Design of a Sub-Slab Depressurization System

TRC implemented vapor intrusion mitigation activities at a minimal cost while meeting the client’s expedited schedule. The indoor air pathway was successfully mitigated, allowing the real estate transaction to proceed as planned.



Confidential Industrial Maintenance Facility

Project Location


The vadose zone soils and fill beneath the building slab of a maintenance facility were affected with chlorinated volatile organic compounds (CVOCs) that caused indoor air quality issues, thereby limiting the marketability of the property. Data indicated the presence of CVOC concentrations above state-regulated screening levels inside the building. To address the indoor air issue, the client decided to install a sub-slab depressurization system (SSDS), utilizing data collected during a delineation assessment performed by another consultant. Results of the active gas sampling at 26 locations in the 15,000 sq. ft. building indicated that the average radius of influence was 25 feet for the mostly sandy subsurface. The client wanted to utilize the data collected during active gas sampling for a basis of design for an SSDS. However, in consultation with TRC, it was determined that utilizing existing data would have resulted in affected soils being remediated versus the stated goal of mitigating indoor vapor. The client requested that TRC conduct a pilot test and assist with the design and build of an SSDS in an expedited manner (three weeks) in order to facilitate a real estate transaction.

Pilot Test
TRC conducted a pilot test while concurrently designing and assisting with the field installation of the SSDS. The goal of this pilot test was to determine the location and number of suction points for system installation. TRC conducted a connectivity pilot test in order to determine the design radius of vacuum influence that could be imparted in the fill material directly beneath the facility’s concrete slab to mitigate vapor intrusion potential into the building. Based on the results of the pilot test, TRC located 10 suction points to cover the required SSDS area (15,000 sq. ft.).
Based on the distribution of the suction points, and due to project time constraints, one leg of the system consisting of five suction points was installed the day after the pilot test data was collected and analyzed. TRC further tested the proposed design through these five suction points. 

System Design
Based on the results of the pilot testing, TRC developed the basis of design for a full-scale SSDS. The objective of the permanent full-scale SSDS is to create a sufficient vacuum below the floor slabs, thereby minimizing the potential for vapor intrusion into the building. The quantity and locations of permanent SSDS suction points were determined based on the results of the pilot tests, client specification, and in consideration of the potential future use of the building.

In order to provide a means to confirm that the SSDS is operating properly, a flow switch and pressure (vacuum) gauge were included near the inlet of each suction fan. Each flow switch activates a remote visual alarm inside the facility if low or no air flow through the fans exists. Also, system valves were included to adjust the vacuum applied at each suction point and volume controlling dampers were included at the outlet of each fan.

Operation and Maintenance
TRC designed a performance test, which was conducted 30 days after completion of construction and startup of the permanent SSDS. Vacuum in the sub-slab relative to the indoor air was measured at each compliance point using a magnehelic differential pressure gauge. Vacuum was also measured at each suction point and on the suction side of each fan. Air velocity was also measured at each header pipe entering the fans to confirm air flow rates. The system operated as specified in the TRC design. TRC also prepared an operation and maintenance plan for quarterly and annual maintenance that includes, but is not limited to, inspection of floor and wall penetration sealing and caulking, as well as checking the fan static vacuum and airflow measurements and operational variances.

Throughout the completion of pilot testing, remedial design, and system installation, TRC has successfully addressed the potential for indoor air issues and worker exposure by designing a system that will successfully mitigate CVOCs in the sub‐slab soil gas using a cost effective targeted approach. TRC’s approach was implemented at a minimal expense and within the client's deadline. If this system was not designed and installed in the expedited manner, the real estate transaction would have been compromised.

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