numerical modelling in geotechnical engineering modelling in geotechnical... · geotechnical...
TRANSCRIPT
Design is defined as the act of conceiving
and producing a plan or model before
construction. The time requirement for
human thinking does not feature in
technological evolution, which is quite
prevalent in numerical modelling for
geotechnical applications. Finite Element
Method (FEM) modelling is a numerical
procedure to determine the stresses and
strains within a complex engineering
problem that can combine structures, soils
and civil infrastructure. This form of
numerical modelling for soil-structure
interaction problems is not a new concept;
however, the widespread prevalence of FEM
for geotechnical design has only recently
been made possible by technological
improvements in software and hardware.
In particular, the user interfaces have
been simplified dramatically allowing
“nonspecialists” access into this once
specialized field. On one hand, this should
be viewed as a highly positive occurrence,
as it facilitates improved accuracy in the
mainstream geotechnical calculations
undertaken for infrastructure and
construction projects. For example, using
advanced numerical techniques, we can
now predict with a high level of accuracy
the impact of new basement construction
on adjacent building foundations and
underlying pipe networks. On the other
hand, we always need to consider if the
accuracy stated within our technical
deliverables reflects the actual accuracy of
such analysis approaches.
Limitations and AssumptionsWhile these techniques open up new
possibilities in terms of engineering
design, it is critically important to
understand the limitations of such
geotechnical software and to be cognisant
of the inherent assumptions in the design
process. Every engineering challenge
brings its own unique assumptions;
FEATURE ARTICLE
DEEP FOUNDATIONS • JAN/FEB 2017 • 69
AUTHORS Paul Doherty, Ph.D., and John O’Donovan, Ph.D., C.Eng., MICE, Gavin & Doherty Geosolutions Ltd.
The past decade has seen technology
improve at an exponential rate, and while
civil engineering occasionally appears slow
to adopt the most recent advancements in
IT, the impact on our industry is impossible
to ignore. Computing power is at a point
where the average smart phone contains
more technology than a supercomputer
from 15 years ago! Data storage and
processing ability is at an all-time high. We
now operate in an environment where day-
to-day communications occur in real-time
regardless of geographical location of the
people involved or of the technical
complexity of the subject concerned. The
major disadvantage of this process is that
with technology changing dramatically,
our expectations also increase and, as a
result, the design/engineering queries are
expected to be resolved instantaneously,
leaving little room to think about the
underlying problem!
Numerical Modelling in Geotechnical Engineering
A flood prone river modelled in the second case study
70 • DEEP FOUNDATIONS • JAN/FEB 2017 DEEP FOUNDATIONS • JAN/FEB 2017 • 71
A.
B.
C.
(A.) Initial failure points close to shaft wall toe, (B.) development of slip circle into the gravel aquifer and (C.) full base failure through heave of the soil over-lying the gravel aquifer
To achieve confidence in the accuracy of
the predictions of soil stresses and the
resulting soil movements that are
generated, it is critically important to
calibrate the soil model employed in any
numerical analysis. This calibration exercise
typically involves simulating a number of
laboratory and, possibly, field tests within
the FEM environment to determine a
synthetic soil response curve that can be
compared to the real experimental results.
The parameters are typically varied until a
reasonable match is achieved between the
laboratory and simulated soil test results.
This is an iterative process as the same
parameters are then used to simulate
additional laboratory experiments tested
under different conditions. The soil
material parameters are adjusted until a
reasonable match is seen with the full suite
of laboratory tests. This calibration exercise
can lead to improved understanding of the
advantages and disadvantages of different
constitutive models. Typically, as the
constitutive model becomes more complex,
the calibration time required to determine
each of the required parameters becomes
longer. A balance should always be struck
between introducing complexity to a FE
model and the accuracy of the output
required. This balance should be informed
by considering what the FE results will be
used for. For example, a nonlinear soil
model may be required to satisfy tight
movement criteria while underpinning a
structure that is sensitive to deformation,
whereas a linear soil model may suffice for
other applications where movement criteria
are less critical.
Case Study - Deep Access Shaft for Tunnel ConstructionGavin & Doherty Geosolutions (GDG) was
commissioned with the analysis of a series
of deep access shafts used to construct a set
of tunnels for a new sewer network under a
major North American city. The soil
stratigraphy consisted of interbedded
glacial till layers of silty clay with a range of
plasticity values. The clay layers were
underlain by an open gravel deposit, which
was also a high-pressure aquifer with
artesian conditions, and, therefore, base
heave blow-out at the bottom of the shaft
was a serious concern during construction.
