A study on the behaviour of a composite foundation by using different analysis methods

It becomes possible to transfer the building loads to the appropriate soil layers by using deep foundations in cases of inadequacy in the soil carrying capacity or large settlements beyond the tolerable limits. The foundation system designed by using piles is called as ?Pile Foundation System?. In general, the cost of pile foundations is much higher than that of shallow foundations. Therefore, to use only the piles to transfer the vertical loads (coming from superstructure) to the soil, requires a great number of piles which increase the cost. By this reason, a new foundation system called as ?piled raft foundation? was defined which is much more economic. Piled raft foundations are being used to limit the settlement of the foundations where the raft foundatins has enough carriying capacity but the settlement requirements are not satisfied. They are being used to transfer the vertical loads and all the horizontal loads (such as seismic loads?etc.) to the strong soil layers. The load is transferred to the soil by means of both foundation and piles where there is a contact between soil and foundation. In this study, the behaviour of the piled raft foundation (belongs to a building having a tunnel form system constructed in Odunpazarı/Eskişehir) was examined under the effect of vertical loads from superstructure. For the soil profiles, the soil engineering parameters were determined by using both site and laboratory soil data using the soil report performed in the related site. The piled raft foundation system of the building was modelled by Etabs, Safe and Midas GTS, 3-D finite element programs. The results obtained in determining the behaviour of the piled raft foundation by using the finite element methods were compared. As a conclusion; in opposition to the acceptance of the transfer of the vertical building loads to the soil being just by piles (even in case of end-bearing piles); it is known that, the loads are transfered to the soil partly by means of both piles and foundation itself where there is a full contact between soil and foundation. And by the distribution in load transfer, it was seen that, it will be possible to design more economic pile-foundation systems since the loads carried by piles decreased. It was determined that, the increase in the modulus of elasticity of the soil under the foundation, increases the load carriying capacity of the soil. But it was noted, on the other hand, that; any changes in the cohesion and internal friction angle of the soil under the foundation does not affect the foundation vertical deformations, pile axial loads and foundation load carrying capacity. As far as the pile axial loads and displacements were considered; almost the same results were reached at the end of the comparison of the results of studies using different finite element programs. By examining the internal forces, some small differences in moment and shear values were determined. These differences in moment and shear values were mainly resulted from the differences in the method of defining soil rigidity values in programs. While the modulus of elasticity value of the soil is being used in Midas GTS to define the soil rigidity; in Etabs, Safe the soil rigidity is being defined by using just a spring constant in resulting more and more rigid soil profiles.