The value of land in large metro areas has climbed so high that developers investigate developing sites previously filled with trash, garbage, debris, or used by industry or the military. Redevelopment or rehabilitation of these properties is performed in accordance with approved urban renewal plans, and under site-specific environmental agencies. Some of these sites have regulatory agency files but filed when regulations were not as strict as they are today. Other sites are so old that they have not been on regulatory agency radar.
Developers usually pay a lower price for such lands compared to a virgin land, which is rare to find in prime commercial or industrial areas, or land that has a dilapidated building on it, only suitable for demolition. Developers know that they are responsible for handling any environmental issues as soon as they open a file with a regulatory agency for the redevelopment of the property. Environmental due diligence helps determine the costs of addressing environmental issues before purchase.
Another essential part of due diligence is examining the foundation – whatever is below the ground surface, to determine its load-bearing properties for future development. There are several options available to improve load-bearing capability depending on the type, depth, and age of the below-surface material and the load-bearing properties required of the proposed development. Developers hire geotechnical engineers, to factor this, and many more parameters into an evaluation of the site, and to develop options for improving foundation strength.
One option for improving foundation strength is through dynamic compaction, which involves dropping a heavy load from a significant height for a certain number of times on locations identified by a grid pattern. The kinetic energy of the weight at the time of impact on the substandard foundation compresses the material, reduces voids in-between material particles, and increases internal friction or shear strength of the material. The practice has been around for decades, and developers are familiar with the methodology. The design of a dynamic compaction program is best carried out by a geotechnical engineer familiar with site conditions and parameters. Dynamic compaction is a reasonable and cost-effective option for specific vertical development to improve load-bearing foundations.
Another option is installing piles in a grid pattern into the ground, extending into the virgin ground. The piles carry the building load via pile skin friction or point resistance at the tip of the piles. Driving piles is more expensive than the dynamic compaction option discussed above. Piles are characteristically useful for high design loadings. Dynamic compaction is useful to minimize ground settlement around the piles, preventing voids from forming below the building as the ground settles over time. While the building remains at its constructed elevation above piles, dynamic compaction helps avoid problems with utilities below the building slab, including water lines, sewer lines, and electrical lines. Limiting the amount of settlement prevents future vertical shifts in ingress and egress structures, driveways connected to the building, docking ports for trailers, and outside staircases if not located on piles.
The gas vapor barrier system under the building prevents unwanted gas from moving upward from materials in the ground into the building. Minimizing settlement by performing dynamic compaction prevents the barrier from vertically shifting and opening passages for unwanted gas moving into the building. The integrity of the barrier layer is essential in maintaining the building’s protection. These problems are tremendously expensive to fix, and agency officials could deem the structure unsafe for occupation.
A third option is the excavation of unsuitable material then backfilling with suitable soil. Depending on the contamination, it is possible to clean the soil then return it clean as backfill. For the building foundation to have sufficient bearing capacity, a geotechnical engineer oversees the operation. Filling the excavation in dry conditions is less complicated than wet conditions. In sites where excavation is deep and groundwater is high, dynamic compaction of the backfill, placed in the ground in wet conditions, may be necessary to achieve sufficient shear strength to support the proposed development.
Developers and city planners want viable solutions that are financially reasonable. While dynamic compaction may sound like a crude methodology, it plays a vital role in improving substandard foundations. If you are considering redevelopment of a landfill, Brownfield or other property where the foundation is currently unsuitable consider establishing a business relationship now with a reliable dynamic compaction contractor since they are highly in demand and their availability can affect the project schedule.
About the Author: Ali Khatami, Ph.D., PE, LEP, CGC, is a Project Director and a Vice President of SCS Engineers. He is also our National Expert for Elevated Temperature Landfills, plus 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 many followers of his blog series “SCS Advice from the Field” on SCS’s website and social media channels. Send him a question or topic you’d like him to address.
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.