Building Settlement Mystery Solved
Construction of a 1.2 million square foot warehouse in Pennsylvania was nearing completion when portions of the building were observed to be experiencing an alarming amount of differential settlement. When an insurance claim was filed, GEODesign was retained by the insurance company to investigate the cause of the settlement and recommend remediation measures.
Some portions of the building experienced up to 6 inches of settlement. The settlement was sporadic and did not have an obvious pattern. The settlement impacts included separation of exterior wall panels and cracks in the foundations.
The site preparation required a massive cut and fill operation. Rock and soil were excavated from the elevated portion of the site and placed as fill in the lower portion of the site, where the grade was raised by up to 40 feet. The settlement was observed at the maximum fill thickness locations.
Some initial thoughts for the settlement causes were as follows:
- Maybe abandoned coal mines collapsed due to the weight of the fill. A review of the site’s history indicated that coal mines were not present below the building.
- Maybe voids, due to dissolved bedrock (karst topography), collapsed due to the weight of the fill. Geologic information indicates that soluble bedrock is not present in the area.
- Maybe soft soils below the fill settled due to the weight of the fill. Borings performed by GEODesign and others did not encounter any significant soft soil conditions.
GEODesign’s post construction borings encountered very slow drilling conditions and loss of drilling fluid at various depths within the fill. This information and test pit results indicated the presence of large boulders (up to 3.5 feet) within the fill material. A review of construction photographs also revealed that significant quantities and sizes of boulders were clustered together (i.e., nested) within the fill.
Nested boulders can result in significant void space between the boulders. Water flowing through the fill can transport soil particles into the voids, which eventually results in settlement at the ground surface. Therefore, GEODesign determined that the building settlement was caused by nested boulders within the fill. Areas with high nested boulder concentrations will result in greater settlement, which also explains why the settlements did not have an obvious pattern.
The original geotechnical report recommended a maximum particle size of 18 inches. Therefore, boulder sized materials were allowed to be included in the fill. Unfortunately, the original geotechnical report did not include material gradation requirements, which are critically needed to ensure that nested boulders are not present.
GEODesign provided several remediation recommendations including supporting the building on piles drilled through the fill, grouting the voids within the fill, and a combination of high strength geotextiles with compacted fill.
Author: Brendan O’Keefe, Senior Staff Engineer
