







































Abstract: Fluctuation in subgrade moisture content of road subgrades is known to reduce the quality and lifespan 
of the driving surface. This paper examines possible specific sources of moisture fluctuation in subgrade soils. 
Groundwater, precipitation, and seasonal cycles are all investigated to determine their influence on long term and 
continuous moisture fluctuations. While precipitation’s impact is minimal and probably only affects small continuous 
water variations, groundwater can greatly change subgrade moisture. The freezing and thawing experienced in cold 
climates appear to have the most destructive impact on driving surfaces. Minnesota offers an environment where 
all the possible factors discussed in this paper are abundant. Additionally, Minnesota’s soil makes it particularly 
vulnerable to subgrade moisture problems. Minnesota’s roads are in desperate need of renovation and developing a 
method to account for subgrade moisture variations could save time and money. Future testing is recommended to 
determine if a thicker road section would be worth the additional cost to protect roads against subgrade moisture 
variation.

Aisthesis      Volume 10,  201924

Moisture Content Fluctuation in Minnesota Road Subgrades

by Matthew Henderson

Introduction
Overview of the Problem
 There are several likely sources of moisture 
underneath roadways. Each potential source 
manifests as a possible design challenge for civil 
engineers because subgrade moisture is detrimental 
to the long-term performance of the driving surface. 
The degree to which a subgrade’s water content 
changes dictates the level of importance that design 
consideration should receive. However, there is some 
disagreement in the industry over which possible 
sources of subgrade moisture are important and which 
possible sources can be neglected. The importance of 
subgrade moisture design considerations will change 
depending upon the road’s location. In the state of 
Minnesota, the volatility of many potential subgrade 
water sources makes uncovering their importance 
in design a priority. If the source of fluctuation 
in subgrade moisture content can be determined, 
it could increase the life of Minnesota roads. 
Additionally, if a cost-effective solution to premature 
road deterioration could be found, time and money 
could be saved while Minnesota struggles to renovate 
its crumbling transportation network.

Background Information
 Underneath every road’s driving surface are 
several layers of soil, each with specific properties 
that contribute to the desired overall performance. 
The Minnesota Department of Transportation 
(MnDOT) Pavement Design Manual breaks these 
sheets of soil into three main categories: the aggregate 
base layer, the subbase layer, and the subgrade layer. 
Figure 1, taken from the pavement design manual, 
depicts the general structure of a road section [1]. 
 The aggregate base is the soil immediately 
underneath the asphalt or concrete driving surface. 
Typically, the aggregate base consists of class 5 or 
class 6 densely-graded gravels with large aggregates. 
Desired properties of the aggregate base include 



Moisture Content Fluctuation in Minnesota Road Subgrades

Aisthesis      Volume 10,  201925

a construction platform for paving, a filter layer, 
resistance to frost heave, and resistance to the effects 
of moisture. The same design manual describes a 
road subbase as the layer of granular, sand or gravel, 
material beneath the aggregate base. The subbase is 
usually made of granular or select granular material. 
Its primary purpose is to help resist frost heave and 
improve moisture properties [1]. The subgrade layer 
is the bottommost sheet of soil in a road section. The 
subgrade is the in situ material upon which the entire 
pavement structure rests [2]. Unlike the aggregate 
base and subbase, the subgrade is the existing natural 
soil, often compacted or treated with stabilizers. 
Although there is sometimes a tendency to only 
study pavement performance in terms of pavement 
mix design and road section, the subgrade can have 
great influence on how well a pavement performs 
[2].
 Because compacted soil in a road subgrade is 
often sensitive to moisture, moisture in the subgrade 
has long been considered important in pavement 
design [3]. Subgrade load bearing capacity and 
volume changes are important parameters that can 
be compromised by the presence of excess moisture 
in a soil [2]. Moisture content, sometimes referred 
to as water content, is one of the most common soil 
parameters. It is a measure of how much moisture 
is in soil and can be calculated as the ratio of weight 
of water to weight of soils in a sample [4]. Optimum 
moisture content (OMC) is the water content at 
which the soil can be compacted to its densest state 
[4]. Subgrade soil is compacted during construction 
at the OMC. However, the moisture content 
changes over time and eventually reaches a certain 
equilibrium moisture level. This new water content 
can be greater or less than the OMC [5].
 The moisture of a soil is directly related to 
the density of a soil. Density measurements are 
an effective way to measure how well a sample is 
compacted and therefore how well it will support 
loads. For a given amount of energy applied through 
compaction, the maximum dry unit weight of a 
soil can only be attained when the soil reaches its 
OMC. Figure 2 displays the common shape of soil 
compaction curves [6].
 From Figure 2, it can be easily seen that a 
change in moisture will affect the density of a soil. 
The American Association of State Highway and 

