Hidrológiai Közlöny 1972 (52. évfolyam)

3. szám - Illei Vilmos: A munkagödör víztelenítése és a résfalas eljárás alkalmazása a Kiskörei Vízlépcsőnél

Illei V.: A munkagödör víztelenítése Hidrológiai Közlöny 1972. 3. sz. 127 contains several silt strata. The base of the sand layer is formed by a sequence of silt and clay layers of 4 to 10 m thickness, below which a second pervious horizon is encountered. 1. Unwatering the construction pit. With the bottom of the construction pit cutting into the first pervious horizon, groundwater lowering in several stages has been adopted after the careful study of several alternatives, as the solution for unwatering the construction pit. [n this layer water has been assumed to percolate in an approximately horizontal direction. Owing to its low permeability (k =10"

8 m/sec) and the long seepage distance, the discharge in the cohesive top layer is negligibly small. The clay layer separating the first and second waterbearing formations may be regarded as continuous, so that the water rising from the second pervious formation could be safely neglected as far as groundwater lowering was concerned. The horizontal flow of groundwater in the first pervious layer was assumed to be symmetrical about a central point and the construction pit was replaced by a circular well having an area in plan, which equalled the area bounded by the extreme wells in the groundwater lowering system. In the case of the fictitious circular well, the average seepage distance was obtained at 400 m. At the point of origin the boundary condition of computation was the original grounwater level, or at times of high river stages, the watersurface in the Tisza River. With respect to the flood stage of 5% probability the lowered groundwater table varied from —17 to —25 m elevation. For purposes of computation withdrawal by a galery was assumed. At a certain distance the drawdown surface thus created is practically the same as that resulting when the drawdown is created, rather than by a galery, by a row of partially penetrating wells. The length of wells was determined in a manner to allow under the lowered groundwater table the particular depression cone of individual wells to develop and to make sufficient filter surface available for passing the discharge to be withdrawn from individual wells. Filter wells were spaced uniformly on 8.00 m centers, whereas for the vacuum wells a spacing of 1.80 rn was adopted. At this arrangement the design discharge of individual filter wells ranged, depending on distance from the Tisza River, from 0.28 to 0.42 lit/sec, whereas that of vacuum wells from 0.06 to 0.10 lit/sec. Down to El. 82.00 m AOD the construction pit was excavated by pumping. The first stage comprising the filter wells was sunk from El. 82.00 m, while the second stage, involving the vacuum well points, from El, 78.00 m. Owing to differences in the foundation levels of individual structures and in their different horizontal dimensions, the third stage of the groundwater-lowering system formed no uniform pattern and consisted of a combination of filter- and vacuum wells. The water withdrawn from the wells is collected in a main header of 250 mm diameter at El. 85.50 AOD, from which it is discharged into the Tisza River through a measuring weir. The discharge from the entire construction pit, recorded at the measuring weir, varied from 20 to 65 lit/sec, depending on the stage in the river and the position of the groundwater table. The groundwater-lowering system adopted has functioned perfectly since 1967 and adjusting always to the progress of construction work ensured dry conditions for various activities. The long and very high flood of 1970 in the Tisza River proved to be the most severe test, which the system withstood to full satisfaction. 2. Slurry-trench walls at the Kisköre Barrage. For reducing seepage under and around the foundation of the barrage and for minimizing uplift pressure on the base slab, it was found necessary to apply means of seepage control. In view of the great thickness of the layers requiring to be cut off, steel sheet piles, or slurry-trench walls were the only potential alternatives. Difficulties in obtaining the large quantities of steel sheet piling material directed attention to the use of slurry trench walls. During the construct ion of the barrage, it was necessary to complete 18,000 sq.m of slurry trench cutoff wails so that extensive laboratory and field tests and experiments were performed before work was started. The objective of these studies was to select the most suitable slurry material and to gain practice with the method of construction. The slurry trench walls have a threefold purpose, namely that of cutting off the path of seepage, foundation and of providing temporary support to the sides of the construction pit. Their dimensions and designs vary with the purpose served. The trench walls were completed before the pit was excavated to the designed bottom. After complete excavation, the upper, about 50 cm high part of the wall of poor quality was removed and replaced by a cast in situ reinforced concrete beam. A ribbed plastic strip was cast into this beam to ensure watertight connection between the wall and the foundation slab of the structure and at the same time the possibility of relative displacement between the two. The trench wall was excavated partly by rotary drilling, partly by hydraulic clamshell bucket, the trench being supported by a clay slurry. The material of the wall is concrete, plain, or reinforced depending on the purpose served. Aláálványozott átmenő darupálya betonozása Fotó: Kotányi Ottó Pouring concrete for the continuous crane girder on scaffolding Jobb parton az erőműhöz csatlakozó darupálya építése (1971) Fotó: Kotányi Ottó Construction of crane girder on the right-hand side connection to the power station ( 1971)