ku 52nd geotechnical engineering conference: designs ......aashto lrfd bridge design specifications,...
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KU 52nd Geotechnical Engineering Conference: Designs Updates for Mechanically Stabilized Earth Walls in 2020 AASHTO
Daniel Alzamora, P.E.FHWA – Resource CenterSenior Geotechnical Engineer
November 12, 2020
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Changes to How Overall (slope) Stability is AddressedAASHTO LRFD Bridge Design Specifications, 9th Edition, 2020
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Changes to How Overall (slope) Stability is Addressed
Summary of What is Required in the LRFD Specifications (2003 – 2018)
• Consider FS output by slope stability design programs to be a resistance factor
• = 1/FS = 1/1.3 0.75 (geotech. parameters well defined, does not support structural element)
• = 1/FS = 1/1.5 0.65 (geotech. parameters not well defined, or supports structural element)
• The analysis is based on Service I limit state• Focus of overall stability is on the soil shear strength needed for stability versus the soil shear strength
available (i.e., Fv = Fh = 0, Mc = 0) • This resistance factor for overall stability is combined with an overall stability load factor (p, i.e., EV) of 1.0
(Table 3.4.1‐2)
• All other loads, including transient loads, have a load factor of 1.0
• Therefore, external loads such as loads due to foundations are essentially unfactored
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Solution Reflected in New Specifications
• Move overall stability into the strength limit state, as overall stability is a collapse, not deformation, scenario
• Consider FS output by slope stability design programs to be the reciprocal resistance factor. This is not a change other than what is shown below.
• = 1/FS = 1/1.3 0.75 (geotech. parameters well defined, does not support structural element)
• = 1/FS = 1/1.5 0.65 (geotech. parameters not well defined, or supports structural element)
• Continue to use p (i.e., EV) for overall stability equal to 1.0• Use standard load factors for Strength I if have foundation load on top of slope
• For example, for Strength I, load group is EV = 1.0 for soil loading, DC = 1.25 for foundation dead load, LL = 1.75 for live load acting on foundation, etc.
• For soil resistance, = 1/FS = 1/1.3 0.75 whether or not slope supports external loads such as a foundation (i.e., rather than decreasing to 0.65 if foundation load is present), since foundation loads will now be factored using Strength I values
Changes to How Overall (slope) Stability is Addressed
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
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Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Changes to How Tmax is calculated AASHTO LRFD Bridge Design Specifications, 9th Edition, 2020
Collaborative effort
Stiffness Method
• Tony Allen, Washington DOT
• Richard Bathurst, GeoEngineering Centre at Queen's‐RMC
Limit Equilibrium
• Dov Leshchinsky, University of Delaware
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Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Why add new design methods?
• The Simplified Method has not been good at predicting reinforcement loads as compared to measured loads, particularly for extensible reinforcements.
• Goal of the changes are to update and improve the requirements for internal stability design of MSE walls.
Allen, T.M. and Bathurst, R.J. (2015). Improved simplified method for prediction of loads in reinforced soil walls. ASCE Journal of Geotechnical and Geoenvironmental Engineering 141(11): 04015049.
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
How did we get here?
• AASHTO T15/FHWA MSE Task Force• Started 2012 – one focus area was MSE internal stability• Composed of MSE leaders in academia, consulting, and the industry• Assessed best path forward (limit equilibrium, and new stiffness method)• Recommended research needed and eventual adoption
• AASHTO T15 and COBS• Beginning in 2014, continued this assessment• Used formal AASHTO COBS process through T15 mid‐yr meetings, the annual AASHTO COBS meetings, and e‐mail
• Final decision: • adopt the Stiffness Method for geosynthetic walls• allow the continued use of the Simplified Method• Use limit equilibrium for situations that are beyond empirical basis for the other methods, and for compound stability
• Adopted in the AASHTO LRFD Specifications as the “Stiffness Method” in 2019 (published 3‐2020)
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Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Changes to MSE wall design
Change in Tmax calculation
• Existing methods• Simplified Method• Coherent Gravity Method (inextensible Reinforcement)
• New methods• Stiffness Method• Limit Equilibrium • These methods are limited in AASHTO to extensible reinforcements only
Change in Overall Stability Calculation
• Service limit vs strength limit
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
What is Tmax?
Z
Sv
Reinforced FillUnit weight (γ)Friction angle (ф)Active earth Pressure (ka)
σv
Tmax is the force acting on the MSE reinforcement at any given depth.
Tmax is a function of the:• vertical stress• strength of the soil• spacing of the reinforcement• Reinforcement stiffness• Facing
Tmax
Source: FHWA
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Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Simplified Method
VHMAX SσT
SV is vertical reinforcement spacing, for equally spaced reinforcements
VaH σ)/(Kσ ar KKZ
Sv
Reinforced FillUnit weight (γ)Friction angle (ф)Active earth Pressure (ka)
σv
...... Zγσ rV
Source: FHWA
Source: FHWA NHI‐10‐024
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Coherent Gravity Method
VHMAX SσT
SV is vertical reinforcement spacing, for equally spaced reinforcements
VaH σ)/(Kσ KaKr)2/(σV eLW
Source: FHWA
Z
Sv
Reinforced FillUnit weight (γ)Friction angle (ф)Active earth Pressure (ka)
σv pa
We
L
ZrγW
Source: AASHTO LRFD Bridge Design Specifications
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Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
New Methods to calculate Tmax
2020 AASHTO – Incorporates the use of these new design methods for extensible reinforcements.
• Limit Equilibrium
• Stiffness Method
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Stiffness Method ‐
• Allen, T.M. and Bathurst, R.J. 2015. An improved simplified method for prediction of loads in reinforced soil walls. Journal of Geotechnical and Geoenvironmental Engineering 141(11): 752 04015049.
