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The Design of Filter Materials and their Importance in Geotechnical Engineering
La conception de matériaux filtrants et leur importance en géotechnique
Messerklinger S.
Consulting Engineers, Poyry Energy Ltd., Zurich, Switzerland
ABSTRACT: The presence of water has a major influence on the design of soil structures as it reduces the effective stresses and
hence shear resistance, and applies seepage forces in case of flow. This key topic is well known to every geotechnical engineer and
the design principle for soil structure is to drain groundwater, infiltrated surface water or seepage water in a controlled manner from
the soil. However, for soil structures whose purpose is to retain water, such as embankment dams impounding a reservoir, or dikes for
flood protection along rivers and channels, both sealing and draining have to be ensured by the structures. With simple construction
measures such as filter and drainage zones incorporated in earth structures composed of selected and treated materials, the stability
and safety of these structures can be improved considerably. This paper discusses seepage control measures as well as the selection
and design of appropriate filter materials.
RÉSUMÉ : La présence d'eau a une influence majeure sur la conception d’ouvrages géotechniques, car elle réduit les contraintes
effectives et la résistance au cisaillement et donc applique des forces d'infiltration en cas de débit. Le sujet abordé est bien connu des
ingénieurs en géotechnique. Le principe de conception des ouvrages est de drainer les eaux souterraines, les eaux de surface infiltrées
ou les eaux d'infiltration de manière contrôlée à partir du sol. Toutefois, pour les ouvrages dont le but est de retenir l'eau, comme les
barrages de retenue en remblai ou les digues de protection contre les inondations le long des rivières et des canaux, l'étanchéité et le
drainage doivent être intégrés dans les ouvrages. Avec de simples mesures constructives telles que de des zones de filtrage et de
drainage dans des ouvrages en terre, et composées de matériaux séléctionnés et traités, la stabilité et la sécurité de ces ouvrages est
considérablement améliorée. Le présent document examine les mesures de contrôle d'infiltrations dans les ouvrages et la sélection et
conception de matériaux filtrants appropriés.
KEYWORDS: filter design, seepage control, embankment dam failure
1 INTRODUCTION
1.1
Effect of water in soils
Soils are composed of single particles. The loads are transferred
at the particle contacts with normal and shear forces
1
. The
maximum shear force which can be transferred at the particle
contact is proportional to the effective normal force at the
contact, as defined by the total interparticle force and the pore
water pressure, should the soil be saturated. The porewater
pressures can correspond to the (a) hydrostatic head, should the
soil skeleton be submerged, or (b) to an excess pressure which
exceeds the hydrostatic head. Excess pressures develop for
example (a) in loose deposits of low permeable granular soils,
such as silts and fine sands, during an earthquake event (see e.g.
Messerklinger et al., 2011a) or (b) by the application of an
external load, e.g. during construction work, on compressible
and low permeable soils such as clays and silts.
Summarizing: The water of a submerged soil skeleton
reduces the effective interparticle forces and hence the shear
resistance of the soil.
If the water in the soil skeleton is flowing with a velocity (v)
at a hydraulic gradient (i), forces due to water flow are applied
on the soil particles. These flow forces on the soil particles act
in addition to the pore water pressures. The flow forces (F) act
in the flow direction. Their magnitude is F=i·
w
·A where
w
is
the unit weight of the water and A is the cross-sectional area (in
1
For clays interparticle forces can act in addition to contact forces.
The contact forces are defined by the self-weight of the soil and the
external loads. Interparticle forces are e.g. (i) electromagnetic
attractions, which are commonly called van der Waals forces or (ii)
electrostatic repulsive or attractive forces at double layers.
flow direction) of the soil body the water is flowing through.
This is the average force on a soil body due to water flow at a
hydraulic gradient of i. However, the flow forces acting on a
single particle vary significantly. The flow velocity of the water
in the pore space depends on the pore diameter and increases
approximately with the square of the pore diameter. If the pore
diameter changes, the pore flow velocity will also change.
However,
in permeability
tests only
the overall
soil permeability,
as defined by
the permeability coefficient (k),
is determined.
Summarizing: In case a hydraulic gradient is applied to the
water in a submerged soil skeleton, the water will flow around
the
single
particles, which
applies
flow
forces
in
flow
direction.
These flow forces depend on the hydraulic gradient and are
independent of the volume of water flowing through the soil.
1.2
Effect of water on natural or man-made soil structures
These effects of water on the soil influence the stability of soil
structures, irrespective of whether they are natural or man-made.
Natural soil
structures
are
for
example: (i)
soil
slopes;
(ii)
in-situ
soils
surrounding
a
man-made excavation or (iii) soil
foundations of buildings or embankment dams. Man-made soil
structures are for example the embankment dams
2
themselves.
Subsequently, the effect of water on the stability and safety of
such structures is discussed.
2
Note: Embankment dams can be used for different purposes such
as road embankments, landfills, off-shore embankments e.g. for sea
water intakes, shore protection or embankments surrounding reservoirs.
