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
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).
* 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 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.”
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 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 SubCommitteee 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
L
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
higiQi ≤ Rr = fRn
hi
gi
Qi
Rr
f
Rn
=
=
=
=
=
=
Load modifier (eta)
Load factor (gamma)
Force effect
Factored resistance
Resistance factor (phi)
Nominal resistance
higiQi ≤ Rr = fRn
Qn
Rn
Probability of
Occurrence
f(g,f)
h g Qn
Q
f Rn
Q or R
R
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
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
Undrained Strength of Cohesive
Soils, su
Vane Shear Test
f=0
su
s
Unconfined Compression
su = qu/2
qu
Typical Values
su = 250 - 4000 psf
Drained Strength of Cohesive
Soils, c’ and f’f
Triaxial Compression
CU Test
Typical Values
c’ = 100 - 500 psf
f’f = 20o - 35o
(modified after Bowles, 1977)
N160
ff
<4
25-30
4
27-32
10
30-35
30
35-40
50
38-43
For N160 = 10, select f’f = 30o
Settlement (in)
Soil Deformation
0
-2
-4
-6
-8
-10
-12
Initial elastic settlement (all soils)
1
10
Primary consolidation
100
Time (days)
1000
10000
Secondary consolidation
Fine-grained (cohesive) soils
Consolidation Properties
Void Ratio (e)
eo
1
sp’ = Preconsolidation
Stress
Cr
Cc
Cs
0.5
0.1
1
10
Log10 sv’
100
Stress Range, 40 – 80 kPa
2.65
2.6
Void ratio (e)
2.55
One log cycle
De=Ca=0.06
2.5
2.45
2.4
2.35
2.3
tp
2.25
0.1
1
10
100
Elapsed Time (min)
1000
10000
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
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
Unconfined Compression, qu
Point Load Test
Typical Values
qu = 1500 - 50000 psi
Length, L
Rock Quality
0.8 ft
Sound
0.7 ft
Not sound, highly weathered
0.8 ft
Not sound, centerline pieces < 4 inches,
highly weathered
0.6 ft
Sound
0.2 ft
Not sound
0.7 ft
Sound
CR = 95%
Core Run
Total = 4 ft
RQD = 53%
CSIR Rock Mass Rating System
This system is based on qu, RQD,
joint spacing, joint condition and
water condition.
Shear stress, t
Rock Mass Strength
C1’
stm
f’i
t
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)
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
In situ modulus of deformation, EM (GPa)
Rock Mass Deformation
90
EM  145,00010
(psi x 106)
RMR10 
40
12
70
10
8
50
30
6
Ea = 2 RMR - 100
4
2
10
10
30
50
70
Rock mass rating RMR
90
GEC 5
FHWA-IF-02-034
Jerry A. DiMaggio P. E.
Principal Bridge Engineer
TEL: (202) 366-1569
FAX: (202) 366-3077
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town! www.fhwa.dot.gov/bridge
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