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TRANSCRIPT
Stability of Natural Slopes and Embankments Underlain
By Weak Clays
Prof. Dr. Yousef M. Masannat*
Abstract
A comprehensive review of the literature concerning case histories of failures of cuts
of natural slopes and embankments constructed on very soft to medium stiff clays was
conducted .The merits and limitations of the different field and laboratory testing
techniques and construction procedures were outlined. The consideration of the
paleogeological history as well as the hydrological and geological characteristics of
the sites of cuts in natural slopes comprised of jointed competent rocks with interbeds
of weak shales, clays and mudstones in establishing the design criteria of the cuts is
emphasized. Also, the proper selection of the investigation and testing techniques of
weak sensitive clays for the determination of their shear strength parameters used in
the stability analysis of embankments constructed on these weak soils is emphasized.
Recommendations, based on experience and judgment concerning the site
investigations and field and laboratory testing techniques and construction procedures
are developed to help designers and practicing engineers in their task of constructing
safe and economic structures.
The targeted factors of safety largely depend on the type of material involved,
level of risk and uncertainty of gathered data used in the stability analysis.
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Stability of Natural Slopes and Embankments
Underlain by Weak Plastic Clays
By
Yousef M. Masannat*
INTRODUCTION
Although advanced recent techniques used in geotechnical testing have
substantially contributed to the maturity of foundation engineering as an independent
discipline, the diversity of challenges in applications manifested by unexpected
failures of presumably safe engineering structures demonstrates that this field is still
in a stage of further development and refinement. This is particularly true in the realm
of risk assessment, forensic engineering, and design analysis of earth structures
founded on soft clay foundations. Discrepancies between the in-situ and laboratory
measured values of soil properties and also between the assumed and actual
stratigraphic sequence of clay deposits often seriously affect the accuracy and thus the
reliability of the stability analyses. This discrepancy between the theoretical analyses
and the actual behavior of embankments founded on soft clays is further exacerbated
.by the negligence of time-scale effect on the behavior of highly sensitive clays.
Boundary conditions, sometimes, seriously affect the results of stability analysis
particularly in the case of non-uniform subsurface groundwater flow conditions.
Records about the actual behavior of embankments constructed over thick
sensitive clays constitute a valuable data source for subsequent analysis by designers.
* Professor, Faculty of engineering and Technology, U. of Jordan.
This would, undoubtedly, contribute to the enhancement of the geotechnical
engineering profession in the design, construction, and monitoring of the actual
performance of embankments and their soft foundations. Of particular importance
also are the documented field records about the improved performance of
embankments through the use of vertical drains at different spacings to enhance pore
water pressure dissipation, incorporation of stabilizing berms in the design to increase
safety and rate of construction, use of pre-loading to decrease post-construction
settlements, and the use of geotextiles to strengthen the foundations and decrease the
potential cracking due to excessive total and differential settlements.
STABILITY OF SLOPES
The assurance of an adequate factor of safety to natural and man-made
earth slopes is the most challenging task of geotechnical engineers. Among the
various methods used to assess the level of safety of earth slopes, the limit
equilibrium method is the most widely used one. This is due to its simplicity
whereby the soil mass is assumed to behave as a rigid plastic body meeting the
Mohr-Coulomb failure criterion and moving along a continuous slip surface. This
method usually leads to practically acceptable results in soils with perfect plastic
behavior but not in brittle very stiff soils. Shear failure of soft to medium stiff
cohesive soil slopes is often preceded by slow time-dependent creep movement i.e.
progressive failure. Tiande et al (1999) in their model of progressive failure of
landslides showed that local plastic failure initiates at the toe of the slope where
shear stress is concentrated. and tension failure occurs at the crest of the slope.
This observation is in conformity with the conclusions of Lo and Lee (1973) in
their finite-element study of the slope stability of strain-softening soils. Local
failure at the toe slice of a slope leads to the transmission
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of stress to the neighboring slices and eventually to the initiation of local failure.
Propagation of local failure continues with continuous creep displacement and
widening of tension cracks at the crest of the slope until it culminates in a catastrophic
landslide unless local failure stops at a certain slice and strength doesn't drop to a post
peak value. In this case the overall factor of safety of the slope is greater than unity.
