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:
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.
Dynamic compaction is a construction technique that increases the density of soil/waste deposits by dropping a heavy weight at regular intervals to consolidate and improve the geotechnical characteristics of the deposit so that it can be suitable for redevelopment. This construction technique can be used to transform otherwise undevelopable property, such as old landfill areas, into developable property.
Most soil types can be improved by dynamic compaction; the method is particularly well suited to non-organic, irregular fill, where variable characteristics such as solid wastes are present. Field conditions and several other parameters are considered when designing and implementing dynamic compaction programs to keep costs in line. The primary considerations include, but are not limited to, waste delineation, distance from the ground surface to ground water, waste thickness, minimum energy, and selection of dynamic compaction parameters.
The following factors and associated costs should be evaluated if dynamic compaction is to be considered:
Major change orders and environmental impacts can be expected if the plan does not address these factors.
If you decide to consider dynamic compaction in your redevelopment project, having onsite construction quality assurance monitoring during the process is important. CQA monitoring will verify that the work is implemented as designed and permitted, and that proper techniques are used to make sure the proper distribution of energy into the ground is taking place. The CQA monitor will also check to see that the final configuration of the fill is achieved, a safe working environment is maintained., and that ground vibrations are monitored near adjacent structures to to prevent structural damage.
For developments involving construction of buildings over a dynamically compacted areas, a combustible gas barrier layer is generally required below the building footprint to safely collect and vent subsurface combustible gases (i.e., typically methane) to the environment. Construction costs associated with a combustible gas barrier layer should include the following:
In summary, dynamic compaction is a proven geotechnical construction engineering method that can be used to improve certain landfill areas to support redevelopment. SCS Engineers has completed many projects of this nature and is ready to serve and help to bring your project in service.
Ali Khatami, PhD, PE, LEP, CGC, is a Project Director and a Vice President of SCS Engineers. He is also our National Expert for Landfill Design and Construction Quality Assurance. He has nearly 40 years of research and professional experience in mechanical, structural, and civil engineering.
Dr. Khatami has acquired extensive experience and knowledge in the areas of geology, hydrogeology, hydrology, hydraulics, construction methods, material science, construction quality assurance (CQA), and stability of earth systems. Dr. Khatami has applied this experience in the siting of numerous landfills and the remediation of hazardous waste contaminated sites.
Dr. Khatami has been involved in the design and permitting of civil/environmental projects such as surface water management systems, drainage structures, municipal solid waste landfills, hazardous solid waste landfills, low-level radioactive waste landfills, leachate and wastewater conveyance and treatment systems. He has also been involved with the design of gas management systems, hazardous waste impoundments, storage tank systems, waste tire processing facilities, composting facilities, material recovery facilities, landfill gas collection and disposal systems, leachate evaporator systems, and liquid impoundment floating covers.