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Hydraulic Heave in Cohesive Soils
Rupture hydraulique du sol en terrain cohérent
Wudtke R.-B.
wudtke geotechnik, Weimar, Germany,
Witt K.J.
Bauhaus-Universität Weimar, Weimar, Germany,
ABSTRACT:
According to DIN EN 1997-1:2009, the shear parameters of the soil are not relevant for stability analysis due to hydraulic heave.
Resistance is only activated by the weight of the soil of the failure area. However, during tests on cohesive soils the failure sequence
was visually detected as a series of pore widening, initial crack formation and hydraulic induced structure decomposition of the soil in
the failure area followed by downstream sudden uplift. On the basis of the test observations a new design approach under
consideration of soil weight, shear parameters and an optionally mobilised earth pressure is proposed. The used reference volume
represents both the construction as well as the flow situation. Processes that occur before the final uplift are transmitted by the
hydraulic gradient i
crit
on the failure area. The final limit state is interpreted as a shear failure.
RÉSUMÉ:
Pour l’évaluation d’une défaillance par rupture hydraulique du sol, les normes DIN EN 1997-1:2009 sur les propriétés de cisaillement
du sous-sol ne sont pas pertinentes. Les résistances sont activées seulement par le poids du sous-sol. Des essais de visualisation du
déroulement de la défaillance en sol cohérent ont montré que le processus se caractérise par une succession d’effets de dilatation des
pores, de formation initiale de fissures, de destruction structurelle du continuum dans la zone de défaillance, suivis d’une rupture
brutale de la surface. Conformément au déroulement de la défaillance, les calculs présentés prennent en compte, en tant que
résistances, le poids mort du sol, sa résistance au cisaillement de rupture ainsi que, le cas échéant, l’existence d‘une contraction du sol.
La mise en évidence se fait sur une zone de défaillance-modèle reproduisant la situation de construction et de courant. Les phases
précédant la rupture finale sont transférées à la zone défaillante par les gradients hydrauliques i
crit
. L’état limite final est interprété
comme rupture par cisaillement.
KEYWORDS: stability analysis, design approach, cohesive soil behaviour, test method
1 INTRODUCTION
Stability analysis against hydraulic heave considers in
accordance with Eurocode 7 (DIN EN 1997-1:2009) exclusively
the soil weight as stabilizing influence. Thus, especially for
cohesive soils the design approach takes not the available shear
strength into account. In accordance with current regulations, a
consideration of shear properties is only possible if special
experience with the material is available.
In order to optimize the design approach of hydraulic heave
for cohesive soils within the constraints of the valid standard, it
is necessary to examine various questions. This includes:
- How to characterize the failure mechanism of hydraulic
heave in cohesive soils?
- Which is the characterizing difference of this for non-
cohesive soils?
- How to describe or determine an appropriate reference
volume for the limit state?
- Which are the controlling material parameters of the limit
state?
- What is the effect of the water content on these soil
properties?
In order to develop a new design approach, it is also
necessary to consider temporal aspects of the failure sequence.
An adaptation of the design approach to the problem of a flow
around excavation wall must be considered for both drained and
undrained soil properties. In addition, a specific consideration of
the supporting effect of the subsoil abutment at sheeting
location is possibly positive.
2 CURRENT DESIGN
Changes in groundwater regime are critical for hydraulic heave
and for particle transport processes in the soil. The stability of
earth structures, excavations and foundations is influenced by
both phenomena during and after construction.
From a global point of view, changes in ground water level
cause changing pore water pressures, which might affect the
general balance within the soil continuum. A violation of the
limit state condition leads in this context to failure. On the other
hand an increased flow of water though the voids leads to
changes in the soil structure due to particle transport. To
evaluate hydraulic heave a limit state condition is critical.
Particle transport processes have only an indirect effect on the
overall stability.
Referred to Eurocode 7 (DIN EN 1997-1:2009) for a
stability analysis against hydraulic heave for each possible soil
prism the limit state condition according to Equations (1) or (2)
has to be evaluated.
dst;d stb;d
u
(1)
dst;d
stb;d
S G
(2)
The limiting criterion in Equation (1) is formulated as a
comparison of the design values of total stresses
stb;d
and pore
