analysis, design, and construction stages of milad geosynthetic

Transcription

ANALYSIS, DESIGN, AND CONSTRUCTION STAGES OF MILAD
GEOSYNTHETIC REINFORCED SOIL BRIDGE ABUTMENT
IN TEHRAN-IRAN1
S. Alireza (Sam) Mirlatifi
Senior Geotechnical Engineer in AECOM Pty Ltd., Sydney, Australia - former Technical Manager in BPI Co.
ABSTRACT
Reinforced soil walls are one the most cost effective options for retaining structures, and are being increasingly used in
recent years around the world. They have also proved that they have performed to an acceptable level under earthquake
loading conditions. This paper presents the analysis, design and construction stages of the first major geosynthetic
reinforced soil bridge abutment built in Iran. The bridge abutment was designed, analysed and constructed by BPI
Company in Tehran, Iran in 2009. This abutment is analysed using both the Limit Equilibrium Method (LEM) for
stability analysis and Finite Element Method (FEM) for deformation analysis under static and seismic loads. The design
has been carried out according to the Federal Highway Administration Manual (FHWA-NHI-00-43, 2001) and National
Cooperative Highway Research Program Report 556-2006 (NCHRP) as a guideline. Deformations of the abutment
were monitored with an accuracy of ±1mm before and after the construction of the concrete sill and are compared with
the outputs from the numerical methods. This project was undertaken on behalf of the Tehran Municipality with the aim
of bringing this new method of abutment construction to the country. This project is likely to be the first of many to
adopt this cost effective solution.
1
INTRODUCTION
Instead of a conventional bridge deck supported on pile-cap or concrete wall abutments, Geosynthetic Reinforced Soil
walls (GRS walls) use alternating layers of compacted fill material and inclusions of geosynthetic reinforcement such as
geogrids or geotextiles to provide support for the superstructure. GRS abutments are similar in principle to GRS walls,
except that GRS abutments are typically subjected to a much higher area load resulting from a spread footing
(commonly referred to as a “sill”), and the loads are close to the wall face. The construction of the reinforced soil walls
with metal strips (RE walls) and GRS walls in Iran dates back to the seventies and early nineties, respectively. Although
many steel strips reinforced bridge abutments (RE system) have been constructed in Iran during the last decades, this
paper presents the first major GRS bridge abutment in Iran, which was completed in 2009.
Generally, GRS bridge abutments are easier and faster to construct and more cost effective (potentially 25–60 percent
less than traditional methods ([FHWA- HRT-11-026, 2011]). The GRS abutments can also omit the use of pile caps and
alleviate the chronic problem of “bumps” between bridge abutments and the approach slab. In addition they are one of
the best solutions in sites with difficult access because they need small tools and light weight equipment. In addition, it
is widely proved that they have acceptable seismic behaviour and high energy absorbing potential against impacts. They
also provide a safer work environment for personnel in work zones. In addition, the technology is environmentally
sensitive and results in minimal environmental impacts. The technology produces a reduced construction and carbon
footprint, eliminates the need for installation of deep foundations or concrete walls, and can be adapted to fit the sitespecific environmental needs [FHWA- HRT-11-026, 2011]. Last but not least, they can have different facings giving
more aesthetic results.
2
GRS CODES, MANUALS, GUIDELINES AND DESIGN METHODS
The current GRS bridge has been analysed and designed according to Federal Highway Administration Manual
(FHWA-NHI-00-43, 2001), which is based on Allowable Stress Design (ASD) method, also known as Service Load
Design (SLD), and National Cooperative Highway Research Program (NCHRP REPORT 556, 2006) as a guideline.
