Design Pamphlet for the Determination of Design Subgrade in Support of the 1993 AASHTO Guide ...

33 Pages·2000·0.2 MB·English

Design Pamphlet
for the Determination of
Design Subgrade
in Support of the 1993 AASHTO
Guide for the Design of
Pavement Structures
PUBLICATION NO. FHWA-RD-97-083 SEPTEMBER 1997
U.S. Department of Transportation
Federal Highway Administration
Research and Development
Turner-Fairbank Highway Research Center
6300 Georgetown Pike
McLean, VA 22101-2296
FOREWORD
A key challenge faced by engineers using the 1993, AASHTO Guide for Design of Pavement
Structures (AASHTO Guide) is the selection of appropriate design values for the subgrade soil
and for the pavement materials. Until now, the information available to help engineers choose
appropriate values has been incomplete. This design pamphlet addresses this problem by
presenting procedures to characterize the subgrade soil. Two companion pamphlets — Design
Pamphlet for the Determination of Layered Elastic Moduli in Support of the 1993 AASHTO
Guide for the Design of Pavement Structures (FHWA-RD-97-077) and Design Pamphlet for the
Backcalculation of Pavement Layer Moduli in Support of the 1993 AASHTO Guide for Design
of Pavement Structures (FHWA-RD-97-076) — p rovide additional, related guidance on selecting
appropriate design values to characterize the pavement materials and interpretation of pavement
deflection data. The procedures presented were developed through the analysis of the Long-
Term Pavement Performance (LTPP) data, documented in the report Analyses Relating to
Pavement Material Characterization and Their Effects on Pavement Performance
(FHWA-RD-97-085).
Application of the procedures and guidelines developed through this analysis will facilitate and
improve application of the AASHTO Guide flexible pavement design procedures. Their use will
provide: (1) improved designs, (2) more realistic estimates of pavement performance, and (3)
more consistent use of the AASHTO design parameters. Furthermore, although the procedures
are specifically developed for use with the 1993 AASHTO Guide, their use will give agencies a
"leg up" on implementation of the design procedures being developed for inclusion in the 2002
AASHTO Guide for Design of New and Rehabilitated Pavement Structures. Thus, this pamphlet
and its companions are critically important to anyone who designs flexible pavements.
Charles J. Nemmers, P.E.
Director
Office of Engineering
Research and Development
NOTICE
This document is disseminated under the sponsorship of the Department of Transportation in the
interest of information exchange. The United States Government assumes no liability for its
contents or use thereof. This report does not constitute a standard, specification, or regulation.
The United States Government does not endorse products or manufacturers. Trademarks or
manufacturers' names appear herein only because they are considered essential to the object of
this document.
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TABLE OF CONTENTS
PAGE NO.
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1
SUBSURFACE CHARACTERIZATION FOR PAVEMENT DESIGN . . . . . . . . . . . . . . . . .. 1
Subsurface Exploration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1
Boring Location and Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4
Sample Recovery for Identification/Classification and Testing of Soils . . . . . . . . . . . . .. 6
SELECTION OF RESILIENT MODULUS TEST SPECIMENS . . . . . . . . . . . . . . . . . . . . . .. 7
Number of Test Specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7
Condition of Test Specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 8
LABORATORY RESILIENT MODULUS TESTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 8
Constitutive Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 8
Test Procedures and Analysis of Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 10
DETERMINATION OF INSITU RESILIENT MODULUS . . . . . . . . . . . . . . . . . . . . . . . . .. 12
DETERMINATION OF DESIGN RESILIENT MODULUS . . . . . . . . . . . . . . . . . . . . . . . .. 15
Seasonal Variation Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 15
Effective Roadbed Resilient Modulus - Serviceability Criteria . . . . . . . . . . . . . . . . . . .. 16
Design Subgrade Resilient Modulus - Subgrade Rutting Criteria . . . . . . . . . . . . . . . .. 17
EXAMPLE PROBLEM AND SUMMARY OF PROCEDURE TO DETERMINE THE DESIGN
RESILIENT MODULUS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 19
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 27
ii
LIST OF FIGURES
Figure Page
1. Example of repeated load triaxial resilient modulus test results of a clay soil . . . . . . . .. 11
2. Chart for estimating effective roadbed soil resilient modulus for flexible pavements
designed using the serviceability criteria (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 18
LIST OF TABLES
Table Page
1. Summary of soil characteristics for supporting pavement structures . . . . . . . . . . . . . . .. 3
2. Summary of repeated load triaxial compression test results for the example
problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20
iii
LIST OF ABBREVIATIONS AND SYMBOLS
AASHTO - American Association of State Highway and Transportation Officials
ASTM - American Society for Testing and Materials
FWD - Falling Weight Deflectometer
GPS - General Pavement Studies
LTPP - Long Term Pavement Performance
SHRP - Strategic Highway Research Program
C - Cohesion of the soil
D - Thickness of the pavement structure and stabilized sub grade layer, if
p
present
D - Depth into the subgrade
s
E - Modulus of elasticity backcalculated from deflection basin measurements
j - Number of seasons
K K , K , K - Nonlinear elastic constants and coefficients of the constitutive equation
1, 2 3 5
and determined by the use of linear regression techniques.