The shafts, which ranged from 25 to 38 m
(82 to 125 ft) in depth, were constructed
using a secant piled starter wall over the
upper 18 m (59 ft) and a structural shotcrete
wall over the lower reaches of the shaft. In
the permanent condition, the base heave
would be resisted by a permanent concrete
however, three general assumptions
common to every soil-structure numerical
model are considered below.
One of the first steps in any soil-structure
interaction problem is to quantify the in-situ
stress regime, i.e., to determine what the
stresses are in the ground prior to con-
struction-induced changes to the stress
regime due to loading (e.g., foundations) or
unloading (e.g., basement excavation). The
in-situ stresses include the vertical and
horizontal stresses, as well as the pore pres-
sures. The vertical stresses can be derived
from the stratigraphy of the site and the
density of the various materials present;
however, the lateral effective stress is a much
more complicated parameter to quantify
accurately. The lateral effective stress is
influenced by the stress history and past
geological events, such as glaciation. A K0
parameter, which is the ratio of the vertical to
horizontal in-situ stress, is often used as an
input in numerical modelling software, and
this parameter can have a dramatic impact
on the results of FEM analyses. Laboratory
tests and correlations from field tests have
been developed to quantify this parameter
and these can be used to select an appro-
priate K0 value for input in the soil-structure
interaction analysis. Further consideration
should be given to the variability of the value
of K0 within a stratum as it may be
influenced by local properties, such as soil
fabric, particularly in cohesionless material.
One of the next steps in the modelling
process is to determine whether the material
is likely to behave as undrained or drained.
This is a critical question as soil material may
behave very differently depending on the
assumption adopted. Undrained behaviour
does not allow for volume changes or the
dissipation of excess pore pressures, and,
therefore, this typically represents a short-
term response to loading/unloading. By
contrast, drained behaviour represents the
fully-equalised conditions where the pore
pressures remain at their in-situ stress state,
and this represents a long-term condition.
Most real soil materials are neither perfectly
drained nor perfectly undrained, but rather
behave somewhere in between, in a partially
drained state. However, depending on the
problem being analysed, employing an
assumption of drained or undrained
behaviour may be more or less valid. For
example, relatively impermeable clay is
likely to behave undrained in short-term
situations, where the stress conditions are
changed, such as basement excavations, but
the same clay may behave drained when
considering the slope stability of a 150-year-
old railway embankment.
The third assumption that needs to be
considered is the choice of constitutive
model and the appropriate stiffness
parameters for the soil within that model.
For example, the Mohr-Coulomb model,
which assumes a linear elastic relationship
up to a linear plastic failure criterion, is one
of the simplest material models available
and is controlled by a single Young’s
modulus which governs the soil stiffness.
This model does not consider the
nonlinearity of the soil material and the
reduction in soil stiffness as the strain level
increases. More advanced models, such as
the small strain hardening soil model, are
available, which capture three soil
behaviour regimes. Initially, an elastic, very
stiff soil response is observed in the small
strain region followed by a nonlinear
elasto-plastic response of the material in
the larger strain zone, and, finally, a fully
plastic response at soil failure. Most soils
exhibit some degree of stiffness anisotropy
between vertical and horizontal stiffness.
Therefore, the possible influence of
stiffness values in different directions
within a soil mass should also be
considered prior to deciding what
geotechnical investigation testing will be
carried out and what numerical analysis
will be completed.
Installation of flood defence works on the river bank Installation of masonry stone wall lining the flood relief channel
70 • DEEP FOUNDATIONS • JAN/FEB 2017 DEEP FOUNDATIONS • JAN/FEB 2017 • 71
A.
B.
C.
(A.) Initial failure points close to shaft wall toe, (B.) development of slip circle into the gravel aquifer and (C.) full base failure through heave of the soil over-lying the gravel aquifer
To achieve confidence in the accuracy of
the predictions of soil stresses and the
resulting soil movements that are
generated, it is critically important to
calibrate the soil model employed in any
numerical analysis. This calibration exercise
typically involves simulating a number of
laboratory and, possibly, field tests within
the FEM environment to determine a
synthetic soil response curve that can be
compared to the real experimental results.
The parameters are typically varied until a
reasonable match is achieved between the
laboratory and simulated soil test results.
