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.