k - At-rest earth pressure coefficient
0
k - Passive earth pressure coefficient
p
M - Resilient modulus of the subgrade or roadbed soil
R
N - Allowable number of load applications for a specific axle weight and
configuration and tire pressure
P - Atmospheric pressure
a
P - At-rest lateral earth pressure in the subgrade at a depth of D
o s
U - Damage factor based on a subgrade vertical compressive strain criteria
rs
u - Relative damage based on a serviceability design criteria
f
2 - Bulk stress
N - Angle of shearing resistance
( - Weighted average unit weight of the pavement structure and stabilized
p
subgrade, if present
( - Unit weight of the subgrade or roadbed soil
s
, - Vertical compressive strain at the top of the subgrade layer
v
F - Deviator stress
d
F - Confining pressure
3
F , F - Normal horizontal or lateral stresses
x y
F - Normal vertical stress
z
L - Poisson’s ratio
iv
DESIGN PAMPHLET FOR THE
DETERMINATION OF DESIGN SUBGRADE MODULI
IN SUPPORT OF THE 1993 AASHTO GUIDE FOR THE
DESIGN OF PAVEMENT STRUCTURES
INTRODUCTION
Resilient modulus is the primary material property that is used to characterize the roadbed soil
for the design of flexible pavements in the 1993 American Association of State Highway and
Transportation Officials (AASHTO) Guide for the Design of Pavement Structures.(1) This value
is simply a measure or estimate of the elastic property of the material at a given stress state (i.e.
assumed to be the modulus of elasticity). Section 2.3.1 of the 1993 Guide overviews the
procedure for determining the "Effective Roadbed Soil Resilient Modulus" that is needed for
flexible pavement design, but there are practical details that are not adequately addressed. The
purpose of this design pamphlet is to provide some additional details and recommendations for
determining the design resilient modulus for flexible pavement design (new construction or
reconstruction).
SUBSURFACE CHARACTERIZATION FOR PAVEMENT DESIGN
Subsurface Exploration
The subsurface investigation should be sufficiently detailed to define the depth, thickness, and
areal extent of all major soil and rock strata that will be affected by construction. Disturbed and
undisturbed samples of the subsurface materials must be obtained for laboratory analyses (and/or
tested in the field) to determine their engineering properties. The extent of the program depends
on the nature of both the project and the site specific subsurface conditions. The standard
penetration and dynamic cone penetrometer tests can be used to determine the insitu strength
characteristics of subsurface soils.
Procedures for the exploration of pavement sites cannot be reduced to a single guideline to fit all
existing conditions. To acquire reliable engineering data, each job site must be explored and
analyzed according to its subsurface conditions. The engineer in charge of the subsurface
exploration must furnish complete data in order that an impartial and thorough study of practical
1
pavement thickness designs can be made. Suggested steps which can be followed are listed
below:
1. Make a complete and thorough investigation of the topographic and subsurface
conditions.
