THE POCKET
Vol. 12 Issue 2
PRACTICAL AND ENTERTAINING SINCE 1997
Spring 2009
Rough road ahead: Why pavement fails
By Ron Shaffer, PE, and Neil Lund, PE
It is a common misconception that pavement
failures, such as potholes, occur solely because
of too much traffic. In fact, traffic is usually only
one contributor to the problem. In Minnesota
and its surrounding states (particularly in Iowa),
Ron Shaffer, PE subgrade (soil) strength, drainage and the
rshaffer@
braunintertec.com climate (more or less in that order) are equal
contributors.
Neil Lund, PE
nlund@
braunintertec.com
Soil Conditions
The Minnesota Department of Transportation
has an index rating for soils ranging from zero
to 70. Coarse, stronger soils that drain well,
such as the sands or sand and gravel mixtures
in Anoka County (MN), rate close to 70. Fine,
weaker silt and clay mixtures that might be fine
for growing crops in Olmsted County (MN), however, rate between
5 to 15. Between these two extremes are soils like silty sand or
clayey sand that offer adequate support under dry conditions and
mixed support when wet.
Drainage
Years ago, a noted professor taught an entire pavement design
course called “Drainage, Drainage, Drainage.” The importance
of drainage cannot be overstated when it comes to working with
pavement. (Paved and landscaped surfaces that are not sloped
to drain well toward area catch basins or ditches can contribute
significantly to subgrade soil saturation.) Poor drainage also
weakens soils. For instance, a soil dry of its optimum moisture
content by only 2 percent has twice the rigidity and loadsupporting ability of the same soil that is 2 percent above its
optimum moisture content. A shift of 4 percent that takes a soil’s
moisture content from moist to wet can result in a loss of half of
that soil’s ability to support pavement loads.
This premature rutting developed because of a soft subgrade
below the pavement.
Climate
The climate in the upper Midwest is as severe as any for
pavement. Worst circumstantially, fractures occur during the
winter months and especially during the spring thaws. Most of
the subgrade soils in this area are frost susceptible, meaning they
heave when they freeze and weaken each time they thaw. Overnight
freezing followed by daytime thawing is a complete freeze-thaw
cycle. A few hundred of these cycles can permanently damage a
pavement.
Effects on pavement
The interplay between soil conditions, drainage, and climate (and
traffic) can contribute to a variety of pavement failures:
1. Heavy cracking, also called “alligator cracking,” commonly
occurs at intersections or in busy parking lots. This kind of
cracking is unevenly spaced and may even be somewhat
muddy as the fine soils beneath the pavement pump up
through the cracks as traffic passes over the failed area.
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See PAVEMENT - Continued on page 2
PAVEMENT - Continued from page 1
Pavement Problems
2. Rutting forms along wheel paths on asphalt-surfaced roads.
Rutting occurs when weakened soils displace away from
vehicles’ wheels. These failures are frequently seen on
rural roads that are under-designed for heavy farm
equipment, or in industrial parks where pavements are
subjected to heavy semi-trailer usage. Rutting can also affect
asphalt pavement if it is of poor quality or compaction.
3. Potholes develop above soils that were poorly prepared
to support pavement. Potholes usually occur in old and brittle
asphalt pavement surfaces. They can often be spotted near
manholes and curbs, along transverse cracks, or where
pavement has been depressed due to settling of underlying
soils, often in conjunction with heavy cracking and rutting,
as described above.
Subgrade improvement
The most common way to improve pavement subgrades is to
densify (compact) the subgrade soils. Compacting the top 3 to
5 feet of the soils will make them more rigid and better able to
support the pavement. If the soil is particularly slow draining, or
if the groundwater table is shallow, a drainage pipe or a layer of
sand might be incorporated into the pavement design to provide a
conduit through which water can escape. If the soils are unusually
weak, they may be sub-cut anywhere between 1 ½ and 3 feet and
replaced with sand or additional aggregate base. Frost susceptible
soils can also be strengthened with a geotextile or geogrid (the
former a woven fabric, the latter an extruded plastic or plasticcoated nylon filament) to separate the soils from the aggregate
base and prevent intrusion of the soils into the aggregate base,
which is caused by pumping action or repetitive traffic. The
geotextile or geogrid also adds additional elements of stiffness.
Alligator cracking
Alligator cracking, potholes and poor drainage
Several improvement methods may be required for poor
soils. For example, a heavily used city street in the clay fields of
western Plymouth, MN, was constructed by first removing the
weak topsoil and some of the clay soils beneath. These soils were
replaced with two feet of sand, and drainage pipes were placed
below the sand on both sides of the road. Finally, an unusually
thick section of pavement was placed on top of the sand to
account for the weak, frost susceptible clay remaining below the
sand.