Tiande et al (1999), in their numerical simulation of failure evolution, clearly showed
that the time between the development of tension crack at the crest of slope and
complete failure is much shorter than that between the initiation of local failure at the
toe of the slope and the development of crack at the crest of the slope. This
emphasizes the importance of observing the initiation of local failure at the toe by
detecting any lateral displacements at the base of the slope at an early stage to take
measures that would prevent further deterioration of slope stability.
Wright et al (1973) criticized the widely used limit equilibrium method of
slope stability for three reasons viz. the negligence of stress-strain characteristics of
soil, the assumption that the factor of safety is the same for every slice, and the
nonsatisfaction of all conditions of equilibrium. Contrary to the assumption that the
factor of safety is the same for each slice in the Bishop's Modified Method, values
vary from one location to another along the shear failure surface when using the linear
elastic stress distribution. To prevent local elastic overstress along the critical shear
surface the required factor of safety varies from 1.44 to 1.49 for purely cohesive soils
having slope angles ranging from 16° to 34° respectively and with
eØ = H tan Ø
c = 0 (i.e. Ø = 0o) where = unit weight of slope material and
H=height of slope. For soils with eØ = 50 the required factor of safety ranges from
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1.12 for a slope angle of 16° to 4.36 for a slope angle of 34°. However, for a wide
range of encountered conditions Wright et al (1973) recommended that a factor of
safety of 1.5 is adequate to prevent local elastic overstress and thus the possibility of
initiating progressive failure in the slope.
Since failure of soft foundations beneath stiff embankments is often of a
rotational type the failure mode is represented by different types of shear tests due
to the rotation of the principal stresses viz. undrained triaxial compression, direct
simple shear, and undrained triaxial extension tests as shown in Figure 1 (Bjerrum
1972). Therefore, it is of an utmost importance to select the undrained strength
value that best represents the average strength along the entire failure surface.
Proper consideration should also be given to the stratigraphic and structural
features of the foundation materials like the types and thicknesses of
intercalations, degree of anisotropy, and orientation of laminations. Usually the
undrained strength from the undrained direct simple shear (DSS) represents the
best average to be used in stability analyses. In case insitu vane shear test values
are to be used Bjerrum (1972) recommended the application of correction factors
for the effect of rate of shearing and for the effect of anisotropy.
In the case of constructing embankments over soft clay foundations there
could be a significant difference in the stress-strain characteristics between the stiff
compacted fill of the embankment and the soft clay foundation. I t is thus
recommended that the undrained strength of both the embankment fill and the
foundation material be reduced by applying the reduction factors RE and RF to the
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undrained strength of the embankment and the foundation materials respectively as
shown in Figure 2 (Duncan and Buchignani, 1975).
STAGED CONSTRUCTION
Ladd (1991) emphasized the importance of controlling the excess pore
water pressure (p.w.p) in the soft clay foundation during all stages of construction on
such soils. This could be done either by slowing the rate of construction or by staged
construction. Staged construction involves suspension of construction at certain
critical sections of embankment for periods that could range from few days to few
weeks. Staged construction aims at enhancing gain in the shear strength of soft clay
foundation through the gradual time-dependent dissipation of p.w.p. Staged
construction often requires the installation of an efficient monitoring system that
allows the measurement of excess p.w.p at different depths beneath different sections
of the embankment as well as the vertical and horizontal displacement The objective
of this system is to enable the designer to carry more reliable stability analyses at the
end of each stage of construction before proceeding to the next stage. Assessments of
the shear strength of clay deposits based on well established relationships with the
vertical effective stresses should be correlated and ascertained with the insitu shear
strengths measured by reliable field testing techniques. This is to ensure the validity
of the assumed shear strength parameters used in the stability analyses and thus the
reliability of the factor of safety computations . Some of the major pitfalls of designers
in their stability analyses is their reliance on the piezometric measurement of p.w.p (to
limited depths and at limited locations) alone or their reliance on the measurement of
insitu shear strength by using some crude methods like the insitu vane shear tests
without any correction for the effects of some factors like anisotropy, rate of shearing,
plasticity, and size of testing apparatus. Stabilizing berms are sometimes used to
improve the stability of the embankment and accelerate the consolidation process.
Pre-loading with fill heights exceeding the final design grade are also used to reduce
the post-construction settlement. To meet construction schedules and reduce post
construction settlements vertical sand or prefabricated wick drains are often used to
provide horizontal drainage and accelerate both the consolidation of the clay
foundations and their strength gain. Settlement calculations require the determination
of soil stratification, soil properties, and its past geologic history within the
significantly stressed zone beneath the embankment.
