UNCERTAINTY AND RELIABILITY - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 108

A crucial issue in society's reaction is the individual risk perception related to
the intrinsic uncertainty. It is about emotion and trust. Uncertainty in geotechnical
engineering may lead to remarkably high risk, i.e. significant vulnerability in areas
prone to hazards. This is commonly not understood nor accepted in society. The
geotechnical engineer should better explain the intrinsic uncertainty in geotechnical
engineering to the outside world. In this chapter, aspects of geotechnical
uncertainties and corresponding limitations and opportunities are elucidated.

Soil investigation
Geo-engineers know that uncertainty of soil properties (stratification and model
parameters) is relatively high with regard to other common building materials like
concrete and steel. Uncertainty for steel it is not beyond ±5%, for concrete it is
about ±15%, for soil it is usually beyond ±50%.
Fig 8.2 shows our limited capability indicating soil conditions by some common
site investigation methods using non-destructive techniques (NDT). The success
rate of our predictability is not more than 30%! In soils this situation is common,
and obviously the engineer dealing with it has to add intuition and experience when
defining relevant soil conditions.
NDT Depth Piles Stone Peat Clay Loam Silt Sand Gas

shaft boulder lens lens lens lens bed pocket

from surface or borehole
CPT + coring N - F +?! +?! + + +? +? + +

Seismic M - F ? ? ? ? ? ? +? +?

Electromagnetic N - M 3 3 +? 3 3 ? ? 3

Ground radar N +? +? ? ?

3
Geoelectric M - F 3 3 + +? ? +? ? ?
Figure 8.2 Capability of site investigation methods in geotechnical engineering.
Legenda: + OK, ? unknown, +? probable, ! damage,

3 3 3

3 not possible;

N: near (1 to 5 m), M: medium distance (5 to 20 m), F: far (more than 20m).

The range of intrinsic and characteristic soil property values gathered from
common field and laboratory tests is well studied. In codes of practice,
corresponding partial safety factors are defined related to actual circumstances and
related risks (see Table 8.2).

Boundary conditions
When modelling the real geometry, a choice has to be made about dimensions
(1D, 2D or 3D), and about the soil stratification. At the borders of chosen domains
suitable conditions are to be chosen with regard to groundwater and soil in terms of
stresses, fluxes and/or displacements. This also holds to some extent for interfaces
within the considered domain; by example, at locations of soil-structure interaction.
For non-linear and time-dependent problems the initial state has to be defined. In
fact, the determination of the initial state is sometimes more difficult than the
actual problem itself.
Fig 8.3a shows the effect of unknown boundary conditions at the edge of a
chosen domain: a two-layered (top clay, bottom sand) soil mass with on top a dike
subjected to a sudden river level rise. The problem arose as a local pore-pressure