Transportation Officials (AASHTO) Guide for 
Design of Pavement Structures recommends that 
any set of specifications for compaction of roadbed 
soils should include a density requirement and soil 
density should be monitored during construction 
[7]. Compaction is necessary to improve a material’s 
engineering properties such as load-bearing capacity, 
stability, stiffness, volume change characteristics, 
resistance to settlement, and frost damage [1].

Effects of Subgrade Moisture Issues
 The moisture content and density of a subgrade 
are crucial factors in the lifetime performance of a 
road. Moisture underneath pavement sections can 
be measured to estimate the mechanistic properties 
of unbound soil layers, which affect pavement 
performances during the service period [5]. Excess 
subsurface water reduces the strength of the soil 
which may cause pavement cracking, rutting, 
deterioration of existing pavement cracks and joints, 
and frost heave [1]. 
 The detrimental effects of excess subgrade 
moisture are universal and well documented. Many 
studies have been performed to test the correlation 
between a soil’s strength and its water content. Lab 
tests have concluded that the resilient modulus of all 
classes of unsaturated granular materials decreased 
to some extent with an increase in moisture content 
[8]. However, the extent to which moisture can 
influence a soil depends on its classification. Edris 
and Lytton concluded that the dynamic properties 



Moisture Content Fluctuation in Minnesota Road Subgrades

Aisthesis      Volume 10,  201926

of fine-grained subgrade materials, silts and clays, 
depend strongly on factors that control moisture 
such as climate [9]. Bayomy and Salem agreed that 
the magnitude of the increase in resilient modulus 
in compacted subgrade materials depends on 
composition, the amount of silt and clay-sized 
particles in the soil, as well as the water content [8]. 
Hicks and Monismith concurred even in coarse-
grained gravels and sand, clean materials (e.g., well-
graded gravel, poorly-graded gravel, well-graded 
sand, poorly-graded sand) are not likely to exhibit 
the same degree of moisture sensitivity as a soil with 
more fines [10]. By definition, fines are soils that pass 
through a #200 sieve [4]. Clay and silt particles are 
often small enough to pass as fines while coarse soils 
like sand and gravel are usually larger. The problem 
of moisture prematurely deteriorating roadways in 
not uncommon. The AASHTO Guide for Design of 
Pavement Structures states that distress in pavements 
is often caused or accelerated by the presence of 
moisture in the pavement section [7].
 Since the numerous problems associated 
with subgrade moisture are so well researched, it 
would be reasonable to assume that it is a primary 
consideration during road section design. However, 
this is not always the case. Bae et al. claim that even 
though it has been agreed that environmental factors 
such as moisture content in the subgrade greatly 
affect pavement performance, the quantitative 
effects on pavement performance have not been well 
documented [3]. The lack of quantitative knowledge 
on this topic is evident in the road design literature 
used in the industry. The MnDOT Pavement Design 
Manual states, “Any construction beneath the 
aggregate base and subbase is at the discretion of the 
District Materials/Soil Engineer” [1]. The AASHTO 
Guide for Design of Pavement Structures also offers 
an ambiguous stance on the topic. It suggests design 
thickness for pavement thickness and aggregate base 
depth but does not specify how the design thickness 
of each layer should depend on subgrade conditions. 
In fact, the guide suggests that local knowledge and 
experience should take precedence over the outlined 
requirements [7]. The vague and imprecise language 
surrounding the importance of subgrade moisture 
can be a source of disagreement on a construction 
site which can cause delays in the building process.

