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What makes Perc rates differ?
In general, sandy soil will absorb more water than soil with a high concentration of clay or where the water table is close to the surface. Fast draining soils like sand and gravel require a smaller system, than slow draining soils like clay.

What can I expect?
Depending on the water table and soil texture hand digging, or a backhoe will dig a series of holes—the shallow holes, usually 3-4 feet deep allow the Soil engineer to perform a drainage test where the new leaching area (or soil absorption system) will be installed. The engineer pours water into the hole and the rate at which the water percolates into the soil will determine the overall size of the leaching area.

Deep holes allow the design engineer to determine the high groundwater elevation which will determine the elevation of the new system. The new system requires a four to five foot separation between the groundwater and the leaching area (or soil absorption system 'SAS'). A raised system is usually a result of high groundwater.

As part of the preliminary work, the lot is surveyed to locate property lines, wells, trees, pools and other setbacks. With this information, the design engineer can draw up a septic design that can be viewed and approved by the local Board of Health. 

Percolation Testing \ Soil Testing 

What is a Perc Test?

A percolation test ('Perc' test) is a test to determine the water absorption rate of soil.

Perc test data is measured in MPI (minutes per inch) and in simplest terms, the test observes how quickly a known volume of water dissipates into the subsoil of a drilled hole, of known surface area. 

Perc tests are a required input for septic design and inform the design, i.e. primarily the size of a leach field.