Asphalt Paving Blog

3 Causes of Unstable Soil (And How to Fix It)

causes of unstable soilWhen starting a paving project, you never know what you may run into. Situations can occur where once the excavation has started, the soil, or base, below the old asphalt is too soft and unstable. Because of this, simply paving new asphalt on top of the base is not an option anymore and a new asphalt solution needs to be explored.

Soil instability can be caused by these 3 major issues:

1. The presence of organics. Topsoil and other plant matter can make it unstable and as it decomposes, can make for a very unstable material - especially in the presence of water.

2.The lack of compaction. Where areas are filled in to bring the elevation up, many times the soils are not compacted in thin enough layers in order to get a uniform, highly compacted soil which results in an unstable condition.

3.The most common reason for instability is composition - the proper balance of mineral pieces, matter, air and water. For instance, a clay soil, with the correct moisture content, can be an incredibly stable soil, almost to the point of concrete. (Bricks can be made out of clay). Introducing too much moisture into clay soil can make it a very unstable soil, which can be tough to deal with and make stable again as it takes on moisture slowly, and releases it slowly as well. Getting a clay soil to dry can take more time and money than planned.

Other soils are sandier and rockier and drain well. Too high of a ratio of sand, however, can make them unstable. Rockier/sandier soils can be very stable and as they are well-draining, tolerate moisture on occasion and do not become unstable easily. There are a variety of soil types in Wisconsin including:

  • Silts
  • Loams
  • Peats 
  • Clays
  • Sand

Combined with varying degrees of moisture present and the ability or lack of drainage for water, soil can come in virtually any combination depending on your location. Additionally, soil types, organics, and compaction can vary on the same property. The cause and solution of an unstable soil must be looked at on a case by case basis and be taken care of and remedied prior to any paving work.

Soil stabilization methods include:

  • Drain excess moisture out of the soils using drainage tile
  • Tilling and turning over the soil to help dry it.
  • Excavation into the dirt subgrade and replacing the soil with a proper base material to help bridge the unstable soils.
  • Soil separation fabric to keep the soils from intermingling with the base material.
  • Reinforcing grid, commonly known as geo-grid, to help the instability by spreading out the loads over a larger area. 
  • Chemical Stabilization, such as lime, fly ash, cement, cement-slag combo, tilled into the soil which changes the soils chemical composition and makes an unstable soil, stable.
  • Foamed Asphalt Injection process where the base material is injected with foamed asphalt oil and tilled with a pulverizer - helping to make the base material mimic the characteristics of a pavement rather than a base material.

Do you have drainage issues or unstable soil on your property? Contact Wolf Paving for more information on a recommended stabilization solution.

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