TESTING UNCERTAINTIES
Uncertainties associated with the insitu measured values of undrained shear
strength of clay foundations as compared with the laboratory measured values should
be dealt with by applying appropriate correction factors depending on the sensitivity
of the soil, sampling disturbance, rate of shearing, type and size of shearing apparatus,
degree of anisotropy, and the effect of progressive failure. Bjerrum (1972) listed 14
cases of embankments that failed although most of them showed a factor of safety
well greater than 1.0 based on the insitu vane shear tests. He introduced a correction
factor, µ, to the undrained strength as measured by the insitu vane shear test (VST) in
the form:
Cu (corrected) = µ.Cu (VST)
where, µ = 1.7 -0.5 log P1 (P1 = plasticity index)
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He also introduced correction factors for the effects of rate of shearing, µR, which
depends on the PI of the clay and for the anisotropy, µA, which depends on the
inclination of the sliding surface and the PI of the clay in the form:
Cu(field)= Cu(VST) µR. µA
He indicated that the effect of anisotropy, µA , is higher in the lean clays of low
plasticity than in the highly plastic clays. It could be also observed from the data listed
in Table II (Bjerrurn, 1972) that the factor; µR, is slightly higher in lean clays than in
highly plastic clays. Bjerrum (1972) also emphasized the effect of progressive failure,
particularly in sensitive clays with strain softening characteristics in initiating failure
in embankments where the computed factor of safety, based on the VST, is well
greater than 1.0. Failure starts at the highly stressed zone beneath the center of the
embankment and gradually extends sideways until a full shear failure plane develops.
Overstressing the sensitive clay causes a sudden drop in its strength to the post peak
value with complete destruction of its structure. When failure occurs the soil will be
differently strained at the different locations along the failure surface. While the soil
strength beneath the highly stressed zone is close to the residual one the soil strength
beneath the less stressed zones at both ends of the failure surface will be close to the
peak one. On one side the soil strength is best represented by the undrained triaxial
compression test while on the other side it is best represented by the undrained triaxial
extension test.
Tavenas and Leroueil (1980) recommended, in the light of the many case
histories of failure where the computed factor of safety far exceeded 1.0 (Figure 3)
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that Bjerrum's approach in the stability analysis of embankments founded on soft
clays be considered only as a crude empirical one.
In the case of non-uniform soil conditions like the presence of intercalations
of silt layers, lenses of sand, or interbeds of gypsum, calcite, or aragonite within the
clay deposit it has been demonstrated by many case studies that the cone penetration
test (CPT) is far more superior than either the VST or the SPT in determining the
variation of strength along the soil profile.
Trial embankments are sometimes constructed and instrumented and
monitored to assess and verify subsurface ground conditions and soil strength and
consolidation characteristics for use in embankment stability and settlement analyses.
They are also used to check for the appearance of any indications of impending failure
that could be employed during the construction of embankment like the development
of longitudinal or transverse cracks. For stability analysis during staged construction
of embankments over soft clays Ladd (1991) recommended the use of the undrained
strength analysis (USA) rather than the effective strength analysis (ESA) because
failures during staged construction often occur under undrained conditions. This
requires the determination of the effective overburden stress (o) and the
preconsolidation stress (c) along the soil profile. This requires running consolidation
tests on undisturbed samples representing the whole soil profile under consideration.
Increments in effective stress during construction are then computed by proper
consideration of stress distribution and pore pressure readings of the installed
piezometers. Using SHANSEP (stress history and normalized soil engineering
properties) approach suggested by Ladd (1974) increments in cu of soil can then be
computed during construction by considering the determined normalized strength
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parameters of the soil and its over consolidation ratio (OCR). This approach is applied
on soils which prove by testing to have a normalized soil engineering behavior.
