Design and Installation of a Sub-Slab Depressurization System from an Industrial Source

TRC designed and installed a sub-slab depressurization system (SSDS) which provided the necessary air flow and vacuum beneath the basement slab to prevent the chlorinated volatile organic compounds from entering the building.  Monitoring of the pressure gradients demonstrated the system performance standards were met and the project cost effectively mitigating the indoor air risk.



Confidential Client

Project Location

Southeast Michigan

Shallow groundwater affected by chlorinated volatile organic compounds (CVOCs) migrated 800 feet off‐site from beneath a 50.5‐acre industrial facility and impacted the groundwater beneath a residential property. Because of the high concentrations in groundwater and highly permeable soils between the groundwater and the building foundation, there existed a risk that gas phase vapors emanating from the CVOC‐affected groundwater could seep into the residential home, impacting the indoor air. Consulting with TRC, the client decided to address the potential migration of CVOC vapors to indoor air by installing a Sub‐Slab Depressurization/Ventilation (SSDV) System. The main challenge with installing the SSDV system was the age of the house, built in 1900, and the structural integrity of the basement walls and floors. The basement floor of the 1,500‐square–foot house was composed of unleveled concrete, and the walls were constructed of grouted field stone. In addition, an unused, but open, water well was located in the basement.

  • A U‐tube manometer located in the basement to monitor differential pressure between the basement and the vent pipe riser;
  • A system alarm, which emits an alarm when differential pressure is not maintained between the vent pipe riser and the basement;
  • A brass ball‐valve sample point located just above the exhaust fan so that system exhaust may be monitored; and
  • A bypass for condensation drainage to prevent freezing or blockage of the exhaust fan.

The monitoring system consisted of a differential pressure gauge, which monitors for the presence of suction in the piping at all times. If the monitoring system does not detect any suction, a visual/audible alarm is activated. In order to measure sub‐slab vacuum, TRC specified two cross slab differential pressure gages to be installed at the two corners of the house furthest from the suction point. The design allowed pressure measurements to be taken from outside the house.

System Installation
The system was installed as designed using a variety of construction techniques. The SSDV system installation consisted of coring a suction hole, installing the fan, installing suction and exhaust piping, and installing other system components. The suction hole was cored through the concrete slab and piping was pitched back toward the suction hole. The fan was positioned on the building exterior, while the alarm was positioned in easily accessible locations in the vicinity of the suction hole. The exhaust fan was hard wired into the house’s existing electrical system to help ensure that the fan remains in continuous operation. For system safety and maintenance, a disconnect switch was located within sight of the fan. To eliminate soil gas intrusion pathways, concrete floors were repaired/replaced as necessary, and all accessible cracks and openings in the foundation walls and floors were sealed. 

System Monitoring and Performance
Following system installation, a system performance test was conducted to confirm that the system was operating within the design specifications. The initial test showed that the system was operating as designed. Quarterly system inspections, which require less the 30 minutes to complete, are conducted with other on‐site work to minimize project costs. A pressure gradient across the building slab has been maintained at acceptable levels for 2.5 years, indicating the subslab‐to‐indoor air migration pathway has been eliminated.

The client, at the agency’s request, completed indoor air sampling during the first year of operation, to show the homeowner that the measured indoor air concentrations were now below indoor air criteria. This indoor air sampling confirmed the SSDV system’s effectiveness. The pressure point monitoring, which establishes that the pathway has been eliminated, demonstrates to regulators that ongoing costly indoor air monitoring is unneeded.

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