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PILE RAFT FOUNDATION WITH PILES ACT AS SETTLEMENT REDUCER

ABDELAZIM MAKKI IBRAHIM

Institut für Geotechnic

Gustav-Zeuner-Straße 1, 09596 Freiberg  – Germany

+4917665580114, E: abdelazimi@yahoo.com 

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 reducersand 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 designcriteria. 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 foundedon 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 ConePenetration Tests (CPT) were conducted. Five layers were encountered and the geotechnical

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 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 siltyclays (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, ).

There are two parameters which define the failure criteria (the friction angle,  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.

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 Parameter Name Pile Raft Unit

Young’s modulus         

Poisson’s ratio    0.2 0.15  

Unit weight   6.0 15  Type of behavior Type Linear, isotropic Linear, isotropic  

Pile type    predefined  

Predefined pile type   Massive circular  

Diameter Diameter 1  

Base resistance      

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 areshown 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 Name Silty clay Sand Unit

General

Material model Model Mohr-Coulomb Mohr-Coulomb  

Drainage type Type Undrained Drained  

Unit weight above phreatic level   16 18  

Unit weight below phreatic level   17 20  

Parameters

Young’s modulus  ′        

Poisson’s ratio    0.34 0.3  

Cohesion   45 5  Friction angle    5 30  

Table 2: Material properties of the soil

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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.