The frost boils are primarily caused by poor earth material in the roadway. Poor material typically consists of weak organic soils that can absorb water, basically dirt. The dirt absorbs water during the summer, then freezes in the road. When it thaws in spring the compaction of the road is lost and is basically mush. If the road has good quality clay structure in it with good compaction, it wouldn’t absorb nearly as much water. The top of the road has a thin layer of gravel. When the frost boils are thawing, traffic pushes the gravel down into the weak soil and the weak soil wants to squish out to the surface. This is why we don’t use a motor grader until the boil has thawed and dried enough otherwise our heavy equipment can sink suddenly into the soft spots creating an even worse situation.
Typical repairs include digging out the frost boil location if budget allocated. If the road has many failures throughout, high expenses can be mitigated by using geotextiles, fabric and weeping tile where possible. The geotextiles and fabric are installed across the whole road about a foot to 2 feet below the surface to separate the poor material from the new good material. The geotextiles adds tensile strength to the road and spreads weight over a larger area, basically the same way a snow shoe keeps you on top of the snow. The fabric then ensures that the poor material can’t squish to the top under the weight of vehicles/equipment wheels. We have been doing this for most asphalt roads that have been replaced in the last several years. When you see asphalt roads also moving and causing block cracking, this is usually for the same reason, poor material and water penetration or ground water. The asphalt provides strength in the road but is heavily reliant on good stable earth below it.