 If factors that affect subgrade moisture were 
better understood, they could be considered in the 
design process and subsequently help engineer better 
roads. Currently, during subgrade construction, 
soil is compacted at OMC to approximately the 
maximum dry density [5]. However, questions 
abound as to whether that standard process is 
enough to ensure moisture issues will not arise. As 
Bayomy and Salem remind us, it is well known that 
certain environmental changes accelerate pavement 
deterioration [8]. If these specific environmental 
changes could be quantitatively accounted for, 
roads could last longer and perform better over 
their lifetime. This research would be particularly 
important in Minnesota, given that the American 
Society of Civil Engineers recently gave its roads 
a D+ grade [11]. In this paper, several important 
factors that could influence subgrade moisture will 
be addressed to quantify how each one contributes 
to driving surface failure in Minnesota.

Factors That Influence Subgrade Moisture Content
Deviation in Moisture from OMC to Equilibrium 
Moisture
 After a road has been built, the moisture in the 
subgrade does not remain at the OMC that it was 
compacted at. On the contrary, the water content 
will migrate towards equilibrium moisture [3], [5], 
[8], [12]. In a previous study by Bae and Stoffels, 
equilibrium moisture content (EMC) varied from 20 
percent wetter to 15 percent dryer than the soil’s OMC 
[5]. Bayomy and Salem concluded that although 
fluctuations in subgrade moisture continue over 
time, the EMC is usually reached within five years 
of construction [8]. The subsequent water variations 
occur on a smaller scale. While some argue continual 
moisture changes only fluctuate by 1 percent, others 
claim fluctuations can vary up to 10 percent [5], 
[8]. It is possible that any change in moisture in the 
subgrade could have consequences for the road. 
These two different categories of moisture variation, 
the initial change from OMC to EMC and the 
smaller continuous changes that last indefinitely, will 
both be examined in this paper. Unfortunately, no 
comprehensive consensus currently exists regarding 
how much moisture content variation is enough to 
compromise the road’s condition. Groundwater, 
precipitation, and seasonal cycles are three of the 
possible causes of moisture fluctuation.



Moisture Content Fluctuation in Minnesota Road Subgrades

Aisthesis      Volume 10,  201927

Groundwater
 Groundwater level has long been considered a 
major factor in determining subgrade moisture [5]. 
Water can easily enter a road section as groundwater 
from an interrupted aquifer, high water table, or a 
localized spring [7]. However, groundwater can 
affect soil that is not in direct contact with the 
moisture through capillarity. Capillarity is the 
upward movement of water in soil due to surface 
tension and it can allow groundwater to rise well 
above the groundwater table [4]. To account for this, 
there must be a safe distance from the top of a road 
subgrade to the water table. Studies conducted by 
Russam as well as Yoder and Witczak reported that 
the effects of the groundwater table on soil saturation 
are significant when the water table is roughly less 
than 6 m below the top of the subgrade [13], [14]. 
Since groundwater levels remain relatively stable 
over time, it is likely that groundwater contributes 
minimally to the continuous moisture fluctuation 
that go on indefinitely in subgrades. However, there 
is general agreement that groundwater could be 
a key source that directs a subgrade’s change from 
OMC to EMC after construction.

Precipitation
 In addition to groundwater, precipitation is 
frequently viewed as a potential source of moisture 
for subgrade soils [5]. While precipitation seems like 
an obvious contributor to water fluctuation, there are 
many studies that argue the contrary. Many studies 
have found the correlation between precipitation and 
subgrade moisture to be minimal or non-existent 
[12], [15], [16], [17]. One study by Rainwater et 
al. interestingly found that precipitation affects 
the unbound aggregate base layer, but the excess 
moisture dissipated before reaching the subgrade 
level [17]. Another experiment found that when 
average rainfall increased, the moisture content also 
rose but when the rainfall dropped, the moisture 
did not drop suddenly. These findings point more 
towards seasonal factors than towards precipitation’s 
influence [8]. Since the bottom of a road section is 
deep beneath the ground, it makes sense that factors 
like groundwater would have more of an effect on 
subgrade water content [5]. If precipitation did 
influence moisture content, it is likely that it would 
only affect the smaller continuous fluctuations and 
not the trend towards an EMC.

