Proceedings of the Institution of Civil Engineers http://dx.doi.org/10.1680/geng.9.00086 Paper 900086 Received 12/11/2009 Accepted 09/02/2011 Keywords: computational mechanics/embankments/failures ICE Publishing: All rights reserved Geotechnical Engineering Deformation and strength of embankments on soft Dutch soil Den Haan and Feddema Deformation and strength of embankments on soft Dutch soil j 1 Evert den Haan PhD Geotechnical Researcher, Deltares, Delft, The Netherlands j 2 Antoine Feddema BEng Senior Consultant/Manager, Deltares, Delft, The Netherlands j 1 j 2 Embankment and dyke stability in the Netherlands has always been evaluated by effective stress analysis. The subsoil of most of these structures is organic, weak and soft, but the internal friction angle of these soils is surprisingly high. Empirical methods are used to obtain acceptable, reduced values of friction angle from triaxial tests for use in stability analyses. It appears possible, however, to do full justice to the peculiar combination of low strength and stiffness and high friction angle by means of the finite-element method using a viscous version of the Cam-clay model. All parameters of the model are found from a single test in a constant-rate-of-strain K 0 oedometer. The approach is illustrated by two case histories, after first providing insight into the peculiar properties of the Dutch soils, and the manner in which they are dealt with. Notation C Æ creep factor c9 cohesion e voids ratio h height h 0 initial height K 0 lateral earth pressure at rest K 0,nc virgin compression value of K 0 k permeability M critical-state strength parameter p9 isotropic effective stress p9 c0 initial equivalent yield stress p9 eq equivalent isotropic effective stress p9 0 initial value of equivalent stress q deviatoric stress s u undrained strength s u =ó 9 p undrained strength ratio t time t age age of deposit t creep creep duration å nat natural strain å vol volumetric strain k* cam-clay swelling factor º* cam-clay compressibility factor ì* creep factor í Poisson ratio ó 9 ax axial effective stress ó 9 p vertical yield stress ó 9 rad radial effective stress ó 9 v0 initial vertical effective stress ö9 internal friction angle 1. Introduction Dutch organic soils and peat, although weak and soft, have surprisingly high values of the effective strength parameters and undrained strength ratios. Their internal friction angles, ö9, can far exceed the 30–358 range that is common in sands, and generally become higher as the organic content increases. For example, peat has ö9 values of up to 908, and in organic clays values of 40–608 are common. ö9 is an important parameter in the Netherlands, as the effective stress approach is used almost exclusively in stability analyses. To obtain reasonable factors of safety, ö9 is determined at strains far below failure, and the true strength of the material is not accounted for. For a realistic assessment it is necessary to combine (low) stiffness and (high) strength parameters in one analysis, and the finite-element method is the obvious means to achieve this. Good results have recently been obtained with finite-element calculations of the deformation of embankments constructed on soft Dutch soils. This will be illustrated by two case histories. The constitutive model used was a viscous version of the modified Cam-clay model. The necessary soil stiffness and strength parameters were determined from oedometer tests in which a constant rate of strain was applied, and lateral stress was measured. The paper starts off by giving the background to the high strength 1
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Deformation and Strength of Embankments on Soft Dutch Soil
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Table 3. Characteristics of IJkdijk peat, Booneschans
10
Geotechnical Engineering Deformation and strength ofembankments on soft Dutch soilDen Haan and Feddema
50454035302520151050
0·1
0·2
0·3
0·4
0·5
0·6
0Days after start of construction (13 August 2008 06:00)
Sett
lem
ent
unde
r cr
est:
m
Settlement plateCalculated
(a)
50454035302520151050
0·200·150·100·05
Allerød
In peat under crestIn top clay under crestIn peat near ditch
Mea
s.C
alc.
IIIIIIInc.
�3
�2
�1
0
1
2
3
4
5
6
7
Days after start of construction (13 August 2008 06:00)(b)
Tota
l hea
d: m
NA
P
�10
�8
�6
�4
�2
0
20
Horizontal displacement, inclinometer no. 53: m
Dep
th: m
NA
P
Full line: measurementSymbols: calculation
0: Start of phase II: End of phase I: filling basin, digging ditchII: End of phase II: ditch deeperIII: Phase III: infiltration in sand coreIIIA: During pause in infiltrationIIIB: Infiltration ends; failure imminent
IIIBIIIA0 III
Peat
Top clay
Clayshell
Pleisto-cenesand
(c)
Figure 12. Results of finite-element calculation, IJkdijk macro-
stability experiment, 2008: (a) construction phase settlements;
(b) pore pressures; (c) horizontal deformations during failure phase
11
Geotechnical Engineering Deformation and strength ofembankments on soft Dutch soilDen Haan and Feddema
The finite-element calculation failed before these rather compli-
cated processes occurred, after 0.33 of the ditch-deepening phase
had been applied. A finite-element calculation with a coarser
mesh of some 800 six-noded elements, however, did not fail even
in phase III, as detailed in an earlier paper (in Dutch) by Den
Haan and Feddema (2009). There, infiltration pressures were
applied by means of wells, and the sand penetration into the peat
was also modelled. A factor of safety (FS) was determined at the
end of phase III by undrained reduction of the Cam-clay M-
parameter, and FS ¼ 1.17 was found, whereas FS ¼ 0.85 would
be expected if the 15% side plane resistance was accounted for.
The coarse mesh has considerably fewer degrees of freedom than
the finer mesh, and this appears to offer additional resistance to
failure. To be sure mesh fineness was sufficient, a very fine mesh
of 4297 15-noded elements was also used. It produced essentially
the same results as reported in this paper.
