guaranteed tensile loads from 220 kN to 490 kN, pressure resistance up to 30 MPa, and a tensile E-modulus of 45 GPa. The following table highlights the main technical data:
| Model | Outer Diam. (mm) | Inner Diam. (mm) | Tensile Load (kN) | Pressure Res. (MPa) | Weight (kg/m) |
|---|---|---|---|---|---|
| H22/230 | 22 | 8 | 230 | 18 | 0.58 |
| H25/220 | 25 | 12 | 220 | 5 | 0.64 |
| H25/300 | 25 | 8 | 300 | 30 | 0.79 |
| H28/320 | 28 | 12 | 320 | 5 | 0.88 |
| H32/360 | 32 | 15 | 360 | 5 | 1.20 |
| H38/490 | 38 | 20 | 490 | 5 | 1.50 |
All models feature a waved thread structure that enhances bonding with grout, and can be supplied in lengths from 2 m to 11 m with left- or right-hand threads. The ultimate strain of 2.0% ensures controlled deformation without brittle failure, a key factor in dynamic ground conditions. The H25/300 model, with its 30 MPa pressure resistance, can inject grout at high pressures to penetrate microfractures, creating a reinforced rock zone. This is especially valuable in tunnel portals where water-bearing faults are encountered. The low weight—as light as 0.58 kg/m for the H22/230—reduces transportation costs and crane loads on site, while the balanced E-modulus provides the stiffness needed to control convergence in soft ground.
The hollow injection bolt is a versatile drill-and-grout bolt suitable for slope stabilization, TBM face support, and confined ground conditions. In open-pit mining, these bolts are used as soil nails to reinforce weathered slopes. In urban metros, they serve as forepoling spiles to pre-support weak ground ahead of the tunnel face. Their cuttability makes them indispensable in mixed-face conditions where TBM cutters may encounter the bolt; GFRP poses no risk to cutterhead integrity, unlike steel. In a recent highway expansion in Norway, contractors used H32/360 bolts to stabilize a 60-meter high cutting in fractured granite. The full grouting column prevented water ingress and frost heave, ensuring decades of maintenance-free service. Similarly, in a Chilean copper mine, H25/300 bolts with 30 MPa injection capacity consolidated a heavily jointed andesite slope, allowing safe bench mining.
As a leading soil nail system manufacturer, SupAnchor also integrates these hollow bar anchors into comprehensive ground support schemes for highway cuts, dam abutments, and underground chambers. The full grouting capability guarantees a watertight seal in permeable strata, enhancing long-term safety and reducing maintenance costs for infrastructure owners. During the construction of the Mumbai Metro Line 3, SupAnchor's GFRP hollow bolts were used to stabilize soft marine clay strata, where corrosion to steel was a major concern. The project's success underscored the bolt's adaptability to urban environments with challenging ground conditions.
With over two decades of experience, SupAnchor has established itself as a trusted ground anchor bolt factory and engineering partner. The company's self drilling anchor systems are manufactured under ISO 9001 and ISO 14001 certified processes, with rigorous testing to meet international standards such as ASTM D7957, EN 14490, and BS 8081. SupAnchor's in-house R&D center in China collaborates with European universities to test and refine anchor technologies, ensuring each product meets the evolving demands of modern geotechnics. From the Crossrail project in London to the Altamira Water Tunnel in Mexico, SupAnchor's geotechnical reinforcement systems have consistently delivered reliable performance in the world's most challenging projects.
Driven by a commitment to Professionalism, Innovation, and Collaboration, SupAnchor continues to advance the frontier of anchor technology. The GFRP hollow injection bolt exemplifies this ethos, combining lightweight efficiency with uncompromising strength and durability. As infrastructure demands grow more complex, SupAnchor remains the partner of choice for engineers seeking safe, fast, and sustainable support solutions.
For more information on the SupFRP® GFRP Hollow Injection Bolt and other self drilling anchor systems, visit SupAnchor's official website or contact their technical support team.
Breakthrough GFRP hollow injection bolt from SupAnchor delivers high tensile strength, corrosion resistance, and full grouting for long-term tunnel and mine stability.
SupAnchor's self-drilling anchor system utilized in a challenging transportation project, delivering efficient ground stabilization and geotechnical reinforcement in difficult geology.
SupAnchor’s self-drilling anchor system was deployed in a European highway expansion to stabilize steep slopes amid difficult geology, showcasing advanced geotechnical reinforcement.
SupAnchor's T thread self-drilling hollow anchor system delivers integrated drilling and grouting, high bonding strength, and robust anti-corrosion options for tunneling, slope stabilization, and foundation reinforcement worldwide.
A major European highway expansion project overcame unstable ground conditions using SupAnchor’s self drilling anchor system, ensuring long-term slope stability and fast installation.
