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. braunintertec.com 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 braunintertec.com 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 braunintertec.com 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
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