Impact of water ingress and tunnelling operations on ground deformations: Insights from the Uma Oya Multi‐Purpose Development Project

The Uma Oya Multipurpose Development Project (UOMDP) in Sri Lanka provides a comprehensive case study on the interactions between tunnel excavation, groundwater ingress and ground deformations. During the construction of a 15 km headrace tunnel using a double shield tunnel boring machine (DS‐TBM), unexpected groundwater inflows exceeding 290 L/s above background triggered extensive aquifer drawdown, surface settlements up to 40 cm and drying of over 340 domestic wells across a 15 km area. Remote sensing, particularly persistent scatterer InSAR, helped characterise the spatial and temporal evolution of settlements. These phenomena were attributed mainly to the complex geological conditions, characterised by high‐grade metamorphic rocks, intensely fractured zones and fault structures that enable rapid water migration. Despite extensive post‐excavation grouting, long‐term subsidence persisted, underscoring the need for proactive management of hydrogeological conditions. Comparisons with international case studies, such as the Gotthard Base Tunnel, reinforce the necessity of integrated water management and risk mitigation in tunnel projects that traverse fractured rock masses.