v Specification BackgroundNow!

AASHTO LRFD: Structural Foundations and Earth Retaining StructuresWhats Happening

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Limit States, Soil and Rock Properties Deep Foundations Shallow Foundations Earth Retaining StructuresJerry DiMaggio, P. E., Principal Bridge Engineer (Geotechnical) Federal Highway Administration Office of Bridge Technology Washington D.C.

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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)

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).

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). v * 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 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).

LRFD IMPLEMENTATION STATUSGeotechnically, 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.

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 agencys 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 AASHTOs 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 AASHTOs Bridge and Structures SubCommitteee in June 2005. The revised Proposed Specification is used in the NHI LRFD Substructure course which currently available.

Principles of Limit State Designs

Fundamentals of LRFD

* 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 StatesL I S T

* Strength Limit State * Extreme Event Limit State * Service Limit State * Fatigue Limit State

Strength Limit State

Extreme Event Limit State

Service Limit State

Service Limit State

Rn / FS Q i iQi Rr = Rni Load modifier (eta) = i Load factor (gamma) = Qi Force effect = Rr Factored resistance = Resistance factor (phi) = Nominal resistance R =n

i iQi Rr = RnQnf( , )

Rn

Probability of Occurrence

Q

Qn Rn

R

Q or R

Subsurface Materials* * * * Soil Rock Water Organics

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

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, qu = 50-1500 psi * Hard rock, qu > 1500 psi

* Rock mass properties

Gravel321 3/4 3/8 4 6 10 100

Sand20 40 60 100200

Silt

Clay

US Standard Sieves

% Finer by Weight % Finer by Weight

80

60

40

20

Uniform Well Graded

0

100

10 1 0.1 Grain Diameter (mm) Grain Diameter (mm)

0.01

0.001

Atterberg LimitsThe water content at which a soil changes state PI = LL - PLSolid SemiSolid Plastic Liquid

PI SL PL LL Increasing water content

Effective Stress Spring Analogy = u = effective stress = total stress * u = pore pressure

P

X

u

Soil Shear Strength n r a r c n Strength envelope f

a

= c + n tan f

Undrained Strength of Cohesive Soils, suVane Shear Test

su Unconfined Compression su = qu/2

=0 quTypical Values su = 250 - 4000 psf

Drained Strength of Cohesive Soils, c and f

Triaxial Compression CU Test

Typical Values c = 100 - 500 psf f = 20o - 35o

Drained Strength of Cohesionless Soils, fq c=0Friction angle is correlated to SPT results. Typical Values f = 25o - 45o

f

Standard Penetration Test (SPT)

For N160 = 10, select f = 30o

Soil DeformationSettlement (in)0 -2 -4 -6 -8 -10 -12 1

Initial elastic settlement (all soils)

10

100

1000

10000

Time (days)

Primary consolidation

Secondary consolidatio

Fine-grained (cohesive) soils

Consolidation Propertieseo 1 p = Preconsolidation Stress Cc Cs 0.5 0.1 1 Log10 10 v 100

Void Ratio (e)

Cr

Stress Range, 40 80 kPa2.65 2.6 2.55

Void ratio (e)

2.5 2.45 2.4 2.35 2.3 2.25 0.1 1 10

One log cycle e=C=0.06

tp100 1000 10000

Elapsed Time (min)

Typical Consolidation Properties

Elastic Properties of SoilYoungs Modulus, Esn

Poissons Ratio, n n

Typical values, 20 2000 tsf Typical values, 0.2 0.5 Typical values, Es / [2 (1 + )]

Shear Modulus, G

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

Rock PropertiesLaboratory 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

Unconfined Compression, q Point Load Test Typical Values qu = 1500 - 50000

Rock Quality0.8 ft Sound Not sound, highly weathered Not sound, centerline pieces < 4 inches, highly weathered Sound Not sound Sound

Length, L

0.7 ft 0.8 ft

0.6 ft 0.2 ft 0.7 ft

Core Run Total = 4 ft

CR = 95% RQD = 53%

CSIR Rock Mass Rating SystemThis system is based on qu, RQD, joint spacing, joint condition and water condition.

Shear stress,

Rock Mass Strength i 3 1

C1 tm

Effective Normal Stress, i = tan-1 (4 h cos2[30+0.33sin-1 (h-3/2 )]-1)-1/2 = (cot i cos i)mqu/8 h = 1 + 16(m n+squ)/(3m2qu)

Rock-Mass Quality and Material ConstantsValues of the parameters m and s are determined based on empirical correlation to rock type and RMR

Intact Rock Deformation, EiTypical values range from 1000 to 13000 ksi

Poissons Ratio, Typical values range from 0.1 to 0.3

Rock Mass DeformationIn situ modulus of deformation, EM (GPa)90 70 50 30 10 10 30 50 70 90

EM = 145,000 10

( RMR10)40

(psi x 106) 12 10 8 6

E = 2 RMR - 100

4 2

Rock mass rating RMR

GEC 5 FHWA-IF-02-034

Jerry A. DiMaggio P. E. Principal Bridge Engineer TEL: (202) 366-1569 FAX: (202) 366-3077The best Geotechnical web site in town! www.fhwa.dot.gov/bridge

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