There are many advantages to keeping septic tanks, pump tanks and other advanced pretreatment units (aerobic treatment units and media filters) shallow. During the design and installation process, potential disadvantages can be accounted for.

As tanks and other components are buried deeper in the ground, there is more downward force on the element from the weight of the soil. Every tank and advanced treatment unit has a specific load-bearing capacity dependent on its material and geometry. The manufacturer should indicate this maximum load as a pressure rating in pounds per square foot or a maximum burial depth.

Depending on the material (concrete, plastic or fiberglass), the component can handle varying amounts of force. Concrete tanks are known to withstand the most downward pressure, but they have limits as well. For instance, standard concrete septic tanks are only intended to be installed under vehicle traffic loads if designed for traffic loading. Unless the tank is traffic-rated, measures may be needed to prevent traffic over components exceeding the load-bearing capacity.   

All tank materials can be damaged if they are buried deeper than their designs specify. The allowable installation depth should be verified with the manufacturer of your chosen tanks and advanced pretreatment units.  

Another concern with deeper burial depths is buoyancy. All buried components have the potential of being lifted out of the ground due to forces acting on them in saturated soil. Saturated soils can exist or develop around the elements because of shallow groundwater, water filling the excavated volume and flooding over the components. Seasonal groundwater can surround the element and rise to a level equal to the original groundwater level in the soil.

If the unit weighs less than the force of water displacement, it will float — particularly when empty — like a fishing bobber.

WATER PRESSURE

On many sites, the deeper you go in the ground, the more likely you will encounter a water table, which could cause an outward force on your tank. The backfill in an excavation may become saturated due to the backfill material being able to accept and transmit water at a greater rate than the native soil. Rainfall can infiltrate the excavated area, filling the pore space in the backfill material with water.

Additionally, a site with a clay pan or limiting layer relatively close to the soil surface can have a seasonal flow of water moving across the limiting condition. An excavation passing through that limiting condition can allow the flowing water to enter the excavation and cause saturation. A limiting layer may sometimes prevent laterally flowing groundwater from coming to the surface. When the hole passes through the limiting layer, the seasonally flowing water below the limiting layer can rise into the backfill material in the excavation.

Pretreatment units placed in floodplains or flood-prone areas can have water flowing into the soil when the site is saturated during a flood event. The floodwater can infiltrate the backfill in the excavation.   

A buoyancy analysis must be conducted to ensure a component will not float when in saturated soil. To carry out the analysis, you must know the weight of the empty tank, the weight of the minimum amount of water in the tank (typically assumed to be zero), the weight of the soil directly above the tank and the weight of the maximum volume of water that is displaced (the buoyant force). There is a benefit here to burying your tank deeper if the soil above the tank is not saturated. In this case, the soil on top of the tank can help balance the upward buoyant force.

If water is encountered during excavation and the design plan does not account for this with anti-floatation protection, the designer of record should be contacted and made aware of this risk. Measures should be taken to eliminate floatation potential.

AVOID INFILTRATION

Remember all excavations are possible areas for collecting rainwater and groundwater — they act like sumps, especially under high rainfall events and when porous bedding material is used. Even on sites with deep water tables, it is still possible to float a component. This is especially true when using soft, noncompacted backfill or porous bedding material, and when mechanical compaction is impossible around fiberglass and poly filter containers. Using proper and approved anti-floatation measures or manufacturer-supplied components minimizes this risk considerably.

Increased concerns about water at deeper depths also come with increased problems for infiltration into pretreatment components. Groundwater infiltration can disrupt the settling, treatment and storage of solids by reducing detention time. Infiltration also results in hydraulic overload of downstream elements in the treatment train by increasing flows beyond design capacity.

Components should be manufactured and installed to prevent infiltration, but it is also less likely to be an issue if there is water around the unit to test joints, seams, penetrations and inlets/outlets.

Finally, shallow pretreatment components are much easier to service and maintain. Many pretreatment components require access to grade, but the bottom depth of the unit can impact serviceability. The deeper the element is buried, the more difficult it is to maintain.

In cold climates, there may be concerns about shallow components freezing. In these cases, 2 feet of soil should provide enough insulation for operation. Tanks buried at shallow depths (less than 2 feet of soil cover) may require additional insulation. In most situations, just the tank lid will be insulated with foam board, but be sure the insulation is designed for burial. Spray-on insulation is now available and provides an additional sealant for seams coated in the insulation. 

EMPLOYING PUMPS

In some projects — homes with basements where wastewater is generated, for example — keeping pretreatment components shallow may require a pump. These pumps are typically grinder or ejector pumps, which create a slurry to move the solids that may be present. If this is part of the system, the design should account for it. Some potential design solutions include:

Limiting the volume pumped — the smaller the dose, the better to minimize turbulence.

Pumping into the sewer line versus the septic tank to minimize turbulence.

Upsizing the septic tank by 50% and having multiple compartments/tanks to allow for more settling.

Utilizing a larger effluent filter that has more storage, such as a 6- or 8-inch diameter to catch solids in suspension.

Having the pump only handle the wastewater from the lower elevation in the structure and letting the flow from higher elevations run at gravity to minimize turbulence.

The benefits of keeping components shallow well surpass the disadvantages. Some designs need to account for cold climates and elevation challenges. Keeping pretreatment elements shallow will reduce the overhead load and likelihood of groundwater infiltration and facilitate maintenance.