Modeling Vacuum Cconsolidation with Finite Elements

Chapter 1
Introduction
1.1 Ground Improvement Techniques
One of the first and the more important considerations in most of civil engineering
projects is the selection of a suitable site for construction. A suitable site usually
means, from a geotechnical point of view, one where the foundation material has
sufficient strength and stiffness to carry the loads imposed without unacceptable
large deformations or stability problems, or else has drainage properties suitable
for the particular construction application. In practice the most suitable site is
determined by social or economic requirements, rather than purely engineering
considerations and in many cases the mechanical properties of the soils (strength,
stiffness or hydraulic conductivity) need to be improved through different engi-
neering methods called ground improvement techniques.
Due to increased urbanization there is now a growing shortage of available
land in many cities which leads to the search for ways to build on land previ-
ously thought unsuitable. Indeed for example there are airports, sea ports or even
skyscrapers that have been developed on land reclaimed from the sea. Another
example of applications for which ground improvement is almost inevitably re-
quired occurs in railway and highway construction. Because these transportation
routes often pass through regions underlain by deposits of soft or weak soils. The
mechanical properties of these softer soils often have to be improved in order to
adequately support the associated earth structures and to reduce the residual set-
tlements that occur during the useful life of the transport infrastructure. Those
are the main reason for which many ground improvement techniques have evolved
during the years. All those techniques seek to improve those soil characteristics
that match the desired results of a project, such as an increase in density and
shear strength to aid problems of stability, the reduction of soil compressibility,
influencing permeability to reduce and control ground water flow or to increase
the rate of consolidation, or to improve soil homogeneity.
In ground improvement, distinction is made between methods of compaction
or densification and methods of soil reinforcement through the introduction of
additional material into the ground. Bergado et al.
[1]
classified the various ground
improvement methods according to whether or not additives are used to directly
1
Chapter 1: Introduction
enhance the strength and stiffness of the ground. A slightly modified version of
their classification scheme for these methods is found in the book of Chai et al.
[2]
:
1. Work on the soil only (primarily to reduce the voids content):
• Densification by applying external forces to coarse-grained soils:
– Surface compaction;
– Vibro-flotation and vibro-compaction;
– Dynamic compaction;
– Resonance compaction.
• Improvement of the drainage of fine-grained soils (often combined with
preloading):
– Sand drains;
– Prefabricated vertical drains (PVDs;)
– Horizontal drains.
2. Addition of other materials into the soil deposit:
• Soil reinforcement:
– Mechanically stabilized earth (MSE);
– Sand compaction piles (SCP);
– Geo-piers (stone columns and granular piles).
• Use of chemical admixtures:
– Deep mixing method (DMM) using lime and cement;
– Chemical piles.
Preloading in combination with the installation of PVDs and cement deep mixing
(CDM, normally in order to form soil-cement columns in the ground) are widely
used to improve the performance of soft clayey deposits. The idea behind the
preloading is to start the consolidation process before the project itself is con-
structed so as to reduce the settlements that necessarily occur whenever a load
is applied to the soil. When preloading is used, the loads can be applied either
directly to the soft ground as a surcharge (by the placement of embankment fill)
or by applying a vacuum pressure to the soil. In this report we will focus focus
our interest on different vacuum consolidation techniques associated to the use of
vertical PVDs drains, horizontal drains and without drains.
1.2 Vacuum Pre-loading
Preloading is a commonly used method for improving the mechanical performance
of soft clayey deposits. The preload to the soil can be imposed by the weight of fill
material, or by a vacuum pressure applied to the soil, or also by some combination
of both. Use of atmospheric pressure as an alternative to fill preloading was first
proposed by Kjellman in 1952. With vacuum consolidation, the equivalent stress
2
1.2 Vacuum Pre-loading
of a 4.5 emphm conventional pre-loading fill can be applied to the clayey deposit
in four to six months, as a function of the horizontal coefficient of consolidation of
the soil and the vertical drains spacing. The increase in vertical effective stress is
due to the decrease in pore pressure, so the vacuum consolidation can be applied
in one single step, even on very soft soils.
Preconsolidation by vacuum consists in applying vacuum on a deposit by pump-
ing air and water from a grid of vertical and horizontal drains, decreasing pore
pressures inside the deposit. Today the use of vacuum preloading is spread all
over the world and a number of successful case histories have been reported in the
literature. This method has several advantages over embankment loading:
• Significant time saving can be achieved: Vacuum consolidation can be fully
effective as early as two weeks after the starting of vacuum operation.
• No fill material is required.
• There is no need for heavy machinery.
• No input of any chemical admixtures into the ground is required, so it can be
considered an ‘ecologic’ ground improvement method.
• If applied to the soft soil through the vertical drains, it results in an isotropic
increase in effective stress in the soil mass and so can be applied with full
intensity in a single stage (an equivalent fill preload is applied in stages to
avoid instability and bearing capacity failure).
Figure 1.1: Classical scheme of a vacuum preloading system: Swedish method (Kjellman, 1952).
Some disadvantages of the method occur when the underlying ground consists
of permeable materials: the cost of the technique will be significantly increased due
to the requirement of cut-off walls into non-permeable layers to seal off the vac-
uum. Moreover, in any vacuum consolidation system, various instrumentations are
necessary to control the system operation as well as to monitor the performance of
the treatment including vacuum pressure, optimal pore water pressure, discharged
water volume, settlement and lateral displacement, which are useful in judgment
of the time to stop the vacuum pump, and controlling stability of embankment
3
Chapter 1: Introduction
construction. This method results therefore technologically more complicated and
the efficacy is not always guaranteed due to some technical problems that might
occur: mainly in maintenance of effective vacuum pressure during treatment.
