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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
(a)
(b)
(c)
Figure 6. (a) Factor of soil arching reduction (Heitz et al. 2008); (b) stress conditions in the subgrade due to moving load on the pavement surface;
(c) pavement deformation due to hard-point effects associated with the presence of rigid inclusions.
Regarding to the structural bearing capacity (STR), a
minimum compressive strength of 7 MPa has to be adopted, and
no shear stress is allowed for unreinforced columns smaller than
30 cm. Besides, if tension can develop, for Domain 1 the rigid
inclusions have to be reinforced, whereas for Domain 2 only an
adequate tensile strength of concrete could be adopted.
On the other hand, Katzenbach et al. (2012) have compared
the safety checks outlined in the ASIRI recommendations with
other guidelines for similar foundation systems usually used in
Germany (CSV, CRPF), according to the partial safety factor
approach. They reported that ASIRI has lower values of safety
factors than those the compared guidelines indicate.
4.2 Low-height embankment
In the case of embankments with heights less than 3 meters,
the design is usually aimed to guarantee the SLSs, according to
the Domain 2. Basically, the geometry of the CSE systems has
to be set to avoid excessive deformation in the surface of the
embankments, in order to allow an adequate traffic operation.
For this objective Lawson (2000) proposed the chart depicted in
Figure 4b, for the design of the height and geosynthetic-
reinforcement of LTP layers considering the columns as hard
points, and according to typical thresholds adopted in transport
projects related to differential settlements.
The differential settlements also depend on the LTP strength.
Figure 4c shows the analysis of Jenck (2005) related to the
influence of the height of the embankment and the strength of
unreinforced LTPs in terms of friction angle. Results indicate
that efficiency factor E increase with height of embankments
until a maximum value similar to the critical height H
C
. Also, it
can be seen that when LTP is composed by materials with
friction angle less than 20 degree the efficiency factor is
drastically reduced, and practically negligible when
= 0.
So far it is not fully analyzed the behavior of CSE against the
cyclic loading of traffic. Heitz et al. (2008) have demonstrated
that the arching mechanism to transfer load of LTP can only be
formed in a very limited extent if geosynthetic reinforcement is
not placed. Based on laboratory model tests under cyclic
loading, they proposed a soil arching reduction factor, k.
Figure 6a shows this factor depending on the ratio of fill
height and column spacing h/s, the frequency
f
and amplitude of
the cyclic load
c
.For rigid inclusion application negative
influence of the traffic loading has to be considered during
construction and operation stages. Figure 6b illustrates that
cyclic loading of traffic can generate the rotation of principal
stresses in the subgrade layers, which could cause severe
damages to the rigid inclusions and pavement serviceability in
the long term, especially for low-height embankments.
Finally, Figure 6c shows an example of pavement
deformation due to a combination of the effects mentioned.
5
CONCLUSIONS
The influence of columns stiffness commonly used on the
Column Supported Embankment (CSE) systems has to be
rigorously investigated in order to establish the implications on
the safety and serviceability issues. The facts that indicate the
higher risks of rigid inclusions compared with flexible ground
improvement methods like stone columns are exposed,
especially when diameters of rigid inclusions are smaller than
30 cm. Moreover, the requirements of LTPs in terms of strength
and thickness, has to be more strict for rigid inclusion
comparing with stone columns, in order to ensure the arching
load transfer in the long term behavior of the CSEs, for both
static and cyclic loading.
6
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