Geotechnical Engineering

SOIL MOISTURE CONTENT

Weather is in the news every day. Oft en rain, snow, blizzards, hurricanes, and fl oods are the headline of the day. While the news may sensati onalize the weather, periods of adverse weather can dramati cally aff ect earthwork operati ons, not just the sensati onal events. Moisture-saturated soils present problems of instability or “pumping subgrades” during constructi on of foundations, slabs, and pavements. Often contractors throw up their hands, and owners throw down their check books, and ask “What can I do?

If a soil is too dry, water can readily be added to adjust the moisture content to a workable level. Most of the soil moisture problems come from soils that are too wet. Getting the excess moisture out can be tough, especially in the winter months and during periods that do not lend themselves to drying of soil.

Unfortunately for projects caught by wet weather with exposed soil subgrades, there are few construction options that don’t cost money. You can shut down the site and wait for sufficient drying to occur, but with interest costs, tenant deadlines, and lost revenue, that opti on is usually not practical.

It is very important for the construction team to work together to arrive at an effective solution. Resolving difficult earthwork problems requires common sense and a practical approach to the problem. In developing alternative solutions, we often use a “decision tree” approach in the following order:

  • Natural Drying - Let nature resolve what nature created (a.k.a.: do nothing).
  • Mechanical Drying - Accelerati on of the natural process by discing and turning the soil, including someti mes blending drier soils into the wet soils.
  • Chemical Drying - Changing or improving soil moisture content by mixing with lime, cement, or other additi ves.A Quality Control (QC) plan is required for most FDR projects and is considered to be an integral part of the FDR project’s success. It outlines the types of inspecti on and testi ng required during constructi on, the intervals required for that testi ng, and who will be providing the testi ng.
  • Replacement - Removal of the wet soil and replacement with drier material from on or off -site sources.

Although these options are generally ranked from least expensive (natural drying) to most expensive (off -site borrow source), there are many conditions that will affect the final decision, including:

  • Available Time - The least expensive solution generally takes the most time.
  • Magnitude of Problem - How wet is the soil?
  • Prevailing Weather Patterns - The best drying conditions require extended sunshine, cloudless conditions, low humidity, and brisk winds.
  • Environmental Constraints - Cement and lime are both dusty and caustic, and require special precautions.
  • Available Space - Does the site have space to dry the soil, blend additives, or to store borrow soil and spoils?
  • Project Location - Is the site near borrow or disposal areas?
  • Soil Type - Simply, clay and silt are more difficult to dry, and sand and gravel are easier to dry.

Good construction practices can reduce the magnitude of the problem. These actions include:

  • Sealing the surface of the subgrade daily with a smooth drum roller to impede infiltration of water into the prepared subgrade.
  • Provide a slight slope to the subgrade to promote runoff and discourage infiltration, although this may not be practical for a finished slab subgrade.
  • Use a compacted, well-graded aggregate to provide better protection against moisture intrusion than can be achieved with an open graded aggregate layer.
  • Control site access and minimize traffic on wet subgrades.
  • Intercept surface and subsurface water flow with trenches and French drains.

Obviously, correcti ng problems with wet soils requires the expenditure of funds, and/or ti me. However, with a proper understanding of the opti ons available, and an appreciati on of the factors that are a functi on of your site, expenses can be reduced, and the site work can be expedited.

We hope this “Lessons Learned” will help you arrive at practi cal and cost-effective soluti ons to problems with wet soils.
ECS Corporate Services, LLC