Seasonal Cycles
 Like groundwater and precipitation, seasonal 
conditions have been studied as possible influencers 
of subgrade conditions. Studies have been able to 
make many connections between seasonal cycles 
and subgrade moisture content. The most significant 
factors are the freeze/thaw cycles that can affect 
soil. According to Bayomy and Salem, the freezing 
of soil moisture can transform a soft subgrade into 
a rigid material while thawing the material can 
produce a softening effect such that for some time 
after thawing, the material has a resilient modulus 
that is only a fraction of its pre-freezing value [8]. 
This point is reinforced in a study by Bae et al. that 
concluded the moisture value decreased when frost 
action occurred in winter while nonfreezing sites 
showed less seasonal variation [3]. When spring 
thaw occurs, moisture moves back into the subgrade 
and can cause failure. In a study, White and Coree 
discovered that 60% of AASHTO road test failures 
occurred during the spring season [18]. Bayomy 
and Salem looked into the specific issues caused 
by spring thawing and found that structural ruts 
in roads were the result of permanent deformation 
occurring in the section layers and subgrade. The 
study concluded these ruts evolve rapidly during 
spring when the pavement structure is weakened by 
excessive moisture released from thawing soils [8]. 
 Fine soils, while often experiencing less change 
in moisture due to lower hydraulic conductivity 
values, are more affected by freeze/thaw cycles. 
Frost susceptible soils have pore sizes that promote 
capillary rise which facilitates the growth of 
frost lenses. Therefore, the amount of fines in a 
subgrade is an important factor in determining its 
frost susceptibility [1]. One study by Bergen and 
Monismith focused on observing freezing and 
thawing in a clay subgrade to discern how fine soils 
handle the seasonal cycles. In the clay subgrades, 
they found the variation in resilient modulus of the 
clay after one freeze/thaw cycle to be 52 to 60 percent 
of the initial value [19]. From these studies, it can be 
determined that much of the fluctuation in subgrade 
moisture is attributed to seasonal changes. Bae et al. 
claim that seasonal variation of moisture can be the 
dominant factor in evaluating pavement roughness 
progression for sites that experience freezing [3]. 
Bayomy and Salem even claim that the combination 
of freezing/thawing temperatures in moist pavement 



Moisture Content Fluctuation in Minnesota Road Subgrades

Aisthesis      Volume 10,  201928

subgrade soils can cause more severe effects than 
the effects of any other water content change [8]. 
Seasonal cycles might contribute to the subgrade’s 
transition to EMC and it can be concluded that they 
are a driving force behind the continuous moisture 
fluctuations.

Subgrade Moisture Fluctuation Implications for 
Minnesota
Groundwater in Minnesota
 All of the aforementioned factors that can 
influence subgrade moisture should especially be 
considered in Minnesota. For starters, the “Land of 
10,000 Lakes” not only has access to an abundance 
of surface water, but also an adequate amount of 
groundwater. Figure 3 from the Minnesota Pollution 
Control Agency illustrates that groundwater is 
present in most of the state [20]. Additionally, for 
about half of the state, groundwater is either readily 
available or available near the surface from glacial 
sands.
 The typical depth of groundwater in Minnesota 
also makes it an important factor to consider. 
The MnDOT Pavement Design Manual calls for 
a minimum pavement section of 30 inches for 
roads with less than 7 million 20-year Bituminous 
Equivalent Single Axle Loads (BESALs) and 36 inches 
for roads with more than 7 million 20-year BESALs 

for new pavements [1]. A Minnesota Department 
of Natural Resources study found the water table 
elevation in the state is commonly within 10 to 30 feet 
of the land surface [21]. Figure 4 displays the water 
table elevation throughout Minnesota. In the figure, 
lower water table elevations are represented by dark 
blue colors while higher water table elevations are 
represented by orange. [21].

