الفهرس 
CHAPTER (1) INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
In conditions where the raft foundation has a sufficient bearing capacity but its
settlement exceeds the allowable values, few piles could be placed under the raft to
lower its settlement to an acceptable level. In these cases, the piles are mainly used
as settlement reducer elements rather than load bearing members. However, the
small number of the settlement reducing piles could cause high axial stresses,
bending moments and shear forces in the pile heads where the piles are structurally
connected to the raft, and this may lead to the structural failure of the piles. To avoid
such structural problems related to the structurally connected settlement reducing
piles, a new foundation system named disconnected piled raft (DPR) has been
introduced over the last two decades. In this foundation system, piles are
disconnected from the raft by placing a layer of granular soil (cushion) between the
piles and the raft.
This research aims to investigate the performance of disconnected piled raft
(DPR) foundations rested on homogeneous and layered sand soils subjected to
uniform vertical loads. Two different soil profiles are considered to examine the
effect of floating and end bearing piles on the performance of DPR foundations. The
homogeneous soil profile consists of a single loose sand layer while the layered soil
profile consists of a loose sand layer underlain by a deeper dense sand layer.
To achieve this aim, three-dimensional finite element analyses using Plaxis 3D
software have been performed to simulate the complex interactions of the DPR
system taken into account the soil nonlinearity. Different types of foundation systems
including unpiled raft, piled rafts (PRs) and disconnected piled rafts have been
numerically modeled to investigate the performance of both disconnected and
connected piled raft foundations in terms of load transfer mechanism, foundation and
soil settlement and load sharing ratio. In addition, the structural performance of PR
and DPR foundations are compared in terms of bending moments and shear forces
generated in the raft and the axial load, bending moments and shear forces generated
in the piles. Moreover, the influence of the key parameters governing the
performance of DPR foundation is studied in details The results of finite element analysis show that the cushion layer in DPR
foundation has a considerable effect on adjusting the load sharing between the raft
and the piles as it reduces the stress concentration at pile heads which leads to
significant decrease in the load shared by piles and better use of the bearing capacity
of shallow subsoil between piles. The results also show that DPR system is quite
efficient in reducing the settlement of the unpiled raft and this efficiency is higher
for end bearing piles than that of the floating piles. Moreover, it is found that DPR
system can effectively reduce the bending moments and shear forces those generated
in the raft with connected piles and this efficiency is more obvious in case of end
bearing piles. In addition, the results indicate that using disconnected piles under the
raft results in reducing the pile axial load, shear forces and bending moments
compared to the connected piles. In other words, the results of this study show that
the DPR system can be effectively applied as an economical foundation system in
sandy soil to support structures subjected to uniform vertical loads.
Based on the numerical results, the influence of various efficient parameters,
which include the thickness and material of cushion, raft thickness, pile diameter and
pile length, on the performance of DPR foundation is investigated in a
comprehensive manner to achieve optimal design for DPR systems.