AASHTO LRFD:AASHTO LRFD:Structural Foundations and Earth Structural Foundations and Earth

Retaining StructuresRetaining Structures Specification Background Specification Background What’s Happening What’s Happening

Now!Now! Limit States, Soil and Rock PropertiesLimit States, Soil and Rock Properties Deep FoundationsDeep Foundations Shallow FoundationsShallow Foundations Earth Retaining StructuresEarth Retaining Structures

Jerry DiMaggio, P. E., Principal Bridge Engineer Jerry DiMaggio, P. E., Principal Bridge Engineer (Geotechnical)(Geotechnical)

Federal Highway AdministrationFederal Highway AdministrationOffice of Bridge TechnologyOffice of Bridge TechnologyWashington D.C.Washington D.C.

? New Legal Load

AASHTO Specification Background: AASHTO Specification Background: Geotechnical Engineering PresenceGeotechnical Engineering Presence

* TRB/ NCHRP Activities (A LOT!)* TRB/ NCHRP Activities (A LOT!)* * Geotechnical Engineering does NOT have a Geotechnical Engineering does NOT have a

broad based presence on AASHTO broad based presence on AASHTO SubCommittees and Task Forces as do other SubCommittees and Task Forces as do other technical specialties.technical specialties.

* SubCommittee on Construction (guide * SubCommittee on Construction (guide construction specs) construction specs)

* SubCommittee on Materials (specs on materials * SubCommittee on Materials (specs on materials

and testing standards) and testing standards)

* SubCommittee on Bridges and Structures * SubCommittee on Bridges and Structures (specs on materials/ systems, design, and (specs on materials/ systems, design, and construction) construction)

History of AASHTO: Design & History of AASHTO: Design & Construction Specifications for Construction Specifications for Bridges and StructuresBridges and Structures* First structural * First structural “Guideline Specification”“Guideline Specification” early 1930s early 1930s (A code yet NOT A code!).(A code yet NOT A code!).* First “significant” Geotechnical content * First “significant” Geotechnical content 1989.1989.* First LRFD specification 1994 (Current – 2004, 3* First LRFD specification 1994 (Current – 2004, 3rdrd

edition).edition).* * First REAL Geotechnical involvementFirst REAL Geotechnical involvement in Bridge in Bridge

SubCommittee activities @ SubCommittee activities @ 1996.1996. (Focus on mse (Focus on mse walls).walls).

* Technical advances to Standard Specifications * Technical advances to Standard Specifications STOPPED STOPPED in 1998in 1998 to encourage LRFD use to encourage LRFD use (secret).(secret).

* Major rewrites needed to walls and foundations * Major rewrites needed to walls and foundations sections (NOW COMPLETE).sections (NOW COMPLETE).

““Geotechnical Scope”:Geotechnical Scope”: AASHTO AASHTO Design & Construction Specifications Design & Construction Specifications for Bridges and Structuresfor Bridges and Structures

* * Topics Included:Topics Included: Subsurface Investigations, Subsurface Investigations, soil and rock properties, shallow soil and rock properties, shallow foundations, driven piles, drilled shafts, foundations, driven piles, drilled shafts, rigid and flexible culverts, abutments, rigid and flexible culverts, abutments, WALLS (cantilever, mse, crib, bin, anchor).WALLS (cantilever, mse, crib, bin, anchor).

* * Topics NOT addressedTopics NOT addressed: integral abutments, : integral abutments, micropiles, augercast piles, soil nails, micropiles, augercast piles, soil nails, reinforced slopes, and ALL SOIL and ROCK reinforced slopes, and ALL SOIL and ROCK EARTHWORK FEATURES. EARTHWORK FEATURES.

Standard and LRFD AASHTO Standard and LRFD AASHTO SpecificationsSpecifications* Currently AASHTO has 2 separate * Currently AASHTO has 2 separate

specifications: Standard specs 17specifications: Standard specs 17thth edition and LRFD, 2004 3rd edition.edition and LRFD, 2004 3rd edition.

* Standard Specifications use a * Standard Specifications use a combination of working stress and load combination of working stress and load factor design platform.factor design platform.

