3D Modelling of Frozen Ground and Associated Volume ... - Plaxis

3D Modelling of Frozen Ground and Associated Volume Expansion
Joseph Sopko, Ground Freezing Services, Moretrench
Plaxis was contracted by Moretrench to provide assistance in setting-up 3D finite element models for the analysis
of how ground freezing process and associated volume expansion would impact existing slurry wall panels. Thanks
to PLAXIS Expert Services, valuable results have been obtained in terms of displacement and integrity assessment
for the existing slurry walls.
What is ground freezing?
The ground freezing process converts pore
water into ice by the continuous circulation of a
cryogenic fluid within a system of small diameter,
closed-end pipes installed in a pattern consistent
with the shape of the area to be treated. The
frozen pore water acts as a bonding agent, fusing
together particles of soil or rock to significantly
increase compressive strength and impart
impermeability.
Freeze pipes are typically installed vertically in the
soil and connected in series-parallel. The coolant
is pumped down a drop tube to the bottom of the
freeze pipe and flows up the annulus, withdrawing
heat from the surrounding soil. When the soil
temperature reaches 32°F (0°C), ice begins to form
around the pipes in the shape of vertical, elliptical
cylinders. As the cylinders gradually enlarge with
time, they interconnect to form a continuous wall.
Once the frozen wall reaches design thickness, the
freeze plant is typically operated at a reduced rate
to maintain the condition.
“PLAXIS professionals assisted us tremendously in moving forward our
analytical tools and ability to understand the mechanics of our project... “
Unlike other groundwater cut-off and excavation
support techniques, ground freezing is a
minimally invasive technique that requires
only limited physical penetration into the
ground and propagates thermally, rather than
by displacement. The ground remains largely
undisturbed during freeze pipe installation and the
freezing process and, in most instances, reverts
to its original state once freezing is discontinued.
Ground freezing provides both excavation support
and groundwater control. While the use of ground
freezing for the construction of deep shafts
through water-bearing soils is the most common
application, the technique is also appropriate for
mining of cross passages and connector tunnels,
stabilization of mixed-face tunnels, frozen-arch
canopies, and containment of contamination.
Finite element modelling
The dimension of the FE model is 100m*50m*60m.
Only half of the geometry has been modelled
due to symmetry conditions. The model contains
a total of 64,000 elements and 90,000 nodes (i.e
FE mesh
Calculated displacement after ground frozen
20 Plaxis Bulletin l Spring issue 2013 l www.plaxis.nl

270,000 dof). Each finite element has 10 nodes.
The slurry wall panels have been defined around a
11m diameter shaft. The frozen pipes have been
placed along a series of concentrated circles.
In PLAXIS the frozen process is modelled in
sequence by applying an isotropic volumetric
prescribed strain of 1.5 % (so 0.5% in X-direction,
0.5% in Y-direction, 0.5% in Z-direction ) followed
by replacing the properties of the swollen clusters
by those of frozen soil after each expansion phase.
The model consists of the following construction
sequence:
• Initial stress definition and initial water level
• Slurry wall construction
• Excavation
• Ground freezing part I at diameter D1
• Ground freezing part II at diameter D2>D1
• Ground freezing part III at diameter D3>D2
• Ground freezing part IV at diameter D4>D3
Vacancy: Advisor PLAXIS Expert Services
Plaxis bv is currently looking for an advisor to
strengthen our expert services team.
The advisor is expected to plan, execute
and report projects and results on his/her
own. He/she also acts as project leader and
maintains contacts with clients, universities
and other external parties.
“The implementation of PLAXIS to applications of artificial ground freezing
has significantly advanced the analysis of complex problems... “
For more information please visit:
www.plaxis.nl/jobs
PLAXIS Expert Services added value:
• Quick start on the job
• Set-up of fully optimized and ready to run 3D
FE models
• Regular model review
• Next business-day advanced technical
assistance
Customer quotes
The implementation of PLAXIS to applications
of artificial ground freezing has significantly
advanced the analysis of complex problems,
particularly the mechanics associated with ground
expansion as well as the decrease in frozen soil
strength with time under load.
Applying PLAXIS to this problems was a
major undertaking for our engineers. PLAXIS
professionals assisted us tremendously in
moving forward our analytical tools and ability to
understand the mechanics of our project as well as
being able to present the results to our client with
incredible output graphics.
The PLAXIS consultants eagerly accepted the
challenge, fully understood our concerns and
rapidly and efficiently assisted us in generating
the models that accurately simulated the complex
issues with stress and deformation on existing
structures. Their assistance has enabled us to
make PLAXIS a readily available tool to analyse
a wide arrange of ground freezing challenges.
Their knowledge, accuracy and efficiency are
unsurpassed in this industry.
About Moretrench
Moretrench is geotechnical contractor delivering
geotechnical solutions to the underground,
industrial and environmental remediation
industries. Moretrench offers a wide range of
specialized services with skilled and experienced
engineering teams in the field of dewatering
and groundwater control, ground freezing,
earth retention and anchors, deep foundations,
underpinning, grouting and ground improvement,
environmental remediation.
Stress development in the slurry panel
www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 21