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help show what procedures and documentation are recommended to result in successful
construction of a subgrade. Various methods of subgrade enhancement are presented in Section
4.5.; Enhancement of in-place soils using proper design of drainage and good compaction,
modification using lime, bituminous materials and chlorides, stabilization using fly ash., and use
of geosynthetics for separation and reinforcement. General design considerations along with
factors affecting of geosynthetic lifespan are also presented.
Substitution using various higher quality granular and lightweight materials is presented
in Section 4.5.6. The granular materials are Select Granular and Breaker Run Limestone. Design
and construction procedures along with specifications are presented. Design and construction of
lightweight fills using Wood Chips, Shredded Tires and Geofoam are also covered.
Summaries using each of the materials and procedures are presented for design and
construction control. Specifications for materials and procedures to use in Minnesota along with
contacts for further information are presented.
Based on a review of the literature, questionnaires and interviews with Mn/DOT and
county engineers and review of specific projects recommendations are made for when and how
the various procedures should be used. The parameters used for the recommendations are “Grade
above Water Table” and “Moisture Conditions”. There are essentially no conditions
recommended for soil enhancement for granular soils. Methods of Modification, Stabilization,
Separation and Reinforcement are recommended for various conditions in the tables.
Table 4.17 lists the conditions including “Thickness of Peat” for which the various
lightweight fills are recommended.
A database has been developed to document installations using the procedures listed.
Projects were located during visits to the cities and counties during the summer, 2002. Sixty five
projects have been identified. It recommended that the projects identified be reviewed about
every three years and the location and parameters for additional projects be added to the
database. In this way actual performance of the various methods of subgrade enhancement can
A subsequent study will look at the various methods of modification, stabilization and
reinforcement as they can be used with the MnPAVE mechanistic-empirical design procedure.
The methods of evaluating the various layers of a pavement section are presented in
Chapter 5. The materials discussed are Select Granular, Granular Subbases and Bases,
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Salvaged/Recycled Aggregates and Hot-Mix Asphalt Mixtures. The specifications used to define
and construct these materials are MnDOT 3149, 3138, 2360/2350 respectively (9). The design
parameters, which are recommended for each of the materials for each thickness design
procedure, are presented.
Field control procedures needed to meet the specifications are also presented in Chapter 5.
The Inspector’s Job Guide for Construction (11) sections for base and HMA construction are
summarized to present items that will help field personnel to give them checklists to properly
construct the pavement layers. Again, in order to realize the performance predicted by the
respective design procedures both in terms of strength (stiffness) and durability the specifications
must be followed carefully.
The remainder of Chapter 1 is a summary of Chapters 2, 3, 4, and 5. The chapters cover the
following items: Chapter 2, the three design procedures, Chapter 3, the Traffic Factors
definitions and determination, Chapter 4, Subgrade Design and Construction, and Chapter 5,
Pavement Layer Design and Construction.
1.2. Minnesota Thickness Design
1.2.1. Soil Factor Design Procedure
The Soil Factor Design is shown in Figure 2.1. It is published in the MnDOT State Aid
Manual (4). The chart uses seven categories of traffic based on the projected 20-year two-
way Annual Average Daily Traffic (AADT) and Heavy Commercial Daily Traffic
(HCADT). The procedures for predicting AADT and HCADT are presented in Sections 3.2
and 3.3. General flow maps are available for the entire state; however, it is recommended that
a District Traffic Engineer or the Office of Transportation Data and Analysis be contacted to
make the 20-year design predictions. These values will be dependent on future development
planned for the area.
The soil is defined using the soil factor, which is dependent on the AASHTO
Classification of the material represented on the particular project. Section 4.2 reviews
methods for determining the appropriate soil that represents the embankment conditions on
the project. The soil classification system is presented in Section 18.104.22.168. and the relationship
between the soil class and soil factor is given in Section 22.214.171.124.
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The thickness for the Soil Factor design is given in terms of the Granular Equivalent
defined in Section 126.96.36.199. Granular Equivalency factors are assigned to materials based on
the specification that they pass. For instance a Specification 3139 class 5 or 6 material has an
equivalency factor of 1.0. A Class 4 material has a factor of 0.75 because it has a less
restrictive gradation band. The relevant specifications for the other pavement materials are
listed in Figure 2.1. Minimum bituminous and total granular equivalent are also shown for
each traffic category. The thicknesses shown in Figure 2.1 represent a reduction in subbase
thickness for granular type soils (soil factor less than 100%) and an increase in thickness for
soil factors greater than 100% (heavy clay and some silty soils).
The soil factor recommended thicknesses have changed somewhat throughout the years
because of changes in traffic levels and construction procedures.
The construction specifications and procedures presented in Chapters 4 and 5 for the soil
and pavement section materials respectively must be followed to realize the design life
predicted by the design procedures.
