It was almost 60 degrees at Kemp Station yesterday. Does that mean spring is here? Probably not quite yet, but it does mean that our roads have moved into the spring thaw. Spring thaw may not impact most of us outside slushy snow and muddy roads, but it does play a significant role for large trucks and how much weight they can haul. You will soon see signs with posted weight limits on town and county roads. These signs help protect the infrastructure during the spring thaw. Let’s look at frost and what role it plays on our winter and spring roads.
First, let’s look at how our roads freeze in the fall. As air temperatures drop below freezing, the water between the soil particles, pebbles, and rock freezes into ice. If you have ever tried to dig into frozen ground, you will see how the ice “glues” the ground together. The surface layer of soil cools and freezes first, with the deeper layers gradually freezing as the cold penetrates further into the ground over time; this is because the surface is directly exposed to colder air temperatures.
Our WISCONET weather station at Kemp Station measure soil temperature. As of Monday morning, the soil temperature was at 31.3 degrees at 2, 4, and 8 inches deep; 32 degrees and 20 inches deep; and 33.8 degrees at 40 inches deep. The frost line is somewhere between 20 and 40 inches. Note that these probes are in a field under a layer of snow, with the snow providing a significant insulating affect. Our melted bare gravel roads were below 32 degrees Monday morning after the nightly freeze but were soft or thawed under the surface frost. They were certainly thawed on the surface by end of day Monday give the warm daytime temperatures, but frost was still in the ground further down as the gravel was wet with puddling caused by the frost. The melted snow will not drain until the frost is fully thawed throughout its depth profile.
The spring thaw works in the same direction as the fall freeze. The ground thaws from the top down, as the surface receives the most direct heat from the sun and surface air temperature, causing the top layers to thaw first while deeper layers remain frozen until the heat penetrates further down. This is when damage can be done to our roads. In the wet thawed layers above deeper frozen layers, the water can’t drain, and the roads can be very sponge-like. For example, we have a section of gravel driveway at Kemp Station that we must block off with orange cones ever spring thaw. The area turns to a pudding-like consistency capturing the misguided vehicle that strays off course.
This same process is at work under our paved roads in the Northwoods. If the gravel substrate is spongey during the spring thaw, this movement cause damage to the concrete or blacktop surface. This is why road weight limits are enforced during spring thaw to protect the road integrity until the roads and road substrates are full thawed and stable.
Wisconsin Department of Transportation issues a frozen road declaration in the fall when the ground freezes to a depth of at least 18 inches. This declaration allows for increased weight limits for specific types of cargo, primarily logs and salt/sand used for winter road maintenance. The purpose is to facilitate the efficient transportation of these essential materials during winter. 91 frost tubes are used across the state to measure the depth of frost beneath our roads. Frost tubes are liquid-filled devices that are installed vertically under the pavement. WisDOT personnel manually read the frost tubes to determine the frost and thaw depths. The Frozen Road Declaration is normally in effect from mid-December to late February or early March. This year, the Frozen Road Period ended across the state last Friday, February 28, 2025 at 12:01 a.m. CST.
This time of year is always hectic for Wisconsin’s forest products industry. Our forestry professionals must watch the weather forecast and plan ahead to move logs and pulpwood from the forest to the mill. Once the spring thaw starts and road weight limit bans are in place, even an empty log truck is too heavy to drive on certain municipal roads. Depending on the region and road conditions, spring thaw road bans are usually removed between late April and mid-May.
Wisconsin's system of ground frost monitoring and seasonal weight restrictions is designed to balance the needs of transportation while maintaining our road infrastructure. All part of the larger process of managing our sustainable natural resources in the Northwoods.