The Cu for a normally consolidated (NC) clay could be estimated from the
equation suggested by Skempton (1957):
where ’o = effective overburden stress
Jamiolkowski et al (1985) suggested the following equation for lightly over
consolidated (OC) clays:
cu / ’c = 0.23 ± 0.04 (’c =effective pre- consolidation stress)
Mesri (1989) suggested: cu / ’c = 0.22
Ladd et al (1977) suggested the following relationship between the strengths of OC
clays and NC clays:
Mayne and Mitchell (1988) suggested the following equation for estimating ’c
for a natural clay deposit
They also suggested that OCR can be estimated from the Cu (field) of the natural clay
deposit in the form:
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OCR = B Cu( field )
σ ' o
where B=22 (PI)-0.48
Tavenas and Lerouiel (1980) suggested a procedure for using the Cu (VST) in
stability analysis. This method involves the reduction of the measured undrained shear
strength values in the zone of the weathered top clay crust and the use of the measured
values beneath this zone (Fig. 4) The computed factor of safety, assuming full
mobilization of strength in the embankment (La Rochelle et al. 1974), is then divided by
the factor Ff which is a function of the liquid limit and sensitivity of soil to get the
corrected factor of safety (Figure 5). The top curve in Figure 5 is for highly sensitive
clays and the bottom one for clays of low sensitivity
It is worth noting that soil disturbance during testing in the laboratory often
leads to the underestimation of the maximum pre-consolidation stress, soil
compressibility, and soil coefficient of permeability (Rixner, 2001). However, all the
above empirical equations are approximate and the strength-stress relationships
should, for important or sensitive structures, be verified by actual testing. This is
particularly true in the case of construction on thick highly compressible and sensitive
clays with high liquidity index, whereby careful monitoring and interpretation of
instrumentation d a t a by highly qualified and experienced geotechnical engineers is
considered crucial to the safety and successful completion of construction works .
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The philosophy of staged construction and proper monitoring and
interpretation of geotechnical data could also be applied to projects that involve cuts
in natural slopes with possible daylighting of sensitive plastic clays. The following
case studies are presented to demonstrate the importance of proper selection and
interpretation of site investigations and soil testing and the scheduling of works
accordingly.
LANDSLIDES AT WADI ES-SIR SEWAGE
TREATMENT PLANT
The Wadi Es-Sir Sewage Treatment Plant (WESTP) is located about 10 kms
to the south west of Wadi Es-Sir town and about 20kms to the east of the Jordan-Dead
Sea Rift (Fig. 6). The site is characterized by its relatively steep topography with an
average inclination of about 15%. It is also characterized by its dry hot summer and
its moderately cold winter with an average annual rainfall ranging between 250mm
and350mm.
During the construction works for the lagoons the site was affected by four
landslides at different dates, namely 8 September, 1993; 28 November, 1993; IO
July
1995; and 23 February 1997 (Fig. 7).
The investigation works that were carried out in the site at different stages
indicated that the geologic cross-section generally consists of (from top to bottom):
i. Loose heterogeneous man-made fill.
ii. Colluvium consisting of sandy silty clay intermixed with variable
percentages of graved -to boulder- size fragments of limestone and
marlstone.
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iii. Bedrock consisting of poor quality and disturbed intercalations of
reworked clayey marl, marlstone, and limestone affected by tectonic
movements and old landslides. It belongs to the Naur Formation (Al-2)
of the lower Ajloun Group which is known in Jordan as the formation
most susceptible to landslides due to the presence of the wet plastic
weak clayey marl.
iv. Yellowish brown to yellowish green clayey marl sometimes intermixed
with variable percentages of gravels and cobbles of limestone and
marlstone.
Figure 8 shows features of the July 10, 1995 landslide which took place during
excavation inspite of the flattening of the cut slopes to IV:4H after the occurrence
of the November 28, 1993 landslide. The clayey marl forming the sliding surface
has a LL of 77.I and a PI of 48.1. The geomorphological features adjacent to
the slide attest to the fact that the region is plagued with multiple old landslides
forming slip surfaces where the shear strength is close to the residual one.
Figure 9 shows the effects of the February 23, 1997 landslide on its adjacent
July 10, 1995 slide area after it was further flattened to IV:5H and provided with
gabion walls and surface drainage ditches. The slide caused extreme disturbance to
the area with destruction and dislocation of the gabion walls and drainage ditches.
The post-failure investigations of the above landslides indicate that the main
causes of slides were:
(a) The presence of pre-existing slip surfaces manifested by the
geomorphological features of the region which indicate that the region had
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experienced intensive tectonic disturbance and many old landslides in its past
geologic history.