The recent new version of the FHWA manual (FHWA-NHI-10-024 &025, 2010) and also FHWA-HRT-11-026, 2011
for GRS Integrated Bridge Systems (known as GRS-IBS) have been revised to Load and Resistant Factor Design
(LRFD). The LRFD method is the latest advancement in transportation structures design practice in the USA. The
LRFD method in various forms is now being applied throughout the world. For example, EuroCode uses the Limit State
Design (LSD) method, which is very similar to the LRFD method. Moreover RTA specification for Design of
1
This paper was presented at the Sydney Chapter YGP night 2011
Australian Geomechanics Vol 47 No 3 September 2012
125
ANALYSIS, DESIGN, AND CONSTRUCTION STAGES OF MILAD GEOSYNTHETIC REINFORCED SOIL BRIDGE
ABUTMENT IN TEHRAN-IRAN
ALIREZA (SAM) MIRLATIFI
Reinforced Soil Walls (R57) in Australia and Code of Practice for strengthened/reinforced soils (BS8006) in UK, are
based on LSD method. Although there are several other GRS construction guidelines provided by different authorities
such as the American Association of State Highway and Transportation Officials, AASHTO (1998), the National
Concrete Masonry Association, NCMA (1997), the Federal Highway Administration, FHWA (Elias and Christopher,
1997), the Colorado Transportation Institute, CTI (Wu, 1994), the Swiss Association of Geotextile Professionals, SAGP
(1981), and the Japan Railways, JR (1998), still the lack of a rational and reliable design methods for GRS abutments
and the lack of well-developed guidelines and specifications for constructing the structures is clearly being observed.
3
REVIEWED CASE STUDIES
Depending on the facing rigidity, GRS bridge-supporting structures can be grouped into two types: “rigid” facing and
“flexible” facing structures. A “rigid” facing is typically a continuous reinforced concrete panel, either precast or cast in
place. A “flexible” facing, on the other hand, typically takes the form of wrapped-around geosynthetic sheets, concrete
modular blocks, timbers, natural rocks, or gabions. GRS bridge-supporting structures with a flexible facing have been
the subject of several studies (e.g., Gotteland et al.,1997; Adams, 1997; Ketchart and Wu, 1997; Miyata and Kawasaki,
1994; Werner and Resl, 1986; and Benigni et al.,1996), and recently have seen actual applications in the United State
and other countries, including the Vienna railroad embankment in Austria (Mannsbart and Kropik, 1996), the New
South Wales GRS bridge abutments in Australia (Won et al., 1996), the Black Hawk bridge abutments in Colorado,
(Wu et al., 2001), the Founders/Meadows bridge abutments in Colorado, (Abu-Hejleh et al., 2000) and Ilsenburg bridge
abutment in Germany (Andreas Herold, 2000). These structures have shown great promise in terms of ductility,
flexibility, constructability and costs. The Milad bridge abutment is also a GRS bridge abutment with a flexible facing.
4
MILAD GRS BRIDGE ABUTMENT IN TEHRAN
Milad Tower is located in Tehran, Iran and is the sixth tallest tower in the world at the moment and stands 435 m high
from base to tip of the antenna. Milad GRS bridge abutment provides an access to Milad Tower. This bridge abutment
carries the load of 20 m single span of a 114 m long cable-stayed bridge on west side of Milad Tower. The length of the
sill is 23.7 m and the bridge consists of 4 lanes and the maximum height of the GRS abutment to below the sill (lower
wall) is 3.5 m and the total height of the abutment to top of the pavement is 7.5 m. The geogrid length in lower wall is
9.5 m for GRS abutment section. Also, southern wing wall has 8 m height with geogrid length of 7 m in perpendicular
direction to the GRS abutment section. The natural soil description of the location consists of very dense (Nspt>50)
cemented mixture of gravel and sand with clay (GC/SC). The location of the bridge and abutment is shown in Figure 1
and 2. The reinforced geosynthetic material is Armatex M 80/30, 55/30 (Kordarna) with flexible facing which is
wrapped geogrid. For aesthetic purposes, a layer of gabion has been added to the geogrid wrapped around the face. The
Milad GRS bridge abutment characteristics are summarised in Table 1.
Table 1: Milad GRS bridge abutment characteristics in Tehran/Iran
Case
Height
Back fill
Reinf. type
Milad
GRS
Bridge
abutment
in Tehran
3.5 m
(lower
wall),
7.5 m
total
Sand, gravel
with clay
(GC/SC)
c = 10kPa,
f=35
g = 20 kN/m3
(>98% of T99)-
Kordarna,
Armatex
M(80/30) for
lower wall
Tult = 80 kN/m
@e = 6%
and M(55/30) for
upper wall
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Reinf.
