FINITE ELEMENT ANALYSIS OF LANDSLIDE IN DREDGED SLOPE
S. LI
Geotechnical Engineering Institute, Tianjin University, Tianjin, China
Z.Q. YUE, L.G. THAM, C.F. LEE
Department of Civil Engineering, The University of Hong Kong, Hong Kong, China
Abstract
This paper presents a finite element analysis of a landslide that occurred in a newly dredged
submarine slope for port development. The landslide was a multiple retrogressive landslide.
The marine slope comprised recent marine and river mouth delta deposits. The landslide
occupied a plan area 200 m long and 150 m wide. A volume of about 240,000 m3 soil
slipped into a newly dredged open space in the sea. The finite element analysis indicates
that the backfill pre-loading significantly increased the failure zone in the dredged slope and
the soil lateral displacements toward to the open space in the sea.
Keywords: Submarine slide, dredged slope, soft soils, finite element analysis
1. Introduction
In this paper, we intend to present a finite element analysis of a landslide that occurred at a
port development near Tianjin, China. The finite element analysis focuses on the main
factor causing the dredged slope unstable. This main factor was found to be the backfilling
of a thick general soil and sand cushion for vacuum preloading of the hydraulic fill in the
reclaimed land where the dredged slope was formed. Details of the landslide investigation
and associated stability analyses can be found in a full paper submitted to Canadian
Geotechnical Journal (Li, et al. 2002).
Landslide site
Figure 1. Location of landslide on Bohai Sea near Beijing, China.
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2. Background
Figure 1 shows the landslide location on the western coastal line of Bohai Bay, China. It
was about 130 km east to Beijing. The coastal region is a typical mud plain and has
experienced a continued land depression and sedimentation for millions years. The marine
and river-mouth alluvial deposits are more than one thousand meter thick.
The landslide occurred during the construction of a long piled wharf. As other wharfs in
this region, the wharf was being built on a reclaimed land that was formed by hydraulic
filling. The berth base elevation of the wharf was designed to be –13.8 m below a standard
sea level. The reclaimed land top elevation was designed to 6.0 m above the standard sea
level. The shoreline of the wharf was 252.0 m long and 434.5m into the sea. The excavation
in the sea for the berth and slope was carried out using dredging. Figure 2 shows a design
cross-section for the wharf development and dredged berth and slope.
Figure 2. Typical cross-section of the dredged slope before landslide.
The original ground was extremely flat and was at the standard sea level. For the
reclamation using hydraulic fill, a trapezium reclamation dam was constructed on the
original ground in 1991. The dam was made up of the concrete was about 4.4 m high. It
located about 50 m behind the designed berth. Reclamation on the land within the dam was
then carried out and completed in 1993. The land within the dam was hydraulically filled
about 4.0 m above the standard sea level.
The construction of the wharf commenced in 1997 four years after the reclamation. The
reclaimed land was backfilled up to 6.85 m in one month. Preparation works was also
carried to strengthen the hydraulic fill (soft clay) via the application of the vacuum
preloading technique. It is noted that vacuum preloading has been widely used for
stabilizing soft clay in this region. The dredged excavation in front the dam was carried out
between July 25 and August 13, 1997.
On August 15, 1997, the dredged slope gradient was found to be too gentle to satisfy the
requirements of the barge for driving piles for the wharf structures. Hence, August 28 to
September 4, additional dredge excavation was carried out and the slope gradient was
amended (see Figure 2). During this period, a 1.0 m thick soil temporary road was built
behind the dam. From September 5 to 15, pile driving construction was carried out in
Finite element analysis of landslide in dredged slope
71
western portion of the dredged slope. No pile driving construction was carried out on
September 16, 1997.
3. Landslide Event
The landslide occurred in the dredged slope at about 09:00 in the morning of September 17,
1997. It lasted for about one hour and resulted in about volume of 240,000 m3 soil that
slipped into the dredged open space in the sea. The landslide occupied a land area of 200 m
long along the coastal line and 150 m wide behind the dam.
