May/June 2023 - 81

Case Study: Rigid Inclusions
at an Oil Refinery
Five large-diameter water tanks were
proposed at a Gulf Coast oil refinery. A
loose, submerged sand with 10-20%
fines layer exists from depths of
roughly 15-37 ft (5-11 m). This layer
was found by the geotechnical engineer
to be susceptible to liquefaction. Below
the sand layer, there is a stratum of soft
clay extending to a depth of about 58 ft
(18 m), which overlies a medium dense
sand layer.
The tanks would generate a surface
pressure of up to about 3,700 psf
(18,065 kg/m2) in operational conditions,
with larger values for temporary
or extreme loading cases. Although the
settlement tolerances were relatively
ample, they would be exceeded unless
ground treatment was carried out
under the footprint of the tanks. The
owner also sought to mitigate liquefaction
of the submerged sand stratum
and liquefaction-induced settlement.
Displacement rigid inclusions was
the technology selected to mitigate
settlement under sustained loads, as
well as mitigate settlement associated
with the occurrence of liquefaction.
Mitigation of liquefaction
altogether, thus precluding
the associated settlement,
was not a design objective per
s e ; howe v e r, t he po s tinstallation
CPT campaign
was planned to assess the
improvement of factor of
safety against liquefaction to
inform the design of the tank
by its designer. The design
developed by GEI Consultants
consisted of a grid of 16 in
(400 mm) diameter rigid
inclusions.
The design for sustained,
static loading was performed
using standard numerical
analyses procedures, with a
focus on careful selection of
suitable, nonlinear model
inclusions in the soil mass. It was
decided to verify the effectiveness of
the system using post-installation CPT.
Response to Installation of
Rigid Inclusions
Layout of rigid inclusions
Post-installation CPTs were installed at
several locations within the footprint of
each tank, at the diagonal and direct
midpoints between rigid inclusions
(nodal locations). The locations for postinstallation
CPTs were selected at, or
very near, the location of a preinstallation
CPT probe.
Comparison of pre- and postPost
rigid inclusion CPT pattern
parameters for the soils that capture
variat ion in soi l modulus wi th
confining stress. Verification of the
liquefaction potential of the improved
soil was performed separately by hand,
through the estimation of the increase
in average relative density of the sand
after the introduction of the rigid
installation CPT probes demonstrates
that there is a significant increase in the
penetration resistance through granular
layers measured after installation of the
rigid inclusions. This was the case for
each of the nodal, post-installation CPT
probes performed at the site.
In the soft clay, pre- and post3D
finite element model of rigid inclusion system
installation penetration resistance
values were almost identical. However,
dynamic pore pressure measurements
did show a significant change in
reaction of the soil to advancement of
the CPT probe within the soft clay.
Dynamic pore pressures generally
decreased in the soft clay after
rigid inclusion installation.
The magnitude of dynamic
pore pressure drop, which
often reached over 20 psi, is
significant considering the
potential increase in static
pore pressure resulting from
instal lat ion of the rigid
inclusions. This was observed
in all nodal post-installation
CPT probes performed. It is
important to note, however,
that dynamic pore water
pressure measurements may
not be repeatable from one
project to another, or even
within the same project, due
to variations in the porous
stone material, operator and
fines content of the soil.
DEEP FOUNDATIONS * MAY/JUNE 2023 * 81
May/June 2023

Table of Contents for the Digital Edition of May/June 2023
