If the plans and specifications do not clearly state the means and meth- ods of construction or the intent to protect the soils, then the owner may be in for an expensive change order. The intent should be clear in the project documents to protect the soils thereby leaving open to the contractor to be creative with the means and methods. How Soils Play the Leading Role Effluent, which has received primary treatment from the septic tank(s) and possibly advanced treatment is discharged to the leachfield through a distribution piping network. The leachfield consists of individual trenches or a bed, depending on the site, the size of the system, and state or local code. The dispersal field may also be known as a drain- field or leachfield, and to reflect its true function could be referred to as the “Soil Treatment Unit.” Research has demonstrated that the soil treatment processes include physical (filtration), chemical, and bio- logical processes. Disease causing pathogens and virus can be starved or poisoned by soil bacteria, fungi, and other microbes that produce antibiotics. There is also predation. Soil is a hostile environment for pathogens outside of the human host where significantly different conditions exist such as temperature, moisture, and acidity. Because of these processes the Soil Treatment Unit archives tertiary treatment and provides natural disinfection. Whereas onsite systems of the past have mainly been concerned with hydraulics (e.g. make the water go away), today’s large decentralized sys- tems are designed for hydraulics and treatment. Not only must the primary treated effluent be adequately dispersed and absorbed, but sufficient treat- ment must also be provided to protect public health and the environment. Preparing and Excavating the Wastewater Treatment Site The main goal is to protect to soil, therefore approach each phase of construction with that in mind. 1. Site clearing and grubbing: If there are trees to be cut and stumps to be removed then evaluate methods for the activity to occur without impacting the soil. For example, if installing a shallow dispersal system in sensitive soils (low permeability silt and clay soils), it may be best to specify tracked equipment only for stumping and dragging out logs. Or it may be practical to cut shrubs and trees flush and leave the stumps. The soil plays an important role in disposing and treating effluent, the project plans and specifications should define methods to protect the soils during construction.
2. Soil moisture content: It is not recommended to install a system in wet conditions or in overly moist soils, as this causes machinery to smear the soil interface, which can affect system performance. It is recommended that the contractor meet with the design engineer prior to the installation to review soil conditions and determine if the conditions are appropriate for installa- tion. Plans should be detailed to excavate in phases that can be completed daily to minimize exposure to rain events. 3. Construction staging in the drainfield area: The drainfield area should not be utilized as a staging area for construction equipment or materials before or after construction of the wastewater treatment system. Temporary fencing, warning tape, barriers and/or appropriately located signs are recom- mended to prevent unauthorized traffic from damaging the drainfield area. 4. Construction traffic: Construction traffic should never be directed over the system area before or following installation. This includes dump trucks and wheeled vehicles. Prior to installation of the wastewater treatment system, the contractor should review the plans and devise an installation process to minimize construction traffic over the disposal areas. The specified product for disposal can influence the performance of the system. Here the dust and fines of an aggregate system can be seen, which will limit the native soils’ permeability. And the placement will cause compaction and embedment.
5. Erosion and Sedimentation Control: Silt and clay can be carried as sediment during storm events. The small particles can plug up soil infiltration
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november 2020
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