Determination of differential settlements of high-rise building foundations in soft soil conditions

Abstract

Currently, the settlements of high-rise buildings are calculated based on two methods. The single pile settlements (s1), are generated due to mobilised shaft friction and pile tip resistance. These settlements are determined with analytical methods (i.e. NEN 9997-1, 2017). Settlements related to pile group behaviour in soil layers below pile tip level (s2) are calculated with the equivalent raft approach i.e. Tomlinson (1977). However, an essential aspect for differential settlement calculations is the redistribution of loads by soil-structure interaction mechanisms, which are not currently incorporated in the calculation approach. A missing factor for a soil-structure interaction calculation is the lack of soil- displacement installation effects in the numerical methods for single pile behaviour. Therefore, this research is focussed on the implementation of a soil displacement single pile behaviour in PLAXIS 3D and thereby towards integration of the structural and geotechnical design in one calculation method for appropriate soil subgrade reactions and load redistribution. In this research numerical methods are used in combination with the embedded beam approach applied in PLAXIS 3D. To approximate the NEN9997 load-displacement behaviour of soil displacement single piles, the behaviour of the embedded beam in combination with the multi-linear shaft friction is fitted in PLAXIS 3D. The fit is conducted by increasing the stiffness of a soil volume below pile tip level. Which does account for the residual stresses after pile installation. The solution of this fit consists of two parts: improvements on behalf of the new embedded beam formulation after Smulders (2018) and a soil stiffness fit below pile tip level. Regarding the new embedded beam formulation, improvements have been determined on mesh-dependency and a stiffer load-displacement response. The fit method performs well in combination with the embedded beams while approaching the load-displacement response of soil-displacement piles according to NEN9997. However, the ultimate bearing capacity of the embedded pile should still be calculated with D-Foundations (analytical method based on NEN9997). In the pile group calculation, the behaviour of single piles is extrapolated to all piles in the PLAXIS 3D model. A plate element is modelled on top of the pile heads to achieve a redistribution of loads due to the overlying superstructure. In this way, the single pile behaviour and bending stiffness of the superstructure is implemented in the PLAXIS 3D model, and the soil-structure interaction calculation is possible. However, group effects due to the soil displacement installation method are only partly covered in this approach. The reduction or increase in skin resistance in pile groups is not covered since, the predefined maximum skin friction capacity is manually inserted as an input parameter and it is not an outcome of the PLAXIS 3D calculation. Afterwards, the proposed framework is applied to a practical case study including measurement data of a high-rise project in Amsterdam. The proposed framework turned out to capture soil-structure interaction properly by taking into account a part of the superstructure bending stiffness and single pile behaviour. Compared to current calculation approaches, complex soil behaviour is captured in redistribution of the loads. However, the calculated settlements are still deviating from the measurement data. A probable explanation for the discrepancy is the shift in applied load over time in PLAXIS 3D relative to the real construction process. In conclusion, an improvement in terms of modelling soil-structure interaction has been achieved by incorporating single pile behaviour and superstructure bending stiffness in PLAXIS 3D. Therefore, the possibilities of linking the structural design to the geotechnical design in an early stage of the design process will be feasible. Combining the designs in an early stage, the structural dimensions of columns and beams are calculated with appropriate soil subgrade reactions and load redistributions by the soil. However, it is recommended to apply more investigation on the behaviour of soil- displacement pile (groups) in combination with embedded beams because, group effects are still not completely incorporated.