Whether a pavement is concrete or asphalt, the design must take
into consideration how subgrade soil conditions, drainage and
climate will affect its performance.
Pothole forming in brittle pavement
2
braunintertec.com
Diving into dredging
MPCA issues new dredged materials guidelines
By Doug Bergstrom, PG, CHMM
When precipitation and sediment run into
lakes and streams and other bodies of surface
water, it may include contaminants. These
contaminants oftentimes stem from what’s
occurred in the area, and could include
dbergstrom@
braunintertec.com chemicals from nearby industrial, commercial
or residential activities. The recent trend for local, state and
federal agencies to impose increasingly stringent stormwater
management practices is intended to reduce the impact of runoff,
but unfortunately cannot eliminate the problem completely.
Contaminated runoff impacts the aquatic environment when
it enters surface water and poses additional threats to the
environment when the resulting sediment is excavated. Not
only does excavation disturb and potentially re-release the
contaminants to the water by exposing contaminated sediment
and stirring it up, it may also contaminate land areas based on
how the excavated materials are stored, disposed of or reused.
Dredging generally refers to the act of excavating materials
underwater, such as to improve navigation, restore pond capacity,
or as part of construction activities. The Minnesota Pollution
Control Agency (MPCA) recently revised its guidance for testing,
storage and disposing of materials dredged from streams, rivers,
lakes and stormwater ponds in Minnesota. The reason for this
change is that MPCA officials and other regulators of solid waste
have seen additional evidence of the presence of contaminants
in stormwater-derived sediments, some of which are significantly
contaminated.
Regulated solid waste
Dredged materials are defined in Minnesota Rules as material
excavated at or below the ordinary high water level (OHWL) of
lakes, ponds, rivers or streams. In Minnesota, once the material is
excavated from water bodies it is also regulated by the MPCA as
solid waste.
Testing requirements
One of the new requirements is that the materials to be
dredged must be tested before dredging begins. If the area to be
dredged is an urban stormwater pond, the guidance requires,
This field
technician has
ventured into a
pond to collect
environmental
samples.
at a minimum, testing for copper, arsenic and an additional 25
separate Polycyclic Aromatic Hydrocarbon (PAH) compounds.
These contaminants are most common in urban runoff and are
derived from chemicals from nearby industrial, commercial or
residential activities. If the area to be dredged is not an urban
stormwater pond, such as a river or lake being dredged to promote
navigation, then testing is to be performed for a minimum of 10
heavy metals, four inorganic or nutrient substances, PCBs and total
organic carbon. Prior to that testing, past industrial activities that
occurred near the body of water must be evaluated to determine if
they may have contributed other contaminants in the water. Thus,
it is necessary to know the site’s history when determining the
laboratory testing parameters.
Sediment in most urban stormwater ponds and some lakes
and rivers is usually rich in silt- or clay-sized particles, and the
testing requirements described above generally assume that the
contaminants are bound up in these small particles. However, when
93 percent or more of a sediment sample is larger than silt (greater
than 0.003 inch), the MPCA has determined that such sediments
are unlikely to be significantly contaminated and therefore chemical
testing is not required prior to reuse or disposal. Such sizing criteria
are easily measured by running samples through a series of sieves,
much like that performed for construction soil testing.
Sampling
The number of locations to be sampled is mostly a function of the
volume of sediment to be dredged, and generally varies from three
to eight locations. However, small ponds where less than 1,000
cubic yards will be dredged may require only one location to be
sampled and tested.
See DREDGING - Continued on page 4
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3
DREDGING - Continued from page 3
At each location, samples must be taken at 2-foot vertical
intervals through the materials to be dredged, as well as 2 feet
into the underlying materials for proper characterization of the
sediments. The reason for sampling the underlying materials is to
evaluate whether there will be any newly-exposed contaminated
sediments once the dredging is completed. Also, if distinct layering
of the sediments is observed, the guidance requires that each layer
be sampled and tested.
Permitting
It is important to notify the MPCA at least 30 days before
beginning dredging activities. An individual permit from the MPCA
is usually not required (only notification must be given) if 3,000 or
fewer cubic yards of sediment will be dredged. The same is true if
more than 3,000 cubic yards of sediment will be dredged and the
material will be disposed of at an appropriately-permitted solid
waste facility.
Dredging projects not falling into either of the two categories
listed above may require an individual National Pollution
Discharge Elimination System (NPDES) permit from the MPCA; an
application for this permit must be submitted at least 180 days
prior to beginning the dredging activities.
