PILE RAFT FOUNDATION WITH PILES ACT AS SETTLEMENT REDUCER
ABDELAZIM MAKKI IBRAHIM
Institut für Geotechnic
Gustav-Zeuner-Straße 1, 09596 Freiberg –Germany
+4917665580114, E: [email protected]
Abstract
In this paper the behavior of pile raft foundation supported by un-identical piles is examined
by use of commercial software program PLAXIS 3D based on finite element method. The
effect of piles length, diameter and piles spacing on reducing overall settlement was
determined and the contribution of raft in increasing load carrying capacity was evaluated
and detailed analysis was undertaken. Attention has been also focused on the improvement of
the foundation performance due to the raft provide a reasonable measure of stiffness and load
resistance.
Introduction
The main purpose of the piles in piled raft foundation (PRF) is to act as settlement reducers
and the load carried by piles is considered as secondary issue in design (Chow 2007). Piled
raft foundations provide an economical foundation when raft alone does not satisfy the design
criteria. In pile raft foundation plies support for control settlement and raft provide additional
capacity at ultimate loading and hence reduce the required numbers of piles and also the raft
may provide redundancy to the piles and thus reduces the potential influence of affected piles
(if any) on the foundation performance, under such circumstances, the presence of the raft
allows some measure of re-distribution of the load from the affected piles to those that are not
affected Poulos et al (1994). Piles may also reduce the differential settlement when raft alone
exceed the allowable settlement and the raft may increase the lateral stress between the
underlying piles and the soil, and thus can increase the ultimate load capacity of a pile as
compared to free-standing piles (Katzenbach et al., 2005). The settlement reducing piles are
therefore introduced in the centre of the raft to reduce differential settlement. Pile and pile
raft foundation have been extensively studied and good contributions was made by Fellenius
(2004).
Statement of the problem
The study area is in Khartoum state-Sudan. The infrastructures in the study area are founded
on old buried channels which comprise of a very weak soils consists of clayey soil, silty clay
and fine sand with some inclusions of lenses and pockets of gravel with high water content.
The presence of these problematic soils and inclusions of gravel pockets creates serious
threats and adverse effects for foundation design. These soil problems lead to excessive
settlement collapse and tilting of many building.
Methods
Detail site characterization has been made and the subsoil layers model was established from
3 boreholes data extend to 25 m from ground surface to the dense sand and 2 Cone
Penetration Tests (CPT) were conducted. Five layers were encountered and the geotechnical
parameters for these layers are estimated from (CPT) tests, Triaxial and Oedometer tests. The
least favorable soil condition was used as the main basis for geotechnical model.
Geotechnical characteristic of subsurface soils
The boreholes revealed existence of alternating layers of very stiff low to high plasticity silty
clays (CL to CH) and very stiff low to high plasticity silts (ML to MH) in the upper 15meters.
This is underlain by medium dense silty sand and sandy silt (SM or SP-SM) layers extended
down to 13 to 15 meters and this layer overlain mudstone layer which underlain by
sandstone, Figure 1 shows cross section of the subsurface layers in the study area.
Figure 1: Cross section of the subsurface soil layers in the study area
Finite Element Methods (FEM)
The main purpose of using finite element in this study is to investigate the effect of adding
piles to the base of the raft acting as settlement reducer.
Selection of constitutive soil model
Mohr-Coulomb model is an elastic-perfectly plastic model is used in this study to model soil
behavior. In general stress state, the model’s stress-strain behaves linearly in the elastic range,
with two defining parameters from Hooke’s law (Young’s modulus, E and Poisson’s ratio, ߥ).
There are two parameters which define the failure criteria (the friction angle, I and cohesion,
c) and also a parameter to describe the flow rule (dilatancy angle, \ which comes from the
use of non-associated flow rule, is used to model a realistic irreversible change in volume due
to shearing.
Raft model
The raft is assumed to be square in shape with the extension of 30 meters and 50 cm thick of
concrete. The load of the building is transferred to the raft by columns each one bearing load
of 10000 kN . The material properties of the raft are shown in Table 1.