(b) The presence of highly plastic beds of clayey marl underlying the jointed and
weathered strata of limestone and marlstone topped with loose colluvium and
man-made fill. The marly beds are dipping unfavorably out of the slope at
angles between 8 and 12 degrees which are considered critical to the stability
of these slopes dominated by highly plastic marls. The strength along these
dipping beds is close to the residual one with their residual shear strength
parameters experimentally estimated at a cohesion of 3 to 8 kN/m2 and
angle of friction of 6 to 9 degrees.
(c) The high rate of water infiltration during intense rainstorms through the highly
penneable beds of loose colluviurn and jointed rocks leading to sudden rise in
pore pressures and softening of underlying clayey marl beds .
(d) The steepness of the ground and the low factors of safety adopted for the slope
cuts leading to creep and progressive failure of the marls that are susceptible to
strain-softening.
(e) The poor drainage conditions allowing the saturation of the fill, colluvium and
underlying poor bedrock materials in the absence of the vegetative cover
causing a substantial increase in the driving forces and a decrease in the
resisting forces.
DIKE 19-ARAB POTASH PROJECT
DEAD SEA-JORDAN
Dike 19 is an 8.3 km long embankment and forms with dike 20 one dike with
a total length of 11.6 km enclosing a salt pan with a storage capacity of 71.3 Mm3
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(Fig. 10). It has a crest width of 8 m and varies in height from 8 to 14 m. The
upstream and downstream embankments slope at 2.5 H:IV with berms whose width
increases with the increase in the dike's height according to certain formula specified
by the designer. The compacted reworked lisan marl constitutes the major portion of
the dike's body (Fig. 11). Construction of the dike commenced in March, 1998 and
ended in November, 1999. On September 22, 1999, Variation Order No I was issued
including the reduction of the height of the dike by 2m and increasing the thickness of
the berms by about lm to satisfy requirements dictated by the stability analysis of the
dike and the decision of impounding the salt pan in January, 2000. On March 22,
2000 a sudden partial failure of the dike occurred causing a rapid release of about 56
Mm3 in about 30 minutes. Investigations indicated that failure started near Chainage
6+000 and caused a 2.3 km wide gap in the body of the dike between Ch 4+600 and
Ch 6+900. The design criteria for dike 19 relied heavily on the experience gained
from the construction of a trial dike and dike 18 which were constructed near the site
of dike 19. However, it was soon discovered at the early stages of construction that
the foundations of dike 19 were more compressible, more sensitive, and less
permeable than those of dike 18. The foundation materials generally consist of very
soft to medium stiff thick to very thick bluisk grey thinly laminated silty clay with
stronger inerbeds of gypsum and aragonite and occasionally with organic debris.
Staged-construction design using the observational approach was adopted in the
construction of the dike to ensure the safety of the structure through the control of the
rate of construction and modification of design features of the dike. The
instrumentation system installed at 1 km intervals along the dike comprised pneumatic
piezometers to measure pore pressures in the foundation soils, standpipe piezometers
to measure long-term water levels in the built dike and its foundation after
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impoundment, survey monuments to measure displacements, and horizontal magnetic
extensometers to measure the vertical settlements of the foundation materials.
Control of construction rate
The main criterion that was used for controlling the rate of construction in dike
19 was the ratio of the excess pore water pressure, u. to the corresponding increment
in the total vertical stresse, , namely, B-bar, i.e. B = ∆u
∆ σ v. It was originally
specified that the maximum values of B are 0.7 beneath the central portion of the
dike and 0.5 elsewere. However, it was noticed that due to the very low penneability
of the foundation soils the dissipation of pore pressures was very slow. Therefore, in
order to avoid the anticipated delay in the completion of the works and the consequent
delay in impounding the pan the maximum values of the B were gradually relaxed
(in increments) to 0.95 below the central portion of the dike and to 0.7 elsewhere. The
designer considered the end of construction state as the most critical. To meet the
contemplated date of completion of construction the required factors of safety at end
of construction was also relaxed from 1.3 to 1.25. With the reduction of the height of
the dike by 2m (Variation Order No 1) it became possible to complete the
construction of the dike on the contemplated date as was originally planned.
