UNCERTAINTY AND RELIABILITY - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 120

or more precisely, irremediable damage. For the built environment, the criterion is
often aesthetic damage, or worse, user damage and structural damage. Because of
the lack of knowledge about long-term effects, the determination of an appropriate
damage threshold is troublesome. In some cases, a lower boundary can be based on
historic evidence, provided the natural system has not changed.
The principle of precaution is combined with the concept of reversal of the
burden of proof. The provoker should demonstrate that his mitigation measures
exclude or sufficiently reduce the uncertainties and related risks, within his
capability and according to the state of art.
gel layer
Figure 8.8 Drainage system, tram-tunnel construction, The Hague
Complex tunnelling by on-line monitoring
In the busy shopping centre of The Hague an underground tram station and car
parking has been built. The construction consists of two diaphragm walls to a depth
of 28 m in mainly sandy soils, a covering roof and internal floors, vertically
anchored. Inflow of groundwater during construction was restricted by a jet grout
arch (a strut for the diaphragm walls and support against uplift) in some places and
a silicate gel layer formed by permeating grouting on other places. Unfortunately,
scour from under the arch (sand boils) and local collapse caused a delay of the
works. After two years - devoted to judicial disputes more that technical
discussions - construction was resumed in 2000 using excess air pressure of 1.14
bar in an air-proof underground building pit, which imposed difficult labour
conditions. A heavier floor, additional ballast and anchors were applied to
guarantee stability against uplift. Soon it appeared that the dewatering system
above the silicate gel layer did not work properly: serious well clogging was
occurring. Heave and subsequent excessive deformation of the walls could
jeopardize the entire project.
The contractor decided to ask for specialist advice from geotechnicians,
geobiologists and geohydrologists. They discovered that due to the long delay gel
material had caused a locally high pH (Luger). This dissolved organic matter
locally that precipitated near the well filters, where the pH was lower. The unusual
clogging problem was solved by applying smart instrumented sand columns and a
temporary isolation system (PVC-liner) making full profit of a thin semi-pervious
peat layer (Fig 8.8). About 100 piezometers were installed and the records every 10