Civil Engineering Reference
In-Depth Information
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Figure 13.19
Three-dimensional finite element analysis of a pile load test and correlated results of the group settlement
given stage of a basement excavation the propping strategy may
need to be modified. Similarly the soil-structure model could
be adjusted to revise the predicted movements for the next stage
of construction. This provides a sometimes critical level of con-
trol during the construction of projects within close proximity
to sensitive structures. A typical definition of trigger levels for a
project is presented in
Table 13.2
, together with example actions
required from the monitoring specialist and contractor on site.
The review of monitoring results also brings significant ben-
efits to the engineer and in some cases the developer, particu-
larly on phased projects. In situations where a multi-phased
development extends along the route of an underlying tunnel,
next to an adjacent road, railway or series of sensitive struc-
tures, the monitoring results observed during construction
of the initial phases can be used to justify the predictions of
later phases. Without this opportunity, and with a lack of site-
specific historical knowledge, an element of conservatism is
often applied to the soil-structure modelling process to reduce
the risk of under-prediction. This can lead to elevated con-
struction costs that could otherwise have been avoided if site-
specific data had been available.
Monitoring can also be used to protect a developer. For
example, if a claim is made by a third part asset/adjacent prop-
erty owner that construction activities resulted in the cracking
of a nearby structure, the results of the monitoring can often
be used to check whether the on-site activities and movements
were of such magnitude that they could have resulted in the
claimed damage.
Even with the advantages listed above, there are still limited
amounts of movement monitoring undertaken as the short-term
cost savings are often judged to outweigh any long-term benefits.
Figure 13.20 Installation of automated monitoring equipment
within a London Underground tunnel underlying a new commercial
development
assigned to predefined magnitudes of movement that corres-
pond to a percentage of the maximum tolerable displacements.
Actions are assigned to each level which may vary from 'organ-
ising a review' to 'complete cessation of all work on site'.
The trigger level system allows the engineer to re-evaluate
the movement predictions from their soil-structure interaction
model as the construction process progresses. For example,
if retaining wall movements are in excess of predictions for a
