Consideration of groundwater pressure on linings of pressure tunnels and shafts

Abstract

In the construction of the Catskill Aqueduct (USA) up to 50 km of 4.3-m-diameter tunnels for a flow of up to 26 mS/see were driven; the internal head h of the water being transported reaches 225 m. A steel facing was incorporated into the tunnel lining; in this case the external groundwater pressure relieving the lining stresses was taken into account. Where the route of the tunnel passed under the bed of the Hudson River the internal head in the tunnel reached 460 m at an external head of the groundwater H = 340 m [1]. During construction of the diversion pressure tunnel for the Tereblya-Rika hydroelectric station (h = 24-28 m) with an inside diameter of 2.5 m and 3610 m long, calculated for a maximum groundwater head H =48 m, groundwater seeped through the joints and cavities of the lining into the tunnel at a rate of 300 liters/see under a considerable head. The percolating water scoured the grout injected behind the lining and the gunite applied on it. To remove these groundwaters about 1600 holes with a depth from I to 2 m were drilled through the lining into the rock. Into all holes were inserted 38-ram-diameter pipes, threaded for attaching removable pressure gauges. The external groundwater pressure was measured by these gauges; the pressure reached 3-4 technical arm and in sections where it was measured exceeded the design internal water pressure. The presence of an excess groundwater pressure made it possible to eliminate completely or partially the reinforced gunite from the entrance portal to the 19 + 80 m station. More than 600 active drain holes, some discharging up to 1 liter/see were left throughout the entire tunnel length. The remaining holes, with insignificant discharges, were sealed with grout and plugged. Observations showed that during tunnel operation leakage from it did not occur. Conversely, the groundwater flowed through the drain holes into the tunnel. This was confirmed by long-term observations of control holes drilled on the exit slope along the route of the penstock in the immediate vicinity of the tunnel and by direct measurement of the water level in the surge shaft with the gates closed at the start and end of the tunnel when this level rose 7 m above the water level in the upper pool [2]. In connection with taking into account groundwater pressure in calculations of linings in soft clays, F. F. Gubin [3] notes that in cases of deep tunnels, when the external earth pressure and groundwater pressure compensate for the internal water pressure in the tunnel, reinforcement of the lining can apparently be omitted. The problem of the distribution of head and, consequently, of pressure in the presence of groundwaters and seepage through the concrete linings of tunnels and the grouting zones surrounding them was first posed and analyzed by G. M. Lomize in his monograph [4, pp. 186-190]. Many conclusions reached in this monograph retain their significance today, especially in connection with the increase in heads and dimensions and depths of underground structares being built in recent years.