The Application of Self Drilling Anchor Bolt at the Tala Hydro Project for Tunneling in Poor Rock Mass Conditions

Time:2024-07-30View:

The Tala Hydroelectric Power Station in Bhutan stands as a remarkable feat of engineering, harnessing the power of the Wangchu River to generate clean and sustainable energy. However, the construction of this project faced significant challenges due to the poor rock mass conditions in the area. Fortunately, the implementation of self-drilling anchor bolts revolutionized the construction process, offering a robust solution to overcome the demanding geological conditions.

tala-hydroelectric-power-station

Project Background

Tala Hydroelectric Power Station is a run-of-the-river type hydroelectric power station on the Wangchu River in Chukha District, Bhutan. Managed by the Tala Hydroelectric Project Authority (THPA), it is the region's largest high-head project, situated at an elevation of 860 meters. The station consists of a 92-meter (302 ft) tall gravity dam which diverts water through a 22-kilometer (14 mi) long headrace tunnel to the power station.

Originally slated for completion in June 2005, the project faced significant geological hurdles, ultimately delaying its commissioning until March 2007. The rock formations, primarily Thimphu gneiss, exhibited challenging characteristics, including joint sets, fault lines, and varying thicknesses of foliation shears.

Geological Challenges at Tala

Rock Mass Conditions

The rock type within the project site is predominantly biotite gneiss with bands of quartzite, mica schist, and sericite schist with amphibolite. These geological formations presented a major challenge for the construction team. The rock conditions, rated 3 - 4 on the NGI Q system, were particularly poor in areas such as C2, C-3, and all of C-4, necessitating urgent stabilization measures.

Variability and Instability

During excavation, the powerhouse and penstocks revealed a wide variance from the anticipated rock classification. The fragmented strata, prone to instability, posed a significant threat to the progress and safety of the construction efforts. The presence of joint sets and fault lines further complicated the situation, making it difficult to maintain the integrity of the tunnels and chambers during construction.

The Need for an Innovative Solution

To overcome these geological challenges and ensure the structural integrity of the Tala Hydro Project, a pioneering solution was needed. Traditional methods of rock stabilization were insufficient due to the complexity and instability of the rock formations. This necessitated the adoption of a more advanced and reliable method to stabilize the rock mass and facilitate safe tunneling operations.

Introduction to Self-Drilling Anchor Bolts

What Are Self-Drilling Anchor Bolts?

Self-drilling anchor bolts are a type of rock bolt that combines the functions of drilling, anchoring, and grouting into a single operation. These bolts consist of a hollow steel rod with an integrated drill bit at the tip. The design allows for simultaneous drilling and grouting, which significantly enhances the efficiency and effectiveness of the anchoring process.

self-drilling-anchor-bolt-system

Advantages of Self-Drilling Anchor Bolts

The use of self-drilling anchor bolts offers several advantages over traditional rock bolts:

1. Simultaneous Drilling and Anchoring: This reduces installation time and mitigates the risk of hole collapse, a common issue in unstable rock formations.
2. Increased Efficiency: The integrated design allows for faster installation, reducing overall project timelines.
3. Improved Stability: The grouting process enhances the bonding between the bolt and the rock, providing superior stability and load-bearing capacity.
4. Versatility: These bolts can be used in a variety of geological conditions, making them suitable for complex and challenging projects like the Tala Hydro Project.

Implementation at Tala Hydro Project

Strategic Deployment

The application of self-drilling anchor bolts brought about a seismic shift in the construction approach at the Tala Hydro Project. These innovative rock bolts were employed strategically to stabilize the desilting chamber walls and facilitate tunneling in poor rock mass conditions.

Technical Specifications

The construction team utilized a combination of 114 anchors with 20-meter length and 38mm diameter, spaced at 3 meters, and an additional row of 36 anchors with 24-meter length and 51mm diameter, also spaced at 3 meters. This strategic deployment ensured that the anchor bolts penetrated deep into the competent rock, providing the necessary stability to the chamber walls.

Installation Process

The installation of self-drilling anchor bolts involved the following steps:

1. Drilling: The integrated drill bit at the tip of the bolt allowed for simultaneous drilling and grouting, ensuring that the bolt was securely anchored into the rock.
2. Grouting: Once the desired depth was reached, grout was injected through the hollow rod, filling any voids and fractures in the rock. This enhanced the bonding between the bolt and the rock, providing additional stability.
3. Anchoring: The bolt was then tensioned to the desired load, ensuring that it provided the necessary support to the surrounding rock mass.

Case Study: Desilting Chamber No.3

Challenges Faced

During the construction of Desilting Chamber No.3, extensive cracks were detected in the RCC wall through borehole camera investigations. The fragmented strata posed a significant challenge, making it difficult to maintain drilled holes long enough for the installation of longer rock bolts. This necessitated the adoption of a more advanced and reliable method to stabilize the chamber walls.

Application of Self-Drilling Anchor Bolts

The construction team employed self-drilling anchor bolts strategically to address the challenges faced in Desilting Chamber No.3. By securely anchoring the chamber walls to the deeper competent rock, these bolts provided the much-needed stability and ensured the safe continuation of the project.

Results and Impact

The use of self-drilling anchor bolts in Desilting Chamber No.3 yielded significant results:

1. Enhanced Stability: The bolts provided superior stability to the chamber walls, mitigating the risk of collapse and ensuring the safety of the construction team.
2. Reduced Installation Time: The simultaneous drilling and grouting process reduced the overall installation time, allowing the project to proceed without significant delays.
3. Improved Load-Bearing Capacity: The grouting process enhanced the bonding between the bolts and the rock, providing superior load-bearing capacity and stability.

Broader Applications in Tunneling

Advantages in Tunneling Projects

The successful application of self-drilling anchor bolts at the Tala Hydro Project underscores their potential in other tunneling projects, particularly those involving poor rock mass conditions. The advantages offered by these bolts make them an ideal choice for a variety of tunneling applications:

1. Versatility: Self-drilling anchor bolts can be used in a variety of geological conditions, making them suitable for complex and challenging tunneling projects.
2. Efficiency: The integrated design allows for faster installation, reducing overall project timelines and costs.
3. Stability: The grouting process enhances the bonding between the bolts and the rock, providing superior stability and load-bearing capacity.

Conclusion

The Tala Hydro Project serves as a remarkable testament to the power of innovation and adaptability in the face of challenging geological conditions. Through the application of Self Drilling Anchor Bolts, the construction team successfully overcame the obstacles posed by poor rock mass conditions, ensuring the stability of critical structures and facilitating tunneling operations. This case study highlights the importance of advanced construction technologies and showcases the pivotal role played by Self Drilling Anchor Bolts in the success of complex projects like the Tala Hydro Project.

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