Novel application of geosynthetics in seismic protection of buildings considering soil-foundation-structure interaction
- Publication Type:
- Thesis
- Issue Date:
- 2018
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The structural, geotechnical and earthquake engineering designs under earthquakes are gradually moving from strength-based seismic design to performance-based seismic design (PBSD). Indeed, seismic design of structures is moving from imposing limits on forces and moments acting on the structures and foundations, to performance-based seismic design allowing more sensiable evaluation of building performance during and after earthquakes with different severity levels. Generally, in PBSD, the conventional prohibitions are released to the extent that maximum and permanent displacements and rotations are kept within acceptable limits, while no structural failure or collapse is allowed. Foundation rocking is a common phenomenon observed during earthquakes. The rocking induced foundation uplifting and soil yielding can function as energy dissipaters to absorb seismic energy and prevent it from being fully transmitted to the superstructures. However, the permeant foundation rotation and settlement are the issues produced by this foundation movement. On the other hand, employing end-bearing pile foundations may result in enormous shear forces developed in the structure and at the connection between the foundation slab and pile heads, as the foundation rocking mechanism is prevented.
In this study, a geosynthetic reinforced composite soil (GRCS) foundation system is proposed to resolve the rocking induced issues for shallow foundations. In addition, a geotextile reinforced cushioned pile foundation is recommended to extend the use of foundation rocking as an energy dissipater to pile foundations. Thus, design engineers can have a broader choice of foundation system for the seismic safeguarding of buildings. To evaluate the seismic performance of the proposed foundation systems, a fully nonlinear three dimensional numerical model is developed to perform time history analysis considering seismic soil-foundation-structure interaction employing FLAC3D software. Hysteretic damping of the soil is implemented to represent the variation of the shear modulus reduction factor and the damping ratio of the soil with the cyclic shear strain, while a Mohr-Coulomb constitutive model is used to simulate the plastic deformation of the soil. Free-field boundary conditions and rigid boundary condition are assigned to the lateral boundaries and the bottom boundary of the model, respectively. Appropriate interfaces are considered between foundation (shallow foundation or piles) and soil to capture possible separation/gapping and sliding. Apart from those, soil-geosynthetic interfaces are also modelled to consider possible sliding and pull-out of the reinforcement layers. Real earthquake records are used as input accelerations applied at the base of the model.
Firstly, an investigation about the impact of dynamic soil properties including Plasticity Index and undrained shear strength on the seismic performance of the superstructures supported by a shallow foundation and an end-bearing pile foundation is carried out. The results indicate that extreme care is required to treat these soil properties to obtain reasonable predictions. In addition, the influence on the soil-foundation-structure system brought by the pile configuration is studied and the numerical predictions shows that the response of the system is sensitive to the pile configuration and therefore it should be chosen wisely to optimise the design.
Furthermore, a three dimensional numerical model simulating a mid-rise building resting on proposed geosynthetic reinforced composite soil (GRCS) foundation is developed to evaluate the influence of the proposed foundation system on the seismic response of mid-rise buildings. In addition, a parametric study is conducted to investigate the impact on the superstructures brought by the arrangement of geosynthetic reinforcement layers focusing on the stiffness, length, number and spacing of the layers. The results indicate that the GRCS foundation can enhance the structural seismic performance from unacceptable to acceptable provided that the arrangement of reinforcement layers is well designed. Eventually, the seismic response of a superstructure supported by a geotextile reinforced cushioned pile foundation consisting of a reinforced interposed layer to bridge between the foundation slab and pile heads is studied. The predictions indicate that the proposed cushioned pile foundation can considerably reduce the structural demands of buildings and piles while control the building deformation within acceptable criteria and consequently, the proposed geotextile reinforced cushioned pile foundation can offer an alternative option for the seismic protection of buildings.
Therefore, in practice, Plasticity Index and undrained shear strength of soil should be considered when numerical analysis is required. In addition, pile configurations should be considered carefully to achieve optimised foundation design. Furthermore, a geosynthetic reinforced composite soil (GRCS) foundation system can provide design engineers with an alternative option to limit excessive settlement, and maximum and residual inter-storey drifts induced by seismic loading; this foundation option can be optimised by analysing the arrangement of the reinforcement layers including their material stiffness, length, spacing and number of the layers with great care. Moreover, for buildings requiring pile foundations, a geotextile reinforced cushioned pile foundation can offer design engineers another solution to control the shear forces that develop in a superstructure, as well as reducing the structural demand of the pile foundations.
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