Traditionally, when constructing a bored tunnel, the concrete tunnel segments used are fitted with rubber gaskets onsite. These are often secured in place using adhesive, to ensure a tight seal and cushioned fit between each segment. When in situ, both the seal and segment are placed under significant pressure, so it is imperative that they are able to perform to the highest standards in order to guarantee safety and performance.
Trelleborg explains how, by using a gasket which is pre-cast into the tunnel segment during manufacture, installers can save time and money, as well as ensure a consistently accurate installation and a retained symmetry. However, it is important to take into account the design of the profile, as it can have a significant impact on the effectiveness and sealing quality of the gasket. Specifically, in the corners where the volume of rubber is generally the highest, it’s vital that the profile design is properly thought out and fully tested, to prevent risk of cracking of the concrete or a leakage path at the corner position of the seal.
Risks Associated with Non-cast Gaskets
For many tunneling projects across the globe, tunnel segments are fitted with rubber gaskets onsite and secured using adhesive. But, it is not uncommon for those who use this method to suffer time-consuming and cost-intensive repair work, which in extreme cases can exert negative effects on the outcome of a project.
This is because displacements can occur between segments if they are not placed in the exactly predetermined position during the on-site installation of the gasket. Displacements act either as an allowance for building tolerances or to help steerage, line and level during the construction of the tunnel.
The purpose of the gasket is to seal to the required pressure at the maximum expected gap and displacement. However, as the size of the gap and/or displacement increases, the ability of a gasket to withstand water pressure decreases. Furthermore, the load transference areas diminish, which also leads to an increase in the stresses within the segment.
The Importance of Rubber Quality
The majority of gaskets for tunnel lining segments are made from extruded Ethylene Propylene Diene Rubber (EPDM) rubber. However, much depends not only on the quality of manufacture, but importantly the quality of the rubber.
Elastomeric tunnel segment gaskets, which are located in precast grooves encircling the mating faces of each segment, provide a secure rubber-to-rubber waterproof seal as soon as the segments are bolted together.
Therefore it is vital that the low compression set and stress relaxation values of the elastomeric compound used in their manufacture enable elastomeric tunnel segment gaskets to recover under reduced applied load. This maintains a functional and watertight seal between segments despite ground movement and harsh wet and dry conditions. With that in mind, it is vital that tunnel segment gaskets are precision engineered to meet the precise requirements of individual projects.
For bored tunnels, using a gasket that is pre-cast into the tunnel segment during manufacture removes the need for adhesive on installation and can enable installers to save significant time and money.
This is because a cast-in gasket forges a superior bond connection with the segment to improve installation accuracy and eliminate the need for extra equipment. It is also more environmentally friendly and an altogether cleaner and safer process, as they eliminate solvent fumes and the presence of flammable materials. In fact, cleaning of the groove and correction of groove imperfections can be eliminated, optimizing and shortening the production process.
Cast-in gaskets also provide a faster, cleaner and more accurate alternative for the manufacture of tunnel segments, integrating the gasket as part of the segment production process. Additionally, they prevent air entrapment within the solution while also helping to retain shape, symmetry and integrity when transported.
The profile is designed to fit into the groove within the mold and is incorporated into the production process. Once it is fitted, the mold is filled with concrete and the cast then becomes an integrated part of the final product. The size of the groove therefore has a major effect on the choice of gasket and its subsequent performance. This approach to tunnel segment gasket design also ensures increased water-tight performance, due to complex integration of the gasket into the concrete segment.
The Importance of Correct Corner Configuration
One common concern across the industry when it comes to the performance of cast-in gaskets is the role of gasket corners – where the gasket material has to be mitred and vulcanized at the joint corner. This is because the vulcanizing material filling the holes in the gasket section can cause it to harden at the mitred corners.
Associated with some “rucking” of the gasket during segment installation, this can form a high point of the harder material, making the rubber incompressible. In turn, this makes the segments susceptible to cracking during installation, maximizing the potential for leakages.
Therefore, it is important that the gasket is designed so that it can be compressed but still provide effective sealing properties. For instance, too high a volume of rubber cannot be compressed and as a result the segment can be damaged, with broken corners or inserts that are pulled out. Another challenge is that the profile is not compressed effectively, resulting in holes forming between the seal and the concrete.
One solution is to enhance flexibility by ensuring a symmetrical design with an anchor at the corner position rather than straight sections, reducing the amount of rubber at corners. Finite Element Analysis (FEA) in the design process will help establish the maximum stress on the corners. The seals should then be tested for compression stiffness and water-tightness to ensure they perform as required when in situ.
Ensuring the Right Solution
It’s clear that using a gasket that is pre-cast into the tunnel segment during manufacture provides installers with many benefits, chiefly a consistently accurate installation with increased water-tight performance. However, it is vital that considerable thought is given to the design of the profile to ensure an efficient, watertight seal over the projected service life of the tunnel.
Therefore, finding a manufacturer with a proven track record for durability and reliability is crucial. Manufacturers need to be able to cater to the size and depth of the tunnel, as well as meet stringent pressure and testing requirements in order to eliminate costly remedial work to seal leaks after construction.
A manufacturer with in-house rubber compound technology can produce a tailored solution to meet project requirements and eliminate the chance of failure in situ. Additionally, it is important to turn to a supplier with the ability to perform extensive compression stiffness and water-tightness testing prior to installation.
Due to the complexity and monetary value of many of today’s segmentally lined tunnel projects, the demand for engineering know-how behind their construction continues to increase. This also extends to elastomeric tunnel segment gaskets, as their performance has the ability to hinder or ensure the smooth running of a given project. Working with a global supplier with the right solutions, expertise and local support, means that installers can rest assured they are using high-performance tunnel segment gaskets make projects run smoother.