Take a Load Off
by Dave Rettner
- New System for Placing Load Restrictions Starts this Spring
- New System Based on Mn/ROAD Research
- Proposed Policy
New guidelines for placing spring load restrictions (discussed below) should help prevent potholes from developing prematurely. But some will always be around--so a new LRRB video described later tells us how to repair them. |
New System for Placing Load Restrictions Starts this Spring [ top ]
Springtime is a critical period for Minnesota's roads. Subgrade soils and aggregate base materials are in a weakened state while the frost leaves the ground and immediately afterwards, when it is still saturated. To prevent excessive roadway damage and protect the public investment, it is important to place load restrictions when they are most needed, at the beginning of the thaw.
Pinpointing this time, however, isn't easy. This spring the Minnesota Department of Transportation (Mn/DOT) is implementing a procedure that, along with improved communications, should lead to more timely placement of restrictions.
| To hear the latest updates, call 800-723-6543 or check the Web: http://mnroad.dot.state.mn.us |
New System Based on Mn/ROAD Research [ top ]
Historically the start of the spring posting period has been determined by physical observations of roadways and shoulders, measurements of thaw with frost tubes and resistivity probes, and the experience of district maintenance and materials personnel. While generally very accurate in determining when the thaw is taking place, the delay in reaching a consensus in a frost zone and the typical one-week notification period have allowed considerable pavement damage to occur.
Analysis of pavement strength during the spring thaw period has shown that the damage incurred each day at the beginning of the thaw is equivalent to about one week at the end of an eight-week posting period. Therefore, to make up for the damage caused by a one-week delay in posting roads, the spring restriction period would need to be extended by several weeks.
Through research conducted at the Minnesota Road Research Project (Mn/ROAD) pavement research facility and validated statewide, we are now able to better predict when weakening will occur using air temperatures currently recorded and forecast for most areas of Minnesota. The actual posting dates from the period 1986 to 1998 were compared with those predicted using the new technique developed by Mn/DOT. The comparison showed that there is typically a seven- to ten-day delay from the time that it was determined load restrictions should be placed and the actual placement date.
Damage caused by this delay can shorten the life of a pavement by 5 to 10 percent. This delay causes damage that could be prevented and results in additional costs to government agencies and increased user costs due to vehicle damage and greater travel time due to pavement condition. The potential savings resulting from improved load restriction placement are substantial because statewide there are about 38,000 miles of paved roads that do not meet the 10-ton spring load design standard.
The new procedure uses three-day weather forecasts to predict when the thaw will occur. Using forecasts gives Mn/DOT lead time to notify the public of the placement of load restrictions and still allow postings to be placed before the thaw has created damaging conditions in the roads.
Proposed Policy [ top ]
The proposed policy, to be implemented in the spring of 1999, will consist of the following:
- The districts would submit their roadway segments for the 1999 Road Restriction Map as they currently do.
- The 1999 Road Restriction Map would be mailed to all those on the current Mn/DOT mailing list, along with an explanation of the new procedure for notification of postings. This would be the only mail notification of the new procedure to the general public.
- A toll-free telephone number (800-723-6543) and Internet site (the Mn/ROAD home page, http://mnroad.dot.state.mn.us) have been established to provide information on postings in each frost zone as quickly and conveniently as possible. A three-day notice of restrictions will be provided using a recorded telephone message, with more detailed status reports found on the Internet site. Mailings will no longer be used.
- The start of the load restriction period will be determined for each zone using measured and forecast daily temperatures, obtained from the National Weather Service. The criteria used to determine when load restrictions will be placed will be: a thawing index greater than 25ÁF degree-days and predicted increases in the thawing index based on the three-day forecast.
The intent is to use the three-day advance forecast temperatures to ensure that the postings are on at the beginning of the thaw and at the same time provide three days of notice to the public that the posting period is coming.
- The load restriction period will be eight weeks in duration in each zone. Historically the posting period has averaged 7.9 - 8.6 weeks depending on the zone. It is felt that using a flat eight-week period is easier to implement.
Based on past history, restrictions are expected to be in place by March 8 in the southern area of the state and by March 15 in the central and northern areas of the state; however, earlier dates are very likely.
Mn/DOT projects savings from this new procedure to be in the millions annually, says John Siekmeier, research project engineer. "Our research is paying off and it will continue to pay us large dividends if we have the patience and the insight to make it happen."
If you have any questions or would like more information, contact Siekmeier at john.siekmeier@dot.state.mn.us
--Rettner is with Mn/DOT's Engineering Services.