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National Hog Farmer
Modern Equipment, Old Roads

Modern Equipment, Old Roads

A recently concluded, multi-year road study looked at factors such as axle design and spacing, tire design, pounds/square inch, total weight, hauling speed, time of year, time of day, road design and construction to determine three primary predictors of damage are axle weight, distance from the edge of the pavement and pavement construction/road drainage.


Manure haulers and local officials, especially those in northern tier states, are finding themselves squeezed between tight nutrient-application windows and the ability of local roads to handle equipment needed to move large volumes of manure in a short time, especially in the spring.

Given information in a just-completed, $600,000-plus, multi-year study of the situation, there are no easy answers. The seriousness of the situation is reflected in the fact that the study, though initiated by the Professional Nutrient Applicators Association of Wisconsin, soon had a list of cooperators that included the Minnesota Pork Producers Association, the Professional Dairy Producers of Wisconsin, the Wisconsin Towns Association, manure applicator associations/groups in six other states, Minnesota highway engineers and the Departments of Transportation (DOT) in four upper Midwest states. Five equipment and three tire manufacturers also participated.

Kevin Erb of the University of Wisconsin’s Environmental Resources Center and advisor to the Wisconsin applicators group recently presented highlights of the study, which looked at factors such as axle design and spacing, tire design, pounds/square inch (psi), total weight, hauling speed, time of year, time of day, road design and construction and more.

Much of the technical work was conducted at a specialized Minnesota DOT site north of the Twin Cities. The research spanned from spring 2008 to early winter 2010. Sensor mats were used to measure pressure. Professional applicators operated the equipment to mimic actual conditions as closely as possible. Nearly 5,000 passes were made over test areas.

Eleven different tanker types and designs were involved — two tandem axle straight trucks with 4,400-gallon tanks; four two-axle TerraGators with tanks ranging from 1,800 to 4,200 gallons; and five farm tractor-tanker combinations ranging from 4,000 up to 9,500 gallons. Loaded gross weights ranged from 47,100 to 134,200 lb. Two standard semi truck-tank trailer combinations, one grossing 80,150 lb. and another 103,100 lb., were also tested to provide comparable stresses from typical semis. Various thicknesses of both asphalt and concrete pavements were tested as well. The final report is nearly 550 pages.

The bottom line from all that data: While pavement damage did not occur under all conditions, the concern is real and the three primary predictors of damage are axle weight (vs. gross weight), distance from the edge of the pavement and pavement construction/road drainage.


Axle Weight


The research points to the importance of allocating weight evenly across all axles and having an adequate number of axles. When the rutting potential of axles was compared between two 6,000-gallon tankers, the one designed to place more weight on the rear axle (considered by the manufacturer to be an advantage during wet field conditions) showed far more potential for damaging roads. Similarly, the 9,500-gallon tanker with four axles had less rutting potential than the 7,300-gallon tank with three axles.

Since there is an additive effect of multiple loads over the same area, in assessing the number of passes that could be made before road damage occurred, the data showed more axles translate to more road-friendly passes.


Location in the Road


Distance from the edge of asphalt pavement was an eye-opener. Researchers found that if the center of the tire was 16 in. or more from the edge, asphalt strain/subgrade damage dropped by more than 75% — and in some studies by more than 94% — than if the tire ran closer to the edge.


Tire Pressure and Design


Tire pressure did not significantly affect predicted damage to either asphalt or concrete pavement. In a comparison of radial and flotation tires, radials put slightly higher strain on asphalt surfaces and road subgrades, though the difference was greatest when trucks were empty.

In a tire-related factor, the researchers found that all TerraGator com­binations were more potentially damaging than other equipment, even at comparable axle weights, as the weight was being placed on narrow lugs despite the flotation appearance.


Seasonal, Time-of-Day Differences


Perhaps least surprising to both northern producers and road officials is that significant seasonal differences exist, with the most critical times when frost is leaving the subgrade and when the subgrade is wet. Unfortunately, those conditions closely coincide with the narrow spring window when manure storage needs emptying and weight restrictions are placed on many rural roads.

Less expected was that time of day was a significant factor, as both road surface and subgrade strain from loads increased substantially during the afternoon — no matter the time of year. The finding, unexplainable yet clear, has led the researchers to recommend hauling in the mornings whenever possible, preferably stopping by 10 or 11 a.m.


Speeds, Stops and Turns


Heavy loads can put major stresses on roads when turning or coming to a stop. Slowing quickly from high speeds and sharp turns can wreak havoc on susceptible roads. While the study did not directly research steerable axles, Erb calls them “essential” on larger tankers.