Construction of the dike proceeded by placing the fill material composed of reworked
marl in 0.15 m thick lifts compacted to a minimum degree of compaction of 95
percent of Standard Proctor at an optimum moisture content ranging mostly between
18 and 20 percent. However, due to the high B value the placement of fill was on
many occasions suspended for periods ranging from 3 to 7 days. During construction
and impoundment many longitudinal and transverse cracks developed in the body of
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the dike. The development of these cracks was most probably due to the excessive
total and differential settlements along and across the body of the dike associated with
substantial horizontal displacements at the base of the dike.
The settlement reached about 4m beneath the central partion of the dike before
failure near Ch 6+000 where failure was most probably initiated as deduced from the
post-failure investigations .
Stability analyses
Many stability analyses were frequently carried out at different stages of
construction using data from the insitu vane shear tests (VST) carried down to a depth
of 1Om below the base of the dike particulary at the locations where cracks developed
in the body of the dike. The results of these tests were employed, without correction,
as the basis for stability analyses. The vane apparatus that was used measured
50mmX100mm and 63.5 mm X 127mm vs. the 75mm X 150mm which was used
during the pre-construction investigations.
It was reported (Gibb 1995) that the smaller vane 50mm X 100 mm gave
undrained strength (cu) values 80 percent higher than those obtained using the 75mm
X 150mm vane. The reliance of the stability analysis on the results of the VST
resulted in an overestimation of the factors of safety. Also, the VST showed
inconsistent results regarding both the increase in strength with depth or with time due
to consolidation. It seems that the intermittent presence of salt layers in between the
laminated silty clay layers resulted in the inconsistent strength measurements. The
VST results generally suffer from the following limitations which render them
unacceptable as a reliable source of data for stability analyses unless properly
corrected on the basis of correlation with other tests like cone penetration test (CPT),
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direct simple shear test (DSS), undrained triaxial compression (TC) or extension (TE)
tests:
i) The test results should be corrected for the effect of the size of the vane
apparatus.
ii) The test tends to overestimate the Cu value in anisotropic laminated plastic
clays where the Cu along the laminae is smaller than across them.
iii) The presence of gravels or salt crystals or fibrous inclusions increases the
measured Cu values.
iv) Although a correction factor has been proposed by Bjerrum (1972) to be
applied to the test results, the scatter of the data as noticed by Ladd and
Foott (1974) demonstrated the uncertainty of the results which had little
correlation with the values obtained from back stability analyses of failed
embankments placed on soft clays.
v) The test tends to overtimate Cu value due to the higher strain rate during the
test than that in the actual case.
The Cu value assigned to the compacted fill of the dike body in the undrained
stability analyses was 100 kPa instead of the measured value of 190kPa to account for
the potential cracking of the dike's body. Cracking was expected due to the high strain
incompatibility between the stiff embankment fill (peak strength at about 1.5 to 2%
strain) and the soft clay foundation (peak strength at about 5% to 12% strain).
The unit weight of the compacted embankment fill was wrongly assurmd to
be 15.7 kN/m3 instead of about 18.6 kN/m3 which resulted in an overestimation of the
factor of safety.
All the above factors resulted in an actual factor of safety at the end of
construction considerably less than the presumed one (1.25).
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Impounding the salt pan commenced on January 4,2000 i.e. a short rest period
was allowed between the end of construction and the commencement of
impoundment This short period was not adequate to cause any considerable increase
in the factor of safety. This resulted in the overstressing of some portions of the soft
foundations along the potential failure surface (Wright 1973). The early impoundment
of the salt pan had exacerbated the critical stability of the dike through the
development of significant zones of contained plastic flow leading to progressive
failure (Ladd 1991).
The rate of settlement during impoundment didn't show any noticeable
decrease as compared with that during construction and was as well combined with a
high rise in pore pressure particularly near Ch 6+000. This caused a continuous
decrease in the factor of safety during impoundment until it culminated in a
catastrophic shear failure on March 22,2000 when the F.S dropped to 1.0.
The degree of consolidation for the 10 m to 15 m zone of the foundation
material beneath the dike was less than 20% to 30% and was smaller at the deeper
zones where the soil gained little or no strength during construction. No use was made
either of the cone penetration test, as was the case in the pre-construction
investigation, or of the pore pressure readings in estimating the strength gain of the
foundation material during construction. Reliance was solely based on the uncorrected
insitu VST readings down to a depth of only !Om below the base of the dike to
estimate gain in the underained strength of the foundation materials during
construction. These tests that were carried out to a shallow depth missed the deeper
soft layers of the laminated silty clay through and along which the shear failure
surface most probably have passed. No stability analyses were carried out during the
impoundment stage inspite of the high pore pressure and settlement readings. These
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high readings of pore pressure and settlement, particularly near Ch 6+000, are due to
the fact that the foundation materials consist of more than 60m thick soft to v. soft
sensitive, highly compressible, and relatively impervious laminated silty clays. The
laminations are sometimes disturbed and convoluted, and thus impeding the drainage
and the fast dissipation of pore pressures.