Disposal of dredged material
Once excavated, dredged materials are generally reused,
often as fill, or permanently disposed at licensed landfills. The
suitability for reuse depends upon the texture of the material
and the concentration of contaminants. The MPCA uses Soil
Reference Values (SRVs) to govern reuse and disposal of excavated
sediments. Soil Reference Values specify the concentration of
contaminants allowed for reuse or disposal and form the basis for
three management levels applicable to dredged sediments.
A backhoe is being
used to sample this
river bottom.
It is allowable to reuse Level 1 dredged materials at residential
or recreational properties. Level 2 dredged materials, meanwhile,
may only be reused at commercial or industrial properties. Level 3
dredged materials must be disposed of at an appropriately licensed
solid waste facility. Level 1 is less than residential SRVs; Level 2
is less than industrial SRVs; and, Level 3 is greater than industrial
SRVs. Landfills may also have their own testing requirements in
order to accept the dredged materials.
Author’s note: The summary of MPCA guidance described above
is based on the guidance posted on the MPCA Web site
(www.pca.state.mn.us) as of April 15, 2009. The MPCA has
indicated that they will be issuing revisions to their dredged
materials guidance document in the near future. Affected parties
should review the most current version of the guidance prior to
planning or performing dredged materials work.
Staff members in our Environmental Consulting Group
are experienced in providing the following services to
address the new dredging guidelines:
• Environmental evaluation of commercial and industrial activities within
a watershed
• Sample collection
• Analytical laboratory testing, including urban stormwater pond (MS4)
testing, and river, stream and lake (non-MS4) testing
• Data evaluation, reporting and material disposition recommendations
• Dredging permitting
Braun Intertec opens office in Cedar Rapids, Iowa
In March, Braun Intertec opened a regional
office in Cedar Rapids, Iowa, to serve the state’s
growing industries. Braun Intertec has been
providing services to projects in Iowa for more
than 30 years and this new office is positioned
to provide geotechnical engineering,
environmental consulting, nondestructive
Timothy Wiles, PE examination, and analytical and materials
twiles@
braunintertec.com testing services.
The office is located at 5915 4th Street SW, Suite 100 in
Cedar Rapids, and can be reached by phone at 319.365.0961.
4
“Cedar Rapids and the surrounding area are home to a diverse
range of industries that have grown during the past few years,” said
Braun Intertec Chief Operating Officer Bob Janssen. “Not only do
we want to better serve our current clients who are physically located
in that area, or are planning projects in that area, but we also
want to be in a position to more fully participate in the progressive
development in this region.”
Timothy Wiles, PE, who recently joined Braun Intertec’s Iowa office
as an Associate Principal Engineer, will oversee the Cedar Rapids
operation. Tim has nearly 20 years of geotechnical engineering
experience and has worked in the area for more than 15 years.
braunintertec.com
Infiltration system design and slope erosion
Ask the Professor
By Charles Hubbard, PE, PG
Dear Professor:
The Infiltration Investigation article in your
spring 2008 issue discussed how infiltration
systems work and defined a number of factors
that influence system design. The article
broadened my perspective looking ahead to
future system design, but also got me looking
chubbard@
back at existing systems that haven’t performed
braunintertec.com
as anticipated or whose performance has
declined over time.
The article mentioned groundwater and rock or variable soil layers
as impedances to infiltration, yet some existing systems perform
poorly despite being isolated from groundwater and constructed
in uniform soils. Are there other things going on during or after
construction that are affecting system performance?
Maintain material integrity
Filtration is one process to consider where materials having
dissimilar particle-size distributions or pore spaces will be in contact
with each other. Sandy ponds covered with topsoil to slow infiltration
and gravel trenches lined with geotextile fabrics are two examples
of this situation. Hydraulic conductivity can be compromised if
particles from the finer-grained material cannot be trapped at the
interface of the coarser-grained material or fabric, or if flow cannot
be maintained as particles are trapped and filtration is established.
The United States Department of Agriculture’s Natural Resources
Conservation Service’s Chapter 26, Gradation Design of Sand
and Gravel Filters, provides a clear and comprehensive means of
evaluating material compatibility based on grain size analyses or
developing material specifications to achieve compatibility.
The device on the pond bottom in the picture accompanying the
article also looked interesting. Was it measuring permeability? Is
permeability typically measured through the pond bottom?
- Scratching my chin in Chanhassen
Dear Shaggy PE:
Your intuition has you headed in the right direction. Aggassi had
it all wrong, you know: In the engineering world, awareness—not
image—is everything. Designing an infiltration system is the easy
part. The challenge is to build the system to perform as designed,
and maintain that performance over time.
