During the planning stage of a tunnel, the permissible future water leakage into the tunnel is determined with regard to the hydro-geological conditions and existing structures. Detailed specifications are drawn up concerning tests-and observations to be made during excavation, and the interpretation of their results, as a guide to the execution of the grouting works. A brief account is given of a specific contract on which methods and equipment for drilling and grouting are adapted to the exacting demands made on accuracy and efficiency. Monitoring of water leakage and groundwater levels is to be continued after the completion of the tunnel.
Groundwater lowering is a frequently occurring result of urban development. The reason for this is that the rainfall from house roofs, paved streets and squares is removed through drains; intentional drainage is provided in or around houses; more or less unintentional drainage is achieved in deep pipe work trenches and by leakage into tunnels. Experience has shown that relatively small quantities of water leaking into tunnels can cause a lowering of the groundwater table. These may give rise to damage resulting from settlement of buildings founded on soft clay or on timber piles, the upper sections of which are deprived of the water preventing them from rotting. In Stockholm, considerable damage has occurred to buildings in a number of locations subject to this process, in which tunneling has been the dominant cause. In certain cases, groundwater lowering has been counteracted by various forms of infiltration, but these solutions to the problem have not always shown themselves to be possible or suitable in the conditions prevailing in Stockholm. Previously, tunnels used only to be sealed if the inflow of water during the construction stage required it or if the function of the tunnel demanded that the roof be free from dropping water. The latter case was usually solved by grouting the roof and sometimes the walls. If this was not successful, the water was collected in drains which carried it down to the floor of the tunnel. It was realized gradually that tunnel floors were of the greatest importance as regards the sealing of tunnels. This reluctance was because grouting was felt to be a necessary evil, as it was expensive and technically difficult to combine with the actual tunneling work. The grouting that was carried out was based mostly on subjective assessments on site on the basis of excessive quantities of water pouring into the tunnel.
It is possible to build a completely tight tunnel, but it is extremely expensive. However, it is not possible to avoid leakage and possible damage during the course of the tunneling work. Realistic planning should therefore be carried out, on the basis of the hydro-geological conditions and possible damage to surrounding structures resulting from a possible groundwater lowering, to try to make an objective assessment of the leakage that can be accepted. Taken to the extreme, the assessment could theoretically be based on the principle of keeping the sum of the costs of the grouting work and any damage to structures to a minimum.