2300
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
(Moradi et al 2013)
Figure 14. Pipe response to shear fault displacement in a centrifuge.
3.3
Seabed structures that displace horizontally
The sea bed in deep water is generally soft and often requires
large shallow foundations to support seabed facilities. If some
movement could be tolerated the size could be reduced.
Further, if the structure connects with a pipeline subject to
walking or other axial force, there may be merit in allowing the
structure to slide horizontally to help relieve concentrated load.
Bretelle and Wallerand (2013) examine the design for a
shallow foundation that displaces horizontally in a cyclic
fashion, as might be caused by repeated pipe expansion and
contraction. The influence of soil softening, foundation
settlement and potential change in stiffness with time is
examined through relatively straightforward analysis. The
authors conclude that shallow foundations designed to displace
horizontally could be useful for subsea pipeline networks.
3.4
Large displacement in dense sand
While numerical analysis for large deformation is increasingly
amenable for engineering design, a relatively simple
constitutive model for dense sand that provides stable large
deformation analysis is still subject to study. Li et al (2013)
propose a Critical State Mohr Coulomb (CSMC) model:
deformation up to peak strength is elastic and thereafter dense
sand dilates (including non-associated flow) and reduces in
strength to the critical state angle of friction. The concept of the
state parameter defined by Been and Jeffries (1985) is used.
A key objective is analysis for punch through of a spudcan
footing in dense sand overlying soft clay. Li et al (2013) have
not reached that target. However, development of the model
starkly highlights non-uniform deformation and preferential
shear band formation in dense sand post peak that makes data
acquisition (lab tests) and model calibration such a challenge.
In analysis for bearing capacity of a circular plate on uniform
sand, the authors found that the elastic stiffness of the sand
influences bearing capacity by as much as 50% over the realistic
range, reminiscent of rigidity index in penetration problems.
Stiffness was found to have greater impact than dilation angle.
The analysis for bearing capacity is described in terms of a
combined bearing capacity factor
N
qγ
that applies across the
range from
N
q
alone to
N
γ
. The proposed formula for
N
qγ
includes soil stiffness and dilation angle along with peak
friction angle, foundation size, soil unit weight and surcharge.
4 SEABED SOILS.
The three papers on soil properties cover diverse topics. Ho et al
(2013) describe undrained cyclic triaxial compression tests on
isotropically consolidated Singapore Marine Clay. The focus is
the behavior of the clay when it is sheared monotonically to
failure after cycling. The tests show that when the current mean
effective stress in the sample reduces below half the original
preconsolidation pressure,
p�/p
c
�
≤ 0.5, due to cyclic loading,
some increase in mean effective stress commences at higher
stress ratio in each cycle. At mean effective stress
p�/p
c
�
≥
0.6 (first few cycles) the mean effective stress of the clay
always reduces. This behavior is similar to normally versus
over consolidated clay. The final effective stress path for
monotonic triaxial compression to failure after cycling similarly
depends on the mean effective stress
p�/p
c
�
at the end of
cycling. However, the shear strength is found to be largely
independent of the previous number of load cycles and the
strain amplitude.
Kim and Safdar (2013) report cyclic direct simple shear
tests on compacted silty sand to define the limiting cyclic stress
ratio versus number of cycles for two initial void ratios.
Tyldesley et al (2013) describe site investigation to define
parameters for wind farm foundation design on a deep deposit
of carbonate clayey silt till in Ontario Canada. This onshore site
investigation demonstrates the use of insitu tests and shear wave
velocity measurement, interpreted together with laboratory tests,
to assemble knowledge on soil strength and stiffness properties.
5 COASTAL AND NEARSHORE WORK.
There are three papers on diverse topics. Madrid et al (2013)
describe site investigation, cyclic laboratory tests and numerical
analysis for the stability of a caisson breakwater in about 20m to
25m depth of water. The caissons are founded on a rubble
mound infilling a large zone where the deep underlying soft
clay soil was removed, Figure 15.
CAISSON
RUBBLEMOUND
172.27
-26.00
RIPRAP300kg
SEAWARDSIDE
RIPRAP 4 ton
CONCRETEBLOCK
RIPRAP 300kg
0.00
-21.00
-23.00
-13.75
-15.00
-13.00
-22.00
-18.00
0.00
+11.00
+3.00
(Madrid et al 2013)
Figure 15. Caisson breakwater and stability analysis for wave impact.
There is much detail in the paper on soil testing and soil
properties, loading cases for various phases of project
construction and hydrodynamic testing to determine dynamic
uplift. A good description is provided on the way cyclic
loading and shear strength reduction were treated for design.
Relic footprints from earlier jack-up activity can occur next
to the location for new shallow foundations. Ballard and
Charue (2013) describe a study on a circular zone of remoulded
soft clay (
S
r
= 2) with a diameter equal to the size of the square
mudmat and with soft clay thickness of half that size. The
limiting envelope for combined moment and horizontal
resistance is computed for a range of applied vertical load
(
V/V
ult
), and a range of distance between the mudmat and the
remoulded zone/footprint that causes the moment and horizontal
resistance to be reduced, as well as
V
ult
.
2D and 3D analyses
show very substantial benefit from the 3D geometry in this case.
A detailed design and project record for installation of large
diameter, buried HDPE pipes in a nearshore environment prone
to seismic loading is described by Bellezza et al (2013). Details
for the case history and the various code requirements
considered in design are documented. Initial measurements are
provided on the vertical deflection of the installed pipes.
6 PILE FOUNDATIONS.
A lack of code guidance on capacity, stiffness and displacement
for cyclically loaded piles is being addressed by collaborative
research including the original GOPAL study and the current
SOLCYP project, supplemented by individual research work.
Several papers to this session report on SOLCYP results from
instrumented field tests, calibration chamber and centrifuge
