452
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
load, the equivalent resistance can be determined according to
Equation (6). In stratified soil conditions, a layer-related
determination of the area proportions of the shear body is
relevant.
According to equation (10), the flow force S is determined
depending on area of the shear body A(t
1
), the available mean
hydraulic gradient i
M
at the bottom side of A(t
1
) and the weight
of water
w
.
1
G A( t )
(6)
M w
1
S i
A( t )
(10)
In the excavation area near to the sheet pile after a definite
construction stage, there will be bulging of the surface (uplift
effect), this leads to activation of shear resistances in
dependency of soil shear parameters. Damaged and undamaged
soil areas have to be distinguished considering failure velocity
depending resistance activation. Damaged soil areas, which
have a comparatively lower shear resistance, are considered by
available hydraulic gradient i ≥
‘/
w
(see Equation (3)).
5 CONCLUSIONS
The presented design approach for the stability analysis against
hydraulic heave in cohesive soils explicitly considers the
available resistance due to cohesion. In addition, the
consideration of a ground support effect acting at the sheeting is
integrated.
The permeability of soil and the excavation velocity
respectively load relieving governs the resistance activation of
the soil during hydraulic heave. Due to reduction of soil load in
excavation area, the pore water pressure is released slowly so
that the undrained shear parameters are relevant. In contrast,
drained conditions are relevant for long phases without
excavation progress and relatively permeable soils. In the
stability analysis for drained conditions the effective cohesion c'
and for undrained conditions the undrained cohesion c
u
must be
considered. The friction of soil should be neglected.
The design approach is divided in two steps. A first step
considers the hydraulic induced structure decomposition of the
cohesive soil beneath the foot of the sheet pile. The condition is
essential to determine the reference volume of the failure area.
The critical hydraulic gradient i
crit
depends on the cohesion,
density and stress history of the soil. I
crit
can be determined
experimentally or might be estimated by experience.
For calculating the shear resistance, shear body height t
1
is
divided into two different heights depending on the soil
structure (Equation (7) and Figure 3). For the height of damaged
area, the relevant shear property and for undamaged area its
shear resistance must be assigned.
In the second step the equilibrium of the vertical forces
acting at the reference volume is analyzed. In addition to the
current design approach (DIN EN 1997-1:2009), side forces and
cohesion of the appropriate shape of the failure area are taken
into account. The second part of the design approach
corresponds to the verification of uplift failure.
During hydraulic induced structure decomposition of the soil
the flow conditions will change. Thus an update of the
distribution of pore water pressure should be taken into account.
1 1,u 1,d
t t
t
(7)
v
v,u
v,d
u 1,u
u,d 1,d
C C C c t
c t
(8)
C
v,u
and C
v,d
represent the activated shear resistances in
undamaged and damaged areas. Equation (8) shows in this case
a soil with undrained conditions.
The assumption of resistances from the weight and the
available shear resistance in accordance with current design
practice (DIN EN 1997-1:2009) leads to a possible
classification as design situations HYD and GEO. The
assignment of coherent partial factors is still an open question
under discussion.
6 REFERENCES
Additionally, the soil bracing, supporting system of the pit
wall and other ground supports can be considered as a further
resistance. The consideration of abutment as an additional
resistance effect can be described as a vertical force which is
transmitted by the sheeting system. In accordance with DIN
1054:2010 this represents the difference between the stress state
without soil deformation and the actual or planned stress at the
excavation abutment.
DIN 1054:2010. Baugrund – Sicherheitsnachweise im Erd und
Grundbau – Ergänzende Regelungen zu DIN EN 1997-1.
DIN EN 1997-1:2009. Eurocode 7: Geotechnical design, Part 1: General
rules; German version EN 1997-1.
Wudtke, R.-B. (2013). Hydraulischer Grundbruch in bindigem
Baugrund. Dissertation, Bauhaus-Universität Weimar. Weimar.
Wudtke, R.-B. and Witt, K. J. (2006). A Static Analysis of Hydraulic
Heave in Cohesive Soil. 3rd International Conference on Scour and
Erosion, Amsterdam, 1 - 3 November 2006, 251.
The consideration of abutment resistance requires the
stability of sheeting system. It must be ensured that the
construction has sufficient safety to permit a deformation free
transferring of vertical forces at sheeting foot.
Transferring of vertical forces by the sheet pile is depending
on frictional component of shear strength. The drained or
undrained shear properties must be assigned in relation to the
anticipated load conditions. A definite amount of earth pressure
can be transferred to sheeting foot by consideration of contact
friction between soil and sheeting. The vertical load component
of the active earth pressure is essential for the determination of
the resistance and generally should be estimated conservatively.
If the hydraulic impact does not lead to hydraulic soil damage
the abutment effect F
v
corresponds to the vertical load of the
active earth pressure.
Wudtke, R.-B. and Witt, K. J. (2010). Hydraulischer Grundbruch im
bindigen
Baugrund
-
Schadensmechanismen
und
Nachweisstrategie.
9.
Geotechnik-Tag
in
München
-
Wechselwirkungen Boden - Wasser - Bauwerk, München,
19.02.2010, 33 - 44.
v
a,v
F E
(9)
The approach for determining the resistance of the abutment
effect of sheeting represents a general solution to the problem,
and requires the full activation of the passive earth pressure in
the pit. The most liable solution can be achieved by determining
the difference stress distribution between an unstressed, not
deformed soil and the stress condition of the planned
construction at the excavation abutment.