Because of the presence of some deep cracks few hours in the reclaimed land before the
landslide, construction workers in some temporary shelters on the dam escaped the site in
time. The landslide caused no fatality. According to witnesses, the first slide that slipped
into the sea was appeared in the centre of the reclamation land. It occupied a plan area of
about 80 m long and 30 m wide. It was more than 30 m east to the driven piles. After the
occurrence of this first slide, second and third slides slipped into the sea retrogressively.
The driven piles subsequently either slipped into the sea or toppled in declined angles in the
landslide debris.
4. Site Condition
The coastal region is a typical silt coast plain. The thick marine and river-mouth alluvial
deposits are inter-layering. The river-mouth alluvial deposits are mainly due to the Yellow
River (see Figure 1).
The dredged slope comprised three types of soft soils. The top layer has three sub-layers.
The top sub-layer was the hydraulic fill. The second and third sub-layers were recent marine
soft soil deposits and were under-consolidated. Their sensitivity was 3 to 5. The natural
water contents were usually greater than its liquid limit. The void ratios was usually larger
than 1.0. The bottom layer also had three sub-layers. It was a silty clay that was part of the
river mouth delta deposit due to the sedimentation of the Yellow River. This layer was
lightly compacted and had higher bearing capacity and shear strength than the second layer.
There was a thin interlayer between the top and bottom layers. More detailed explanations
of the soil conditions can be found in Li et al. (2002).
The dredged slope and the reclaimed land had shallow groundwater table. According to
drilling logs, the observed groundwater table was about 4.2 m above the standard sea level.
This water level was quite stable.
The tide in the coastal region was an irregular semidiurnal tide. According to local
information, the tidal level was 0.96 m at 09:00 when the landslide occurred and was
reduced to the lowest level 0.66 m at 10:00 when the landslide completed.
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5. Finite Element Analysis
A detailed landslide investigation has revealed that there were a number of factors causing
the retrogressive landslide (Li, et al. 2002). These factors included the backfilling on the
hydraulic fill for vacuum preloading, the amended dredging excavation for pile driving
barge, the construction of pile driving and the reduction of the tidal level. The landslide
investigation found that the backfilling of a thick general soil and sand cushion for vacuum
preloading of the hydraulic fill in the reclaimed land might have made the dredged slope to
be marginally stable.
In order to have a better understanding on the effect of the backfilling on the stability of the
dredged slope, we have carried out a further investigation on the distribution of stresses and
displacements in the dredged slope before the landslide. In the ensuing, we are going to
present the finite element analysis of the effect of the backfilling on the dredged slope
stability.
It is well known that the finite element method is a powerful numerical method for stress
and deformation analysis of geomechanical problems. This method has been used by many
researchers and engineers for the analysis of slope stability (eg., Sheng et al. 2002). By
comparing with the classical limit equilibrium based slice methods, it has the following
advantages: a) no assumptions need to be made in advance about the shape or location of
the potential slip surface; b) No assumptions need to be made about slice side forces; c) the
solution can give data about deformation and stress distribution in the slope.
The computer program used in the finite element analysis was for two-dimensional (plane
strain) analysis of elastic-perfectly plastic soils with a Mohr-Coulomb failure criterion. It
adopted 4-node isoparametric quadrilateral elements with linear displacement variation. The
discrete point loads, gravity loading and distributed edge loading can be taken into account.
These loads can be applied in a single increment, or in multi-stepped increments that are
controlled by a subroutine program.
To solve the nonlinear problem, a Newton-Raphson method based iterative scheme was
adopted. All the stress and strain components were accumulated from the values in each
iteration. The general procedure was to determine the stress in each element so that the yield
criterion could be satisfied. These stresses were compared with the Mohr-Coulomb failure
criterion. If the stresses were within the Mohr-Coulomb failure envelope, then the
deformation at the corresponding elements were considered to be elastic. If the stresses
were on or outside the failure envelope, then the deformation at the corresponding elements
were considered to be yielded. If a calculated stress for an individual element was greater
than the yielding value, then the additional part was deleted but was then included in the
residual force vectors to maintain static equilibrium. The iterations would continue until the
failure criterion and the equilibrium at all nodes were satisfied within a given convergent
level.
The criterion for judging whether or not a slope would experience a landslide was that a
sufficient number of Gauss points are yielded and those points can allow the development
and formation of a global slip mechanism in the slope.