 While 10 to 30 feet is sufficiently deep to avoid 
penetrating groundwater with a road section, it is 
important to remember that capillary action can draw 
moisture up great distances in fine soils. Additionally, 
thicker road sections than the minimum 30 to 36 
inches bring the top of the subgrade even closer to 
the groundwater table.

Precipitation in Minnesota
 Minnesota experiences a wide spectrum 
precipitation across the state. The Minnesota 
Department of Natural Resources map shown 
in Figure 5 displays this variability in annual 
precipitation [22].
 This data confirms that there is an adequate 
amount of moisture precipitation to consider it 
a factor in road section design. Also, since annual 
precipitation varies greatly across the state, its 
importance in section design could also change 
depending on a road’s location.

Seasonal Cycles
 Minnesota experiences some of the most extreme 
seasonal cycles in the United States. Temperature 
range from an average low of about 0o F across 
the state in January to an average high of about 
80o F across the state in July [23]. These extreme 
temperature swings are accompanied by freezes that 



Moisture Content Fluctuation in Minnesota Road Subgrades

Aisthesis      Volume 10,  201929

penetrate deep underneath roads and intense thaws 
that produce abundant moisture. Figure 6 from the 
Minnesota Department of Transportation compares 
frost depth measurements underneath two paved 
roads on opposite ends of Minnesota [24]. 
 The data shows that frost penetrates deeper and 
lasts longer in northern Minnesota compared to 
Southern Minnesota. While effects of frost might be 
more severe in the northern portion of the state, the 
maximum depth of frost achieved in the 2017-2018 
winter in the southern part of the state easily surpasses 
the minimum depth requirements for subgrades 
outlined by the MnDOT Pavement Design Manual 
[1]. This is a real concern and indicates that seasonal 
variations should be considered in Minnesota road 
section design.

Typical Minnesota Subgrade Soils
 Minnesota’s topsoil classifications vary 
considerably throughout the state. From organic 
material ideal for farming to permeable sandy soils, 
there is a lot of variety. Some engineers use topsoil 
surveys to examine the type of soils that will be 
encountered during a road construction project 
if excavation does not exceed five feet. At that 
depth, the soil is fairly homogenous to the topsoil. 
However, the top of soil subgrades is recommended 
to be a minimum of 30” below the surface and 
thicker sections can be even deeper [1]. Because the 
subgrade layer carries the loads of the road section 
below a depth of five feet, understanding the soils 
encountered at deeper depths could be crucial to 
better understanding how moisture fluctuations 
correlate to road performance. 
 As part of this investigation, soil boring logs 
were examined for all 87 counties in Minnesota. 
Boring soil classifications were recorded at a depth 
of five feet at five locations for each county. The 
boring locations were selected at sites throughout 
each county to give a survey of the county’s soil. 
That representative data was then used to record a 
single generalized classification of soil at a depth of 
five feet for each county. It is important to note that 
soil conditions can vary greatly across a construction 
site and even more so across an area as large as a 
county. This study does not claim that a single soil 
classification is representative of an entire county 
but rather attempts to acquire a representative 
sample of soils encountered throughout the state. 
The soil boring data was collected from the MnDOT 
Electric Geotechnical Database [25]. Out of 87 
counties in Minnesota, only six had generalized soil 
classifications at five feet deep that did not categorize 
the material as peat, clay, silt, or loam. This means it 
is likely almost all of Minnesota’s roads are built on a 
soil that contains at least 10% fines. These widespread 
conclusions that implicate such a large area are 
made based on methods that involve imprecise 
qualitative assumptions. However, many counties 
have subgrade depth soils that contain much more 
than 10% fines so the argument for considering soil 
classification when designing a road section remains 
strong. As mentioned earlier, fines, such as clays and 
silts, retain moisture better than coarse soils and are 
more susceptible to the negative effects of moisture 
fluctuations. 