* LRFD uses a limit states design platform * LRFD uses a limit states design platform with with different load and resistance factorsdifferent load and resistance factors (than LFD).(than LFD).

LRFD IMPLEMENTATION STATUSLRFD IMPLEMENTATION STATUS

Geotechnically, most States still use a Geotechnically, most States still use a working stress approach for earthworks, working stress approach for earthworks, structural foundations, and earth retaining structural foundations, and earth retaining structures. Several States have totally structures. Several States have totally adopted LRFD.adopted LRFD.

Many State Geo/Structural personnel and Many State Geo/Structural personnel and consultants consultants ARE NOT FAMILARARE NOT FAMILAR with the with the content of LRFD 3content of LRFD 3rdrd edition. edition.

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

What are UNIQUE Geotechnical issues What are UNIQUE Geotechnical issues related to LRFD?related to LRFD?

* Strong influence of construction on design.* Strong influence of construction on design.* GEOTECHs strong bias toward performance * GEOTECHs strong bias toward performance

based specifications.based specifications.* Natural variability of GEO materials.* Natural variability of GEO materials.* Variability in the type, and frequency of tests, * Variability in the type, and frequency of tests,

and method to determine design property and method to determine design property values of soil and rock.values of soil and rock.

* Differences between earthwork and structural * Differences between earthwork and structural foundation design model approaches.foundation design model approaches.

* Influence of regional and local factors.* Influence of regional and local factors.* * General lack of data on limit state conditions.General lack of data on limit state conditions.

What Happening Now?* FHWA sponsored a complete rewrite of Section 10 * FHWA sponsored a complete rewrite of Section 10

during 2004. The rewrite was prepared by National during 2004. The rewrite was prepared by National subject matter experts and had broad input from a subject matter experts and had broad input from a number of Key State Dots, (including T-15 member number of Key State Dots, (including T-15 member States), and the Geotechnical community (ASCE - States), and the Geotechnical community (ASCE - GI, DFI, ADSC, PDCA).GI, DFI, ADSC, PDCA).

* During the Proposed spec development @ 2000 * During the Proposed spec development @ 2000 comments were addressed. The Proposed spec was comments were addressed. The Proposed spec was then distributed to all States for review. An then distributed to all States for review. An additional @ 1000 comments were addressed.additional @ 1000 comments were addressed.

* The revised Proposed Specification was advanced * The revised Proposed Specification was advanced and approved by the AASHTO’s Bridge and and approved by the AASHTO’s Bridge and Structures Sub-Committeee in June 2005. Structures Sub-Committeee in June 2005. The revised Proposed Specification is used in the The revised Proposed Specification is used in the NHI LRFD Substructure course which currently NHI LRFD Substructure course which currently available.available.

Fundamentals of LRFDFundamentals of LRFDPrinciples of Limit State DesignsPrinciples of Limit State Designs

* Define the term “Limit State” * Define the term “Limit State” * Define the term “Resistance”* Define the term “Resistance”* Identify the applicability of each of the * Identify the applicability of each of the

four primary limit states.four primary limit states.* Understand the components of the * Understand the components of the

fundamental LRFD equation.fundamental LRFD equation.

A Limit State is a A Limit State is a defined conditiondefined condition beyond which a structural component, beyond which a structural component,

ceases to satisfy the provisions for which ceases to satisfy the provisions for which it is designed.it is designed.

Resistance is a Resistance is a quantifiable value that quantifiable value that definesdefines the point beyond which the the point beyond which the

particular limit state under investigation particular limit state under investigation for a particular component will be for a particular component will be

exceeded.exceeded.

