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Structural Foundations for Bridges

Structural Foundations for Bridges

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    Bridge structural foundations-and_earth_retaining_structures Bridge structural foundations-and_earth_retaining_structures Presentation Transcript

    • AASHTO LRFD: Structural Foundations and Earth Retaining Structures
      • Specification Background What’s Happening Now!
      • Limit States, Soil and Rock Properties
      • Deep Foundations
      • Shallow Foundations
      • Earth Retaining Structures
      • Jerry DiMaggio, P. E., Principal Bridge Engineer (Geotechnical)
      • Federal Highway Administration
      • Office of Bridge Technology
      • Washington D.C.
    • ? New Legal Load
    • AASHTO Specification Background: Geotechnical Engineering Presence
      • * TRB/ NCHRP Activities (A LOT!)
      • * Geotechnical Engineering does NOT have a broad based presence on AASHTO SubCommittees and Task Forces as do other technical specialties.
      • * SubCommittee on Construction (guide construction specs)
      • * SubCommittee on Materials (specs on materials and testing standards)
      • * SubCommittee on Bridges and Structures (specs on materials/ systems, design, and construction)
    • History of AASHTO: Design & Construction Specifications for Bridges and Structures
      • * First structural “Guideline Specification” early 1930s
      • (A code yet NOT A code!).
      • * First “significant” Geotechnical content 1989.
      • * First LRFD specification 1994 (Current – 2004, 3 rd edition).
      • * First REAL Geotechnical involvement in Bridge SubCommittee activities @ 1996. (Focus on mse walls).
      • * Technical advances to Standard Specifications STOPPED in 1998 to encourage LRFD use (secret).
      • * Major rewrites needed to walls and foundations sections (NOW COMPLETE).
    • “ Geotechnical Scope”: AASHTO Design & Construction Specifications for Bridges and Structures
      • * Topics Included: Subsurface Investigations, soil and rock properties, shallow foundations, driven piles, drilled shafts, rigid and flexible culverts, abutments, WALLS (cantilever, mse, crib, bin, anchor).
      • * Topics NOT addressed : integral abutments, micropiles, augercast piles, soil nails, reinforced slopes, and ALL SOIL and ROCK EARTHWORK FEATURES.
    • Standard and LRFD AASHTO Specifications
      • * Currently AASHTO has 2 separate specifications: Standard specs 17 th edition and LRFD, 2004 3rd edition.
      • * Standard Specifications use a combination of working stress and load factor design platform.
      • * LRFD uses a limit states design platform with different load and resistance factors (than LFD).
    • LRFD IMPLEMENTATION STATUS
      • Geotechnically, most States still use a working stress approach for earthworks, structural foundations, and earth retaining structures. Several States have totally adopted LRFD.
      • Many State Geo/Structural personnel and consultants ARE NOT FAMILAR with the content of LRFD 3 rd edition.
      • “ AASHTO and FHWA have agreed that all state DOTs will use LRFD for NEW structure design by 10/07.”
    • What are UNIQUE Geotechnical issues related to LRFD?
      • * Strong influence of construction on design.
      • * GEOTECHs strong bias toward performance based specifications.
      • * Natural variability of GEO materials.
      • * Variability in the type, and frequency of tests, and method to determine design property values of soil and rock.
      • * Differences between earthwork and structural foundation design model approaches.
      • * Influence of regional and local factors.
      • * General lack of data on limit state conditions.
    • What Should I Know and Do?
      • * Become familiar with BOTH the AASHTO standard specifications and LRFD specs.
      • * Develop an understanding of your agency’s current design practice with your structures office.
      • * Develop and compare results for SEVERAL example problems with LRFD and YOUR standard design practice.
      • * Translate your current practice to an LRFD format with your structural office.
      • * Communicate findings of your example problem comparisons to AASHTO’s SubCommitteee members.
    • What Happening Now?
      • * FHWA sponsored a complete rewrite of Section 10 during 2004. The rewrite was prepared by National subject matter experts and had broad input from a number of Key State Dots, (including T-15 member States), and the Geotechnical community (ASCE - GI, DFI, ADSC, PDCA).