It’s not just about permeability
When considering infiltration, it is easy to focus mainly on the
permeability of the infiltration medium. Permeability, or hydraulic
conductivity (k), defines the velocity (in cm/sec, in/hr or ft/day)
at which water moves through an infiltration medium. Hydraulic
conductivity is important because it helps us estimate the volume
of water that can be accommodated by an infiltration system
over time, and thus size the system based on infiltration needs.
Hydraulic conductivity is not a steady-state parameter, however.
Construction activities and post-construction processes can reduce
the hydraulic conductivity of even uniform infiltration media from
what is estimated based on mechanical analysis measured in the
laboratory, or even measured in the field. Designers therefore need
to think not only of how the infiltration system will be configured, but
how it will be built and how it may be affected by post-construction
processes that the design can’t reflect.
Keep the system clean
Sedimentation is another process that occurs both naturally and
as a result of construction activities. Fine-grained soil particles
dislodged by surface runoff from natural slopes, paved surfaces
and unimproved construction grades can be washed into pond
bottoms, trenches or wells. Unprotected pond slopes are also
obvious and potentially significant contributors of sediment. It can’t
be assumed that sediments from such sources can be filtered, and
flow maintained. Some sedimentation is inevitable, of course, and
it is one reason infiltration systems require maintenance over time.
The best time to protect against sedimentation may be during
construction, when the systems are likely most vulnerable. Surface
runoff near infiltration systems in general should be managed
aggressively, and pond slopes in particular should be protected as
soon as possible.
braunintertec.com
See PROFESSOR - Continued on page 6
5
PROFESSOR - Continued from page 5
Tread lightly
Even where material compatibility is not an issue, compaction
can densify infiltration surfaces, which in turn reduces hydraulic
conductivity. The potential for the occurrence of unfavorable
densification can be reduced by specifying a relative density
limit or a relative compaction limit for the infiltration media.
Compaction and even general construction traffic can also cause
particle crushing, altering the texture, grain size distribution and
hydraulic conductivity of infiltration media. “Before and after”
grain size analyses and field infiltration tests can be performed to
help determine if compaction or construction traffic has affected or
could affect performance of the infiltration system.
Have confidence in your hydraulic conductivity (k)
Though hydraulic conductivity isn’t everything, do a sufficient
amount of exploration and testing in advance of design to feel
confident that the k you select is representative of conditions below
and beyond the infiltration surface. Evaluate the consistency of the
infiltration medium by advancing borings or test pits in designated
infiltration areas well below the anticipated maximum infiltration
depth. Borings may be more practical for deeper infiltration
systems and will certainly be able to confirm the presence of
and depth to groundwater or rock. Test pits, however, are often
invaluable for revealing more subtle subsurface layering that could
have a significant impact on infiltration.
developed, and the effect of variations in particle size distribution
weighed against overall system performance. Consider testing
samples from a range of depths above, at and below the infiltration
test depth. If the infiltration system is sufficiently large, tests can be
performed on samples taken from similar depths at other locations
away from the infiltration test location (multiple infiltration tests
should also be considered for large infiltration areas).
Hey Chuck:
I’ve got a question about erosion protection on steep slopes.
Seems there are more issues related to shallow slumping, erosion,
loss of topsoil and patchy vegetation establishment on 2:1, maybe
even some 2 ½:1 slopes. Straw mats aren’t always anchored well,
but they also don’t necessarily hug the slope very well either, and
they don’t prevent bad things from happening right underneath
them. Are there other products or means of protecting these steeper
slopes?
By the way, that toaster you gave my wife and I for our wedding
broke…
- Not thinking about you at all while on vacation
My Dear Newlywed:
Sorry about your toaster. I thought that, since I purchased the
next-to-cheapest brand, it would at least last six months. So it looks
like I’ll have to make it up to you with some technical advice.
Yes, steep slopes are difficult to protect, particularly if comprised
of locally soft or loose soils that are susceptible to erosion or
slumping, or are layered and exposed to periodic or seasonal
seepage. Slopes on the order of 3:1 (horizontal:vertical) or flatter
generally perform well enough with straw mats or straw mats
encased in plastic netting. Slopes on the order of 1 ½:1 or steeper
are typically constructed with layers of geosynthetic reinforcement
that help form the slope face, and likewise perform well. The slopes
in between are the ones that are most troublesome.
