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A case study of embankment retaining wall in Ontario
Conference Paper · October 2016
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Ryerson University
WSP Canada Inc.
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A case study of embankment retaining wall in Ontario
Laifa Cao, Scott Peaker & Shaheen Ahmad
WSP Canada Inc., Toronto, Ontario, Canada
ABSTRACT
This paper presents a case study of an embankment retaining wall. The retaining wall was designed to be up to 6m in
height comprising six layers of rock-filled gabion baskets with base width of up to 4m founded on a very stiff to hard silty
clay to clayey silt till. Three years after wall construction, vertical settlement of up to 138mm and lateral movement of up
to 413mm were recorded at the face of the retaining wall. The significant lateral movement induced cracking in the road
pavement structure. The wall stability and bearing capacity were reviewed. It was found that the wall was underdesigned
for overturning, sliding and global stabilities; and soil bearing capacity was overestimated. The settlement was found mainly
due to the consolidation and overstressing of the founding soils, whereas wall rotation and sliding as well as compaction
and expansion of the stone baskets contributed to the significant lateral movement. Improvements for such retaining wall
design are recommended.
RÉSUMÉ
Cet article présente une étude de cas d’un mur de soutènement de talus. Le mur de soutènement a été conçu pour
atteindre 6 m de hauteur formée de six couches de gabions remplis de roches avec largeur de base jusqu'à 4 m, fondée
sur un très raide d’argile limoneuse dur à limon argileux jusqu'à. Trois ans après la construction du mur, règlement vertical
jusqu'à 138 mm et un mouvement latéral de 413mm ont été enregistrés à la face du mur de soutènement. Le mouvement
latéral significatif induite par fissuration dans la structure de chaussée routière. La stabilité du mur et portance ont été
examinées. Il a été constaté que le mur a été underdesigned pour des stabilités renversement, coulissantes et mondiales;
et sol capacité portante a été surestimé. La colonie est trouvé principalement en raison de la consolidation et sursollicitation
des sols fondateurs, tandis que le mur rotation et glissement ainsi que compactage et l’expansion des ensembles de Pierre
a contribué à l’important mouvement latéral. Améliorations pour une telle structure de mur de soutènement sont
recommandées.
1
INTRODUCTION
Roadway embankments approach a bridge are often
constrained by retaining walls due to limited space in the
right of way. Flexible retaining walls such as reinforced
earth wall and gabion wall are generally used as they are
considered to be tolerant of the ground settlement under
the weight of embankment fill. However, if the ground
conditions are not properly assessed, ground settlement
and lateral movement of such retaining walls could
significantly affect the serviceability of the roadway.
This paper presents a case study of an embankment
retaining wall. The retaining wall was designed to be up to
6m in height comprising six layers of rock-filled gabion
baskets with base width of up to 4m founded on a very stiff
to hard silty clay to clayey silt till over firm to very stiff silty
clay till. Three years after wall construction, vertical
settlement of up to 138mm and lateral movement of up to
413mm were recorded at the face of the retaining wall. The
significant lateral movement induced cracking in the road
pavement structure. The wall stabilities, soil bearing
capacity, ground settlement under the embankment fill, and
wall lateral movement were reviewed. It was found that the
wall was underdesigned for overturning, sliding and global
stabilities; and soil bearing capacity was overestimated.
The settlement was attributed mainly to the consolidation
and overstressing of the founding soil under the weight of
embankment fill, whereas the wall rotation and sliding as
well as compaction and expansion of the stone baskets
contributed to the significant lateral movement. Possible
remedial options for the retaining wall are discussed.
Improvements for such retaining wall design are
recommended.
2
SITE CONDITIONS
CONSTRUCTION
AND
RETAINING
WALL
The site of the approach embankments of a road bridge
(road over rail) is located in Ontario, Canada. Due to the
road bridge replacement and road widening, the approach
embankments needed to be reconstructed. Since the east
side of the embankment was constrained by private
property, a retaining wall was required to be constructed to
retain the embankment fill. A geotechnical investigation (by
others) including borehole drilling, standard penetration
tests (SPT) and laboratory testing of soil index parameters
was carried out to support the design of the bridge and
retaining wall.