This is an iterative process as the same
parameters are then used to simulate
additional laboratory experiments tested
under different conditions. The soil
material parameters are adjusted until a
reasonable match is seen with the full suite
of laboratory tests. This calibration exercise
can lead to improved understanding of the
advantages and disadvantages of different
constitutive models. Typically, as the
constitutive model becomes more complex,
the calibration time required to determine
each of the required parameters becomes
longer. A balance should always be struck
between introducing complexity to a FE
model and the accuracy of the output
required. This balance should be informed
by considering what the FE results will be
used for. For example, a nonlinear soil
model may be required to satisfy tight
movement criteria while underpinning a
structure that is sensitive to deformation,
whereas a linear soil model may suffice for
other applications where movement criteria
are less critical.
Case Study - Deep Access Shaft for Tunnel ConstructionGavin & Doherty Geosolutions (GDG) was
commissioned with the analysis of a series
of deep access shafts used to construct a set
of tunnels for a new sewer network under a
major North American city. The soil
stratigraphy consisted of interbedded
glacial till layers of silty clay with a range of
plasticity values. The clay layers were
underlain by an open gravel deposit, which
was also a high-pressure aquifer with
artesian conditions, and, therefore, base
heave blow-out at the bottom of the shaft
was a serious concern during construction.
The shafts, which ranged from 25 to 38 m
(82 to 125 ft) in depth, were constructed
using a secant piled starter wall over the
upper 18 m (59 ft) and a structural shotcrete
wall over the lower reaches of the shaft. In
the permanent condition, the base heave
would be resisted by a permanent concrete
however, three general assumptions
common to every soil-structure numerical
model are considered below.
One of the first steps in any soil-structure
interaction problem is to quantify the in-situ
stress regime, i.e., to determine what the
stresses are in the ground prior to con-
struction-induced changes to the stress
regime due to loading (e.g., foundations) or
unloading (e.g., basement excavation). The
in-situ stresses include the vertical and
horizontal stresses, as well as the pore pres-
sures. The vertical stresses can be derived
from the stratigraphy of the site and the
density of the various materials present;
however, the lateral effective stress is a much
more complicated parameter to quantify
accurately. The lateral effective stress is
influenced by the stress history and past
geological events, such as glaciation. A K0
parameter, which is the ratio of the vertical to
horizontal in-situ stress, is often used as an
input in numerical modelling software, and
this parameter can have a dramatic impact
on the results of FEM analyses. Laboratory
tests and correlations from field tests have
been developed to quantify this parameter
and these can be used to select an appro-
priate K0 value for input in the soil-structure
interaction analysis. Further consideration
should be given to the variability of the value
of K0 within a stratum as it may be
influenced by local properties, such as soil
fabric, particularly in cohesionless material.
One of the next steps in the modelling
process is to determine whether the material
is likely to behave as undrained or drained.
This is a critical question as soil material may
behave very differently depending on the
assumption adopted. Undrained behaviour
does not allow for volume changes or the
dissipation of excess pore pressures, and,
therefore, this typically represents a short-
term response to loading/unloading. By
contrast, drained behaviour represents the
fully-equalised conditions where the pore
pressures remain at their in-situ stress state,
and this represents a long-term condition.
Most real soil materials are neither perfectly
drained nor perfectly undrained, but rather
behave somewhere in between, in a partially
drained state. However, depending on the
problem being analysed, employing an
assumption of drained or undrained
behaviour may be more or less valid. For
example, relatively impermeable clay is
likely to behave undrained in short-term
situations, where the stress conditions are
changed, such as basement excavations, but
the same clay may behave drained when
considering the slope stability of a 150-year-
old railway embankment.
The third assumption that needs to be
considered is the choice of constitutive
model and the appropriate stiffness
parameters for the soil within that model.
For example, the Mohr-Coulomb model,
which assumes a linear elastic relationship
up to a linear plastic failure criterion, is one
of the simplest material models available
and is controlled by a single Young’s
modulus which governs the soil stiffness.
This model does not consider the
nonlinearity of the soil material and the
reduction in soil stiffness as the strain level
increases. More advanced models, such as
the small strain hardening soil model, are
available, which capture three soil
behaviour regimes. Initially, an elastic, very
stiff soil response is observed in the small
strain region followed by a nonlinear
elasto-plastic response of the material in
the larger strain zone, and, finally, a fully
plastic response at soil failure. Most soils
exhibit some degree of stiffness anisotropy
between vertical and horizontal stiffness.
Therefore, the possible influence of
stiffness values in different directions
within a soil mass should also be
considered prior to deciding what
geotechnical investigation testing will be
carried out and what numerical analysis
will be completed.