8. DiscussionThe two finite-element calculations presented in this paper have
applied a creep model that is a viscous version of modified Cam-
clay, to embankments on soft organic clays and peat, with
parameters determined from constant rate of strain K0 oedometer
tests. The latter include the strength parameter M, which is quite
high in these soils, while cohesion is zero, and strain-dependent
permeability. The finite-element calculations with this model
appear to produce very satisfactory fits to the measured deforma-
tions and pore pressures, and the failure of the IJkdijk case is also
covered satisfactorily.
The determination of the strength parameters of soft organic clay
and peat has long troubled the geotechnical profession in the
Netherlands. The very high �9 value of these soils poses
problems both in laboratory strength testing and in stability
calculations. The procedure used in this paper – interpreting
constant rate of strain K0 oedometer tests within the framework
of a viscous version of modified Cam-clay to produce both
strength and compressibility parameters – is possibly a viable
alternative for Dutch practice.
The ability to predict vertical and horizontal embankment
deformations has a bearing on fill material consumption, on track
or road maintenance, and on deformations of foundations, piles
and utilities buried near the toe of the embankment. The ability
to faithfully predict pore pressures set up during construction can
further reduce the occurrence of failures during construction if
adequate surveillance and feedback are performed. The most
important function of calculations, however, is to predict failure,
and in this respect the adequate indication of failure of the IJkdijk
embankment is encouraging.
Predicting failure of dykes has become an important aspect of
geotechnical engineering in the Netherlands. Dyke authorities are
required to evaluate dyke safety every 5 years, and there are some
17 000 km of such dykes in the Netherlands! Evaluation is in
terms of the factor of safety and probability indices determined
from limit equilibrium analyses. Using the finite-element ap-
proach described in this paper, a dyke can first be built up, from
15:00:0013:00:0011:00:00�10
0
10
20
30
40
09:00:00
(Por
e) p
ress
ure:
kPa
0
20
40
60
80
100
120
140
160
Max
. hor
izon
tal d
efor
mat
ion:
mm
Lower infiltration tube (A)
Infiltration tubes in sandfill (B)
Pore pressure at base of sandfill (C)
Pore pressure 1 m above base ofsandfill (D)
Horizontal deformation
27 Sept. 2008
A
D
CB
50
60
70
80
Figure 13. Development of infiltration pressures and pore
pressures in sand core and of maximum lateral inclinometer
deformation, phase III failure stage, IJkdijk macro-stability
experiment, 2008
12
Geotechnical Engineering Deformation and strength ofembankments on soft Dutch soilDen Haan and Feddema
history as it were, to the present state, taking advantage of the
prolonged creep compression under the dyke and the accompany-
ing increase in shear resistance. Then a factor of safety can be
determined by one of various methods in which failure is
simulated. The most usual approach is the stepwise, undrained
reduction of the Cam-clay strength parameter M until failure
occurs, giving the factor of safety as the ratio of the available and
the reduced value of M.
However, the applied creep model has several limitations that
need to be considered. These are
(a) volume changes and pore pressures set up by rotation of the
stress tensor during construction and loading
(b) anisotropy of creep rates
(c) overconsolidation effects.
More general limitations lie in such matters as not modelling
localising deformations along shear surfaces during failure,
inadequate knowledge of the shape of the shear stress envelope in
principal stress space, the non-uniqueness of solutions when non-
associativity is assumed on the shear stress envelope (e.g. by
assuming zero viscoplastic volume change), and the inability to
model side plane resistance.
Stress tensor rotation occurs during embankment construction as
a result of load spreading. At the toe, for example, the initial
geostatic state with vertical major principal stress rotates over 908
to the passive state, and locations between crest and toe undergo
intermediate amounts of rotation.
In soft soils such rotations usually induce volume decrease and
pore pressure increase. These effects are not dealt with by the
creep model used here. Jardine et al. (1997) describe how, once
consolidation has occurred under the rotated stresses, undrained
loading without further rotation yields high undrained strength,
close to that which is obtained without any rotation. This is due
to the soil’s fabric gradually adapting to the rotated state of
stresses and strains. In dyke safety evaluation the failure loads
stem mostly from rising water levels, and as these are essentially
horizontal, they will induce fresh rotations of the stress tensor,
and appear so quickly as to be essentially undrained. This effect
is probably small, however: the IJkdijk finite-element model was
run with extreme water loading (water level in the basin behind
the dyke raised quickly to crest level), and it was found that
rotations were less than 108 in the zones in which shear failure is
expected to occur.
Modified Cam-clay is an isotropic model in the sense that the
yield ellipse remains orientated along the isotropic stress axis.
Developments are under way in which the ellipse rotates depend-
ing on the relative amounts of isotropic and distortional plastic
strains. Such anisotropic models appear to improve fits to meas-
ured soil behaviour, and have recently been adapted to account
for soil viscosity, as well as the specific behaviour of peat (Leoni
et al., 2010). This may provide an avenue for further improve-
ment of the approach presented here.
The creep model does not deal adequately with the overconsoli-
dated state. On unloading, rates of creep strain reduce strongly
and behaviour becomes essentially elastic, and only critical-state
strength is used. Embankments with a significant passive zone of
highly overconsolidated material may therefore be less amenable
to the approach described here.
The side plane and mesh size effects on the moment of failure in
the IJkdijk case have been noted, and in any calculation of failure
it will be necessary to take these effects into account.
Notwithstanding these limitations, and given that due care is
exercised, the procedure described in this paper should allow
successful application of the finite-element method to the calcula-
tion of deformations and strength of embankments on soft
organic soils.
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