VERTICAL ELLIPTICAL ACCESS-SHAFTS Geometrical optimisation through FE-modelling

University essay from Lunds universitet/Geoteknik; Lunds universitet/Institutionen för byggvetenskaper

Abstract: Urbanisation is causing densification of cities and more and more facilities are being placed in the underground to utilise the urban space as efficiently as possible. Access to the underground is obtained by constructing vertical access shafts. By constructing the shaft with a circular plan geometry, an effective construction is obtained which can carry large earth pressure loads by utilising the arching effect in the construction. In this way, the walls do not need to be supported with struts and the shaft is given a free opening from above. In tunneling projects, tunnel boring machines may need to be lowered into the shaft, which requires a large radius of the circular shaft. By instead constructing the shaft with an elliptical plan geometry, the ground surface can be utilised more efficiently, while long objects can still be transported up and down from the shaft. The purpose of this thesis is to investigate the behavior of elliptical retaining structures and to seek an optimised elliptical shape where the arching effect can be utilised as far as possible in the structure. The study is linked to a tunneling project between Lund and Malmö, where a new sewage tunnel will be built to transport wastewater from Lund to an expanded sewage treatment plant at Sjölunda in Malmö. In addition to the shape of the shaft, the foundation depth of the retaining structure is also assessed on the basis of predetermined water inflow requirements in the shaft. The study is based on drilling data from a previous project where geotechnical surveys were carried out at Sjölunda. From the given survey data, a geo-model with known geotechnical parameters is defined. The tunnel will connect to Sjölunda at a depth of 30 m, meaning that the shaft bottom will be at a depth of 30 m. Also a requirement for a free opening of 11 m was predefined for the shaft. Groundwater related problems, such as the foundation depth of the retaining structure, are investigated in the finite element software SEEP/W. The soil-structure interaction is modelled by using the finite element software PLAXIS. A simplified method, which does not take into account the soil-structure interaction, is also used and modelled in the FE-software Robot. With a requirement to allow a maximum groundwater inflow in the shaft of 0.5 l/s, the foundation depth of the retaining structure is determined to 10 m below the shaft bottom, i.e. 40 m below ground surface. The shape of the shaft is investigated by starting from a circular shaft that was modeled in both PLAXIS 2D and PLAXIS 3D. The models are verified by analytical calculations of the soil pressure and the corresponding horizontal normal forces from the arching effect in the structure. After verifying the model in PLAXIS 3D, a parameter study is performed in which three elliptical geometries, with increasing elongation, are investigated. For one of these geometries, a model is also built in Robot, where the load is applied as a uniformly distributed radial load corresponding to the soil pressure at different depths taken from PLAXIS. By comparing the obtained forces in the structure from both PLAXIS and Robot, it is found that PLAXIS is better at simulating the real behaviour, where the retaining structure’s interaction with the ground generates lower load effects in the wall. Based on the results, PLAXIS is considered to give a more realistic result and these models are further on used to define an optimised geometry. The resulting forces in the structure, obtained for three different geometries in PLAXIS, are compared with the capacity of a predefined cross-section of a diaphragm wall. The comparison showed that the optimum elliptical geometry, given the predetermined requirements of the shaft and the geological conditions in the area, was obtained as a relation between the short and long diameters of an ellipse of 0.45.

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