• Allen, T.M. and Bathurst, R.J. 2018. Application of the simplified stiffness method to design of reinforced soil walls. Journal of Geotechnical and Geoenvironmental Engineering 144(5):756 04018024.
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Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Stiffness Method
Z
Sv
Reinforced FillUnit weight (γ)Friction angle (ф)Active earth Pressure (ka)
σv
Source: FHWA
Tmax = SV [γr H Dtmax + γf (Href/H)S] Kavh Φfb Φg Φfs Φlocal Φc
Φ are empirically determined factors that capture the effect of reinforcement, cohesion, and wall geometry have on Tmax
Tmax = SV σH
σH = σV [Kavh Φfb Φg Φfs Φlocal Φc] [σV Ka (Kr/Ka)] for simplified
σV = [γr H Dtmax + γf(Href/H)S] [γr Z + γf hs] for simplified
• Dtmax is the Tmax distribution factor• Href is the reference wall height of 20 ft
Source: AASHTO LRFD Bridge Design Specifications, 9th Edition, 2020
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Soil Failure Limit State:• Working stress conditions may not apply if
there is a contiguous failure surface through the reinforced soil zone.
• As specified in AASHTO (2020), keep factored peak reinforcement strains in wall < 2% for stiff faced walls, and <2.5% for flexible faced walls, to maintain (soil) working stress conditions.
• These target maximum strains are 0.5% strain more conservative than recommended in the Allen and Bathurst (2018) ASCE paper.
Source: AASHTO LRFD Bridge Design Specifications, 9th Edition, 2020
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Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Source:Allen, T.M. and Bathurst, R.J. (2015). Improved simplified method for prediction of loads in reinforced soil walls. ASCE Journal of Geotechnical and Geoenvironmental Engineering 141(11): 04015049.
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
• Limit equilibrium (LE) analysis has been used in the design of complex structures for decades.
• LEA finds the required tensile force distribution in each layer
• For internal stability –applicable for extensible reinforcement only
• LEA applicable for all reinforcements for global and compound stability
Technical Report Documentation Page 1. Report No.
2. Government Accession No.
3. Recipient’s Catalog No.
FHWA-HIF-17-004
4. Title and Subtitle
5. Report Date
Limit Equilibrium Design Framework for MSE Structures with Extensible Reinforcement
October 2016 6. Performing Organization Code
7. Principal Investigator(s): See Acknowledgements for Authors and Contributors
8. Performing Organization Report
No. Dov Leshchinsky, Ph.D1, Ora Leshchinsky, P.E.1, Brian Zelenko, P.E., John Horne, Ph.D., P.E.
9. Performing Organization Name and Address
10. Work Unit No. (TRAIS)
Parsons Brinckerhoff 1015 Half Street, SE, Suite 650 Washington, DC 20003 1ADAMA Engineering, Inc., 12042 SE Sunnyside Rd., Suite 711, Clackamas, OR 97015
11. Contract or Grant No. DTFH6114D00047-5010
12. Sponsoring Agency Name and Address
13. Type of Report and Period
Federal Highway Administration HIBT-20 Office of Bridge Technology 1200 New Jersey Avenue, SE Washington, DC 20005
14. Sponsoring Agency Code
15. Supplementary Notes FHWA COR – Silas Nichols, P.E. FHWA Alt. COR – Khalid Mohamed, P.E. 16. Abstract
Current design of reinforced soil structures in the U.S. distinguishes between slopes and walls using the batter angle as a criterion. Using a unified approach in limit state design of reinforced ‘walls’ and ‘slopes’ should diminish confusion while enabling a wide and consistent usage in solving geotechnical problems such as complex geometries and soil profiles. Limit equilibrium (LE) analysis has been used successfully in the design of complex and critical (e.g., tall dams) for many decades. Limit state analysis, including LE, assumes that the design strength of the soil is mobilized. Presented is a LE framework, limited to extensible reinforcement, which enables the designer to find the tensile force distribution in each layer required at a limit state. This approach is restricted to Allowable Stress Design (ASD). Three example problems are presented. 17. Key Words
18. Distribution Statement
Mechanically Stabilized Earth Wall Design, MSE Wall Design, Limit Equilibrium, Geotechnical, Extensible reinforcement
No restrictions.
19. Security Classif. (of this report)
20. Security Classif. (of this
21. No. of Pages
22. Price
UNCLASSIFIED
UNCLASSIFIED 120
Form DOT F 1700.7(8-72) Reproduction of completed page authorized
Source: FHWA‐HIF‐17‐004
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Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Source: FHWA‐HIF‐17‐004
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Source: FHWA‐HIF‐17‐004
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Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Source: FHWA‐HIF‐17‐004
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Source: FHWA‐HIF‐17‐004
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Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Source: FHWA‐HIF‐17‐004
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Impact on Design –
Extensible reinforcements
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Illustration of some of the design differences for specific parameters.
Source: FHWA
Updating Designs for Mechanically Stabilized Earth Walls in AASHTO
Impact on Design – Extensible reinforcements
1. More accurate model to represent loading of reinforcement
2. Generally require less reinforcement for walls under 25‐ft
3. Little change for walls over 25‐ft
4. New methods allow us to take into account• Facing contribution
• Variable lengths of reinforcements
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QUESTIONS?