Time-dependent load-deformation behaviour of foundations under alternating loads in overconsolidated clay
Final Report Abstract
In the context of the modernisation and revitalisation of existing buildings, increasingly complex tasks such as the re-use of foundations have to be addressed. An accurate prediction of the loadsettlement behaviour of existing foundations is of particular importance if existing and new foundations are to be combined. In order to take into account the time-dependent deformation behaviour of foundations, high demands on the modelling of the subsoil must be met, since for cohesive soils both consolidation processes and the time-dependent material behaviour (creep) need to be realistically assessed. In addition to the time-dependent deformation behaviour of the soil and changing structural loads arising from demolition and reconstruction phases, construction works in the vicinity also influence the long-term deformation behaviour of foundations. In particular, neighbouring groundwater drawdowns and the associated changes in the groundwater level are responsible for the fact that foundations in urban areas are exposed to alternating loads resulting from changes in uplift and effective stresses in the soil. The research project focused on the long-term deformation behaviour of raft foundations and piled rafts in overconsolidated (oc) clay, taking into account measurements on the foundations of the Messeturm and the SGZ-Bank/Park Tower building complex in Frankfurt am Main extending over several decades. At the University of Pretoria, centrifuge model tests were carried out on raft foundations and piled rafts in oc kaolin, in which the influence of repeated groundwater drawdowns on the one hand and of unloading and reloading of the raft on the other hand were investigated. The evaluation of the in-situ measurements and the experimental investigations were complemented by 3D coupled pore pressure-displacement analyses using the finite element method (3D FEA), where the time-dependent material behaviour of the soil was modelled using the visco-hypoplastic AVISA model. To determine the material properties for the AVISA model, an extensive laboratory test program consisting of oedometer tests as well as undrained monotonic and cyclic triaxial tests on clay samples from the Frankfurt Formation was carried out. On this basis, a parameter set verified and calibrated by means of back analyses of the above-mentioned in-situ measurements was determined that realistically represents the system stiffness of the heterogeneous Frankfurt Formation, taking into account the limestone beds and sand lenses embedded in the oc clay. With this parameter set, a numerical parametric study was then carried out by means of coupled 3D FEA to investigate, among other things, how alternating loads, consolidation and creep affect the load distribution between piles and raft for piled rafts and the design parameters commonly used in engineering practice, namely modulus of subgrade reaction and equivalent pile spring stiffness.
Publications
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Numerische Untersuchungen zum Zeit-Setzungsverhalten von Gründungen im überkonsolidierten Ton. Beiträge der Spezialsitzung "Forum für junge Geotechnik-Ingenieure", 37. Baugrundtagung in Wiesbaden, DGGT, Essen, 55-65.
Ganal, A.
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On the AVISA model for clay – Recommendations for calibration and verification based on the back analysis of a piled raft. Computers and Geotechnics, 154, 105126.
Tafili, M.; Ganal, A.; Wichtmann, T. & Reul, O.
