Application of the observational method in urban tunneling

The paper deals with the application of the observational method to urban tunneling with particular reference to the canopy technique, which provides excavation support in poor soil conditions as an alternative to tunnel boring machines. The canopy is obtained by means of micropiles and/or jet grouting columns plus supplementary techniques such as fibreglass elements, shotcrete and steel ribs. By using this construction technique, tunnel design can be customized accounting for different soil conditions and existing structures. The construction procedure can then be modified in real time in case unforeseen soil conditions are met or excessive settlements are recorded, as it frequently happens in urban areas. However, proper application of the observational method in urban tunnelling requires a special monitoring effort and a well organised sequence of control and adjustment operations, as it is exemplified by reported case histories referring to highway, railway and sewer tunnels.