Geoscience Reference
In-Depth Information
9
OBSERVATION AND PREDICTION
In a world where demands for improving life and work environment are ever
increasing, particularly for densely populated areas and weak soil conditions,
construction in and on the underground is complex. Proper insight in the short and
long-term behaviour of soils and their interaction with structural elements is
inevitable for achieving reliable and sustainable works in an efficient manner. Not
always straightforward design tools are sufficient and sophisticated methods are
required. In the process of design and construction, two aspects are highlighted in
this chapter: physical simulation and numerical simulation. For both, understanding
of the concepts of adopted material behaviour (constitutive models) in a multi-
dimensional fashion is essential.
A PHYSICAL SIMULATION
Besides field observation, either by monitoring a pilot test or at an unexpected
failure case, the use of physical simulation is inevitable for improvement of our
understanding of soil behaviour. It provides valuable information on failures
(liquefaction, landslides, dams), deformations (roads, foundations, tunnels), soil-
structure interface behaviour (piles, anchors), and transport (contaminants, heat).
Physical simulation by means of tests and monitoring provides the means to check
constitutive concepts, which form the fundaments of prediction models. In this
respect physical simulation and numerical simulation are complementary. In fact,
the objective of physical simulation is validation, while the objective of numerical
simulation is prediction. A comprehensive evaluation of the application of physical
testing methods is outlined in Chapter 3 C .
A striking example of the need for physical simulation can be found from 'odd'
behaviour of horizontal soil deformations due to a nearby excavation or landfill.
Elastic numerical simulation will result in predicting the horizontal deformation
quite well in magnitude but completely wrong in direction: the reality will show a
deformation in the opposite direction! Here, the soil behaviour must be simulated
with suitable calibrated constitutive models that include plasticity and/or viscosity.
Physical simulation requires special skills and experience in scaling laws
(centrifuge), fabrication (sample preparation), sensors and transducers, and data
elaboration (visualisation).
Scaling
Before a large, expensive object is constructed, experimentation on models
(modelling) is used to determine the best properties for construction and for its
functioning. One must know how to scale up the results of a model test to a full-
scale prototype. Here, the concept of physical similarity is central. This concept is
satisfied when values of dimensionless parameters that characterise the
phenomenon of interest are equal in model and prototype.
When developing models, it is not possible to take into account all factors that
influence the phenomenon of interest; some should be disregarded and those of
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