Civil Engineering Reference
In-Depth Information
As the project progresses, however, the design becomes
much less susceptible to change and generally by the comple-
tion of RIBA Stage D the foundation and substructural options
will be sufficiently established that more detailed design tech-
niques can be undertaken. It should also be noted that by this
stage a project-specific ground investigation will generally
have been undertaken providing the engineer with a suitable
level of information relating to soil design parameters to carry
out detailed design including any associated analyses.
Soil-structure interaction applications can be considered in
two or three dimensions. For example, it is possible to simulate
the movement behind a retained excavation using 2D analysis
and the propping effects that the corners have on a basement
box can be modelled using 3D methods (
Figure 13.6
).
The elements within the ground that most commonly require
the use of soil-structural models are those that either have the
potential to cause surface or near-surface ground movements
(and hence nearby structures or infrastructure), those that do
not correspond to 'standard' design guidance and procedures
such as piles at close spacing and those structural elements
where load settlement behaviour cannot be satisfactorily estab-
lished using simpler means (
Figure 13.7
).
It is also possible to analyse how a number of different elem-
ents interact with each other, such as the effects of loading a
raft that has been formed within a basement where the retain-
ing walls are supported by ground anchors or floor slabs. Here
the behaviour of each individual element may be predicted
using traditional calculation methods; however, the effect of
incorporating all within a scheme may not be accounted for
when considered in isolation.
Most soil-structure interaction software packages allow the
user to create and manipulate the models relatively quickly. This
can result in an extremely powerful tool when designing and
later optimising construction sequences as the engineer has the
ability to change aspects such as structural stiffness, soil proper-
ties, excavation and construction sequence, retaining wall prop
heights and embedment depths allowing numerous parametric
studies to be undertaken within a short space of time.
Large and complex raft foundations are also frequently mod-
elled using soil-structure interaction techniques to predict col-
umn displacements and differential movements between the
various structural and substructure elements such as cores and
retaining walls. Moment contour plots showing the predicted
magnitudes of forces over the plan area of the raft can also be
obtained to calculate the required reinforcement quantities.
The effects of time on foundation behaviour can also be
simulated using certain pieces of software. When loading or
unloading a saturated clay its reaction to this change in sur-
charge can be said to occur in two stages:
(ii)
short-term - as a result of elastic deformation of the soil
mass under the foundation loading
(ii)
long-term - following the dissipation of porewater pres-
sures that occurs as a result of the change in effective ver-
tical stresses which can take many years depending on the
permeability of the clay.
While with certain formulae and geotechnical software
packages the short- and long-term displacements are generally
estimated by the user by adjusting the soil stiffnesses within the
clay layers, others are able to simulate the long-term behaviour
of the soil by referring to formulae that take into account soil
properties such as permeability in addition to strength, stiff-
ness and the groundwater profile. Such software packages are
also able to predict ground heave which may impact on the
viability of certain substructure solutions such as a ground-
bearing slab.
The ground movement and hence the movement of founda-
tions of existing structures adjacent to a proposed development
Figure 13.6
Two- and three-dimensional finite element models of a basement retaining wall
