1319
Technical Committee 202 /
Comité technique 202
Figure 4. Lateral displacement of a wooden pile. Modeled with
embedded piles, volume pile and plate element.
As the displacements and structural stiffnesses of the piles
are equal, the bending moments should also be similar.
However, notable difference could be found in bending
moments as shown in figure 5. The moment distribution of the
embedded pile is very irregular, indicating inexact values. The
embedded pile with refined mesh whereas produced practically
identical bending moments with the volume pile.
Figure 5. Bending moment of laterally loaded wooden pile. Modeled
with embedded pile, embedded pile with refined mesh, volume pile and
plate element.
The outcome of the analysis is that the element mesh should
be refined around the embedded pile, if accurate structural
forces are important to find out. Inaccuracy of the embedded
pile element will probably be emphasized in actual design
projects, where coarser element mesh is used. Other outcome
was that the different pile elements produced very similar
displacements and bending moments, if the element mesh
around the piles was similar.
In figure 6 the safety analysis conducted for the different pile
element types is shown. Initial settlement of 60 mm is caused
by 70.0 kN/m
3
train load. One should notice that none of the
pile elements have a failure criterion as they are purely elastic.
Therefore the safety analysis is not reliable for large
displacements as the bending moment of the pile increases
beyond the structural capacity of the pile. As the bending
moment capacity of a d200mm wooden pile is known to be
approximately 15 kNm (Ranta-Maunus 2000), it was further
analyzed at which displacement level structural failure may
occur. Accordingly the bending moment capacity is reached
when the settlement of the embankment is approximately 0.15
m.
It is shown in figure 6 that the safety factor without
reinforcements is F=1.23. The volume pile and the embedded
pile with refined mesh produces similar safety factors, F=1.29
and F=1.28 respectively for the displacement level of 0.15 m.
Safety factor with the plate element is slightly smaller, equal to
F=1.26. The embedded pile with the original mesh gives higher
safety factor than the other. The factor was found to be F=1.33
indicating that the element can overestimate the stability
conditions if the analysis is made without mesh refinement
around the pile.
0
0,05
0,1
0,15
0,2
0,25
1
1,1
1,2
1,3
1,4
1,5
Settlement of embankment [m]
Overall safetyfactor ΣMsf
no pile
plate element
embedded pile(refinedmesh)
volulme pile
embedded pile
Figure 6. Safety analysis of different piles as a function of
embankment settlement.
In general it can be said that the different structural elements
produced similar results under operational loading conditions.
Embedded pile was influenced by the coarser mesh even
thought the magnitudes of forces were correct as an average. In
all cases the mobilized forces are clearly smaller than the
structural capacity of the piles. The value of maximum
mobilized bending moment was 3.78 kNm, when corresponding
lateral displacement was 33.4 mm.
A reason for this kind of behavior is a failure mechanism,
where the piles are tilting with the soil mass. The foot of the pile
has a hinged joint with soil, which causes smaller forces
compared to a rigid connection that would be plausible if piles
are driven deeper into the dense soil layers.
The installation effects or the effect of interface elements
were not taken into account in this study. Obviously these
effects should be considered if the piles are used near the
railway track. One should also notice that even if the soil
behavior is well known due to failure test, the study considering
piles is theoretical as no piles were installed for the conducted
failure test.
4 SHEET PILE WALLS
Permanent sheet pile walls are used occasionally for the
stability improvements. The reason for using this method is
usually the lack of space around the embankment and therefore
a counter weight berm is not possible.
In the following, a case study from western Finland near
Seinäjoki town is presented. A double track was supported with
sheet pile walls anchored through the embankment as shown in
figure 7. Sheet piles are installed through the soft clay layer
(+27…+38) to the hard soil layer. There are no triaxial test
results available from this site and therefore the FEM analysis
are conducted using typical effective strength parameters of soft
Finnish coastal clays. Thus the real stability conditions of this
specific site can differ from the factors presented here.
