Septic systems are site specific and no one design or type is suitable for every situation. In many cases there is only one legally permissible system type but it may not be feasible from a financial or long term maintenance expense viewpoint.
The major factors that need to be considered for a residential structure are:
- Square footage of residence and number of bedrooms or rooms that could be used as bedrooms.
- Depth to bedrock, impervious or restrictive horizons, groundwater seepage, and quality of soil structure.
- Soil type.
- Available area and legal lot status.
- Slope, easements, and other setback requirements
The square footage of a residence and the number of bedrooms determines the minimum daily water flow required to be used in a design. If a room can be used for a bedroom it is considered a bedroom even without a closet or bathroom. It is well documented that larger square footage homes use more water than smaller homes. In either case the factor requiring the larger water flow is used. The number of bathrooms has no impact on daily water flow or size of the septic system.
Depth to bedrock or restrictive horizons determines which system types are permissible under regulations. Test excavations must be made with a backhoe in the proposed septic drainfield area. These are typically 2’ wide by 5’ deep or until the restrictive horizon or bedrock is encountered. In most cases at least four excavations are necessary although two are the minimum required by regulation. It is necessary to perform additional excavations over the entire area so that during construction no unsuitable soils are encountered. The quality of the soil structure must be such that heavy equipment can excavate clean trenches and beds without destroying the undisturbed material required to be left between the excavations. In some cases there is adequate depth but the soil structure will not allow construction of the system
Soil type determines how large the required system type must be. In a few cases there are certain soil types for which no system may be permissible. Soil types are determined by the texture of the soil based upon particle size. Class IB, II, and III soils are acceptable for most systems depending upon the depth of the restrictive horizon or bedrock. Class IV soils mandate the largest areas and eliminate some otherwise suitable system types. Class IV soils are clay soils typically found east of IH-35 but are found routinely in pockets above bedrock horizons west of IH-35. When class IV soils are found on shallow restrictive horizons such as bedrock the resulting systems tend to be large, high maintenance, and expensive to install and operate for the long term. The most common soils west of IH-35 are Class III soils which are typically known as caliche. Caliche is an undefined material and the use of this descriptive is very broad. Class III soils are calcareous in nature due to the widespread limestone west of the Balcones Escarpment which roughly parallels IH-35. Class III soils may or may not be suitable for septic installation depending on depth to bedrock, presence of large gravel or rocks, and general structure. Class IB & II are sand with less than 30% gravel and sandy loam type soils respectively. These are excellent soils but are found typically only around the Colorado River flood plain in alluvial deposits. These soils allow the largest number of system choices and smallest size drainfield areas. Typically these are small simple systems that are the least expensive and the easiest to operate with minimum long term maintenance costs.
Available area and legal lot status are the most difficult aspects of planning an OSSF. First the lot must be legally subdivided in accordance with local regulations. If it is not legally subdivided it still must be a legal “meets & bounds” type tract. If a large tract has been surveyed or divided up and sold without the subdivision recorded, permits may not be approved or attainable. These are called illegal tracts or subdivisions. Many of these tracts are listed on the tax rolls under the newer illegal subdivision or owner name. Because these are listed on the tax rolls in this manner does not mean they are legal. The appraisal district has a fiduciary duty to collect taxes from the owners of the property regardless of how it is divided up.
Many lots were legally subdivided many years ago when septic requirements were less stringent and required little area. In many cases today multiple lots will be required to build a structure and OSSF. Often easements encumber all boundaries of these lots and must be vacated prior to permit approval to construct. Because another nearby property has been developed does not mean an adjacent or nearby tract can be developed. Once all easements, improvements, structures, and other concerns are considered there must be available area to install the system without variances. Regulatory jurisdictions are not permitted to grant any variances on new construction. If a project cannot be completed without a variance it must be downsized or it may not be permitted. Existing structures requiring replacement systems may be granted variances but only in certain circumstances and only if the structure has not been modified beyond the limits of the original septic permit provisions. There are some variances that are never granted such as setbacks to potable water lines and wells. New lots may have special restrictive covenants or subdivision requirements that dictate particular system types or require replacement area be available if the existing system fails.
Slopes, easements and other setback requirements affect the net available area and the feasibility of septic installation. Septic systems must meet minimum setback distances from easements, wells water lines, property lines, foundations, structures, improvements, lakes and creeks, rock ledges, drop-offs/cliffs, grade elevation changes, and others not listed here. New construction must meet all required setback distances. Replacement system construction may be granted certain variances on a case by case basis. Percent slope is defined under the formula “rise over run”. For example a slope with 15’ vertical rise or drop over 100 feet would be considered a 15% slope. Most systems must be installed with 15% or less slope where the drainfield is located. Some systems are permissible where slope is greater than 15% but less than 30%. These are usually high maintenance type systems and are typically expensive to construct and maintain. Steep slopes also are very difficult to construct with heavy equipment and usually require hand labor which drives the costs significantly higher. The time required to install these systems is often doubled or tripled. Once these factors are considered the net available area can be calculated and feasibility determined.
In order to evaluate a site to determine septic system feasibility the follow steps must be taken:
- Formulate an ideal plan on paper of how the lot is to be developed including future improvements such as workshops or swimming pools. Generally a survey of the property is necessary and often must include topographic contours at 1’ elevation intervals and a tree survey. Sketch the location of the house, driveway, water line, utilities and water wells on a copy of the survey. Other information may be needed as well.
- Meet on site with the designer to discuss the layout and possible option for septic system placement. At this time the locations for test excavations can be marked.
- Call 1-800-DIG-TESS. This is the local “one call” number that must be called prior to excavation. Dig-Tess will contact all entities within the state that may have buried utilities. They will mark the utilities within 300’ of the proposed excavation. They mark only known utilities in public right of way. No private lines are marked. This is a legal requirement subject to a minimum fine of $10,000.00 for non-compliance. Any damaged utilities will be the responsibility of the property owner or contractor.
- Have test excavations performed in the designated areas.
- Once the test excavations are performed the designer can determine the exact size and system types permissible for the conditions on this individual property.
Mandated documents include:
- Proof of ownership
- Survey showing all platted easements and generally will show topographic contours at 1’ intervals plus major trees or other significant features on the property.
- Subdivision plat
- Floor plan
- Flood plain verification forms
- Application for Permit to Construct/Authorization to Construct OSSF
- Local building permit applications or development applications
- Driveway permit where applicable
- Locator map with detailed travel directions
Additional documents may include:
- Notarized affidavits for systems requiring maintenance contract recorded on the deed records of the appropriate county
- Maintenance contract with duly licensed service providers
- Miscellaneous affidavits for specific situations
- Notarized letters for specific situations
- General checklist of other jurisdictional requirements such as Balcones Canyon land requirements, Golden Cheeked Warbler permits From the Department of Fish & Wildlife, TXDOT permits for driveways on TXDOT roads, etc.
- Vacation of easements
- Other items on a case by case basis for specific issues.