Moisture Content Fluctuation in Minnesota Road Subgrades

Aisthesis      Volume 10,  2019

Recommendations
 While research suggests that certain factors 
have been more influential than others on subgrade 
moisture changes, they should all be considered in 
road section design. The conditions in Minnesota 
make groundwater, precipitation, and seasonal cycles 
all relevant design considerations. Additionally, 
many of the soils found in Minnesota are conducive 
to causing moisture issues. One way the longevity and 
performance of Minnesota roads could be improved 
is by forcing these conditions to be considered by 
design engineers. The current language in highway 
design literature is vague and might not account for 
all the factors related to excessive moisture damage. 
Thicker road sections are a proven solution to deal 
with problematic subgrades [3], [7]. However, 
these improvements come with a cost. While the 
specific additional costs of building a more moisture 
resistant road depend on many factors such as the 
additional depth required and project location, any 
added costs must always be properly justified on 
public infrastructure projects. More research should 
be conducted to see if the added costs of reducing 
subgrade moisture related failures would be worth 
it. The relationship between subgrade moisture 
effects and road section thickness should be further 
researched in hopes of providing more answers to 
this question.

Conclusion
 Excessive subgrade moisture and subgrade 
moisture fluctuations are detrimental to the 
performance of a road’s driving surface. Cracking, 
rutting, swelling, and roadway deterioration are all 
results of reduced strength in a road section caused 
by subgrade moisture. There are several factors 
that can contribute to a soil’s change from OMC to 
EMC or to continuous moisture fluctuations that 
occur over the lifetime of a road. Groundwater level, 
precipitation, and seasonal freeze/thaw behavior are 
some of the most influential forces. Groundwater can 
cause changes from OMC to EMC and precipitation 
can influence continuous smaller changes in 
moisture content. Seasonal freezing and thawing are 
likely the biggest culprit in causing moisture related 
subgrade problems due to the extreme changes 
it produces each year. While groundwater level, 
precipitation, and seasonal variation are variables 

that should be considered throughout the United 
States, they are particularly relevant in the state of 
Minnesota. The state has many aquifers capable of 
influencing subgrades through capillary action. It 
also experiences significant annual precipitation 
and extreme seasonal variation. Additionally, the 
soils at subgrade depth throughout the state could 
promote the entrapment of moisture which amplifies 
the importance of considering how a subgrade could 
behave in the road section design process. Based on 
these findings, more research should be done to reveal 
if increasing the thickness of road sections would be 
an effective way to neutralize the problems associated 
with inevitable subgrade moisture fluctuations in 
Minnesota. Future research should examine possible 
solutions to help transportation and geotechnical 
engineers deal with the complex issue of subgrade 
moisture fluctuations in Minnesota.

Acknowledgements
 The analysis in this paper would not have been 
possible without the resources provided by several 
MnDOT soil engineers. I would like to thank Edward 
Welch, Kristi Olson, Scott Zeidler, Nathan Bausman, 
and Chris Dulian for their contributions.

References
[1] Minnesota Department of 

Transportation,  “Pavement Design Manual,” 
Minnesota Department of Transportation,  2018. 
[Online]. Available: https://www.dot.state.
mn.us/m aterials/pvmtdesign/docs/newmanu-
al/Pavement%20Design%20Manual.pdf. [Ac-
cessed: Jan. 24, 2019].

[2] “Subgrade,” Pavement Interactive. [Online]. 
Available: https://www.pavementinteractive.
org /reference-desk/design/design-parameters/
subgrade/. [Accessed: Jan. 29, 2019].

[3] A. Bae, S. M. Stoffels, C. E. Antle, and S. W. Lee, 
“Observed Evidence of Subgrade Moisture 
Influence on Pavement Longitudinal Profile,” 
Canadian Journal of Civil Engineering, vol. 35, 
no. 10, pp. 1050-1063, Oct. 2008. [Online]. 
Available: doi: 10.1139/L08-047. [Accessed: Jan. 
28, 2019].