Because of this disadvantages, for several decades after Kjellman, the method
had not widely been applied due to various difficulties. With improvement in
the methodology and producing high quality vertical drains and airtight sheets,
vacuum consolidation method became popular in many countries. Furthermore,
together with clarifying the actual mechanism of vacuum preloading, improving
construction techniques for drain installation and vacuum pumping operation,
monitoring process, as well as developing analytical methods for designing al-
lowed the method be applied effectively and economically and capable to conduct
in various site conditions whether on-land or underwater. Even that, researches
are continuing to try to make the method more convenient, increase the efficiency
and to extend it to various application fields. Various international symposiums on
vacuum consolidation have been conducted with the target of better understand
the technique and the technology transfer.
The mechanism of vacuum consolidation is different from that which occurs
when a conventional surcharge load is applied to the ground surface to induce
soil consolidation. With the load originating from the weight of embankment fill
there will be consolidation settlement resulting from volume change in the soil
as well as shear stress deformations which will be manifested basically as lateral
displacement of the ground under the embankment. Instead, the basic principle
of vacuum consolidation is to remove atmospheric pressure from an isolated soil
mass to produce a reduction in the pore pressure. This reduction generates a pore
pressure gradient between the soil mass and the point of application of vacuum
by starting the consolidation process and providing an increase in effective stress.
The vacuum pressure will cause only isotropic stress increments in the ground.
Generally this will induce settlement as well as inward lateral displacement of the
ground, towards the center of the vacuum loaded region of soil.
Furthermore, the effects of the drainage boundary conditions are generally dif-
ferentinbothcases. Forexample, forasoildepositwithtwo-waydrainage, vacuum
pressure can be applied at one boundary (upper boundary) or also internally via
drainsinstalledintheground. Atthelowerdrainageboundarynovacuumpressure
can be applied effectively. Then there will be always a head difference between the
two boundaries of the deposit and the final condition will correspond to a steady
flow of water through the layers towards the upper drainage boundary at which
the vacuum pressure is applied. In contrast, if a conventional surcharge load is
applied to the surface of a soil layer with two way drainage, the flow of porewater
will move towards both the top and bottom drainage boundaries, driven by the
gradients of excess porewater pressure that vary throughout the layer.
It is worth to notice that, as reported by Dam et al.
[3]
, the effective vacuum
pressure inside the soil (under-sheet vacuum pressure) is normally lower than the
vacuum generated in the pump. For on-land systems, efficiency of vacuum loading
of 70-80% is evident. Designed vacuum pressure in Menard MVC system (ap-
pendix B.3) is around 75 kPa, while a value of 80 kPa is used in China (section
4
1.3 Aim of Research
B.2). In some design cases greater values of efficiency (80-95%) are reported. How-
ever, a minimum value of 60 kPa used to be recommended for design. Sandanbata
et al.
[26]
assert that together with using highly airtight membrane and highly per-
meable drain material, the introduction of a special designed air-water separation
vacuum pump system allows attaining a stable effective under-sheet vacuum pres-
sure as high as 90 kPa during treatment. In practice, various vacuum consolidation
systems have been developed for a number of applications. Some relevant methods
are described in details in appendix B.
1.3 Aim of Research
Numerous applications, in which the vacuum consolidation method is used to
improve soft clayey deposits, have been reported in literature. The latest version
of Plaxis 2D AE released in February 2014 contains new facilities in order to allow
for the analysis of vacuum pre-loading.
The aim of this report is to analyze the different vacuum consolidation tech-
niques that are used in practice, to describe a standard procedure to implement
vacuum drains and to give to the users some practical guidelines to follow. This
will be done simulating vacuum loading in the new version of Plaxis and comparing
it with analytical solutions that are found in literature. Therefore an evaluation of
the major modeling features is performed to see how the different parameters influ-
ence the results. Then, a case study will be elaborated and a comparison between
experimental data and F.E.M. results will be give. In conclusion, the purpose is
to give an answer to some open issues that are still in those new features. Some
of the questions that we set out to answer are:
• Do we need to model the horizontal vacuum drain collectors that connect the
vertical drains?
• Does the initial saturation of the sand layers (after activation of the vacuum
drain) influence the process?
• How to create a vacuum in the top sand layer (important for the stability)?
• What happens if we turn off vacuum drains, we can have inflow? It is that
acceptable?
• Is the approach to determine an equivalent horizontal permeability still valid
for vacuum drains?
1.4 Assumptions and Limitations
Inthisworkwewilluseatwo-dimensional(2D)finiteelementanalysis. Thegeneral
procedures for the creation of a geometry model, the execution of a finite element
calculation and the evaluation of the output results are described here in detail.
Thepurposeofthisreportisnottogivedetailedinformationabouttheconstitutive
5
Chapter 1: Introduction
models, the finite element formulations or the non-linear solution algorithms used
in the program. For detailed information on these and other related subjects, it
is possible to consult the Scientific Manual and the Material Models Manual of
Plaxis as well as the various papers and books listed in the bibliography.
6