Resistance can be defined in Resistance can be defined in terms of:terms of:

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

* Stress (normal, shear, torsional)* Stress (normal, shear, torsional)* Number of cycles* Number of cycles* Temperature* Temperature* Strain* Strain

Limit StatesLimit States

* Strength Limit State* Strength Limit State * Extreme Event Limit * Extreme Event Limit

StateState * Service Limit State * Service Limit State * Fatigue Limit State* Fatigue Limit State

LIST

Strength Strength Limit Limit StateState

Extreme Event Limit Extreme Event Limit StateState

Service Limit StateService Limit State

Service Limit Service Limit StateState

Rn / FS Rn / FS QQ

iiiiQQii ≤ R≤ Rrr = = RRnn

ii ==ii ==QQii ==RRrr = = ==RRnn ==

Load modifier (eta)Load modifier (eta)Load factor (gamma)Load factor (gamma)Force effectForce effectFactored resistanceFactored resistanceResistance factor (phi)Resistance factor (phi)Nominal resistanceNominal resistance

iiiiQQii ≤ R≤ Rrr = = RRnn

ff((,,))QQnn RRnn

QQ RR

QQnn RRnn

Q or RQ or R

Prob

abilit

y of

Pr

obab

ility

of

Occu

rrenc

eOc

curre

nce

Subsurface MaterialsSubsurface Materials

* Soil* Soil* Rock* Rock* Water* Water* Organics* Organics

10.4SOIL AND ROCK PROPERTIES10.4.1Informational Needs10.4.2Subsurface Exploration10.4.3Laboratory Tests10.4.3.1Soil Tests10.4.3.2Rock Tests10.4.4In-situ Tests10.4.5Geophysical Tests10.4.6Selection of Design Properties10.4.6.1Soil Strength10.4.6.1.1Undrained

strength of Cohesive Soils10.4.6.1.2Drained Strength of Cohesive Soils10.4.6.1.3Drained strength of Granular Soils10.4.6.2Soil Deformation10.4.6.3Rock Mass Strength10.4.6.4Rock Mass Deformation10.4.6.5erodibility of rock

Soil CharacteristicsSoil Characteristics

* Composed of individual grains of * Composed of individual grains of rockrock

* Relatively low strength* Relatively low strength* Coarse grained (+ #200)* Coarse grained (+ #200)

* High permeability* High permeability* Fine grained (- #200)* Fine grained (- #200)

* Low permeability* Low permeability* Time dependant effects* Time dependant effects

Rock CharacteristicsRock Characteristics

* Strength* Strength* Intermediate * Intermediate

geomaterials,geomaterials,qquu = 50-1500 = 50-1500 psipsi

* Hard rock, * Hard rock, qquu > 1500 psi > 1500 psi

* Rock mass * Rock mass propertiesproperties

Undrained Strength of Undrained Strength of Cohesive Soils, sCohesive Soils, suu

Unconfined CompressionUnconfined Compressionssuu = q = quu/2/2

Vane Shear TestVane Shear Test

ssuu

qquu

=0=0

Typical ValuesTypical Valuesssuu = 250 - 4000 psf = 250 - 4000 psf

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

Triaxial Triaxial Compression Compression

CU TestCU Test

Typical Valuesc’ = 100 - 500 psf

’f = 20o - 35o

For NFor N116060 = 10, select = 10, select ’’ff = 30 = 30oo

(modified after Bowles, 1977)(modified after Bowles, 1977)NN11

6060 ff

<4<4 25-3025-3044 27-3227-321010 30-3530-353030 35-4035-405050 38-4338-43

Soil DeformationSoil Deformation00

-2-2-4-4-6-6-8-8

-10-10-12-1211 1010 100100 10001000 1000010000

Time (days)Time (days)

Sett

lem

ent

(in)

Sett

lem

ent

(in)

Initial elastic settlement (all soils)Initial elastic settlement (all soils)

Primary consolidationPrimary consolidation Secondary consolidationSecondary consolidation

Fine-grained (cohesive) soilsFine-grained (cohesive) soils

Consolidation PropertiesConsolidation Properties

LogLog1010 vv’’

Void

Rat

io (e

)Vo

id R

atio

(e)

pp’ = ’ = Preconsolidation Preconsolidation StressStress

CCss

CCrr

CCcc

0.10.1 11 1010 1001000.50.5

11eeoo

One log cycleOne log cyclee=Ce=C=0.06=0.06

0.10.1 11 1010 100100 10001000 1000010000Elapsed Time (min)Elapsed Time (min)

