NCHRP Progress Review. Seismic Analysis and Design of. Embankments, and Buried Structures. January 22, 2007

Transcription

1 NCHRP Progress Review Seismic Analysis and Design of Retaining i Walls, Slopes and Embankments, and Buried Structures January 22, 2007

2 Objectives of NCHRP Project Develop analytical methods and recommended LRFD specifications for seismic design of retaining i walls, buried structures, t slopes, and embankments The specifications shall be compatible and consistent with the philosophy and format of the AASHTO LRFD Bridge Design Specifications Ref: NCHRP Research Project Statement Project 12-70, FY 2004

3 Background NCHRP Project Builds on other projects involving development of LRFD specifications NCHRP completed in 2003 NCHRP completed in 2006 Scope of NCHRP & NCHRP Limited to bridges and components directly attached (wing walls and abutments) Included determination of ground motions Excluded retaining walls, buried structures, slopes, and embankments

4 Need for NCHRP Project Difficulties with retaining wall designs M-O method blows up with steep back slopes and high pga s Uncertainty in selecting seismic coefficient Limited guidance for some walls (e.g., soldier pile, tieback, and soil nail walls) Lack of written guidelines for slope stability Appropriate seismic coefficient Analysis method (i.e., FS versus deformations) Liquefaction effects

5 Need for NCHRP Project (cont.) Lack of methods in Section 12 of LRFD on seismic design of buried structures (i.e., drainage culverts and pipes, box culverts, pedestrian tunnels) Transient versus permanent ground displacement Permanent ground displacement (flotation, lateral spread, & settlement) Flexible versus rigid pipe

6 Goals for Project Improve existing methods or develop new methods that overcome shortcomings Optimize design approach for routine design and for special cases Avoid hidden conservatism Ensure applicability to WUS and CEUS Include no seismic i design provision i Be consistent with AASHTO LRFD Bridge Design Specifications Revisions to ground motions from NCHRP Project Other changes

7 Status of Project Work completed (April 2004 Dec. 2006) Reviewed literature, identified issues & proposed methodologies Prepared 1 st Interim Report (Jan. 2005) Prepared 2 nd Interim Report (Mar. 2006) Developed preliminary specifications, commentaries & example problems Prepared 3 rd Interim Report (Nov. 2006) Work to complete (Jan July 2007) Prepare 2 nd draft specifications, commentaries & example problems (May 2007) Finalize report, specifications and commentaries & example problems (July 2007)

8 Overview of Proposed Approach Ground motion recommendations Initial screening Adjustments for wave scattering Determination of PGV from S 1 Revised displacement charts/equations Guidance on soil property selection Undrained strengths Importance of c and Φ Design recommendations Method of analysis (simple & generalized) Design Checks (global, external & internal)

9 Overview of Proposed Approach Design of retaining walls Method of active pressure determination Didn t abandon M-O. Introduced (1) revised seismic coefficient for scattering, and (2) effects of c Recommended generalized slope stability approach for many situations (layered soil profiles, geometric constraints, etc.) Passive pressures from log spiral with cohesion Slopes & Embankments Adjustments for wave scattering & displacement Buried Structures Procedures for treating transient ground displacements

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11 Seismic i Loads & Load Factors Ground motions USGS developed new maps for AASHTO 7% in 75 year design basis PGA, S s, and S 1 NEHRP site factors CD with coordinate look-up Proposed screening for No Analysis at nonliquefiable sites

12 Seismic Loads & Load Factors Height-dependent scattering adjustments k max = α PGA α = 1 + H/100 [(0.5/β)-1] where H = fill height β = F v S 1 /PGA β=1.5 β=1.0 β=0.5

13 Seismic Loads & Load Factors Displacement-related seismic coefficient 0.5 k max for design permitted Assumes 1-2 inches of displacement acceptable Newmark displacement estimates d=f(k y /k max, PGA, PGV) PGV=f(S 1 )

