Civil Engineering Reference
In-Depth Information
Distance from Excavation
d
Max. Excavation Depth
H
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.1
0
Heave
0.1
0.2
0.3
0.4
0
0.1
0.2
0.3
Zone for High Horizontal Stiffness of Support
(Multi-Level Propped Excavation)
0.4
0.5
0.6
Zone for Low Horizontal Stiffness of Support in
London Clay (Free Cantilever or Soft Prop/Anchors)
0.7
0.8
Figure 13.5
Vertical and horizontal displacement envelopes based on research undertaken by Clough and O'Rourke (1990)
Finally, it is recommended that the relevant results obtained
from numerical software packages are compared with those
obtained from empirical correlations.
of computer software packages has increased exponentially in
popularity with most graduate engineers able to use a number
of products competently.
The applications of this type of software are many, with
packages available that do everything from allowing the user
to model the entire structural frame of a building to simulating
the interaction of piles located within close proximity to each
other, with the latter using some form of soil-structure inter-
action calculation.
When progressing the design of foundation or substructure
elements the initial part of this process often involves review-
ing the client's and design team's aspirations and developing a
solution that responds to their scope which is both economical
and can be safely constructed within a given time period.
In the early design stages there is generally insufficient
information available to justify any amount of detailed ana-
lysis and therefore the various elements are assigned approxi-
mate sizes often based on experience or simple formulae. A
varying level of conservatism is also exercised at this stage
which relates to the level of information available at that point
in time. It should also be noted that due to the fluid nature of
most projects in their early stages of design (e.g. RIBA Stages
B/C and possibly D) the client's brief is susceptible to poten-
tially significant changes and therefore undertaking time con-
suming numerical analysis to size foundation and substructure
elements and quantify likely ground movements around them
may well end up being superseded.
13.3 Applications and limitations of soil-structure
models
Soil-structure models have commonly been used with a view
to get a better understanding of the interaction between struc-
tural elements and a founding material (i.e. soil and ground-
water) and in particular predicting the resulting structural and
foundation movements. These models are commonly used to
assist with the design of foundations (raft, pile groups, foot-
ings, etc.) and subsurface structures (retaining walls, tunnels,
anchors, etc.) as well as to quantify the level of risk associated
with the impact of certain construction sequences and methods
on neighbouring structures and foundations.
Appreciation of the limitations of what such models are cap-
able of is as important as understanding their application. The
following sections highlight both the instances where the use
of these models can benefit a project and where their limita-
tions preclude any worthwhile application.
13.3.1 Applications
Historically, foundation designs were progressed using estab-
lished design methods that were based on case-study data,
proven design formulae within standards, codes and guidance
and even trial and error. As technology has improved, the use