Spacing/
length
0.4 m/9.5
m lower
wall,
4 m upper
wall
Facing type/
connection
Geogrid/
Wrapped
with Gabion
facing, with
angle of 68˚
(2H:5V)
Maximum
Lateral
Movement
of Wall Face
<1 mm, both
vertical and
lateral after
Sill placement
Sill
1.2 × 3.3 m
with 1.35 m
clearance
from the
face
Site Location
Site Location
Milad Tower
The GRS abutment
Figure 1: The location of the Milad GRS bridge
abutment in Tehran (Courtesy of Google Map)
Figure 2: Completion of the bridge deck in 2011 (Courtesy
of Google Map)
SIL
L
Figure 3: The view of the Milad GRS bridge abutment
with gabion facing
5
Figure 4: A cross section from Milad GRS bridge
abutment (lower wall=3.5 m, upper wall=4.0 m, total
height=7.5 m)
CONSTRUCTION MATERIALS
The main elements of GRS bridge abutments include geosynthetic inclusions for reinforcement, suitable soil and
drainage material. Armatex® M is used as a geosynthetic soil reinforcement material in this project. Armatex® M is a
woven geogrid, made of high tenacity Poly Vinyl Alcohol (PVA) yarns with PVC coating. Some of the advantages of
this type of geogrid are high tensile strength at low elongation (at 2, 3 and 6% strain), high resistance to chemicals in
soils, acid and alkali earth (PH 0-14), high level of microbiological resistance, high resistance to damage during
installation, optimal interlocking with coarse-grained soils, high pull-out resistance and low values of creep which
ensure long-term stability. The material is usually delivered as 5m wide roles. The ultimate tensile strength (Tult) of
Armatex M (80/30) which has been used in lower wall is 80 kN/m in 6% strain and Armatex M (55/30) in upper wall is
55 kN/m in 6% strain. According to the reinforcement manufacturer (Kordarna) datasheet, the total reduction factor to
evaluate the design strength (Ta) of this product with consideration of creep (A1), installation damage (A2), connections
(A3), acid and alkaline effects (A4) is 1.6 for design life of 120 years (Ta=Tult/A1×A2×A3×A4 × 1/γ).
According to the laboratory testing results, the in situ soil of the area has been suitable material for soil reinforcement
purposes. The soil consists of clayey gravel with sand or clayey sand with gravel (GC/SC) with Plasticity Index (PI) of
lower than 15% and fine content of less than 20%. In addition, a non-woven 300 gr/m2 geotextile has been used as a
barrier in facing. Hot-dip galvanized steel mesh has been used for gabion in facing. 300 mm thick granular material
with size of 19-25 mm with slotted PVC pipe has been used as drainage.
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ANALYSIS AND DESIGN CRITERA
Milad GRS Bridge abutment has been analysed and designed according to the Federal Highway Administration Manual
(FHWA-NHI-00-43, 2001) which is based on Allowable Stress Design (ASD) method and National Cooperative
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Highway Research Program (NCHRP REPORT 556, 2006) as a guideline. Table 2 summarises the minimum required
factors of safety for GRS abutments in ASD method based on the FHWA (2001).
Table 2: Milad GRS bridge abutment design criteria based on FHWA2001 in static (1) condition
External stability
Overall
slope
stability(2)
External/
Internal/
Compound
1.5
1.
2.
3.
Internal stability
Bearing capacity
Deep
seated
Lateral
squeezesoft soils
Connection
Overturning
Sliding
Pull
out
Rupture
Eccentricity
Wrapped
Return
length(3) >1m
The above factor of safety should be reduced to %75 in seismic condition. The PGA of the area is 0.35 g
for 475 years earthquake.
The factor of safety against overall stability for bridge abutments is 1.6 in RTA standard (R57).
The return lengths of geogrids are 1.6 m and the only upper geogrid layer beneath the sill is 3.5 m in this
project with wrapped connection.
2.5
(NA)
e>L/6
2
1.5
1.5
1.5
Based on field studies of actual structures, AASHTO (1996) suggests, that tolerable angular distortions (i.e., limiting
differential settlements) between abutments or between piers and abutments should be limited to the following angular
distortions:
•
0.005 for simple spans and 0.004 for continuous spans.
