2933
Estimation of undrained shear strength of soft soil obtained by cylinder vertical
penetration
Estimation de la résistance au cisaillement d’un sol mou en conditions non-drainées obtenue par la
pénétration verticale d’un cylindre
Equihua-Anguiano L.N.
UPAEP University, Puebla, Mexico
Orozco-Calderon M.
Geotechnical consultant, Ciudad del Carmen, Campeche, Mexico
Foray P.
Laboratoire 3SR,Grenoble, France
ABSTRACT: Determination of undrained shear strength (
s
u
) in cohesive soils is essential for the design of the offshore
pipelines/flowlines, especially when burial depth is small (in the order of twice of the pipeline diameter). This paper presents
experimental results of the undrained shear estimation, obtained by the vertical penetration of a cylinder in a soft normally clay with
deep water characteristics. Results obtained were compared with analytical solution for a cylinder embedded in a cohesive soil, as
well as with experimental
s
u
values obtained using laboratory vane test. The results between the two techniques are very close
RÉSUMÉ : La détermination de la résistance au cisaillement en conditions non-drainées des sols est nécessaire dans le design des
conduits/
flowlines offshore,
particulièrement dans le cas où profondeur d´enfoncement est de l´ordre de deux fois le diamètre du
conduit. Cet article présent les résultats d´une étude expérimentale conduisant à la détermination de la résistance au cisaillement non
drainée d´un sol argileux très mous, en utilisant l´enfoncement d´un cylindre. Enfin, concernant l´aspect modélisation, une solution
analytique correspondant à l´enfoncement d´un conduit dans un sol cohérent a été utilisée. Les résultats ont été comparés avec ceux
obtenues avec un scissomètre du laboratoire et montrent une bonne approximation.
KEYWORDS: undrained shear strength, deep water, offshore geotechnics, pipeline, vane test..
1 INTRODUCTION.
Deep water soils present special characteristics with respect to
the other soils. Examples of the particular parameters
correspond to undrained of strength and plasticity. In field, high
plasticity values (liquidity index close to 1) and low undrained
shear strength (
s
u
) have been found, this last parameter varies
approximately from 1 to 3 kPa on the seabed surface.
Undrained shear strength parameters is main in the marine
pipeline conception as well as, its variation with depth,
especially in the first tens centimeters. In practice, CPT, T-bar
and recovery of intact samples with the Box corer and mini-
Tbar are used
in situ
(ISSMGE-TC1 2005, Puech et al. 2010) to
obtain geotechnical design parameters. In the same way,
laboratory test are done for
s
u
determination to little depth in the
soil, using experimental techniques well known as: vane shear
laboratory (AFNOR, 1995), fall cone (Hansbo 1957, Orozco
Calderon & Mendoza 2002), T-bar penetrometer (Stewart &
Randolph 1991), among others. These techniques allow
obtaining
s
u
to the surface level, at a point or continuously with
depth. In this context, the article presents an option to measure
s
u
from testing cylinders in laboratory as is described below.
1.1
Objective
An alternative to estimate experimental undrained shear
strength variation with depth was developed. Procedure and
laboratory tests were done penetrating a cylinder in a soft soil
with deep water characteristics. First, the cylinder was placed
horizontally in the soil surface and then, it was penetrated
vertically into the soil. The vertical displacement (
z
) was
applied with an electromechanical actuator and the vertical
force (
F
v
) was recorded and were compared with a theoretical
solution, in order to determine the undrained shear strength.
Finally, the results were compared with
s
u
values obtained with
a laboratory vane. Figure 1 shows the schematic cylinder and
nomenclature used.
z
F
v
Cylinder model
Soil
r
0
Figure 1. Schematic cylinder penetration.
2 PARTIAL PENETRATION OF A CYLINDER
2.1
Theoritical solutions
A theoretical solution to the failure load or collapse for a pipe
on the soil was given by Murff et al. (1989) analyzing a pipe
partially embedded on the soil and the undrained shear strength
(
c
) uniform with the depth. The solution takes into account the
type of surface roughness of the pipe through the concept of
adherence
a
d
=
c
, and consider a value of
=0 as smooth
surface and
as rough surface.
Vertical force on the cylinder (or a pipe) to a certain depth
z
on a soil with horizontal surface, is given by equation (1):
0
cos
2sen
1
2 cos
2
2
2cos
2sen
2
v
F
r c
(1)
where:
a
d
= adhesion at the interface soil-cylinder;
c
=undrained
shear strength;
=sen
-1
;
=sen
-1
(1-
z
/
r
0
);
r
0
=cylinder radius;
z
=penetration depth of the cylinder.
