New Method to Design Reinforced Concrete Lining for Pressure Tunnel/Shaft

Designing the lining of pressure tunnels is a complex engineering task, requiring careful consideration of both hydraulic and mechanical factors and the complex reaction between the lining and the surrounding rock mass. The design must minimise head losses, prevent excessive leakage, eliminate the risk of hydraulic fracturing or displacement of the rock, and ensure long-term structural integrity during filling, commissioning, operation, and de-watering phases. In reinforced concrete-lined pressure tunnels or shafts, when the internal water pressure exceeds the external groundwater pressure, the differential pressure in distributed between the lining and the surrounding rock mass. The load distribution depends on the stiffness and hydraulic conductivity of both the undisturbed and disturbed rock mass zones, as well as the properties and behaviour of the lining. In general, stiffer rock masses will absorb a larger portion of the differential water pressure, reducing the likelihood of lining cracks. When designing a reinforced lining, the primary objective is to determine the most economic thickness of the lining and the density of reinforcement, while also ensuring that:

1.      The width of cracks caused by differential water pressure remains within a specified limit. 

2.      Water loss through leakage is minimal/below acceptable range.

3.      Reinforcements stress way below yield.

This post presents the presentation file (which has been presented in SANCOT, 2026 Symposium, South Africa) for a new method for the design of reinforced concrete linings for pressure tunnels and shafts, developed as an extension of the Fernandez (1994) method. The proposed approach retains the key advantage of the Fernandez method—namely, the use of tensile strain as the cracking criterion for the lining—while enhancing it through more reliable crack spacing and crack width calculations based on Eurocode EN 1992‑1‑1 (2004). In addition, a calculation spreadsheet has been developed to implement the proposed method for practical design applications. The spreadsheet also incorporates the original Fernandez (1994) method, enabling users to directly compare results. As expected, the Fernandez method typically predicts a higher degree of cracking with smaller crack widths.

I’d love to hear your thoughts or questions on this updated design approach. Please contact me through following email:

info@mzgeotechnics.com