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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)

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AASHTO LRFD: Structural Foundations and Earth Retaining Structures

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Aashto lrfd structural foundations and earth retaining structures l.jpg

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.


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New Legal Load

?


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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)


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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, 3rd 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).


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“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.


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Standard and LRFD AASHTO Specifications

* Currently AASHTO has 2 separate specifications: Standard specs 17th 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).


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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 3rd edition.

“AASHTO and FHWA have agreed that all state DOTs will use LRFD for NEW structure design by 10/07.”


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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.


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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.


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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.


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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.


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Resistance can be defined in terms of:

* Load/Force (static/ dynamic, dead/ live)

* Stress (normal, shear, torsional)

* Number of cycles

* Temperature

* Strain


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Limit States

* Strength Limit State

* Extreme Event Limit State

* Service Limit State

* Fatigue Limit State

L

I

S

T


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Strength Limit State


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Extreme Event Limit State


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Service Limit State


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Service Limit State


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Rn / FS  QShigiQi≤ Rr = fRn

hi =

gi =

Qi =

Rr =

f =

Rn =

Load modifier (eta)

Load factor (gamma)

Force effect

Factored resistance

Resistance factor (phi)

Nominal resistance


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ShigiQi≤ Rr = fRn

Qn

Rn

f(g,)

 Qn

Probability of Occurrence

 Rn

R

Q

Q or R


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Subsurface Materials

* Soil

* Rock

* Water

* Organics


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10.4SOIL AND ROCK PROPERTIES

10.4.1Informational Needs

10.4.2Subsurface Exploration

10.4.3Laboratory Tests

10.4.3.1Soil Tests

10.4.3.2Rock Tests

10.4.4In-situ Tests

10.4.5Geophysical Tests

10.4.6Selection of Design Properties

10.4.6.1Soil Strength

10.4.6.1.1Undrained strength of Cohesive Soils

10.4.6.1.2Drained Strength of Cohesive Soils

10.4.6.1.3Drained strength of Granular Soils

10.4.6.2Soil Deformation

10.4.6.3Rock Mass Strength

10.4.6.4Rock Mass Deformation

10.4.6.5erodibility of rock


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Soil Characteristics

* Composed of individual grains of rock

* Relatively low strength

* Coarse grained (+ #200)

* High permeability

* Fine grained (- #200)

* Low permeability

* Time dependant effects


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Rock Characteristics

* Strength

* Intermediate geomaterials,qu = 50-1500 psi

* Hard rock, qu > 1500 psi

* Rock mass properties


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Undrained Strength of Cohesive Soils, su

Vane Shear Test

f=0

su

s

qu

Unconfined Compression

su = qu/2

Typical Values

su = 250 - 4000 psf


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Drained Strength of Cohesive Soils, c’ and f’f

Triaxial Compression

CU Test

Typical Values

c’ = 100 - 500 psf

f’f = 20o - 35o


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Guided Walk Through

For N160 = 10, select ’f = 30o


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Soil Deformation

Initial elastic settlement (all soils)

0

-2

-4

-6

-8

-10

-12

Settlement (in)

110100100010000

Time (days)

Primary consolidation

Secondary consolidation

Fine-grained (cohesive) soils


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Consolidation Properties

eo

p’ = Preconsolidation Stress

1

Cr

Void Ratio (e)

Cc

Cs

0.5

0.1

1

10

100

Log10v’


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Stress Range, 40 – 80 kPa

2.65

2.6

2.55

2.5

2.45

2.4

2.35

2.3

2.25

One log cycle

De=Ca=0.06

Void ratio (e)

tp

0.1110100100010000

Elapsed Time (min)


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Elastic Properties of Soil

  • Young’s Modulus, Es

    • Typical values, 20 – 2000 tsf

  • Poisson’s Ratio, u

    • Typical values, 0.2 – 0.5

  • Shear Modulus, G

    • Typical values, Es / [2 (1 + u)]

  • Determination by correlation to N160 or su, or in-situ tests


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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


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Intact Rock Strength

Unconfined Compression, qu

Typical Values

qu = 1500 - 50000 psi

Point Load Test


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Rock Quality

0.8 ft

Sound

0.7 ft

Not sound, highly weathered

Not sound, centerline pieces < 4 inches, highly weathered

Length, L

0.8 ft

Sound

0.6 ft

Core Run

Total = 4 ft

0.2 ft

Not sound

Sound

0.7 ft

CR = 95%RQD = 53%


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CSIR Rock Mass Rating System

  • This system is based on qu, RQD, joint spacing, joint condition and water condition.


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Rock Mass Strength

Shear stress, t

f’i

t

C1’

stm

s3

s

s1

Effective Normal Stress, s’

f’i = tan-1(4 h cos2[30+0.33sin-1(h-3/2)]-1)-1/2

t = (cot f’i – cos f’i)mqu/8

h = 1 + 16(ms’n+squ)/(3m2qu)


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Intact Rock Deformation, Ei

  • Typical values range from 1000 to 13000 ksi

    Poisson’s Ratio, u

  • Typical values range from 0.1 to 0.3


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Rock Mass Deformation

(psi x 106)

90

70

50

30

10

12

10

8

6

4

2

In situ modulus of deformation, EM (GPa)

Ea = 2 RMR - 100

1030507090

Rock mass rating RMR


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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 l.jpg

Jerry A. DiMaggio P. E.Principal Bridge Engineer TEL: (202) 366-1569FAX: (202) 366-3077

The best Geotechnical web site in town! www.fhwa.dot.gov/bridge

WOW! FREE STUFF FROM THE FEDERAL GOVERNMENT!


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