14 Gravity / Semi Gravity Walls Seismic active and passive earth pressure determination Determine active pressures using either Simplified M-O equation for active pressure Generalized limit equilibrium methods Determine Passive Pressures from log spiral charts M-O active earth pressure cut backslope criteria Slope Angle Flat 3H:1V Slope of Active Wedge 1.5H:1V 2H:1V Figure X.7-2 Application of M-O Method for Non-Homogeneous Soil

15 Gravity / Semi Gravity Walls Active earth pressures for c-φφ soils Use where backfill soils are cohesive or failure plane in native backcut Capillary stress in silty soils max. c=50~100 psf Figure B-1 Seismic Active Figure B-1 Seismic Active Earth Pressure Coefficient for φ = 30

16 Gravity / Semi Gravity Walls Seismic passive earth pressure for c- φ soils Use log spiral methods rather than M-O OEquations Capillary stress in silty soils max. c=50~100 psf Figure B-1 Seismic Active Earth Pressure Coefficient for φ = 30 Figure B-3. Seismic i Passive Earth Pressure Coefficient based on Log Spiral Procedure

17 Gravity / Semi Gravity Walls Generalized equilibrium methods Use where M-O approach not suitable: soil conditions or backslope geometry/steep cut slopes Earth pressures applied as boundary force in commercially available slope stability programs Wall displacement analysis Use where D/C Ratio > 1 (FS < 1) Allowable sliding displacements > 1 to 2 inches, supporting larger reductions in seismic coefficient

18 Non-Gravity Cantilever Walls Seismic passive earth pressure e Limit equilibrium method (free- or fixed-earth pressure) for most analyses Use log spiral method with adjustment t for soil inertial effect Wall displacement analysis / numerical methods Beam column approach Software available (P-Y Wall, L-Pile, COM624, BMCOL) P-multiplier developed FE or FD modeling allowed

19 General MSE Walls Initial static design meets AASHTO Specifications Adequate performance: Global/External/Internal Stability Method of Analysis Performance Criteria No excessive sliding or rotation of structural No structural failure of reinforcing strips or facing elements Analyses Eliminate A m = (1.45 A) A factor (Segrestin and Bastick, 1988) ) Displacement-based design (if necessary)

20 Method of Analysis Anchored Walls Assume seismic active pressures can be mobilized due to anchor stretch. Seismic active earth pressures Use of generalized limit equilibrium methods (layered natural al c φ soils) Use k=k max Use total stress strength parameters c φ Assume uniform active pressure distribution Key product is location of critical failure surface for anchor location

21 Soil Nail Walls Design and analysis procedures are similar to those for MSE Walls No AASHTO LRFD Specifications currently exist for soil nail walls 2005 NCHRP Project Report may form the basis for a specification Currently, designs follow guidelines in FHWA G.E.C. No. 7 (2003) which describes two computer codes, GOLDNAIL and SNAIL Use AASHTO MSE Wall acceleration factor Allow displacement-based design

22 Slopes & Embankments Methodology Recommends screening level (nonliquefiable sites) 3H:1V: pga > 0.3g 2H:1V: pga > 0.2g Estimate seismic coefficient similar to retaining walls Includes adjustment for scattering (H > 20ft) and for cohesion Reduce seismic coefficient by 50% if several inches of deformation acceptable Other Considerations Proposes screening for liquefaction (liquefaction unlikely if N value > 5 bpf and pga < 0.15g) Includes use of numerical modeling methods

23 Buried Structures General Ground Shaking (TGD) Detailed Procedures Ovaling/Racking Deformations Axial/Curvature Deformations - not required due to limited length Ground Failure (PGD) General Methodology Analysis Simplified Procedures Ovaling & Racking Numerical Modeling Methods (Recommended Alternative) Complex Geometry Long Span Critical Structure Highly Variable Subsurface Conditions High Seismic Areas Under High Embankment

24 Buried Structures General Vulnerability Screening Factors Level of Shaking Soil Conditions Structure Properties No Analysis Requirements (Screening Level) S 1 0.2g (or PGV 10 in/sec) for D > 1.0 m S 1 0.3g 03g(orPGV 15 in/sec) for D 10m 1.0 PGV (in/s) = x C 1) C 1 = LogS (2.3 LogS ) 2