Analysis and improvement of the support in ancient railway tunel

At Onix Underground, our commitment to safety and innovation drives us to engage in challenging projects within the mining and underground infrastructure sectors. A clear example of this is our recent collaboration in the rehabilitation of an old railway tunnel, where we provided technical assistance to optimize the fortification elements used in the project.

THE CHALLENGE OF REHABILITATING OLD TUNNELS

Railway tunnels built in the past present a series of structural challenges due to their excavation and construction methods. Commonly, these tunnels were excavated using the “drill and blast” technique, resulting in over-excavations and irregularities that were later filled with rubble in the lining framework. The lining, made of brick, ashlar, or masonry, was constructed with the help of wooden frameworks, leaving a backfill which, depending on the terrain conditions, could be filled with debris or mortar.

In this context, any structural reinforcement intervention must consider the geomechanical conditions of the environment, especially when dealing with support elements anchored in such a heterogeneous and porous material as backfill rubble.

ANALYSIS AND ON-SITE TESTING

In November 2024, our team conducted a technical visit to the site to assess the performance of the support elements used by our client. Tensile tests were performed on two types of bolts:

• Rebar bolts (25 mm), anchored with resin or mortar.
• Expandable rock bolts (240 kN), anchored by friction.

The pull-out tests revealed that the rebar bolts failed to achieve effective anchorage due to the loss of fixing material within the porous backfill rubble. In contrast, the expandable rock bolts demonstrated excellent performance, achieving secure fixation through friction with the heterogeneous surrounding materials.

Both types of bolts have the same tensile resistance capacity (240 kN), which allowed us to determine that the most viable solution was to replace the rebar bolts with expandable rock bolts, ensuring the stability of the tunnel and the safety of the project.

For summary purposes, the following tables present the types of bolts tested and the results obtained:

Expandable Rock Bolts – EMC 240 kN, Length 3 m

Table 1: Summary Table of Tensile Tests for Expandable Rock Bolts

NOTES:

• Based on the test results, it is concluded that the tensile load was applied in three stages until reaching 50% of the nominal load.
• Elongations are measured, and it is verified that the bolt is not pulled out.
• Under these conditions, the tested bolt is considered suitable for execution.

Rebar Bolts ɸ25mm. Length 3 m

Table 2: Summary Table of Tensile Tests for Corrugated Bar Bolts

NOTES:

• Based on the test results, it is concluded that the bolts failed due to pull-out:
  • (*) In Tunnel 12, the maximum recorded pull-out load is 13.15 kN, and the minimum recorded pull-out load is 0.629 kN.
  • (**) In Tunnel 17, the maximum recorded pull-out load is 12.76 kN, and the minimum recorded pull-out load is 0 kN.

The Technical Report includes the results of the previously mentioned tests, concludes that:

• “The results of the tests performed indicate that the EMC expansdable rock bolt satisfactorily meets the requirements established in the project. However, the corrugated bar bolts do not reach the necessary load, which is attributed to the characteristics of the terrain in which they are installed.
• Therefore, it is recommended to prepare an additional technical report that considers supplementing the area where the corrugated rebar bolts have been installed with expandable rock bolts. This action will ensure compliance with the safety and functionality standards required for the success of the project.”

MODELING AND TECHNICAL JUSTIFICATION

To technically support this decision, a finite element modeling was carried out, allowing for the analysis of stress distribution and demonstrating the feasibility of the change. With this solution, the project execution was optimized without compromising the safety of the structure or the personnel.

GROUND CONTROL SYSTEMS INSTALLED IN TUNNELS 12 AND 17

In these tunnels, two types of supports, referred to as Type I and Type II, were employed with the following elements:

TYPE OF TUNNEL WHERE BOLTS AND SHOTCRETE APPLICATION IS USED:

Type I: Tunnel 12, L = 3 m, mesh 2 x 2 m, 30 MPa, thickness 5 cm, RMR > 55.

Type II: Tunnel 12 and Tunnel 17, L = 3 m, mesh 1.5 x 1.5 m, 30 MPa, thickness 15 cm, RMR = 45 – 55.

As can be seen, Tunnel 12 presents both types of support, and Tunnel 17 presents Type II.

COMMITMENT TO SAFETY AND INNOVATION

This project is yet another example of our dedication to safety and efficiency in underground environments. At Onix Underground, we not only develop high-performance products but also accompany our clients through every phase of their projects, providing technical solutions and ensuring transparency through our Traxlink system.

We will continue to innovate and contribute our knowledge to improve safety and efficiency in the mining and underground infrastructure sectors.

 ONIX UDNERGROUND – THE POWER OF SAFETY