This criterion, suggests that for a 20 m span for instance, differential settlements of 80 mm for a continuous span or 100
mm for a simple span, would be acceptable, with no ensuing overstress and damage to superstructure elements. On an
individual project basis, differential settlements of smaller amounts may be required from a functional or performance
criterion.
During last decades, general agreement has been reached that a complete design approach should consist of the
Working Stress analyses, Limit Equilibrium analyses and Deformation Evaluations. All of the external and internal
factors of safety in static and seismic condition have been satisfied in Working Stress analyses and checked with Limit
Equilibrium method by Slide (6.0). The LEM results show that the factor of safety for overall stability in critical section
has been assessed about 2 in the static condition and 1.65 in the seismic condition. The compound pseudo static overall
stability analysis in the seismic condition is shown in Figure 5 as an example.
Figure 5: Compound pseudo static overall stability analysis in seismic condition, Fs=1.65>1.1
The finite element method by Plaxis 2D V9.0 also has been undertaken to evaluate deformations, geogrids load
distribution and the performance of the abutment and the relative shear stresses and plastic points are shown in Figures
6a and 6b as examples.
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Figure 6a: The relative shear stresses after
construction of the bridge deck and traffic loading,
7
Figure 6b: Plastic points after the construction of the
bridge deck and traffic loading
CONSTRUCTION STAGES AND MONITORING
The construction period of lower GRS wall was about two months. However, since the construction process applied in a
GRS abutment is inherently simple, the construction time could be significantly reduced. First, the foundation
preparation carried out by proof rolling and the gaps filled with lean concrete. Although the facing has been designed
flexible, a shallow small strip foundation has been constructed beneath the facing to alleviate any possible differential
settlement and to increase the construction accuracy. A large size gravel material has been used as a first layer to
facilitate drainage. Then the second layer of geogrid material has been spread on the first layer and fixed with steel Ushape hooks. As the facing system is wrapped, steel cage with L-shape bars have been used in the facing to reduce
facing deformation during the construction. Then the first 150 mm layer of suitable soil compacted with heavy vibratory
roller. The soil has been compacted in 150 mm layers thickness with compaction of more than 98% of AASHTO T-99,
and ±2% of optimum moisture, wopt. Compaction within 1 m of the back face of the wall including drainage material
and backfill soil has been undertaken by at least three passes of a lightweight vibratory compactor. After the second 150
mm layer, the geogrid has been wrapped around and fixed with the steel hooks. Then the final layer of soil is compacted
to finish the first 0.4 m reinforced mass (three 150 mm uncompacted soil is roughly about 400 mm compacted soil).
This operation has been repeated to reach to the level of the sill. The geogrid sheets have had a minimum 300 mm
overlap with each other in each layer. After finishing the construction of GRS bridge abutment, the gabion facing has
been employed for aesthetic purposes with hot-dip galvanised steel mesh 50×50×5 mm.
Deformations of the abutment from 28 points on facing were monitored in three dimensions with an accuracy of ±1 mm
with the Total Station during and after the construction. The summary of the monitoring results is shown in Table 3
below. The results show that the performance of the GRS bridge abutment is within the design criteria and the
reinforced abutment mass has a high stiffness and a high capacity for bearing heavy loads as anticipated. However,
more accurate numerical calibrated models are going to be built to study the long term performance of the GRS
abutment in more detail.
Table 3: The summary of the monitoring results of Milad GRS bridge abutment in compare with the predicted values
after construction of sill foundation and the upper wall [1]
Case
δH -maximum lateral disp. on the
δV -maximum vertical disp. of the
facing
Sill
Monitoring Results
<1 mm
<2-3 mm
Predicted by the FEM analysis
3 mm
9.6 mm
Design Criteria
80 mm
80 mm
[1] The abutment is not under service loads yet and further monitoring has been scheduled after construction of
the bridge and during the traffic loading.
