08-10-2012, 12:03 PM
Numerical Analysis of Piled-Raft Foundation Considering Sand Cushion Effects
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ABSTRACT
The piled-raft foundation is a new design concept as one of the
effective methods of foundation to reduce settlements of structures. On
the other hand, an alternative piled-raft system with disconnection gap,
sand cushion, between the pile and raft was investigated to compare the
influence of ultimate bearing capacity and settlement. Load-settlement
relation curves are used to evaluate the ultimate bearing capacity. In the
numerical analyses, a plane strain elasto-plastic finite element model
(Mohr-Coulmb model) is used to present the response of the piled-raft
foundation.
INTRODUCTION
As Korean government has proposed a plan of new land development
in the Nakdong river deltaic plan near the Busan, the second largest city
in Korea, infrastructures such as industrial and commercial facilities
have been, and are still being constructed since the early 1990’s. In this
area, the soft clay layer about 20 to 30m thickness has been deposited
over the sandy soil or directly on the undulated bedrock surface. On the
top of the soft clay layer, typically about 5 to 10m’s of a sand layer is
placed. For the structure foundation under the soft clay condition, piles
are customary embedded directly on the bedrock to control the
settlements in this area. But the effectiveness of foundation is needed to
be studied in the bearing capacity consideration. To satisfy the design
consideration with respect to the settlements and ultimate bearing
capacity, alternative foundation needs to be required.
In this paper, two different piled-raft foundations with or without the
sand cushion were compared in the ultimate bearing capacity and
settlements. The ultimate bearing capacities of raft foundation which
had various raft widths were firstly evaluated by using the loadsettlements
curves. Secondly, the piled-raft foundations which had the
specified sand cushion thickness and had the various numbers of piles
were investigated to check the improvement of ultimate bearing
capacity. Thirdly, the improvement of ultimate bearing capacity of the
piled-raft foundations which had various sand cushion thickness were
investigated at the certain displacements. Finally, the total and
differential settlements of piled-raft foundations were compared in the
various numbers of piles and sand cushion thickness.
ANALYSIS METHOD
Fig. 1 shows the typical results of laboratory tests of the soil samples in
this area. Considering the behavior of piled-raft systems, plane-strain
nonlinear analyses were performed by using the finite element code
called PLAXIS (Verneer and Brinkgreve, 1995). This geotechnical
finite element program can analyze the behavior of a raft and piles on a
layered soil profile. Since the problem is symmetrical, only the right
half is modeled. As for the element type, six node triangular elements
were used. The side resistance was modeled by using interface
elements between the piles and soil layers where the piles were
available. The contact between the raft and the soil was assumed to be
frictionless.