In general, the subsurface conditions encountered at
the site consist of 3.0 to 5.6m thick, loose to compact sandy
silt fill extending to El. 96.0 to 94.4m, underlain by about
5.5 to 7.6m thick, very stiff to hard silty clay to clayey silt till
extending to El. 90.6 to 86.8m, which in turn is underlain by
firm to very stiff silty clay till to El. 79.7 to 79.2m. The firm
to very stiff silty clay is underlain by hard clayey silt till or
very dense silt till to sandy silt till. The water contents were
9% to 17% for the sandy silt fill, 12% to 16% for the very
founded on the very stiff to hard silty clay to clayey silt till at
El. 95.5 to 94.1m. Soil parameters including friction angle,
unit weight, and coefficient of lateral earth pressure were
also recommended in the geotechnical investigation report.
Based on the soil bearing capacity and parameters
recommended in the geotechnical investigation report, a
gabion retaining wall was designed to be up to 6m in height
comprising six layers of gabion baskets with base width of
up to 4m founded at El. 94.3m on the very stiff to hard silty
clay to clayey silt till. A typical cross section of the gabion
wall is shown in Figure 2. The wall face and base had an
inclination angle of 6o (i.e. 1H:9.5V) in order to increase
the base resistance and resisting moment. The wall was
designed with 1m height of fill in front of the wall toe. A
drainage pipe was also proposed behind the wall. It is
noted that the wall was designed to retain a 1V:2H slope
backfill above the wall. The new fill height at the road centre
was about 2.8m (above the existing level prior to bridge
replacement) and the maximum new fill height at the edge
of the road was about 7.8m.
The gabion retaining wall was constructed in the
summer of 2007.
Figure 1. Soil profile and SPT N-values at the site
stiff to hard silty clay to clayey silt till, and 17% to 18% for
the firm to very stiff silty clay till. It is noted that the pocket
penetrometer tests on the firm to very stiff silty clay till only
showed the undrained shear strengths of about 25 kPa,
indicating a relatively weak condition of the firm to very stiff
silty clay till. Field vane shear strength testing was not
undertaken by the geotechnical consultant. The
groundwater level was at El. 87.8 to 86.9m during the
investigation. Figure 1 shows soil profiles and SPT Nvalues.
The geotechnical report recommended an allowable
bearing capacity of 200 kPa and factored ULS (ultimate
limit states) bearing capacity of 300 kPa for a retaining wall
Figure 2. A typical section of designed gabion wall
3
FIELD OBSERVATIONS
Shortly after the gabion wall was constructed and the road
was opened, longitudinal pavement cracks were observed.
In response, a movement monitoring program of the gabion
retaining wall was initiated. The monitoring points were
steel bars driven into rock-filled gabion baskets along the
top of the retaining wall. The measurements started in
November 2007, a few months after the completion of the
retaining wall construction. The development of vertical
settlement of up to 138 mm and horizontal deflection of up
to 413mm had been recorded by July 2010.
Photo 3. Tensile cracks above wall and tilted guide rail
Photo 1. Wall tilt near abutment (the top of fence post was
pushed laterally)
A field visit of the site was carried out in June of 2010
to examine the condition of retaining wall. There were
visual signs that both vertical and lateral movements were
being experienced by the retaining wall and the approach
embankment. The maximum height of the wall was
measured as 4.8 m instead of 5.0 m above the natural
ground level. The tilt of wall was observed as shown in
attached Photo 1. Uneven settlement was observed in the
CSP culvert underlying the north approach fill as shown in
attached Photo 2. Longitudinal cracks had developed in
the pavement and granular fill along the side of the gabion
wall as shown in attached Photo 3.
4
REVIEW OF RETAINING WALL DESIGN
The stability of the east retaining wall including bearing,
sliding, overturning and global stabilities, vertical
settlement and lateral movement were reviewed using the
available data. A parametric approach to the analysis,
using a plausible range of soil shear strengths and unit
weights was applied. The unit weight of the foundation soil
and backfill was taken as 20 or 22 kN/m3. The friction angle
was taken as 30o or 32o for the foundation soil, 40o for the
friction between gabions, 15o or 16o for the friction between
the wall and the backfill, and 22o or 29o for the friction
between the wall and foundation soil. The lower bound
values were generally similar to those proposed in the
geotechnical investigation report for this project.
Analyses of the following four cases were carried out:
(1) 6m high gabion wall as designed with lower bound
of soil parameters;
(2) 6m high gabion wall as designed with higher
bound of soil parameters;
(3) 5.8m high gabion wall measured on July 23, 2010
with lower bound of soil parameters;
(4) 5.8m high gabion wall measured on July 23, 2010
with upper bound of soil parameters.
For each case, two groundwater level (GWL) scenarios
were modelled, one with GWL assumed at El. 87.8m
(lower GWL, the groundwater level measured in the
monitoring well during the geotechnical investigation) and
another with GWL at the base of wall (higher GWL).