1.2.2. R-Value Procedure
Figure 2.2 is the R-Value design chart currently used by MnDOT for design of HMA
pavement sections. The chart is in Reference 5. The embankment soil R-Value is determined
by a standard laboratory test procedure that is run in the MnDOT Maplewood Laboratory.
The procedure is outlined and discussed in Section 188.8.131.52.
The R-Value can also be predicted from the AASHTO Classification of the soil as shown
in Table 4.5, which is in Section 184.108.40.206.
The traffic for the R-Value procedure is defined in terms of Equivalent 80-kN (18,000-lb)
axle loads (ESALs). ESALs represent the effect of various axle loads and configurations on
the performance of a pavement. Methods for estimating ESALs for a given location are
presented in Section 3.4. ESALs are calculated from the total traffic predicted in a design
lane (Section 3.4.1), the vehicle type distribution (Section 3.4.2.) and the average effect of
each vehicle type in terms of ESALs per passage of that vehicle (Section 3.4.3.). Methods of
taking into account predicted growth are given in Section 3.4.4. A spreadsheet to make the
calculations is presented in Section 3.4.6.
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The thickness for the R-Value procedure is given in terms of Granular Equivalent
thickness using the same concepts as for the Soil Factor Design. The G.E. factors are listed in
The three thicknesses obtained from Figure 2.2 are the total G.E., the bituminous plus
base thickness G.E. and the minimum bituminous G.E.
An alternate R-Value Design in terms of full depth HMA is presented in Figure 5-3.7 of
Reference 5. MnDOT no longer uses this “full depth” design chart unless a 1-m (30-in.) layer
of select granular material is used under the surface layer. Some cities and counties use full
depth design where there is limited vertical clearance or there is a severe aggregate shortage.
If this procedure is used for design it is very important that the subgrade be compacted well
and uniformly to adequately support construction equipment and the design traffic for the
1.2.3. MnPAVE Procedure
The Beta Version 5.009 of MnPAVE is now available (6). MnPAVE is a mechanistic-
empirical based procedure, which uses relationships from MnROAD to predict the
performance of a pavement. Elastic layer theory is used to calculate the critical strains in the
system, which are correlated with fatigue cracking and development of rutting. In order to
calculate strains, the resilient modulus of each layer including the subgrade must be
determined and used along with the thicknesses of the pavement layers. The design then
involves the determination of the thickness required to keep the strain low enough to
withstand the calculated repetitions.
MnPAVE is set up so that the year can be divided into five seasons defined in Section
220.127.116.11. These can be adjusted for special situations. This makes MnPAVE much more
versatile than the others.
Currently, MnPAVE uses ESALs as input for traffic. The ESALs are calculated using the
procedure presented in Section 3.4 just as for the R-Value procedure. For the mechanistic
calculations the traffic is defined using Load Spectra, which represents the distribution of
loads on various axle configurations.
The subgrade is defined using the Resilient Modulus (M
) as it is predicted to vary
throughout the year. The resilient modulus can be determined in the laboratory with a
repeated load triaxial test using the test conditions given in Section 18.104.22.168. However,
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laboratory triaxial testing has only been performed on a limited number Minnesota soils. The
correlations given in Table 4.5 should be used to estimate the resilient modulus either from
the R-Value or the AASHTO Classification. These correlations result in five moduli
representing the five seasons defined at MnROAD.
The resilient moduli of the pavement layers are determined based on the specifications
that the granular material or mixture passes. The moduli listed in Table 5.2 in Section 5.3.3.
were measured from in-place testing at MnROAD. The high values for each layer in the
winter represent frozen conditions and the other moduli represent the variations measured
with the Falling Weight Deflectometer (FWD).
Section 2.4 summarizes the draft of an operating manual being developed for MnPAVE
(6). The manual includes the Setup, Startup, Input and Output for the software. The results
will give the operator the predicted life based on the design parameters assumed for a given
1.2.4. Procedure(s) to Use in 2001-03?
The three design procedures available in Minnesota have been summarized in Chapter 2.
More complete descriptions of Soil Factor and R-Value procedures are given in References 4
and 5 respectively. These procedures have been used around Minnesota for the past 25 plus
years on roads with all levels of traffic. The MnPAVE software is now being developed (6).
The MnPAVE program makes it possible to account for many factors that could not be
directly considered previously. The potential for improved design with MnPAVE is very
great. However, it needs to be used for various design situations to develop confidence in the
performance prediction equations. Designs with different types of materials such as stabilized
or reinforced subgrades or bases should be tried to see what is predicted from MnPAVE
compared to performance observed in the field. When new procedures or materials are used
the resulting pavement section should be simulated with the MnPAVE model.
It is recommended that if a pavement is being designed with either the Soil Factor or R-
Value procedures that a corresponding design be done with MnPAVE. A comparison
between the two designs should be made. We ask that the Minnesota Road and Research
Section be informed of the results of these comparisons. A form summarizing the
comparisons of the designs should be completed so that the experience with MnPAVE
relative to the current designs can be documented.
Documents you may be interested
Documents you may be interested