Installation of flood defence works on the river bank Installation of masonry stone wall lining the flood relief channel
Paul Doherty BE, Ph.D., C.Eng., is the managing
director of GDG, a specialist geotechnical engineering
consultancy in Dublin, Ireland, providing innovative
geotechnical solutions across a broad range of civil
engineering sectors. John O’Donovan Ph.D., C.Eng.,
MICE, a senior engineer leads the urban construction
sector at GDG specialising in the design of founda-
tions and basement structures and in the assessment
of ground movement on existing buildings.
plug at the bottom of the shaft, which was
also toed into the side walls using a shear
key type construction. While this concrete
plug provides resistance in the long term,
base heave was a significant concern
immediately after excavating to the final
depth, before the plug could be constructed
and the concrete achieve its maximum
compressive strength.
A 2D numerical model of the shaft was
developed that considered the soil layer
response and the stiffness of the structural
elements forming the shaft walls. The
model was calibrated using oedometer
tests, triaxial results and falling head
permeability experiments. Three separate
analyses were considered: (i) an undrained
type soil response, (ii) a drained soil
response and (iii) staged construction
considering all of the consolidation phases.
In the first analysis, which was relevant
to the short-term condition, the soil was
considered to behave undrained. This
analysis showed that immediately after
excavating to the base of the shaft, the
unloading of the soil created negative pore
pressures below the base. These negative
pore pressures can also be thought of as “soil
suctions,” and provide added stability to the
soil below the shaft, which ensured the shaft
was stable immediately after the excavation.
The second analysis considered the soil
to act as fully drained, and, therefore,
represented the soil condition following
dissipation of the excess negative pore
pressures. This analysis showed the shaft to
be completely unstable with the soil plug
heaving upwards and the shaft collapsing
as the failure surface intersected the gravel
aquifer, resulting in a catastrophic failure.
The final analysis, which was much
more complex, considered the time taken to
build the shaft and modelled the partially
drained consolidation during the construc-
tion process. Once the final dig was com-
plete, the analysis then considered the time
dependent change in the pore pressure
regime in the soil as the material transitioned
from undrained to drained conditions. This
analysis predicted the factor of safety as a
function of time following excavation to the
target base elevation. The development of
the failure mechanism was apparent and the
time to failure was shown to be in the order
of four months. This type of analysis allowed
a pragmatic construction programme to be
developed that relied on a relatively quick
construction of the base plug. A suite of
monitoring points was also established on
the shaft walls and base to allow the heave to
be assessed and compared with the predic-
tions to provide confidence in the residual
safety of the shaft at any moment in time.
Case Study - Flood Defence EmbankmentsThe Office of Public Works is responsible
for identifying urban areas in Ireland that
are at risk of periodic flooding, and, in one
such location, the local town and county
councils had developed a flood defence
scheme to protect life and property from
both severe high tides and river flooding
due to extreme rainfall events. GDG was
commissioned to complete the design of
the flood defences along a 500 m (1,640 ft)
long cul-de-sac to the north of a tidal stretch
of river and adjacent to a golf course that
has been identified for future development.
The defences that were required in this area
were a combination of flood walls to
contain the river and a flood relief channel
to relieve flood waters that may threaten a
nearby low-lying residential area.
The proposed design required a
reinforced concrete gravity retaining wall to
complete the flood defences. A detailed
hydrogeological analysis and numerical
seepage study was undertaken. GDG also
provided the designs for the relief channel
to alleviate flooding, for a road access to the
golf course, for the relocation of services and
for a pedestrian access from the nearby road
to an underpass at the adjacent rail line.
ConclusionsDespite the advancement of computing
power and the improved efficiency of
numerical modelling software, we must
remain diligent in our search for robust,
reliable and efficient engineering solutions.
These solutions are not driven by increased
computing power or user-friendly software
interfaces — they are provided by engineers
with relevant experience and a will to
dedicate time to critically think about
challenging problems. Real value is
provided when these traditional engi-
neering ideals are combined with the most
up-to-date numerical modelling capa-
bilities. A tradesman relies on their tools to
complete a job to a high standard and so
does a geotechnical engineer; however, the
final finish is driven by the engineer and not
the tools available to them. While numerical
model l ing sof tware has improved
dramatically, the final solution adopted for
construction should be one that recognises
(a) the underlying limitations of the
software, (b) the level of calibration, (c)
simplifications in the soil behaviour and
geometry that are modelled, and (d) the
experience of the user in analysing the
problem at hand. Engineering should always
be done by engineers and not computers!
Seepage analysis of the proposed flood defence system
72 • DEEP FOUNDATIONS • JAN/FEB 2017