      • * During the Proposed spec development @ 2000 comments were addressed. The Proposed spec was then distributed to all States for review. An additional @ 1000 comments were addressed.
      • * The revised Proposed Specification was advanced and approved by the AASHTO’s Bridge and Structures Sub-Committeee in June 2005.
      • The revised Proposed Specification is used in the NHI LRFD Substructure course which currently available.
      • Fundamentals of LRFD
      • Principles of Limit State Designs
      • * Define the term “Limit State”
      • * Define the term “Resistance”
      • * Identify the applicability of each of the four primary limit states.
      • * Understand the components of the fundamental LRFD equation.
    • A Limit State is a defined condition beyond which a structural component, ceases to satisfy the provisions for which it is designed. Resistance is a quantifiable value that defines the point beyond which the particular limit state under investigation for a particular component will be exceeded.
    • Resistance can be defined in terms of:
      • * Load/Force (static/ dynamic, dead/ live)
      • * Stress (normal, shear, torsional)
      • * Number of cycles
      • * Temperature
      • * Strain
    • Limit States
        • * Strength Limit State
      • * Extreme Event Limit State
      • * Service Limit State
      • * Fatigue Limit State
      L I S T
    • Strength Limit State
    • Extreme Event Limit State
    • Service Limit State
    • Service Limit State
    • Rn / FS   Q  i  i Q i ≤ R r =  R n
      •  i =
      •  i =
      • Q i =
      • R r =
      •  =
      • R n =
      Load modifier (eta) Load factor (gamma) Force effect Factored resistance Resistance factor (phi) Nominal resistance
    •  i  i Q i ≤ R r =  R n f(  ,  ) Q n R n Q R  Q n  R n Q or R Probability of Occurrence 
    • Subsurface Materials
      • * Soil
      • * Rock
      • * Water
      • * Organics
    • 10.4 SOIL AND ROCK PROPERTIES 10.4.1 Informational Needs 10.4.2 Subsurface Exploration 10.4.3 Laboratory Tests 10.4.3.1 Soil Tests 10.4.3.2 Rock Tests 10.4.4 In-situ Tests 10.4.5 Geophysical Tests 10.4.6 Selection of Design Properties 10.4.6.1 Soil Strength 10.4.6.1.1 Undrained strength of Cohesive Soils 10.4.6.1.2 Drained Strength of Cohesive Soils 10.4.6.1.3 Drained strength of Granular Soils 10.4.6.2 Soil Deformation 10.4.6.3 Rock Mass Strength 10.4.6.4 Rock Mass Deformation 10.4.6.5 erodibility of rock
    • Overview of Soil and Rock Materials
      • * Apply the principle of effective stress to computation of vertical effective stress
      • * Use the Mohr-Coulomb equation to determine the shear strength of soils.
      • * Understand the difference between drained and undrained strength
      • * Know what field or laboratory test should be performed to obtain the required soil or rock properties.
      • * Understand the difference between the intact properties of rock and the rock mass properties.
    • Soil Characteristics
      • * Composed of individual grains of rock
      • * Relatively low strength
      • * Coarse grained (+ #200)
        • * High permeability
      • * Fine grained (- #200)
        • * Low permeability
        • * Time dependant effects
    • Rock Characteristics
      • * Strength
        • * Intermediate geomaterials, q u = 50-1500 psi
        • * Hard rock, q u > 1500 psi
      • * Rock mass properties
    • % Finer by Weight Uniform Well Graded Gravel Sand Silt Clay Grain Diameter (mm) 100 100 10 1 0.1 0.01 0.001 US Standard Sieves 3” 2” 1” 3/4 ” 3/8” 4 6 10 20 40 60 100 200 80 60 40 20 0
    • Atterberg Limits
      • The water content at which a soil changes state