Pile model
Circular massive concrete bored pile has been chosen in this study, the material properties of
the pile have been taken from the literature Table 1.
Parameter
Young’s modulus
Poisson’s ratio
Unit weight
Type of behavior
Pile type
Predefined pile type
Diameter
Base resistance
Name
‫ܧ‬
‫ݒ‬
ߛ
Type
െ
െ
Diameter
‫ܨ‬௠௔௫
Pile
͵‫଻Ͳͳݔ‬
0.2
6.0
Linear, isotropic
predefined
Massive circular
1
ͳǤͷ‫Ͳͳݔ‬ସ
Raft
͵‫଻Ͳͳݔ‬
0.15
15
Linear, isotropic
Unit
݇ܰȀ݉ଶ
െ
݇ܰȀ݉ଷ
െ
െ
െ
݉
݇ܰ
Table 1: Material properties of embedded pile and the raft
Structural model of the raft and pile raft
Three dimensional model was created shows the load distribution, raft extension. Thirty six
piles are used to support the raft. The piles length are systematically decreases form 20 meter
at the center to 8 meter to the edge of the raft. The raft and the arrangements of pile are
shown in Figure 2. Because of symmetry of the problem only quarter of the model is used to
reduce the time of the calculation.
Figure 2: 3 Dimension model of the raft with un-identical piles
Geotechnical parameters of subsurface soil
The parameters of the soil use in this study are obtained from the results of Cone Penetration
Test (CPT), Standard Penetration Test (SPT) and triaxial and oedometer laboratory tests. The
material properties of the soil are shown in Table 2.
Parameter
Material model
Drainage type
Unit weight above phreatic level
Unit weight below phreatic level
Young’s modulus
Poisson’s ratio
Cohesion
Friction angle
Name
Silty clay
General
Model
Mohr-Coulomb
Type
Undrained
16
ߛ௨௡௦௔௧
17
ߛ௦௔௧
Parameters
ͳǤͷ‫Ͳͳݔ‬ସ
‫ܧ‬′
0.34
‫ݒ‬
45
ܿ
5
I
Table 2: Material properties of the soil
Sand
Unit
Mohr-Coulomb
Drained
18
20
െ
െ
݇ܰȀ݉ଷ
݇ܰȀ݉ଷ
ʹǤͷ‫Ͳͳݔ‬ସ
0.3
5
30
݇ܰȀ݉ଶ
െ
݇ܰȀ݉ଶ
‫݋‬
Results and conclusion
The total displacement is shown in Figures 3 and 4 for un-pile raft and pile raft respectively,
these figure show decreasing of total settlement from 49.71 mm to 39.42 mm when piles are
add to the raft. These figures show the improvement of foundation performance and
decreasing of total settlement compare to the shallow foundation (raft alone), this may be due
to the high pressure subjected to the soil surrounding the pile shaft, which is result of the
consolidation of the soil and thus improvement of the soil bearing and deformation behavior,
also due to the reduction of water content as the result of increasing density of the soil and an
increase in strength and stiffness of the soil, the figures also show that the settlement of the
raft is found to be maximum in the centre and minimum at the corner. A reduction of
maximum settlement up 20% is achieved when piles are introduced compare to a shallow
foundation (raft alone).
Figure 3: Un-pile raft
Figure 4: Combined Pile Raft
References
Chow, H. 2007: Analysis of pile-raft foundations with piles of different lengths and
diameters. PhD thesis University of Sydney.
Fellenius, B. H., 2004: Unified Design of Piled Foundations with Emphasis on Settlement
Analysis. Journal of the Soil Mechanics and Foundations Division, ASCE, 125(GSP), 1-23.
Katzenbach, R., Bachmann, G., Ramm, H 2005: Combined Pile Raft Foundation(CPRF):
An Appropriate Solution for Foundation of High-Rise Building. Sloak Journal of Civil
Engineering (SJCE), Page 19-29.
Poulos, H.G., 1994.: An approximate numerical analysis of pile-raft interaction, Int. Journal
for Numerical and Analytical Methods in Geomechanics, 18: 73–92.