4.1
Photo 2. Separated and sagged culvert below approach fill
Bearing Capacity
It is well known that the soil bearing capacity of an earth
retaining structure is not only controlled by the founding soil
properties but is also affected by the lateral load acting on
a retaining structure. The soil bearing capacity for a
foundation with a lateral load will be significantly smaller
than that without a lateral load. However, this is often poorly
communicated in geotechnical investigation reports.
The soil ultimate bearing capacity was estimated based
on the formula proposed by Vesic (1973, 1975), in which
the influence of lateral load on the bearing capacity is
considered. Table 1 summarizes the soil ultimate bearing
for the four cases under higher GWL and lower GWL
conditions. Taking the vertical bearing resistance factor of
0.5 as recommend in Canadian Foundation Engineering
Manual (2006), the factored ULS bearing capacity was
calculated and is shown in Table 1. It was found that the
factored ULS bearing capacity was lower than 300 kPa
Table 1. Soil bearing capacity
Case
Ultimate
Lower Higher
GWL
GWL
1 351
256
2 534
395
3 338
248
4 512
381
Unit: kPa.
Factored ULS
Allowable
Lower Higher Lower Higher
GWL
GWL GWL
GWL
176
128
117
85
267
198
178
132
169
124
175
83
256
191
173
127
as recommended in the geotechnical investigation report
for all cases. At the higher GWL, the estimated ULS
bearing capacity with the consideration of the lateral earth
pressure was only 43% to 66% of that recommended in the
geotechnical investigation report, in which the influence of
lateral load to the bearing capacity was probably not
considered. At the serviceability limit states (SLS), the
bearing capacity is typically defined by settlement criteria
under the load and resistance factor design (LRFD).
However in common practice, the bearing capacity at the
SLS is often taken as the same as the allowable bearing
capacity in the traditional working stress design (WSD).
Taking a global factor of safety of 3, the allowable bearing
capacity was calculated and is shown in Table 1. Again the
estimated allowable bearing capacity with the
consideration of the lateral earth pressure was only 43% to
66% of the recommended allowable bearing capacity in the
geotechnical investigation report.
It is noted the vertical stress at the retaining wall base
level ranged from 176 to 194 kPa at the location of the road
centre and ranged from 116 to 132 kPa at the location of
the retaining wall. If the influence of the lateral earth
pressure was not considered, the allowable bearing
capacity of 200 kPa recommended in the geotechnical
report met the vertical load requirement. However, under
the action of the lateral earth pressure, the average working
load on the effective base of the retaining wall ranged from
206 to 212 kPa, which was greater than the allowable
bearing capacity recommended in the geotechnical report.
It seems that the working load was underestimated and
soil bearing capacity was overestimated without the
consideration of the influence of lateral earth pressure on
the work load and the soil bearing capacity during the
retaining wall design for this project.
4.2
Stabilities
A retaining wall should be designed for overturning, sliding
and overall (global) stabilities. Although the LRFD has been
widely used for structure design, the WSD is still used for
retaining wall design in the checking of the overturning,
sliding and overall (global) stabilities. Table 2 summarizes
the factor of safety (FOS) against these stability failures
under the working stress condition for the four cases.
Figure 3 shows a typical result of overall stability analysis
using commercial limit-equilibrium software.
The analyses indicate that the overturning and overall
stabilities of the gabion wall are marginally acceptable, but
less than the generally accepted requirements of a FOS of
2 for overturning and FOS of 1.5 for overall stability. The
stability of the gabion wall in 2010 was worse than the
Figure 3. A typical result of overall stability analysis
Table 2. Factor of safety against stability failures
Case
1
2
3
4
Min. FOS Min. FOS
against
against
overturning
sliding
1.81
1.85
1.69
1.72
1.36
1.96
1.15
1.51
Min. FOS against
overall
Lower
GWL
1.39
1.48
1.38
1.47
Higher
GWL
1.35
1.44
1.34
1.38
designed wall as the FOSs for the wall in 2010 were less
than the designed wall under the same condition. This is
primarily due to the loss of batter, i.e. the wall became
vertical in 2010.
The factor of safety against basal sliding for the wall in
2010 was in the critical condition with a FOS of 1.15 when
the groundwater level was modelled to lie at the base of the
gabion wall if the soil parameters proposed in the
geotechnical report were used.