Void

ratio

(e)

Void

ratio

(e)

2.652.65

2.62.6

2.552.55

2.52.5

2.452.45

2.42.4

2.352.35

2.32.3

2.252.25

Stress Range, 40 – 80 kPaStress Range, 40 – 80 kPa

ttpp

Elastic Properties of SoilElastic Properties of Soil

Young’s Modulus, EYoung’s Modulus, Ess Typical values, 20 – 2000 tsfTypical values, 20 – 2000 tsfPoisson’s Ratio, Poisson’s Ratio, Typical values, 0.2 – 0.5Typical values, 0.2 – 0.5Shear Modulus, GShear Modulus, G Typical values, ETypical values, Ess / [2 (1 + / [2 (1 + )])]Determination by correlation to Determination by correlation to N1N16060 or s or suu, or in-situ tests, or in-situ tests

Rock PropertiesRock PropertiesLaboratory testing is for small intact Laboratory testing is for small intact rock specimens rock specimens Rock mass is too large to be tested Rock mass is too large to be tested in lab or fieldin lab or fieldRock mass properties are obtained Rock mass properties are obtained by correlating intact rock to large-by correlating intact rock to large-scale rock mass behavior – failures scale rock mass behavior – failures in tunnels and mine slopesin tunnels and mine slopesRequires geologic expertiseRequires geologic expertise

Intact Rock StrengthIntact Rock Strength

Point Load TestPoint Load Test

Unconfined Compression, qUnconfined Compression, quu

Typical ValuesTypical Valuesqquu = 1500 - 50000 psi = 1500 - 50000 psi

Rock QualityRock QualityLe

ngth

, LLe

ngth

, L

0.8 ft0.8 ft

0.7 ft0.7 ft

0.8 ft0.8 ft

0.6 ft0.6 ft

0.2 ft0.2 ft0.7 ft0.7 ft

SoundSound

Not soundNot sound, highly weathered, highly weathered

Not soundNot sound, centerline pieces < 4 , centerline pieces < 4 inches, highly weatheredinches, highly weathered

SoundSound

Not soundNot soundSoundSound

Core RunCore RunTotal = 4 Total = 4 ftft

CRCR = 95% = 95% RQD RQD = 53%= 53%

CSIR Rock Mass Rating CSIR Rock Mass Rating SystemSystem

This system is based on qThis system is based on quu, RQD, , RQD, joint spacing, joint condition and joint spacing, joint condition and water condition.water condition.

Rock Mass StrengthRock Mass Strength

CC11’’

Shea

r st

ress

, Sh

ear

stre

ss,

Effective Normal Stress, Effective Normal Stress, ’’tmtm 33 11

’’ii

= (cot = (cot ’’ii – cos – cos ’’ii)mq)mquu/8/8’’ii = tan = tan-1-1(4 h cos(4 h cos22[30+0.33sin[30+0.33sin-1-1(h(h-3/2-3/2)]-1))]-1)-1/2-1/2

h = 1 + 16(mh = 1 + 16(m’’nn+sq+squu)/(3m)/(3m22qquu))

Intact Rock Deformation, EIntact Rock Deformation, Eii

Typical values range from 1000 to Typical values range from 1000 to 13000 ksi13000 ksi

Poisson’s Ratio, Poisson’s Ratio, Typical values range from 0.1 to 0.3Typical values range from 0.1 to 0.3

Rock Mass DeformationRock Mass Deformation

EE = 2 RMR - 100 = 2 RMR - 100

40

10RMR

M 10145,000E

9090

7070

5050

3030

1010

In s

ituIn

situ

mod

ulus

of d

efor

mat

ion,

E m

odul

us o

f def

orm

atio

n, E

MM (GP

a) (G

Pa)

1010 3030 5050 7070 9090

1212

1010

88

66

44

22

(psi x 10(psi x 1066))

Rock mass rating Rock mass rating RMRRMR

GEC 5GEC 5FHWA-IF-02-034FHWA-IF-02-034

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

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www.fhwa.dot.gov/bridgewww.fhwa.dot.gov/bridge

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