Visual confirmation of a layered infiltration medium or even a
uniform sand is not enough on which to base a design. In addition
to borehole testing or double-ring infiltrometer testing to determine
the hydraulic conductivity at the desired infiltration depth (yes,
the device on the pond bottom in the picture accompanying our
article is a double-ring infiltrometer), material samples should be
subjected to mechanical analyses so that a relationship between
hydraulic conductivity and particle size distribution can be
6
Straw mats are used successfully on steep slopes, but I agree
with you that poor contact between the mat and slope face makes
the slope much more vulnerable to erosion and other forms of
instability. Getting topsoil to “stick” to steep slopes, and slopes
subject to seepage or composed mainly or entirely of sand, can
also be difficult. There are more robust alternatives to straw mats,
however, that we have been pleased with throughout the years.
braunintertec.com
See PROFESSOR - Continued on page 7
PROFESSOR - Continued from page 6
View across and up the slope at partially installed and filled
webbing. The townhomes are set back approximately 15 feet
from the top of the slope. This slope was later covered with
sod and planted with scattered trees.
For slopes with limited vertical relief and located adjacent to open
water, there is always riprap or other armoring. For taller slopes,
and where a more robust surface treatment is desired, there is also
cellular confinement.
Cellular confinement systems consist of plastic webbing, solid
or perforated, that is stretched out over the slope face like an
accordion, fixed with anchors between 1- and 2-feet long, and filled
with topsoil. The webbing, which can be ordered with different web
sizes depending on slope gradient, helps prevent the topsoil from
sliding, and the anchors and optional geotextile separation fabric
Analytical Lab earns Seal of Excellence
Looking downslope at anchored and partially filled
webbing. The perforations are intended to help infiltrating
surface drainage flow freely through the system (as opposed
to becoming trapped within it). The geotextile fabric beneath
the webbing is pressed close to the underlying soil as the
topsoil is placed.
enhance the stability of the underlying soils as well. The webbing
is easily anchored at the top of the slope and pulled down the face
of the slope. The webbing can generally be filled from beyond the
top or bottom of the slope with a backhoe, and the topsoil can be
smoothed by hand. Shrubs and trees can also be planted through
the webbing.
If you want to go steeper than 2:1, I might recommend a
geosynthetic reinforced slope with the geosynthetic forming the
slope face or contained within stacked galvanized metal forms.
Summer hours announced
Recently, the Braun Intertec
Analytical Laboratory was recognized
with the Seal of Excellence award from
the American Council of Independent
Laboratories for the fourth consecutive
year. The lab also was recognized
for being one of the top five laboratories providing customer
satisfaction.
The Braun Intertec Analytical Laboratory will be open to receive
samples on Saturdays from 8 a.m. until noon throughout the
summer, beginning May 2 and extending through Oct. 31. The lab
will be closed the following holiday weekends, however: Memorial
Day, Independence Day and Labor Day.
ACIL’s program is based on client feedback about laboratory
services. The program is unique because clients submit their
feedback directly to the ACIL. The Seal of Excellence program is a
mechanism for evaluating environmental testing laboratories by
assessing the integrity of data, meeting customer’s quality needs,
and setting the standards of performance for the testing laboratory
industry.
As always, if you have a special project requiring sample receipt
during off hours, please contact your project manager and we’ll
accommodate your needs.
Normal receiving hours are from 8 a.m. to 5 p.m. Mondays
through Fridays, and Summer Saturdays from 8 a.m. until noon.
braunintertec.com
Braun Intertec Analytical Laboratory
11001 Hampshire Avenue S
Minneapolis, MN 55438
952.995.2600
7
11001 Hampshire Ave. S
Minneapolis, MN 55438
Minneapolis
Albertville
Bismarck
Cedar Rapids
Detroit
Fargo
Hibbing
LaCrosse
Lakeville
Mankato
Rochester
Saint Cloud
Saint Paul
800.279.6100
763.497.4159
701.255.7180
319.365.0961
248.388.2403
800.756.5955
800.828.7313
800.856.2098
952.469.3644
800.539.0472
800.279.1576
800.828.7344
800.779.1196
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Questions, Requests
and Comments
Charles Hubbard, PE, PG
Braun Intertec Corporation
1826 Buerkle Road
Saint Paul, MN 55110
Phone: 651.487.7060
Fax: 651.487.1812
[email protected]
This newsletter contains only
general information. For specific
applications, please consult your
engineering or environmental
consultants and legal counsel.
At Braun Intertec we are committed
to being your full-service professional
geotechnical and environmental
consultant. With 13 office locations
throughout the Midwest, we can
provide you with a mix of services
to meet your needs in the most cost
effective, efficient and timely manner.
Our experience includes:
• Design and Construction Services
• Site Selection and Planning Services
• Facility Management Services
• Laboratory Services, including
Analytical, Materials Testing, and
Nondestructive Examination
©2009 Braun Intertec Corporation
Providing engineering and environmental solutions since 1957