Septic Soil

Why rock isn't “suitable soil”.

There is much confusion being created by practitioners in the OSSF industry classifying rock as some form of suitable soil into which they can, therefore, install soil absorption systems for septic effluent. There is a need, therefore, for clarification on how to comply fully with the rules, 30TAC285.

This paper, then, will look only at the rules and how to apply these rules to soil classification under 30TAC285. The intention here is to help Designers, Installers and Regulators understand how to properly apply the rules when classifying sites with rock or rocky soils. As stated above, there is a great need for this understanding.

Discussions about the functions of rock, fractured rock, weathered rock, and even the grossly misused term “caliche” in treating wastewater are outside the scope of this discussion. Even professionals such as Geologists, Engineers, and Soil Scientists are in varying forms of agreement and disagreement on this subject.

30TAC285 defines soils with particle sizes in excess of 2 mm (about 5/64 of an inch, or about the diameter of a student's No. 2 pencil) to be gravel. Look at the site as a whole, not as a single handful of soil carefully extracted from between rocks or rock layers. If the site soils contain rock, cobbles, layers or any shape pieces of rock that are larger than 2 mm, then that portion of the soil is gravel. If that gravel portion of the soil is more than 30% of the volume, then that soil is unsuitable for a standard absorption field.

A layered rock structure is a restrictive horizon at the top, and throughout the structure.

For support on the above discussion refer to Tables V and VI in 30TAC285, as well as section 285.30(b)(1) (A), (B), and (C).

In 285.33(d)(4) and (5), we are told that in fractured rock, or gravel we can either use soil substitution, or secondary treatment with disinfection. One should also be aware that we could also use a full depth mound, E.T., or any of those other solutions where we physically or vertically separate the disposal field from a limiting condition.

The common practice of searching about for a place to dig a profile hole that looks like suitable soil when the balance of the disposal field is rock is obviously an attempt to circumvent the rules. Clearly, we are directed to design systems for the soils into which they are installed. If any portion of the disposal field is in rock, then the disposal field must be considered to be in rock.

What about if we blast the rock?

Blasting the rock increases the fissures through which effluent could pass too quickly, which brings one right back to 285.33(d)(4) and (5).

Rock in it's various forms, therefore, is not “suitable soil” under 30TAC285.