Post-construction investigations
The post construction investigation didn't disclose any geological features or
imperfections that could have caused the failure of the dike. Failure couldn't also be
attributed to piping erosion in the foundation materials due to their cohesiveness and
low permability and the low hydraulic gradient, or due to piping through the body of
the dike which was well compacted and provided with affective drainage control
measures. The partial collapse of the dike was found to be due to the inadequate
bearing capacity of the foundation material which gained little strength during
construction and due to the destabilizing effect of the impounded water behind the
dike and within the upstream longitudinal cracks.
The three boreholes which were drilled in the failed section after failure have
defined the location of the failure surface as being that which separates the disturbed
zone above it from the undisturbed zone beneath it (Dar/Harza 2001). The failure
surface near Ch 6+000 is most probably a rotational one that starts at the junction
point of the upstream berm with the dike and exits at about 27m from the downstream
toe of the dike with a maximum depth of about 20m beneath the dike (Fig. 12).
Longer rest period between the end of construction and commencement of
impoundment and control of the rate of impoundment, based on pore pressure
20
readings and stability analyses, could have saved the dike as shown in the illustrative
sketch (Fig. 13).
CONCLUSIONS AND RECOMMENDATIONS
1- The factor of safety that should be adopted for cut slopes and
embankments underlain by soft clay layers should be commensurate with
the degree of uniformity of ground conditions, anticipated changes in the
environmental and stress conditions, and the severity of the adverse
economic, social, and environmental consequences of any potential failure.
A factor of safety ranging between 1.4 and 1.6, depending on the sensitivity
of soils, is generally adequate to avoid overstressing and thus progressive
failure of slopes.
2- For construction of embankments on soft clays it is recommended to adopt
staged construction based on the observational approach by monitoring the
pore pressures and the vertical and horizontal displacements during and for a
reasonable period after construction. The undrained stability analysis is
recommended for construction on soft clays with low permeability.
SHANSEP approach recommended by Ladd could be used to establish the
soil strength profile if the soil proved, by testing, to have a normalized
behavior. The CPT supported by DSS tests on undisturbed samples is far
better than the VST in defining soil stratigraphy and evaluating strength gain
during construction.
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3- Rapid relief of stresses by deep excavation in slopes dominated or underlain
by overconsolidated plastic clays could lead to shear failures under undrained
conditions. Staged excavation with proper monitoring of vertical and
horizontal displacements would allow good evaluation of the stability and
control of construction activities.
4- Proper consideration of the past geologic history supported by careful
examination of the geomorphological features of slopes would allow early
detection of the presence of pre-existing slip planes that could form potential
failure surfaces due to the low shear strength on such planes.
5- In the design of cut slopes underlain by layers of plastic clayey marls inclined
unfavorably towards the excavation utmost care should be excercised not to
daylight such. strata or even be close to them. Adequate confinement is
needed in order to avoid overstressing of such soils that are often susceptible
to strain-softening.
6- In the design of stiff embankments over soft clay foundations it is strongly
recommended to introduce correction factors to the undrained strength of
both the embankment fill and the clay foundation as suggested by Duncan
and Buchignani (1975) to account for the high stress-strain incompatibility
between the stiff embankment fill and the soft clay foundation and thus to
avoid the initiation of progressive failure.
7- Undrained direct simple shear test better represents the average undrained
strength of the clay foundations underlying stiff embankments than either the
undrained triaxial compression or extension tests. Correction to the results of
this test, however, should be introduced in case evidence exists that the layers
22
of clay foundations experienced, in their past geologic history, strong
disturbance either by liquefaction or slippage.
AKNOWLEDGMENTS
The author expresses his deep appreciation for the University of Jordan in
general and for the Deanship of Scientific Research in particular for their moral and
financial support which enabled him finish this research on time during his sabbatical
leave from the faculty of Engineering and Technology in the year 2006-2007.
23
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