129
Monitoring Points
Monitoring Points
Figure 7: The monitored points on GRS bridge abutment facing
(b)
(a)
Figure 8: (a) The density test during construction – (b) geogrid layers with minimum 300 mm overlap
Heavy weight roller
Light weight roller
near facing
(a)
(b)
(c)
Figure 9: (a) The compaction method by heavy and light weight roller- (b) The detail of the wrapped around connection
to the gabion facing- (c) The view of the final gabion facing
8
CONCLUSIONS AND RECOMMENDATIONS
The Geosynthetic Reinforced Soil (GRS) is one of the most appropriate solutions for bridge abutments in the view of
the abutment performance, construction costs, time and safety in comparison to other conventional methods. The most
effective factors in performance of the GRS walls are the quality of the compacted soil with the correct selection of the
reinforcement materials according to the author’s experience. To reduce the cost and elapsed time of the construction in
future projects it is recommended to omit the concrete sill, provided that suitable analysis and design concept with
appropriate details is applied. This system is called Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS)
and has been presented in FHWA-HRT-11-026.
9
ACKNOWLEDGEMENT
This project is the result of many engineers’ and technicians’ efforts. In addition, I acknowledge the BPI Co., Ardam
Consulting Engineers Co., Band Construction Co. and Tehran Municipality, as the project main client, for their support
and understanding to accept this new technology in the bridge and road industry. Special thanks to Mr. Salehabadi the
130
CEO of the BPI Company and Mr. Shahabi, the construction manager. I would also like to commend Mr Clinton
Every’s supports and valuable contributions in preparation of this paper.
10
REFERENCES
Abu-Hejleh, N., Wang, T., and Zornberg, J. G. (2000). “Performance of geosynthetic-reinforced walls supporting
bridge and approaching roadway structures.” Proc., ASCE Geotechnical Special Publication No. 103, GeoDenver 2000, Denver, 218–243.
Adams M., Nicks J., Stabile T., Wu J., Schlatter W., and Hartmann J. (2011), “Geosynthetic Reinforced Soil Integrated
Bridge System, Interim Implementation Guide”, Federal Highway Administration, U.S. Department of
Transportation Washington DC. Publication no. FHWA-HRT-11-026.
Elias V., Christopher B.R. and Berg R.R. (2001), “Mechanically Stabilized Earth Walls and Reinforced Soil Slopes
Design and Construction Guidelines”, Federal Highway Administration, U.S. Department of Transportation
Washington DC. Publication no. FHWA-NHI-00-043.
Herold, A. (2002),"The first permanent road-bridge abutment in Germany built of geosynthetic-reinforced earth" on
http://www.ibh-herold.de/de/inhalt/publikationen/download/2002-Herold-The first permanent road-bridge...Nizza.PDF
Mirlatifi S. A., Fakher A., Ghalandarzadeh, A. (2010), “The Deformation Study of Reinforced Earth Walls against
Earthquake ", Journal of Civil and Surveying Engineering (Volume: 44, Issue: 5), 2010, 705-711.
Mirlatifi S. A., Fakher A., Ghalandarzadeh A. (2007) "Seismic study of reinforced Earth Walls by Shaking Table Model
Tests", 4th International Conference on Earthquake Geotechnical Engineering, 25-28 June 2007, Thessaloniki,
Greece, paper No. 1253.
Salmanzadeh Z. A., Mirlattifi, S.A, Rahmani I. (2010), “The case study of Milad Tower access bridge abutment, the
first Geogrid reinforced soil abutment in Iran”, 4th International Conference on Geotechnical Engineering and
Soil Mechanics in Iran, Tehran, 2010, Paper Code: (BMPSAL) 691.
Sam M.B. Helwany, Jonathan T.H. Wu, and Akadet Kitsabunnarat, (2007), “Stimulating the Behavior of GRS Bridge
Abutments”, Journal of Geotechnical and Geoenvironmental engineering, 10, 1061/ (ASCE) 1090-0241.
S.R. Lo, (2004), “Application of Numerical Modelling to the Design of Reinforced Soil Walls for Infrastructure
Projects - Some Australian Experiences” GeoAsia2004: 3rd Asian Regional Conference on Geosynthetics:
Now and Future of Geosynthetics in Civil Engineering, June 2004, Seoul, Korea, p193.
Wu, J. T. H., Lee, K. Z. Z., Helwany, S., and Ketchart, K. (2006), “Design and construction guidelines for geosynthetic
reinforced soil bridge abutments with a flexible facing.” NCHRP Rep. No. 556, Transportation Research
Board, National Research Council, Washington D.C.
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analysis, design, and construction stages of milad geosynthetic

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