Finite element analysis of landslide in dredged slope
73
In the finite element analysis, the Mohr-Coulomb yield model was used. The soil parameters
are the total cohesion c, the total internal friction ϕ , the deformation modulus E, the
Poisson’s ratio ν, and the soil unit weight. The values of the soil parameters used in the
finite element analysis are given in Table 1.
In the finite element analysis, we examined the stress and displacements in the dredged
slope by imposing the backfilling as a surcharge load. The results are then compared with
those obtained under the assumption that the backfilling was not constructed.
Furthermore, we used the shear strength
τf
stress level below.
Shear stress level =
as the strength parameter to define a shear
τ
τf
(1)
where τ is the shear stress calculated by the computer program at each element Gauss point.
This stress level was used to show the extent and distribution of the calculated shear stress
in the dredged soil slope.
Table 1. Soil parameter values used in the finite element analysis.
Soil layer
Silt
Layer
Total
friction
angle ϕ
( q)
Deformation Poisson’s
Total
Ratio
cohesion c Modulus E
ν
MPa
kPa
1-1
17.9
1.04
7.7
1.9
0.40
1-2
16.5
1.13
9.0
2.6
0.35
1-3
17.5
2.08
14.2
3.3
0.35
19.6
17.5
17.9
6.9
0.30
2-1
19.7
20.9
21.0
8.0
0.30
2-2
18.7
8.4
28.0
4.6
0.35
2-3
20.2
29.1
22.0
9.8
0.25
Interlayer
Clay
Layer
Unit
weight
γ
(kN/m3)
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Figure 3. Distribution of the shear stress level in the dredged slope with backfilling surcharge.
Figure 3 presents the distribution of the stress level in the dredged slope with the backfilling
surcharge loading whilst Figure 4 presents the distribution of the stress level in the dredged
slope without the backfilling surcharge.
Figure 5 presents the distribution of the displacement vector in the dredged slope with the
backfilling surcharge loading whilst Figure 6 presents the distribution of the displacement
vector in the dredged slope without the backfilling surcharge. Comparing the results in
these four figures, we can observe that the backfilling surcharge loading enlarged
significantly the failure zone in the dredged slope and caused large lateral displacements in
the dredged slope toward to the open space in the dredged berth region.
Figure 4. Distribution of the shear stress level in the dredged slope without backfilling surcharge.
Figure 5. Distribution of the displacement vector in the dredged slope with backfilling surcharge.
Finite element analysis of landslide in dredged slope
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Figure 6. Distribution of the displacement vector in the dredged slope without backfilling surcharge.
Furthermore, Figure 7 presents the variations of horizontal displacements with depth at
three horizontal locations behind the dam for the case with the backfilling surcharge whilst
Figure 8 presents the similar results for the case without the backfilling surcharge. It is
evident that the backfilling surcharge loading significantly increased the lateral
displacements in the dredged slope toward to the sea and caused the lateral displacement at
12 m behind the slope crest to be larger than that at the crest (0 m). So, the soils behind the
slope crest pushed their front soils into the open space in the sea.
Figure 7. Variations of horizontal displacements with depth at three horizontal distances to the dredged
slope crest
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7. Conclusions
In the above, we have briefly discussed the landslide that occurred in a newly dredged slope
for a wharf construction in the coastal region of Bohai, China. We presented a non-linear
finite element analysis on the effect of the backfilling surcharge on the stability of the
dredged slope. From the analysis, we have further confirmed that the backfilling surcharge
was one of the factors causing the dredged slope unstable.
8. Acknowledgements:
The authors would like to thank the financial supports from the Research Grants Council of
Hong Kong SAR Government and the Hong Kong Jockey Club Charities Trust.
9. References
Li, S., Yue, Z.Q., Tham, L.G., Lee, C.F. and S.W. Yan, (2002). Practical approach for analysis of slope stability and
landslide in under-consolidated soils for port development, paper submitted to Canadian Geotechnical
Journal. August 2002. p.63.
Sheng, Q., Yue, Z.Q., Lee, C.F., Tham, L.G. and Zhou, H. (2002). Estimating the excavation disturbed zone in the
permanent shiplock slopes of the Three Gorges Project, China, Int. J. of Rock Mechanics and Mining
Sciences. Vol.39: 165-184.