Road Construction


The ability of roads to handle heavy loads is directly related to how well they are constructed, especially below the surface. A high-quality subgrade, good drainage and surface thickness are all factors.

While all roads spread weight in an inverse triangle configuration from the surface down to the subgrade, concrete spreads it considerably wider. Roads constructed with good subgrades and 5½ in. of asphalt were pretty much bullet-proof as well, but finding those qualities in rural roads originally designed to handle a B John Deere tractor and ground-drive spreader is rare at best.


The “Million Gallon” Question


In order to determine whether it’s better for roads to move the same amount of manure by making more trips with a smaller unit or fewer trips with larger equipment, the researchers used a computer model based on the data collected in the study. They modeled loads on two different thicknesses of concrete pavement as well as two different thicknesses of asphalt.

The number of passes needed to move the million gallons (using water weight of 8.3 lb./gal) ranged from a high of 238 for the 4,200-gallon TerraGator to a low of 105 for the 9,500-gallon tanker. On the 3½-in.-thick asphalt, the design most likely approximating rural roads (though exceeding many), all comparisons showed some road surface damage. However, only a couple showed greater projected damage than an equivalent number of passes from a typical 80,000-lb semi.

The picture changed substantially when subgrade damage was projected, with the semi being much less damaging to the subgrade than any of the equipment alternatives. While there were moderate differences in the projections between the manure hauling options, none appeared to show a clear advantage for the “more-but-lighter” or the “heavier-but-fewer” options. Total time required to move the million gallons was not considered in the comparison, leading one to believe that haulers, legal implications aside (see “Flirting with the Law” sidebar), will opt for getting the job done as quickly as possible.


Farmer/Hauler/Municipal Cooperation


Beyond these equipment-related factors, Erb offers several potential short- and long-term solutions, most needing cooperation between farmers and local officials.

1.         Get the weight off the shoulders. This may require temporarily posting roads as one-way so that hauling equipment can run in the center. While it means prior agreement with local officials, it is both easier on roads and safer. Emergency services are usually exempted, but all others must comply. These postings are usually only for a day or two; experience has shown that enforcement may be needed at the start.

2.         Select routes that will minimize potential damage to local roads and bridges, while also taking into account traffic, topography and safety. At times, this may require a different route for loaded tankers vs. empty returns. And the route may not be the shortest. Again, haulers should work with local officials, even though this usually won’t require one-way postings.

3.         Install approach lanes, paved pads and 60-ft. culverts at field entrances. These options, generally done at farmer expense, can take much of the slowing/turning stresses off local roads by allowing for better approaches and wider turns. In any case, entry and exit off roads should be done only at established entrances, not through road ditches.

4.         Temporarily close roads. Again requiring official approval, this option has been used to allow semi-tankers to park in the center roadway, off the shoulders, while transferring manure to field tankers. Another alternative is to install, at farmer expense, an extended shoulder with a paved transfer pad.

5.         Maintain existing roads and look for longer-term solutions during major reconstruction projects. This falls to the local municipality. Roads need adequate drainage. Ideally, the bottom of ditches should be one foot below the rock subgrade and the rock should daylight to the ditch. Culverts should be kept open. Potholes, etc., need to be repaired promptly. Subgrade weaknesses should be corrected during major patching and reconstructs, and municipalities might consider options such as thicker pavement in critical areas and installation of turn approaches as part of these same projects, when limited budget resources can be used most effectively.

6.         Pump vs. haul to the field. Some producers and haulers, with the cooperation of local officials, are investing in pumps and hoses to move manure up to several miles from storage to field application equipment. Permission is required to temporarily place hoses in road rights of way and through culverts, etc., but officials often find that preferable to the over-the-road alternative. Dustin Ransom, custom hauler and president of the Wisconsin applicator’s group, reports his company has even invested in directional boring to put transfer pipes under roads, usually surfacing at field fencelines.


Bonds and Other Options


While some local municipalities are considering bonds or similar financial assurances from haulers as a way to protect their taxpayers against road damage, the option has its drawbacks. One is that many commercial haulers don’t have adequate assets to meet bonding company requirements, meaning they couldn’t operate in affected areas.

Determining road damage, if any, can also be difficult. Before-and-after pictures may not be adequate. Erb points out that most damage occurs from the bottom, not the top. The subgrade can be affected immediately, with related surface damage not becoming visible until months later.

A better solution is for everyone to work together. “Knee-jerk reactions are not going to solve the problem,” he says.  .


Bill Gnatzig is a member of the Wisconsin Pork Association board of directors and a freelance writer from Roberts, WI.



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