FILLING STATION EXCAVATION

Pinnacle Engineering was retained to conduct environmental assessment services for a property in south Minneapolis that was slated for redevelopment.  The property was initially developed as a gasoline service station with underground storage tanks, hydraulic hoists, and an oil-burning furnace; portions of the site had been used for automobile sales and repair.   Fuel tanks had been removed in the 1970’s.

Pinnacle completed a Phase I Environmental Site Assessment, pre-demolition inspections, a geotechnical investigation, a Phase II Environmental Site Assessment, and a Limited Site Investigation.  Environmental and geotechnical soil borings revealed 12 feet of fill overlying swamp deposits on the north end of the property, and low-permeability glacial till on the south end.  Significant petroleum contamination was found in the fill, the swamp deposits, and in groundwater on the northern portion of the site.  

The proposed development included a level of underground parking, which required the excavation of several thousand cubic yards of petroleum-impacted soil.  Pinnacle prepared a Development Response Action Plan (DRAP) for the excavation and disposal of this soil, oversaw implementation of the DRAP, and conducted construction material testing.  On behalf of the client, Pinnacle secured over $200,000 of investigation and cleanup funding from Hennepin County and the Metropolitan Council for environmental site investigation and remediation of the property. 

Due to the presence of the swamp deposits, it was necessary to excavate approximately one foot below the groundwater table on the north end of the property to remove soils that were unsuitable for building support.  During excavation, an area approximately 25 feet wide, similar to a filled-in channel, was found under the swamp deposits, and dewatering was required to effectively excavate this material.  The project plan and budget did not include dewatering and treating/disposing of petroleum-impacted groundwater, so several feet of fill was placed in the northern section of the excavated area to act as a dam.  Contaminated water was pumped from the channel area into the northern area, behind the dam, lowering the water sufficiently to excavate the channel.  Recycled crushed concrete was then placed in the base of the excavation, under the water table, to create a stable base for placement and compaction of imported fill to bring the Site to the construction grade.  Once the channel was excavated and filled, pumping was stopped, and water flowed through the crushed concrete and stabilized at its original level.  Since soil and groundwater were already impacted on the northern end of the Site, this temporary dewatering had no adverse environmental effect to the property. 

Pinnacle also designed and oversaw the construction of a passive soil vapor system under the new building to manage soil vapor from the impacted groundwater on the north end of the property. 

Based on the investigation and completed remediation, the petroleum release was closed by the Minnesota Pollution Control Agency.