soft soil

October 30, 2017

Discovering unexpected pockets of soft soils at the time of construction can delay your project and drive up costs for landfills, support features, and many other types of construction. If you don’t find them, building over them can result in unexpected settlement affecting a structure or building, or cause a slope stability problem for a berm or stockpile. You can avoid both of these scenarios with early investigation and appropriate construction planning.

While landfill development investigations typically require numerous soil borings within the proposed waste limits of the landfill, it’s common to overlook perimeter areas. Pockets of soft soil deposits can be associated with nearby existing wetlands, lakes, or rivers; with wind-blown silt or ancient lake deposits from periods of glaciation; or with fill placed during previous site uses.

The landfill perimeter areas may contain tanks for leachate or fuel, buildings, perimeter berms for screening or landscaping, stockpiles, and other features. A tank or building constructed over soft soils could experience unexpected settlement affecting the performance and value of the structure. The potential for a slope stability problem can increase for a large berm or stockpile built on soft soils.

The first step to avoid these problems and identify problem soils is to include perimeter areas in your subsurface investigation. Perform soil borings or test pit excavations at the locations of the proposed perimeter features such as tanks or berms. If you encounter soft soils, address them like this:

• If the deposits are relatively shallow, excavate the soft soils and replace them with compacted engineered fill.
• If the deposits are deeper and there is sufficient time in the project schedule, pre-load the soft soil area to reduce future settlement and increase soil strength before construction, and monitor the pre-loading with instrumentation such as vibrating wire piezometers and settlement platforms to confirm when the pre-loading design goals have been achieved. Preloading can be accomplished with temporary soil fill placement that is later removed when the pre-loading is completed or by staged placement of fill for a permanent fill feature such as a berm.
• If the project schedule doesn’t allow for pre-loading and the soft deposits are deep, consider a ground improvement method such as GeopiersTM to improve soil strength and stiffness in place. You can then proceed with constructing tanks, buildings, berms, or other structures over the improved soil area without special foundations. You may also use a deep foundation system such as piles or drilled piers to build over a soft soil area.

Contact SCS’s geotechnical engineers for more information on how to find and test soft soil areas early in a landfill’s project schedule, so you can effectively address associated construction issues in a way that considers cost and minimizes unexpected project delays.

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