2719
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Consolidating Soil-Pile Interaction
Interaction pieux-sol en cours de consolidation
N. El-Sakhawy & A. Nassar
Engineering College, Zagazig University, Zagazig, Egypt
ABSTRACT: Soft clay is consolidated upon any slight change in the effective vertical stress. Consolidation causes a downdrag
movement to the shaft piles that penetrate this layer. The downdrag movement adds additional loads to the already loaded pile. This
force is expressed as a negative skin friction. Negative skin friction extends to a depth depending on the consolidating soil-pile
interaction. This depth is referred to as the neutral plan.
This paper presents a study on the behavior of soil-pile interaction during soil consolidation. Experimental work was developed to
analyze the negative skin friction and the location of the neutral plane on a single pile embedded in soft clay layer. The pile is ended
either in clay or sand soil, or is floated. The clay layer was allowed to consolidate. The study concluded that the neutral plane is
located closer to the end of the pile as the end bearing increases. The depth of neutral plane increases by increasing the embedded
length of the pile in the clay layer. Closed form equations of Shong, 2002, predict the experimental results in very well agreement.
RÉSUMÉ :
Une argile molle se consolide sous l’effet d’une augmentation de la contrainte verticale effective. Cette consolidation
provoque un déplacement vertical des pieux qui pénètrent la couche de sol. Ce mouvement ajoute des charges supplémentaires au pieu
déjà chargé. Cette force peut être exprimée en frottement latéral négatif et s’étendre en profondeur en fonction de l’interaction sol-
pieux en cours de consolidation. Cette profondeur est définie comme étant le plan neutre.
Cet article présente une étude sur le comportement de l'interaction sol-pieux lors de la consolidation des sols. Un travail expérimental a
été développé pour analyser le frottement négatif et l'emplacement du plan neutre sur un seul pieu fondé dans une couche d’argile. La
couche d’argile peut se consolider. L'étude montre que le plan neutre se situe près de l’extrémité du pieu ou la charge augmente. Ainsi,
la profondeur du plan neutre augmente en même temps que la longueur du pieu. Ces résultats sont en accord avec ceux de Shong,
2002.
KEYWORDS: soft clay consolidation, pile foundation, negative skin friction, down drag movement neutral plane, floating piles
1 INTRODUCTION
Deep foundations are installed in different soil stratification.
Usually the penetrated soil stratifications offer considerable
resistance for the pile shaft deformation upon loading. This
resistance is called shaft resistance or skin friction. This soil
behavior can be reversed drastically if the penetrated soil
encompasses soft clay.
Soft clay is consolidated upon any slight change in the
effective vertical stress. Consolidation and settlement of soft
clay surrounding a pile usually drag the pile downward (Walker
and Dravall 1973, Sharif 1998, Leung, et al. 2004). The
downdrag movement adds additional loads, downdrag force, to
the already loaded foundation. Dragload heavily depends on the
interface properties (friction coefficient, and adhesion), surface
loading, and axial load (Sangseom Jeong, et al. 2004).
This force is the most common problem in the design and
construction of pile foundations in soft soil (Maugeri et al.
1997, Van Der Veen 1986). (Briaud 2010) pointed out
situations where downdrag force should be considered in the
design.
The downdrag force can be expressed as a negative skin
friction (N.S.F) as it acts in the opposite direction for the
normally skin friction. The negative skin friction will be
mobilized in the upper portion starting from the pile head to a
neutral depth, ND, after which positive skin friction is
mobilized in the lower portion. ND can also be defined as the
depth at which the relative displacement between the pile and
the soil is zero (Fellenius 1989, 2004, and 2006). (Bozozuk
1972) suggested that ND depends only on the embedded length
of the pile in the clay layer. However, (Poorooshasb et al.
1996) stated that ND is not highly influenced by the
surcharge. But that the presence of a strong soil layer at
the tip of the pile would have a significant influence.
(Shong 2002) proposed closed form equations to locate the
neutral plane depending on the elasticity of the soil and the pile.
According to this approach, the depth of neutral plane, ND, to
pile penetration length, L is:
) -1(
2
u
d
s
u
Q
Q
Q
Q
L
ND
(1)
Where
Q
u
= ultimate pile capacity = Q
s
+ Q
t
(2)
Q
s
, the pile shaft resistance over the whole shaft length, is:
(3)
dz
Dz
Q
L
z
s
∫
)
(
0
'
L = length of the pile,
D = pile diameter,
Q
t
, = the pile toe resistance:
Lz
t
t
t
ANQ
`
. .
(4)
A
t
= the toe area of the pile,
Q
d
= imposed load at pile top
Shong suggested a range of
varies between 0.25 - 0.35 for
clay, and a value of 3 for N
t
, toe bearing capacity coefficient.
Consolidating Soil-Pile Interaction
Interaction pieux-sol en cours de consolidation
El-Sakhawy N., Nassar A.
Engineering College, Zagazig University, Zagazig, Egypt