2. Conduct exploratory borings at a spacing and depth prescribed by the engineer.
The spacing and depth of these borings are dependent on the variability of the
existing soil conditions, both vertically and horizontally. These borings should also
be used to determine the water table depth. Take sufficient and appropriate auger,
split tube, or undisturbed samples of all representative subsoil layers. Prepare
boring logs and soil profiles.
3. Classify all soils using the AASHTO (or Unified) soil classification system. Table
1 relates the Unified soil classification of a material to the relative value of a
material for use in a pavement structure. A moisture-density test should be used to
determine the compaction characteristics for soil and untreated pavement
materials. AASHTO T99 should be used for coarse-grained soils and aggregate
materials, and low plasticity fine-grained soils; whereas, AASHTO Tl80 should be
used for medium to high plasticity fine-grained soils. The degree of compaction
required for the in-place density should be expressed as a percentage of the
maximum density from the specified test procedure.
4. Examine the boring logs, soil profiles, and classification tests and select
representative soil layers for laboratory testing. Determine the insitu resilient
modulus for each major soil type encountered.
5. Use the soil profile along the roadway alignment to relate resilient modulus to each
type of subgrade soil encountered. Select a design subgrade resilient modulus that
is representative of each soil type. The designer may want to consider using
different layer thicknesses and/or pavement structures for each soil type.
2
Table 1. Summary of soil characteristics for supporting pavement structures.
Subgrade Strength
When Not Subject to Potential Frost Compressibility and Drainage
Major Divisions Name Frost Action Action Expansion Characteristics
Gravel GW Well-graded gravels or Excellent None to Very Almost None Excellent
and gravel-sand mixtures, little Slight
Gravity or no fines
Soils
GP Poorly graded gravels or Good to Excellent None to Very Almost None Excellent
gravel-sand mixtures little Slight
or no fines
*d Silty gravels, gravel-sand Good to Excellent Slight to Medium Very Slight Fair to Poor
GM - - silt mixtures
u Good Slight to Medium Slight Poor to Practically
Impervious
CC Clayey gravels, Good Slight to Medium Slight Poor to Practically
gravel-sand-clay mixture Impervious
Sand SW Well-graded sands or Good None to Very Almost None Excellent
and gravelly sands, little or no Slight
Sandy fines
Soils
SP Poorly graded sands or Fair to Good None to Very Almost None Excellent
gravelly sands, little or not Slight
fines
*d Silty sands, sand-silt Fair to Good Slight to High Very Slight Fair to Poor
SM - - mixtures
u Fair Slight to High Slight to Medium Poor to Practically
Impervious
SC Clayey sands, sand-clay Poor to Fair Slight to High Slight to Medium Poor to Practically
mixtures Impervious
*The subdivision of the GM and SM groups is on the basis of Atterberg limits; a suffix d is used when the liquid limit is 25 or less and the plasticity index is 5
or less; the suffix u is used for all other cases.
Table 1. Summary of soil characteristics for supporting pavement structures (continued).
Subgrade Strength
When Not Subject Potential Frost Compressibility and Drainage
Major Divisions Name to Frost Action Action Expansion Characteristics
Silts and ML Inorganic silts and very fine Poor to Fair Medium to Very High Slight to Medium Fair to Poor
Clays LL sands, rock flour, silty or
is less clayey fine sand or clayey
than 50 silts with slight plasticity
CL Inorganic clays of low to Poor to Fair Medium to High Slight to Medium Practically Impervious
medium plasticity, gravelly
clays, sandy clays silty clays,
lean clays
OL Organic silts and organic Poor Medium to High Medium to High Poor
silt-clays or low plasticity
Silts and MH Inorganic silts, micaceous or Poor Medium to Very High High Fair to Poor
Clays LL diatomaceous fine sand or
is
silty soils, elastic silts
Greater
than 50 CH Inorganic clays of high Poor to Fair Medium to Very High High Practically Impervious
plasticity, fat clays
OH Organic clays of medium to Poor to Very Poor Medium High Practically Impervious
high plasticity, organic silts
Highly Pt Peal and other highly organic Not Suitable Slight Very High Fair to Poor
Organic soils
Soils