Effects Of Excess Subsurface Water
The following is adapted from the CLRP Workshop Manual Roadway and Roadside Drainage, available in print or on our website
Frost Action (Heaving)
Frost heave is a major problem in New York State or wherever freezing temperatures occur for prolonged periods. Heaving occurs when there are:
- Freezing temperatures
- Free water available to create ice lenses
- Frost-susceptible soils present
All three must be present to have frost heaving. Since we cannot control the weather, we usually concentrate on eliminating the source of free water or using non-frost-susceptible soils.
Figure 1 shows the effects of frost heaving. Free water is drawn up by capillary action. As the ground freezes from above, a line between the frozen ground above and the unfrozen ground below (freezing front) moves down. Where the freezing front interacts with water drawn up by capillarity, ice lenses grow. As the front moves down, additional lenses grow and the ground heaves.
In the spring, thawing occurs from the top down. Any excess moisture trapped in ice lenses will cause the pavement to be excessively weak. It is critical that we do not use frost-susceptible soils in our bases and surfaces. Frost-susceptible materials are prone to more problems and will be weaker in the spring. Eliminating the source of free water is done by use of intercepting drains, trench drains, or intercepting ditches. Lowering the water table by use of underdrains is of limited effectiveness due to the high level water can be raised from capillary action. Frost-susceptible soils are ones that have both high capillarity and permeability. Clays are not as frost susceptible as silts, but are very weak when wet. They have a high capillary action but pull water so slowly that by the time enough water has been pulled through them to create ice lenses, it is spring. Silts are very susceptible to frost heaving. Gravels and sands are the best materials to use to eliminate the problems of frost heaving. We cannot typically replace the subgrade (native materials) but we MUST use non-frost-susceptible materials in our bases to help reduce problems.
|Low||Clean gravels and washed sands|
|Medium||Unwashed sands with moderate amounts of silty fines|
|High||Dirty gravels and pure clays|
|Very High||Silts and silty materials (including most materials called clay in New York State)|
Note: We cannot eliminate frost heaving subgrades, but we can build roads on them with the use of clean, high quality materials.
One note of caution must be made when we make roadway repairs. Every winter a few culverts seem to “sink” or “settle.” Many of these culverts were actually compacted with non-frost-susceptible materials, and the roadway around them is just heaving up to make them seem lower. When backfilling pipes, we should reuse the material, if possible, in the trench around the pipe to help alleviate this problem. If we use a non-frostsusceptible material around the culvert, it does not heave in the winter. As the road on either side heaves, the culvert appears to settle. See Figure 2.
Pumping is the mixing of layers of soil due to the vibration and loads of traffic. Especially during wet periods, the loads of traffic can cause the soil of the subgrade to be pushed or pumped into the base gravel. If the amount of fines pumped into the base is too great, the material will become weak and fail prematurely. Use of a separation fabric or sand filter layer between the native soil and the base of the roadway will eliminate most of this problem.
Potholes are formed by the interaction of excess water, traffic, and weak materials. Excess water is the most significant culprit. Good materials may still form potholes if excess moisture is present.
Potholes form when the lower layers, softened by excess water, do not provide strength to the pavement above. The surface layer is overstressed and cracks or softens. Traffic pushes this weak material away and potholes are formed. Since we cannot stop traffic and even good materials will eventually pothole, we must eliminate excess moisture by use of free draining bases and interception drains.
Spring thaw is when we get to harvest our most famous crop, potholes. Roads thaw from the top down. Also, the shoulders may remain frozen while the roadway had thaws. This is especially true if the materials in the shoulder have a higher fine content than the base under the road. When this happens, spring thaw is worse than a typical rainy period. With an unfrozen layer over the frozen base and the shoulders frozen, the road acts like a bathtub. If we do not allow for the water to drain with underdrains or daylighted bases, the saturated soils will be much weaker than they should be and will fail much sooner.
Posting roads to restrict heavy vehicles may be necessary to keep roads from failing during the spring thaw period. Roads should be posted for as short a period as possible. Tire pressure restrictions have been used by some agencies to help reduce damage to rural roads.
The thawing of the roadbed in the spring occurs from the top down and usually starts under the center of the roadway. The shoulders stay frozen and trap water in the base and subgrade. This saturated material is very weak and fails prematurely.
Removal of the excess water can be done by either daylighting the base (the daylighted material thaws faster than a dirty material) or installing subsurface drains to help remove the free water.