In regions of the United States where pavements are constructed in freeze-thaw environments, spring load restrictions (SLR) are typically used as a preservation strategy. During the spring, pavement layers are generally in a saturated, weakened state due to partial thaw conditions and trapped water. The critical time for SLR is when the pavement first thaws, thus, proper measurement and prediction of freeze-thaw events is crucial to a successful load restriction strategy.
The past policy used by the Minnesota Department of Transportation (Mn/DOT) was to reach agreement between the districts located in the affected zone that SLR should be placed on the state trunk highway system, and then give a 7-day notice. Counties, townships and municipalities generally followed state recommendations on load restriction posting and removal dates. The duration of SLR was typically 7 to 9 weeks.
The current Mn/DOT policy uses actual and forecasted average daily temperature to determine when SLR should be placed and removed. Current law now specifies that all county, township and municipal roads will begin and end SLR in common with the truck highway system, unless these roads are posted otherwise.
For Minnesota, SLR impacts many more miles of the county, township and municipal systems than the state trunk highway system. Damage incurred to the entire road system is quite substantial if SLR are not placed before the critical thaw period.
- SLR are not new to Minnesota. Although changes were recently made in the way SLR are placed and removed, the concept has been around since 1937.
- As the average air temperature decreases, a pavement structure freezes from the surface, down. In the same manner, as the average air temperature increases, a pavement thaws from the surface, down.
- During the spring the average air temperature may fluctuate above and below freezing and a pavement structure may go through many smaller freeze-thaw events on its way to a complete thaw.
- In Minnesota there are about 39,000 miles of paved roads that do not meet the 10-ton spring load design standard and therefore should be restricted to lower loads during the spring.
- Included in these 39,000 miles are about 1,600 miles of state trunk highways, 23,600 miles of county state aid highways, 2,400 miles of municipal state aid city roads, and roughly 11,000 miles of other local roads constructed and maintained through local funding.
- The damage that occurs during each day in the spring is about 5 times greater than the damage that occurs during each day in the summer.
- Mn/DOT's new SLR procedure results in an approximate annual savings of more than $10,000,000.
Explanation of Process
The process used to place SLR is as follows:
- The Districts submit their restricted roadway segments for the annual ROAD RESTRICTION MAP to the Director of the Office of Materials & Road Research by November 30 of the preceding year.
- The annual ROAD RESTRICTION MAP is available electronically at the Office of Materials & Road Research.
- A toll-free telephone number (1-800-723-6543) and local telephone number (651-366-5400), a web page (Seasonal Load Limits), and by subscribing, an automatic e-mail update have been established to provide information on postings in each frost zone as quickly and conveniently as possible. A 3-day notice of restrictions is provided using a recorded telephone message with more detailed status reports available on the Internet site. U. S. postal service mailings are no longer used, but you can get current and timely notice by subscribing to receive automatic e-mail updates.
- The start of the load restriction period is determined for each zone using measured and forecast daily temperatures for several cities within each frost zone. The criteria used to determine when the load restrictions will be placed is when the cumulative thawing index (TI) for a zone exceeds 25 F degree-days based on the 3-day weather forecast, with predicted increases well in excess of 25 F degree-days.
The intent is to use the 3-day advance forecast temperatures to ensure that the postings are on at the beginning of the thaw and at the same time provide 3 days of notice to the public that the posting period is coming. The calculation is made using the following formula:
TI = Summation (Average Daily Temperature - Reference Temperature)
The reference temperature decreases 1 degree F per week from 29 degrees F on February 1 to 24 degrees F on March 15.
- The Internet site, recorded telephone messages, and automatic e-mail updates are the only means of notification provided on a regular basis.
Frequently Asked Questions
Do spring load restrictions do any good?
It is estimated that a typical low volume asphalt road's life will be increased by about 10% due to implementing improved SLR procedures. A 10% increase in life is expected to result in an annual savings of more than $10,000,000.
What is the most critical time for placing spring load restrictions?
The most critical time for placing spring load restrictions is when the pavement structure has thawed a depth of 6 to 12 inches from the surface. SLR help to preserve the aggregate base layer during this time because excess moisture is trapped in this layer and is unable to drain into the frozen subgrade.
Once SLR are placed, can they be removed when temperatures are cold again?
It is typical in Minnesota to have several freeze-thaw cycles during the spring. SLR are intended to be placed prior to the first significant thaw. If the restrictions were removed during a temporary re-freeze event, pavements would be severely damaged since excess moisture is trapped between the thin frozen surface layer and the still frozen deeper soil layer. The re-freeze events are typically unstable and do not add significant strength to the pavement structure.
What are other state SLR policies?
In regions of the country where pavements are constructed in freeze-thaw environments, states impose SLR to preserve pavement structures when economic constraints prevent reconstructing or overlaying the pavement to withstand greater loads. The most common methods used by these states to place and remove SLR are engineering judgement, experience and visual observation. Other states, including Washington, Alaska, South Dakota and Minnesota, use deflection tests, frost depth sensors, and air and pavement temperature data to predict thawing and place SLR.
Couldn't we simply lower the speed limit rather than reducing vehicle weight limits during spring load restrictions?
When vehicle speed is reduced on an asphalt concrete or aggregate - surfaced road, the damage can actually increase since the load is applied for a longer time.
Which county, township and municipal roads are affected during SLR?
All county, township and municipal roads that have an asphalt concrete or aggregate - surface are restricted during SLR, unless posted otherwise. Any road can be restricted if it is posted by the local agencies.
Why not build or reconstruct all roads to 10-ton capacity?
The cost would be extremely high and the financial resources do not exist.