3298
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
surveys in following years. Underwater observations and
examinations were also carried out.
The processes of internal erosion of the dam body has been
observed and degradation of its structure notified. These
processes were described by W.Wolski and M.J.Lipiński,
A.Furstenberg and T.Barański in “Influence of internal erosion
on safety of old dams” 2000 and they generally consists of
movements of soil particles within soil layers, form one soil to
another, along boundaries of the layers or along the soil contact
with rigid surfaces such as concrete etc.
“These movements of soil grains, irrespective of the
aforementioned situations, are strongly dependent on the force
of seeping water, characterized by the hydraulic gradient. The
bigger the force of seeping water, the coarser the grains which
can be moved. The movement of the soil particles is controlled
by the geometry of the pores in the soil, particularly any
constrictions the particles encounter during their movement.
Certainly, if a particie is bigger than a constriction, the
constriction will prevent its further movement (Kenney & Lan,
1985).
Nevertheless, whichever type of internal erosion takes place, the
effect of the process is to increase the porosity of the soil zone
thus influenced (Lafleur et al., 1993).
The effects of internal erosion most often detected are those
located close to the drainage zones, which are characterised by
higher gradients, particularly where the filters are not efficient.
In these zones a large volume of particles may be washed out of
the dam body or its foundation, thus creating sinkholes or
caverns, often followed by regressive erosion - piping (Wolski,
1987).
Much more hazardous for dam safety, because more difficult to
detect, are the internal erosion “Loose zones", which are
characterized by increased (with respect to initial) porosity of
the soil, which may develop without visible symptoms on the
ground surface or the slopes of the dam. The internally eroded
soil in the loose zones may be contractive during undrained
shearing and therefore it has a high liquefaction potential. The
phenomenon of liquefaction initiated in such loose zones
"hidden" in the foundation or dam body can be triggered by any
dynamic loading, e.g. during flood discharge, thus causing a
flow slide usually with catastrophic consequences”
One of the elements being the subject of consistent
observation and recording is the shape of phreatic level within
the earth dam of Włocławek. This is supported by dense
network of piezometers installed and automatically observed
during the years of operation.
It has been identified that the observed phreatic level within
the earth dam significantly differs from the designed one,
calculated for stable downstream level (W.Wolski & others;
2000). Based on archival data analysis the scheme of low
density material extent has been prepared (W.Wolski.,
J.Mirecki. 2011) within cross-section V – see Drawing 1.
Drawing 2 presents the zones of loose material within dam body
(with D
R
<0,33). Based on the scheme of material zones the
numerical model of the dam has been generated, taking into
account the zones of low density material. Calculations have
been performed using Z_SOIL2011v.11.13 software, the
commonly used software based on final elements analysis
method used for geotechnical, hydrotechnical and
environmental engineering calculations (Data Preparation &
Tutorials Z_SOIL PCv2010).
4 variants were considered for calculation purposes:
Variant 1
– the zones of low density material as shown on
Drawing 3. The academic assumption (based on expert’s
recommendation) was, that low phreatic level within the dam is
being induced by proper work of surface insulation of upstream
slope. The watertight facing adopted within zone of known
upstream slope surface strengthening. The scheme of material
zones as shown on Drawing 3.
Variant
2
– assumptions as per Variant 1. The watertight
screen adopted within zone of known concrete panels location
at upstream slope surface.
Variant 3
– the zones of low density material considered as
per Drawing 3. The upstream slope surface insulation not
considered, due to poor condition of the slope strengthening
elements, qualified for repairs. The piping effect considered
within dam base, as per expert’s recommendations (W.Wolski,
J.Mirecki.2011). Location of the piping effect has been
indicated within area, which was the subject of temporary
partition during construction stage and where strong water flow
was observed (K.Fanti, K.Fiedler, J.Kowalewski, S.Wójcicki,
1972)(page 352 drawing 5-5). Based on this assumption the
zone of loose alluvium has been introduced within the model,
that such effect has not been considered at design stage as
induced during dam erection. This area has been prescribed for
piping effect occurrence. Similarly, another zone of piping
effect may take place in location of oxbow beyond the earth
dam. Material zones scheme for Variant 3 as per Drawing 5.
Variant 4
– all assumptions as for Variant 3. Additionally,
the watertight facing has been adopted within zone of known
concrete panels location at upstream slope surface. Variant 4
assumes necessary repairs of concrete panels made.
At calculation stage, the different up- and downstream water
level configurations were considered. Flooding conditions were
modelled as per water states dated 23.05.2010, and normal
working conditions dated 01.04.2012.
Boundary conditions were established based on both of
records from existing piezometers and up- and downstream
water levels. Two calculation cases were taken into
consideration: normal working conditions and flooding
conditions.
The piezometers network has been modelled, and after
analysis completion, the results were compared with values
observed in real. The seepages at upstream slope were also
analysed, however only the ones having place above the
existing downstream water level were considered. The above
comparison was used for back analysis of the ground
parameters and adequate modifications were made to meet the
best matching to the real situation. The leading case for
parameters verification was the normal working conditions of
the dam.
The analysis results were presented in form of phreatic level
projection across the dam body cross-section. Variant 1 is
shown on Drawing 7. This drawing presents the low
arrangement of groundwater level, which closely reproduces the
one observed at the dam.
The arrangement of phreatic level is due from calculation
based on the assumption that there is an impermeable facing on
upstream slope. The aim of variant one of calculations was to
demonstrate whether it is possible to achieve the low level of
phreatic level as described in professional literature. Despite the
convergence of a solution obtained from the calculations and
the observed in nature above mentioned situation does not occur
in the area of Wloclawek Dam. Structures situated on upstream
slope were qualified to repair and do not meet conditions made
in the calculations for Variant 1.
The results of phreatic level calculation in Variant 2 are
presented on Drawing 8. This variant of calculations was based
on the assumption that there is a watertight facing in the area of
slope with reinforcing concrete panels. The results of numerical
analysis for Variant 2 were not in the compliance with the
position of phreatic level observed in nature.
The calculation results of phreatic level in Variant 3 are
presented on Drawing 9. The appearance of piping effect in the
base of the dam was considered in this variant. The position of
phreatic level obtained in the calculations is the most consistent
with the conditions observed in nature comparing to other
variants. The differences relate to the initial filtration section
