Ah, spring in Northern Minnesota. Days are longer, temps warm up, songbirds return, lake ice melts and the roads deteriorate. Road maintenance turns into triage: frozen culverts, washed-out roadways, frost boils and heaves keep are some of the symptoms of Spring Fever on the Cook County Highway Department system. The main culprit? Water.

Well-constructed gravel roads shed water efficiently. A crown of 2-4% moves water off the road surface to an adequate ditch. In spring, snowmelt and rainfall events increase the amount of water movement. The ground starts to thaw. Culverts that normally accept that water may still be frozen. Water then builds on the side of a roadway at the inlet of the culvert, saturating and moving through the road base. A saturated base can lead to a road washout.

Water also freezes and thaws within the roadbed. As the temperature drops in fall and winter, the layers of the roadway freeze in layers called ice “lenses”.  Differential frost heaves also occur around culverts bedded in poorly drained base materials, forming seasonal dips and heaves. Spring rolls around and the roadway thaws from the top down. Water trapped above the still frozen subgrade “boils up” and finds a path to the surface in the weakest spot. Frost boils (fondly referred to at the Highway Department as “bubblin’ scoobies”) appear as sporadic wet, soft areas on otherwise dry, firm gravel roadways and are often an indication of weak subgrade. Heavy truck traffic can do significant damage to a gravel road if it hasn’t had time to “heal.” Because of this, road restrictions stay on gravel roads in Cook County for longer than the state minimum of 8 weeks. 

The solution for frost-related issues in roadways is to ensure adequate drainage. Chronic problem areas may need reconstruction with less frost-susceptible soils and/or a layer of geotextile. Routine ditching, grading and culvert replacement can help to move water away from the road. But when we’re “in it” during the spring thaw, all we can do is free culverts, fix washouts, and let the spring fever break. 

County Connections is a column on timely topics and service information from your Cook County government. Cook County – Supporting Community.