It is noted that the outlet of the drainage pipes for the
wall in 2010 was located close to the existing ground
surface. When the groundwater level at the wall toe
reaches the ground surface, local sliding failure would
occur. In the case of the groundwater table rising up at the
back of wall due to clogging of filter fabric at the back of
wall, local sliding failure would also occur. This could be
one reason that a lateral movement of 413mm was
recorded. Bulging of gabion baskets may also be a
contributing factor.
4.3
Settlement and Lateral Movement
The monitoring results for vertical settlement and lateral
movement for monitoring points 37 to 45 are shown in
Figures 4 and 5, respectively. Monitoring points 37 to 42
were installed at the wall top with the wall height of less
than 5m. Monitoring Point 43 was installed at the wall top
with the wall height of 5.0m, Point 44 was positioned at the
Figure 4. Vertical movement measured at wall top
wall top with the wall height of 5.8m, and Point 45 at the
corner of the wall near the bridge abutment wall. The
measurements began in November 2007, a few months
after retaining wall construction. The development of
vertical settlement of up to 138mm and horizontal
deflection of up to 413mm was recorded in July 2010,
relative to the November 2007 baseline. The monitoring
results show that the settlement of the wall due to soil
consolidation could be considered as being essentially
complete by August 2008, one year after wall construction
(See Figure 4). The maximum consolidation settlement
was 96mm, which was close to the settlement of the north
approach embankment of about 100 mm. From August
2008 to July 2010, the increment of vertical movement at
points 37 through 42 (wall height of less than 4.5m) was
less than 10mm and remained comparatively stable;
whereas the increment of vertical settlements at points 43
through 45 (wall height of 5.0 to 5.8m) ranged from 22 to
50mm and did not slow down. The significant vertical
settlement of the highest wall sections may be partially due
to overstressing of the founding soils which was induced by
the loss of wall batter.
From November 2007 to July 2010, a maximum lateral
movement of 413 mm was recorded at monitoring point 44
installed in the highest retaining wall section (see Figure 5).
Similar to the vertical movements, the lateral movements
at points 37 to 42 have slowed down since January 2009,
whereas the lateral movements at points 43 and 44 in the
highest wall section did not decline. A relatively small
lateral movement of 50mm at point 45 installed at the
corner of the wall is reasonable as the gabion wall was
butted against 3m high RSS (retained soil systems) wall at
the corner. The significant lateral movement could not be
purely due to the vertical settlement and the wall rotation.
Sliding of the gabions may contribute part of the lateral
movement. Some component of the apparent lateral
movement may also be a result of expansion (bulging) of
the gabion baskets. Workmanship in construction of the
gabion baskets might not have been of the highest quality.
The measurements in 2010 showed that individual basket
heights varied from 800mm to the design filled height of
1000mm, but most baskets were 850 mm to 900 mm in
Figure 5. Horizontal movement measured at wall top
Figure 6. Comparison between predicted and measured
settlements
height. Many baskets appeared to be deformed under the
self-weight load, suggesting that they were not tightly
packed with stones.
Since no consolidation testing was carried out for this
project, an accurate estimation of the consolidation
settlement is impossible. An approximate estimation of the
vertical settlement can be conducted using empirical data
for the glacial tills in Ontario as recommended by Cao, et
al. (2015). Assuming that the compression index (Cc) is
0.037 for the very stiff to hard silty clay till and 0.1 for the
firm to very stiff silty clay till, the recompression index (Cr)
is 0.081 for the very stiff to hard silty clay till and 0.016 for
the firm to very stiff silty clay till, the initial void ratio (eo) is
0.36 for the very stiff to hard silty clay till and 0.46 for the
firm to very stiff silty clay till, and the coefficient of
consolidation (cv) is 20 m2/year, the total ground settlement
was 220mm including 90mm in the first three months after
the construction of 2.8 to 7.8m height of the embankment.
Even through the selected soil parameters were based on
the empirical data, the predicted settlements are in good
agreement with the measured settlements as shown in
Figure 6. The back-analysis indicates that 95% of the
consolidation settlement would be reached by July 2010.
5
DISCUSSIONS ON REMEDIAL WORKS
The monitoring data show that the vertical settlement and
lateral movement of the highest wall sections have not
attenuated. Review of the wall stability indicates that the
local bearing shear failure and sliding failure may occur
when the groundwater level reaches the ground surface at
the toe of the wall. These suggest that remedial work of
the highest wall section may be required.
Overstressing of the foundation soils beneath the
highest sections of the gabion wall appears to be the main
cause of settlement and tilting. The monitoring did not show
an abating trend in settlement or deflection in the highest
sections of the wall.
