3314
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1.2.1
Soil slopes
Natural soil slopes are subjected to the natural groundwater
flow conditions, and are formed of the given in-situ soil
material which may have predefined slip surfaces with reduced
shear strength, due to previous slides. These were subjected to
earthquakes and weather conditions typical in the region and
have correspondingly an overall factor of safety for slope
stability of somewhat above one.
Constructing in or on natural soil slopes either reduces the
resisting forces, e.g. by excavation, or applies driving forces,
e.g. when structures are founded on the slope. For slopes within
or next to man-made structures an overall factor of safety of
well above one is desirable. Hence, the stability of natural
slopes next to or within the construction area normally has to be
improved.
The stability of slopes can be improved by man-made
structures which apply resisting forces such as anchors, piles,
etc. Or the slope stability can be improved by lowering the
groundwater level in the slope to increase the effective stress
and hence shear resistance at the drained soil. The water level
can be lowered by drainage, e.g. with borings filled with filter
and drainage materials and free or pumped outflow (see e.g.
Messerklinger 2012).
1.2.2
Soils surrounding a man-made excavation
The excavation in saturated soil can be surrounded by an (i)
impermeable or (ii) a permeable wall. For an impermeable wall,
both the earth pressure and the water pressure act on the wall,
and the pressure can be in the order of two to three times that
for a permeable wall, on which only the effective earth pressure
is acting. Lowering the water level behind the wall, e.g. by
pumped wells or by drainage into the excavation pit, reduces the
loads on the wall. However, this imposes a hydraulic gradient
on the in-situ soil surrounding the excavation. This hydraulic
gradient applies flow forces on the particles of the soil and these
forces can cause transport of fine soil particles within the coarse
soil particle skeleton for internally unstable soils (criteria see
Chap. 2). At the surface where the water leaves the soil body,
e.g. at the pumped drainage well or at other drainage points, the
soil can be eroded unless the surface is protected with a filter
and drainage material.
1.2.3
Soil foundations of a dam impounding a reservoir
With the impounding of the reservoir a hydraulic gradient and
water pressure are applied on the soil foundation. The increased
hydrostatic water pressure reduces the effective stresses and
hence strength of the soil. The imposed hydraulic gradient
applies flow forces onto the soil particles which can cause
erosion within the soil skeleton or at the surface of the soil body
where the water flows out of the soil. A layer of filter material
at the water exit below a layer of drainage material will prevent
erosion of soil particles and increase the effective stress.
Examples are presented in Messerklinger et al. 2010 and 2011b.
1.2.4
Man-made embankment dams for reservoirs
With the impounding of the reservoir the hydraulic gradient and
the water pressure are applied onto the man-made earth fill.
Man-made earth structures allow for the placement of filter and
drainage zones within the dam body. This is normally supported
by a zone of reduced permeability (e.g. clay, concrete, asphalt,
geomembrane) which reduces the volumes of water flowing
through the structure (see Messerklinger 2011c). The
incorporation of filter materials assures the stability and safety
of embankment dams.
1.3
Summary
Water has a major influence on the stability and erosion
resistance of natural and man-made soil structures as it reduces
the effective stress and hence shear strength of the soil and
applies forces in case water is flowing through the soil.
Hence, draining the water out of the soil structures improves
their stability or the stability of structures built on or in the soil.
However, draining of the soil has to be done in a controlled
manner. The hydraulic gradients and hence flow forces applied
on the soil particles must not erode particles within the soil
skeleton or at the surface.
For natural soils, this is assured by limiting the hydraulic
gradient. For man-made structures the erosion is controlled by
filter zones incorporated in the soil structure. The design of
suitable filter materials is discussed in the next chapter.
2 DESIGN OF FILTER MATERIALS
For the design of state-of-the-art filter materials, the
following
six
aspects
are
considered:
(a) Filter ability (b) Internal stability
(c) Self healing (d) Material segregation
(e) Drainage capacity
(f)
Material durability.
2.1
Filter ability
With the identification of effective stresses in soils by Terzaghi
and his co-workers in the early thirties of the last century,
(Terzaghi 1936)
a
new
era
in
soil mechanical
engineering was
initiated. This was the time when the effects of water on soil
were investigated in depth, and resulted in the development of
the consolidation theory (Terzaghi & Fröhlich 1936).
At the same time, Bertram (1940) proposed the criterion
D
15filter
/d
85base soil
≤ 6 for soil filters based on laboratory
investigations. This filter criterion was later modified to
D
15coarse-side filter
/d
85fine-side base soil
≤ 4 and a drainage criterion of
D
15fine-side filter
/d
85coarse-side base soil
≥ 4 was added by Terzaghi and
Peck (1948), (Fig.1). These filter and drainage criteria were
used for decades and are still today subjects lectured on to the
bachelor and master students.
Mass percent passing [%]
Gradation curves
of suitable filter materials
Gradation curves
of the base or core soil
Grain diameter [mm]
Figure 1: Filter and drainage criteria from Terzaghi & Peck (1948).
The filter design was reconsidered after incidents at and
failures of major dam structures. E.g. after the Balderhead dam
incident, where core material was eroded from an open fracture
in the core zone into the filter material so causing sinkholes at
the dam crest (Vaughan et al. 1970), Peter Vaughan and his co-
workers searched for what they called the “perfect filter”. The
idea was to hold back the smallest grain of a core material even
under severe conditions such as concentrated seepage flow at
high hydraulic gradients through e.g. a crack in the core. The
approach towards the criterion was not via the gradation curve,
such as adopted before by Terzaghi and his co-workers, but by
the permeability coefficient of the filter material. Vaughan
believed
that
“..effectiveness of a filter may be defined by its
permeability with more generality than by its grading.
”
(Vaughan
& Soares 1982, p.17). They proposed a
linear correlation between
the permeability coefficient (k in m/s) and the filtered particle
diameter of k = 6.1E-6 ·
1.42
(
in
m, Note: The particle size of
clays with flocculated structure
is
the
floc-size.).
At the same time, James Sherard was investigating the
cracking and failure of embankment dams built in the United
States (Casagrande 1950, Sherard et al. 1963, Bertram 1967). In
1973 he wrote (p. 272):
“… at present it is well known that
cracks have developed in the impervious sections of many dams
