Over recent decades, the world is witnessing a rapidly increasing demand for tunnels due to their unique characteristics and potential applications such as roadway and railway transportation. However it is not unusual for any forms of risks to occur after tunnel completion and during operation and maintenance stage. This paper reviews all the key major functional risks for tunnelling projects from several case studies that have been carried out by other researchers. As a result, a more effective risk management approach can be added to the current operation and maintenance plan. And from there also, a risk contingency plan can be established or added to current risk management plan to enhance the operation and maintenance of tunnelling projects.