If the horizontal component of the load on the retaining
wall (i.e. the lateral earth pressure) can be significantly
reduced, then the inclination of the resultant force acting on
the base of the wall can be rendered essentially vertical,
without significant eccentricity.
Three methods of picking up or reducing the lateral
earth pressure include:
(i) Soil nailing; or
(ii) Installation of closely spaced layers of horizontal
geogrid; or
(iii) Replacement of the backfill in the active wedge
with light weight fill (expanded slag, expanded
polystyrene or equivalent).
The gabion wire baskets and gabion stones within the
baskets present some practical difficulties as far as the
advancement of horizontal soil nails through the wall is
concerned. Although it might be technically feasible to
vibro-drive steel bar through the gabions, without the
benefit of field-trials to assess the feasibility of advancing
soil nails through the cages and tension tests to assess
their capacity, this method is not considered contractually
viable. For this reason, the second alternative – installation
of geogrid in the backfill soils behind the gabion wall in the
north approach fill is more reliable.
Option (ii) will require reconstruction of most, if not all,
of the north approach fill and roadway infrastructure.
The benefit of the geogrid fill reinforcement option is
that the existing gabion wall is essentially converted from a
gravity retaining structure to a facing wall only. A secondary
benefit is that the upper sloped portion of the approach fill
(which lies above the top of the gabions and below the
asphalt road surface), can be reinforced. This area
exhibited creep and surface erosion, resulting in tension
cracking of the pavement and tilting of the guide rail.
Reinforcement of the north approach fill would require
road (bridge) closure during the full course of the work. The
existing approach fill material can be salvaged and reused
to create the reinforced fill. The face of the fill abutting the
existing gabions wall will require a filter fabric lining,
followed by a wrap of the geogrid. In this way, the
reinforced fill will remain independent of the gabion wall.
The potential conflict with existing reinforcing strips to
the bridge abutment might also apply to Option (iii). This
option involves the sub-excavation of existing fill behind the
retaining wall within the active wedge zone and replacing
this fill with compacted light weight fill, such as expanded
slag. Alternatively, the use of ultra-light weight fill
(expanded polystyrene) could also be considered.
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The last option is to do nothing as the majority of the
consolidation settlement of the underlain silty clay till has
been reached, the soil strength has been improved under
the embankment fill, and the retaining wall is marginally
stable. Continuous monitoring of the wall movement is
required if this option is chosen. More frequent
maintenance of the road pavement will be required as the
tensile cracks in pavement structures could continuously
develop due to the lateral movement of the retaining wall.
6
CONCLUSIONS AND RECOMMENDATIONS
From the review of an embankment retaining wall, the
following conclusions can be provided:
•
The working pressure on a retaining wall base
was higher than the vertical load, due to the
contribution of the lateral earth pressure. For a
preliminary analysis of a gravity retaining wall, the
working pressure can be assumed as 150% to
200% of the vertical load.
•
The bearing capacity for the retaining wall design
was significantly underestimated, probably
without the consideration of the lateral earth
pressure. For a preliminary analysis of a gravity
retaining wall, the bearing capacity can be taken
as 50% of that without consideration of a lateral
load.
•
The significant vertical settlement is attributed to
not only the soil consolidation, but also the
overstressing of the founding soils which was
induced by the loss of wall batter.
•
The significant lateral movement is contributed to
not only the wall rotation and sliding, but also the
compaction and expansion of the stone baskets
under the self-weight load.
For the preparation of a geotechnical investigation
report for a retaining wall, a recommendation for a global
stability analysis is required, and a settlement analysis
must be carried out if the founding soils are underlain by
relatively weak soils. Since details regarding retaining wall
design are often not know at the time of preparation of a
geotechnical investigation report, it is critical that the
geotechnical engineer be engaged at the design stage to
reassess the recommended soil parameters and to assess
the wall stability.
REFERENCES
Canadian Geotechnical Society, 2006. Canadian
Foundation Engineering Manual, 4th ed., BiTech
Publisher Ltd, Richmond, BC, Canada.
Cao, L.F., Peaker, S. and Ahmad, S., 2015. Engineering
characteristic of glacial tills in GTA, 68th Annual
Canadian Geotechnical Conference, Quebec, Canada.
Vesic, A.S. 1973. Analysis of ultimate loads of shallow
foundations, JSMFD, ASCE, 99: 45-73.
Vesic, A.S. 1975. Foundation Engineering Handbook, 1st
ed., Winterkorn and Fang, Van Nostrand Reinhold.