Sülfeld lock
As part of the upgrading of the Mittellandkanal, the southern chamber of the Sülfeld double lock as well as the corresponding water saving basins were replaced by a modern extended shipping lock. To this end, an economic and environmentally compatible excavation and groundwater drainage concept was drawn up. The concept entailed enclosing the entire excavation pit for the lock chamber and the water saving basins with an impermeable excavation pit wall that extended down to the natural layers of basin silt with low permeability below the excavation pit bottom. This layer sealed the excavation pit horizontally from the underlying aquifer consisting of fissured rock. Measurements in additionally drilled groundwater measurement points were filtered in the rock and equipped with pressure transducers. The instruments revealed a high groundwater potential in the rock, partly extending above ground level, particularly at the western end of the excavation pit. The superposition of the weight of the layer from the lower edge of the basin silt to the excavation pit bottom and the groundwater potential present in the rock showed that there was not sufficient safety against uplift in the entire excavation pit for the new lock chamber (see picture 1). It was therefore necessary to lower the groundwater potential in the rock down to approx. 10 m before the pit could be excavated. Numerical groundwater flow calculations based on a 3D model were carried out for dimensioning the necessary installation for groundwater relief. These showed that the necessary measures to control uplift forces in the excavation pit are feasible at relatively moderate costs (12 groundwater relief wells). As the necessary lowering level for the groundwater relief system was approx. 7 m above the planned pit bottom, a passive pressure relief was possible i.e. without pumping and just by discharging the groundwater flowing out of the wells into a collecting tank in the excavation pit. The groundwater was then pumped from the collecting tank into the channel.
After excavating the pit over a wide area down to the discharge level, discharge pipelines were installed at the northern and southern end of the pit enclosure wall and the relief wells were then connected up accordingly. To prevent the discharge system from freezing, the discharge pipes and well connections were clad in insulation. Pictures 2 shows the collection pipe installed at the northern excavation pit wall during the placing of the lower concrete for the lock chamber at the upper head. Picture 3 presents the pumping system in the lowered collecting tank with the pressure pipe to the upper basin. The complete groundwater relief system was put into operation in February 2005, with an initial overall pumping rate of approx. 100 m³/h. Connection of the deep well to the collecting pipe with a free gradient to the collecting tank provided for automatic regulation of the relief system, as the groundwater potential in all wells was lowered to approximately the same level, regardless of the inflow to the individual wells. Several redundant pumps served to pump the groundwater from the collecting tank via a pressure pipe to the upper basin. The groundwater relief system for uplift protection of the excavation pit bottom operated without any disturbances through to August 2006 when it was decommissioned.
Literature
Odenwald, B. und R. Schwab: Grundwasserhaltung beim Neubau einer Schleuse am Mittellandkanal mittels Grundwassermodellierung. Tiefbau, Heft 3, 116. Jahrgang, März 2004, S. 126 128.
Odenwald, B. und E. Kunz: Altbohrungen als Ursache von Wasseraufbrüchen in einer Schleusenbaugrube. bbr, heft 3, 59. Jahrgang, März 2008, S. 40 - 47.