30



Moisture Content Fluctuation in Minnesota Road Subgrades

Aisthesis      Volume 10,  2019

[4] D. P. Coduto, M. R. Yeung, and W. A. Kitch, 
Geotechnical Engineering: Principles and Practices, 
2nd ed. Upper Saddle River, NJ: Pearson, 2011.

[5] A. Bae and S. M. Stoffels, “Evaluation of Pavement 
Subgrade Long-term Equilibrium Moisture 
with Suction Potential,” KSCE Journal of Civil 
Engineering, vol. 23, no. 1, pp. 147-159, 30, Nov. 
2018. [Online]. Available: SpringerLink, https://
link.springer.com/arti cle/10.1007/s12205-018-
1227-8. [Accessed: Jan. 28, 2019].

[6] I. Basheer, “Empirical Modeling of the 
Compaction Curve of Cohesive Soils,” Canadian 
Geotechnical Journal, vol. 38, pp. 29-45, 2001. 
[Online]. Available: file:///C:/Users/User/ 
Downloads/Empirical_modeling_of_the_
compaction_curve_of_cohe.pdf. [Accessed: 
March 3, 2019].

[7] American Association of State Highway and 
Transportation Officials, “AASHTO Guide for 
Design of Pavement Structures,” 1993. [Online]. 
Available: https://habib00ugm.files.wor dpress.
com/2010/05/aashto1993.pdf. [Accessed: Jan. 
29, 2019].

[8] F. Bayomy and H. Salem, “Monitoring and 
Modeling Subgrade Soil Moisture for Pavement 
Design and Rehabilitation in Idaho Phase 
III: Data Collection and Analysis,” University 
of Idaho, Idaho Transportation Department, 
2005. [Online]. Available: https://www.webp 
ages.uidaho.edu/niatt/research/final_reports/
klk459_n04-16.pdf. [Accessed: Jan. 29, 2019].

[9] E. V. Edris and R. L. Lytton, “Climatic 
Materials Characterization of Fine-Grained 
Soils,” Transportation Research Record, pp. 39-
44, 1977. [Online]. Available: http://onlinepubs. 
trb.org/Onlinepubs/trr/1977/642/642-009.pdf. 
[Accessed: March 3, 2019].

[10] R. G. Hicks and C. L. Monismith, “Factors 
Influencing the Resilient Response of Granular 
Materials,” Highway Research Record, pp. 15-31, 
1971. [Online]. Available: http://online pubs.
trb.org/Onlinepubs/hrr/1971/345/345-002.pdf. 
[Accessed: March 3, 2019].

[11] Minnesota Section of the American Society 
of Civil Engineers, “Report Card for Minnesota’s 
Infrastructure 2018,” Infrastructure Report 
Card, American Society of Civil Engineers, 
2018. [Online]. Available: https://www.
infrastructurereportcard.org/wp-conte nt/
uploads/2016/10/Fu l lRep or t-MN2.p df . 
[Accessed: March 3, 2019].

[12] K. D. Hall and S. Rao, “Predicting Subgrade 
Moisture Content for Low-Volume Pavement 
Design Using In Situ Moisture Content Data,” 
Transportation Research Record, vol. 1652, 
no. 1, Jan. 1999. [Online]. Available: Sage 
Journals, https://journals.sagepub.com/d oi/
pdf/10.3141/1652-47. [Accessed: March 3, 2019].

[13] K. Russam, “Subgrade Moisture Studies by 
the British Road Research Laboratory,” Highway 
Research Record, no. 301, pp. 5-17, 1970. [Online]. 
Available: https://trid.trb.or g/view/127427. 
[Accessed: March 3, 2019].

[14] E. J. Yoder and M. W. Witczak, Principles 
of Pavement Design, 2nd ed. Hoboken, NJ: 
John Wiley & Sons Inc, 1975. [Online] 
Available: https://app.knovel.com/hotlink/toc/
id:kpPP DE000N/principles-pavement-design/
principles-pavement-design. [Accessed: March 
3, 2019].

