Civil Engineering Reference
In-Depth Information
simply vary hydrostatically with depth, as in some urbanised
areas groundwater was historically extracted for industrial pur-
poses at depth and underdraining of the material above may
have subsequently occurred.
Methods for determining groundwater levels are discussed
further in the following section.
as pressuremeter and/or high pressure dilatometer tests, CPT
tests and geophysical testing which can all be used to identify
stiffness and strength parameters.
For soil-structure interaction analysis it is also extremely
important to define the groundwater regime beneath a site.
Standpipes and piezometers are generally used to monitor the
variation of groundwater levels and porewater pressures. For
sites close to tidal bodies of water understanding the tidal in-
fluence on groundwater level is also vital.
The final set of parameters adopted for numerical analysis
will be based on a review of all of the test data which may
potentially include some statistical analysis to justify the
assumptions made.
It should be noted that the values adopted from the site in-
vestigation may be subject to change as the design and analysis
process progresses. Modifications to the chosen values may
occur as more information becomes available such as histor-
ical case-study data, if it contains a sufficient level of detail and
is relevant to the mechanism being modelled, or field testing.
Field testing is particularly important where there is little
knowledge of how a proposed foundation solution is likely
to behave or interact with surrounding elements, particularly
where there is a limited amount of case-study data to aid in
predicting the load settlement behaviour of a foundation.
Full-scale testing may be required which may include load
testing of single and multiple elements (
Figure 13.11
). Strain
gauges can be included within the foundations to ascertain how
the loads are shed into the surrounding ground. The results
from these tests should then be compared with soil structural
models with design parameters modified where appropriate in
response to this new level of information.
13.4.3 Site investigation design
Typically, a geotechnical site investigation is designed and
undertaken around the beginning of RIBA Stage D, as by this
point enough is known about the scheme and the site to allow a
geotechnical engineer to predict what type and spread of test-
ing will be required to subsequently undertake a safe and eco-
nomic design and manage the predicted ground-based risks.
Where soil-structural models are likely to be used as part
of the design process it is important that by the time the site
investigation design is being undertaken, all the parameters
that will be required for the modelling are known. Therefore,
if the modelling is to be undertaken by a different engineer, it
is highly important that the modeller and site investigation de-
signer have had sufficiently detailed discussions so that the risk
of data gaps within the final investigation report is minimised.
If both activities are being undertaken by the same person, then
that engineer should be suitably experienced in both modelling
and site investigation techniques or be given suitable technical
support.
When it is understood what parameters will be required the
most suitable exploratory methods must be chosen. The cost of
the various sampling and testing methods should be reviewed
at this stage as it may be hard to justify some of the more ex-
pensive options on less complex projects where only simplistic
levels of modelling are required.
When choosing the depth and spread of exploratory holes,
attention should be given to the layout of the proposed building
and any design constraints. A logical approach is to place the
holes in the locations that will be the subject of the modelling.
These may comprise areas below stability cores, areas where
the site boundary borders sensitive structures or where struc-
tures or services run beneath the site. The depth of the investi-
gation should extend an appropriate distance below the max-
imum anticipated founding depth. For piled foundations this
may often be taken at 2 to 3 diameters beyond a pile toe. For
rafts and shallow footings, depths of between 1 and 2 times the
foundation width may be adopted providing the investigation
extends beyond any layers that may still influence settlement
predictions even at depth.
For complex analyses where high quality samples are
required, the method of forming the boreholes needs to be con-
sidered. For example, triple tube core barrels may be required
to produce a high standard of undisturbed sampling with ap-
propriate laboratory testing such as small strain triaxial and
consolidation tests. It is also important to complement the la-
boratory testing with other methods of defining the properties
of a soil mass. These would likely include
in situ
methods such
13.5 Structural model
13.5.1 Typical structural components
There are numerous foundation and substructural elements
that can be modelled in soil-structure analyses with the most
common being:
raft foundations;
■
pads and strip footings;
■
embedded and conventional retaining walls;
■
pile foundations.
■
This section will discuss how different foundation types are
generally modelled when carrying out soil-structure interaction
analysis. It should be noted that the above list is not exhaustive
and that further reference to simulating other structural com-
ponents beyond those discussed below may be required.
13.5.1.1 Raft foundations
Rafts are generally modelled using finite element analysis
with the raft structure represented using two-dimensional plate
elements. Depending on the method of analysis the raft will