      • PI = LL - PL
      Solid Semi-Solid Plastic Liquid SL PL LL PI Increasing water content
    • Effective Stress – Spring Analogy
      •  ’ =  – u
      •  ’ = effective stress
      •  = total stress
      • * u = pore pressure
       ’  u P
    • Soil Shear Strength  ’ = c’ +  n ’ tan  f ’  ’ a  ’ a  ’ r  ’ r  ’  ’ n  ’  ’ n  ’ f  ’  ’ c’ Strength envelope
    • Undrained Strength of Cohesive Soils, s u Unconfined Compression s u = q u /2 Vane Shear Test s u  q u  =0 Typical Values s u = 250 - 4000 psf
    • Drained Strength of Cohesive Soils, c’ and  ’ f Triaxial Compression CU Test Typical Values c’ = 100 - 500 psf  ’ f = 20 o - 35 o
    • Drained Strength of Cohesionless Soils,  ’ f Standard Penetration Test (SPT) Typical Values  ’ f = 25 o - 45 o Friction angle is correlated to SPT results.  ’ f  ’ q’ c=0
    •  
    • Guided Walk Through For N 1 60 = 10, select  ’ f = 30 o (modified after Bowles, 1977) N 1 60  f <4 25-30 4 27-32 10 30-35 30 35-40 50 38-43
    • Soil Deformation 0 -2 -4 -6 -8 -10 -12 1 10 100 1000 10000 Time (days) Settlement (in) Initial elastic settlement (all soils) Primary consolidation Secondary consolidation Fine-grained (cohesive) soils
    • Consolidation Properties Log 10  v ’ Void Ratio (e)  p ’ = Preconsolidation Stress C s C r C c 0.1 1 10 100 0.5 1 e o
    • One log cycle  e=C  =0.06 0.1 1 10 100 1000 10000 Elapsed Time (min) Void ratio (e) 2.65 2.6 2.55 2.5 2.45 2.4 2.35 2.3 2.25 Stress Range, 40 – 80 kPa t p
    • Typical Consolidation Properties Property Typical Value C c 0.1 to 1.0 C r 10 % of C c C s Approximately C r C  4% to 6% of C c C v 0.01 to 1.0 ft 2 /day
    • Elastic Properties of Soil
      • Young’s Modulus, E s
        • Typical values, 20 – 2000 tsf
      • Poisson’s Ratio, 
        • Typical values, 0.2 – 0.5
      • Shear Modulus, G
        • Typical values, E s / [2 (1 +  )]
      • Determination by correlation to N1 60 or s u , or in-situ tests
    • Rock Properties
      • Laboratory testing is for small intact rock specimens
      • Rock mass is too large to be tested in lab or field
      • Rock mass properties are obtained by correlating intact rock to large-scale rock mass behavior – failures in tunnels and mine slopes
      • Requires geologic expertise
    • Intact Rock Strength Point Load Test Unconfined Compression, q u Typical Values q u = 1500 - 50000 psi
    • Rock Quality Length, L 0.8 ft 0.7 ft 0.8 ft 0.6 ft 0.2 ft 0.7 ft Sound Not sound, highly weathered Not sound, centerline pieces < 4 inches, highly weathered Sound Not sound Sound Core Run Total = 4 ft CR = 95% RQD = 53%
    • CSIR Rock Mass Rating System
      • This system is based on q u , RQD, joint spacing, joint condition and water condition.
    • Rock Mass Strength C 1 ’ Shear stress,  Effective Normal Stress,  ’  tm  3  1    ’ i  = (cot  ’ i – cos  ’ i )mq u /8  ’ i = tan -1 (4 h cos 2 [30+0.33sin -1 (h -3/2 )]-1) -1/2 h = 1 + 16(m  ’ n +sq u )/(3m 2 q u )
    • Rock-Mass Quality and Material Constants
      • Values of the parameters m and s are determined based on empirical correlation to rock type and RMR
    • Intact Rock Deformation, E i
      • Typical values range from 1000 to 13000 ksi
      • Poisson’s Ratio, 
      • Typical values range from 0.1 to 0.3
    • Rock Mass Deformation E  = 2 RMR - 100 90 70 50 30 10 In situ modulus of deformation, E M (GPa) 10 30 50 70 90 12 10 8 6 4 2 (psi x 10 6 ) Rock mass rating RMR
    • Read More About It GEC 5 FHWA-IF-02-034
    • Jerry A. DiMaggio P. E. Principal Bridge Engineer TEL: (202) 366-1569 FAX: (202) 366-3077 The best Geotechnical web site in town! www.fhwa.dot.gov/bridge WOW! FREE STUFF FROM THE FEDERAL GOVERNMENT!