2739
Bearing capacity of displacement piles in layered soils with highly diverse strength
parameters
Capacité portante des pieux de deplacements battus dans les sols stratifiés avec des paramètres
fortement differés de la resistance
Gwizdala K., Krasinski A.
Gdansk University of Technology, Gdansk, Poland
ABSTRACT: Reliable prediction of pile bearing capacity including the installation effects is still a serious research and engineering
problem. Computational analyses should be related to the load-settlement characteristics comprising entire range of loads, up to the
limit state. European standard Eurocode 7 includes only the general rules and some calculation concepts. In the analytical method of
pile bearing capacity calculation a commonly applied static formula, based on pile base and shaft unit resistances, is proposed. It is
also recommended that the calculating procedures and the values of unit soil resistances used in the calculations would be verified by
field tests done on real piles. In the paper the cases of bridge structures founded on driven displacement Vibro and Franki piles are
presented. The design assumptions and computational results were verified by static load tests of piles (SPLT). Soil profiles and
geotechnical parameters of soil layers have been determined by CPTU tests. One of static load tests was carried out on instrumented
pile equipped with extensometers.
RÉSUMÉ : Prediction rèliable de la capacité portante des pieux avec les effets d’installation constitue toujours un problème important
de l’ingénièrie et de la recherche. Analyse devrait inclure les caractéristiques charge-tassement dans le cadre entière de la charge
jusqu’au l’état limit. Le standard Eurocode inclue seulement des règles et une concéption générales de calculs. Une méthode analytic
de la capacité portante est proposée par la formule statique considerant unitaires resistances de base et du frottement latérale. Il est
récomandé que la procedure de calcul ansi que les valeurs unitaires de la résistance au sol soit verifier par un essais de chargèment
d’un pieu réel. Quelques cas des fondations de ponts posés sur les pieux battus Vibro et Franki sont présentés. Les assumptions de
dimmensionement et les résultats de calcul sont vérifiés par le chargement statique des pieux (SPLT). Le profil du sol et les
paramètres de couches du sol sont détèrminés par les essais CPTU. Un essais de chargement statique est réalisé avec un pieu
instrumenté avec éxtènsomètres.
KEYWORDS: pile bearing capacity, settlement of piles, displacement piles
1 INTRODUCTION
Reliable prediction of bearing capacity and settlements of piles
is still quite a difficult task either from theoretical as well as
from practical point of view. The analysis of the transmission of
loads by the construction into the subsoil in terms of full
relation between load and settlement (
Q
–
s
) is more adequate.
Currently in Europe and also in Poland general Eurocode 7 rules
are applied.
According to the Eurocode recommendations, a design of
pile foundations should be made using one of the following
approaches:
a) based on the static load test results, the consistency of which
with other comparable experiences has been proved by
calculations or in some other way;
b) based on empirical or analytical calculation methods, the
reliability of which has been confirmed by static load tests
in comparable conditions;
c) based on dynamic load test results, the reliability of which
has been revealed by static load tests in similar soil
conditions and for the same type of piles;
d) based on the observation of the behavior of comparable pile
foundations, provided that the data were verified by field
tests (site investigations and soil testing).
It should be pointed out that basic method for the evaluation
of
Q
-
s
curve are static load tests. An example of the relation of
shaft, base resistances, total load and pile shortening are
presented in Fig. 1. According to Eurocode principles, the
conventional ultimate load
Q
u
, corresponding to the settlement
equal to 10% of pile diameter (
s
= 0,10
D
) has been indicated. In
such approach, partial coefficients for loads and partial
coefficients for shaft and base resistances should be correlated
with assumed ultimate load level determined from
Q
-
s
curve.
Characteristic bearing capacity of the pile determined on the
basis of geotechnical parameters from ground test results can be
calculated from the following formulae:
R
c;k
=
R
b;k
+
R
s;k
(1)
R
b;k
= A
b
·
q
b;k
(2)
R
s;k
=
∑ A
s;i
· q
s;i;k
(3)
where:
q
b;k
– characteristic value of the unit resistance under the pile
base,
q
s;i;k
– characteristic value of the unit resistance over the pile
shaft in subsequent soil layers,
A
b
– calculated area of the pile base,
A
s;i
– calculated area of the pile shaft in the subsequent soil
layers.
All over the world there exists quite a lot of methods for the
calculation of pile bearing capacity. Also in Poland proposals of
q
b;k
i
q
s;i;k
calculation in relation to various technologies of
piles have been elaborated, see e.g. Gwizdala 1997, Gwizdala
2011, Gwizdala and Steczniewski 2007, Gwizdala et al 2010,
Krasinski 2012. When the pile bearing capacity is concerned,
the range of settlement curve for the given method should be