[15] H. C. S. Thom, “Quantitative Evaluation of 
Climactic Factors in Relation to Soil Moisture 
Regime,” Highway Research Board, no. 301, pp. 
1-4, 1970. [Online]. Available: https://tr id.trb.
org/view/128231. [Accessed: March 3, 2019].

[16] G. Cumberledge, G. L. Hoffman, and A. 
C. Bhajandas, “Moisture Variation in Highway 
Subgrades and the Associated Change in Surface 
Deflections,” pp. 40-49, 1974. [Online]. Available: 
ht tp : / /on l inepubs . t rb.org/On l inepubs/
trr/1974/497/497-004.pdf. [Accessed: March 3, 
2019].

[17] R. N. Rainwater, R. E. Yoder, and E. C. 
Drumm, “Comprehensive Monitoring Systems 
for Measuring Subgrade Moisture Conditions,” 

31



Moisture Content Fluctuation in Minnesota Road Subgrades

Aisthesis      Volume 10,  2019

Journal of Transportation Engineering, vol. 125, 
no. 5, Sept. 1999. [Online]. Available: ASCE 
Library, https://ascelibrary.org/doi/abs /10.1061/
(ASCE)0733-947X(1999)125:5(439). [Accessed: 
March 3, 2019].

[18] T. D. White and B. J. Coree, “Threshold Pavement 
Thickness to Survive Spring Thaw,” Tapir 
Publishers, pp. 41-51, 1990. [Online]. Available: 
https://trid.trb.org/view/360732. [Accessed: 
March 3, 2019].

[19] A. T. Bergen and C. L. Monismith, 
“Characterization of Subgrade Soils in Cold 
Regions for Pavement Design Purposes,” 
Highway Research Board, no. 431, pp. 25-37, 
1973. [Online]. Available: https://trid.trb.org/
view/126479. [Accessed: March 3, 2019].

[20] Minnesota Pollution Control Agency, “The 
State of Groundwater,” Minnesota Pollution 
Control Agency. [Online]. Available: https://
www.pca.state.mn.us/water/state-groundwa ter. 
[Accessed: March 4, 2019].

[21] R. Adams, “Water Table Elevation and Depth 
to Water Table,” Minnesota Department of 
Natural Resources, Minnesota Hydrogeology 
Atlas Series Atlas HG-03, June 2016. [Online]. 
Available: https://files.dnr.state.mn.us/waters/
groundwater_section/mapping/m ha/hg03_
report.pdf. [Accessed: March 4, 2019].

[22] Minnesota Department of Natural Resources, 
“1981-2001 Normal Precipitation Maps – Annual 
(January – December),” Minnesota Department of 
Natural Resources, State Climatology Office DNR 
Division of Ecological and Water Resources, 
August 2012. [Online]. Available: https://www.
dnr.state.mn.us/climate/summaries_and_
publications/pr ecip_norm_1981-2010_annual.
html. [Accessed: March 4, 2019].

[23] Minnesota Department of Natural Resources 
State Climatology Office, “1981-2010 Normals 
Map Portal,” Minnesota Department of Natural 
Resources, PRISM Climate Group, Oregon State 
University, Feb. 2017. [Online]. Available: https://

www.dnr.state.m n.us/climate/summaries_and_
publications/normalsportal.html. [Accessed: 
March 4, 2019]. 

[24] Minnesota Department of Transportation, 
“Frost and Thaw Depths,” Minnesota Department 
of Transportation, MnDOT Materials and Road 
Research, March 2019. [Online]. Available: 
http://dotapp7.dot.state.mn.us/research/
seasonal_load_limits/thawindex/frost_t haw_
graphs.asp. [Accessed: March 4, 2019].

[25] Minnesota Department of Transportation, 
“MnDOT Geotechnical Investigation 
Information Interchange Internet Interface 
(GI5),” Minnesota Department of Natural 
Resources. [Online]. Available: http://www.dot.
state.mn.us/materials/gi5splash.html. [Accessed: 
March 5, 2019].

32


