System Position On The Lot Is Critical To A Successful Onsite Installation

Soil and landscape characteristics work together to ensure the right system is installed in the optimal location.

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Properly locating the soil treatment area not only involves knowing the soil characteristics (texture, structure, consistence and color) of the site, but also how the area relates to the landscape where an onsite system is being installed. Landscape – along with soil characteristics – will affect surface and subsurface flow of water into, across and out of the site.

Because of this impact, most states require identifying the hillslope position and the slope shape as a part of the site evaluation process to locate and design an individual sewage treatment system. As we have mentioned many times, an installer should take note of these characteristics, and questions about location relative to water movement should be resolved with the designer before installation begins, and probably before you bid the job!

PROPER TERMINOLOGY

There may be some terminology relative to hillslopes and landscapes encountered in the design and system layout. This terminology originates from classic soil and geomorphic landscape studies. The terminology is explained in detail in the Field Book For Describing and Sampling Soils available online from the Natural Resources Conservation Service (USDA-NRCS).

There are five recognized areas along the profile of a hill. Think of them as segments of a line transect that runs up and down the slope. From the top of the hill to the bottom, these segments are the summit, shoulder, backslope, footslope and toeslope. As mentioned above, knowing the location can tell a lot about surface and subsurface drainage patterns.

Areas that are convex or linear in shape typically have good surface and subsurface drainage characteristics. On our transect, the shoulder and backslope areas have convex or linear shapes and are good places to locate the soil treatment area. Those at the bottom of the slope, footslope and toeslope would be concave in shape and are areas where water collects and causes wet soil conditions; so these would be areas to avoid.

The interpretation can go either way for summit areas. If the summit location does not have much slope and the soils are heavier in texture – clay or clay loams – internal drainage will be slow and drainage away will be slow, so an installer could expect those soils to be wet. On the other hand, if the materials are coarser in texture or there is some slope to the area, drainage can be good.

In all cases, the property that helps determine if we are in an area of wet soils is the soil color and whether the soil has redoximorphic features present. Redoximorphic features are represented by specific kinds of soil mottles caused by concentration or depletion of the soil minerals contributing to the soil color.

SLOPE DESCRIPTION

There is one more concept to consider and understand. Think of facing the vertical transect where the hillslope position (also called landscape position) has been defined and then run a line parallel to the contours across the slope. Ask yourself if this line is roughly linear, convex or concave. So each point on the side of the hill can be defined by a combination of two of the conditions. This provides nine possible combinations with the first word from the hillslope description and the second from the plane perpendicular to the slope: linear-linear; linear-convex; linear-concave; convex-linear; convex-convex; convex-concave; concave-linear; concave-convex; and concave-concave.

If the proposed soil treatment area is at the bottom of any concave type landscape, the design should take this into account by providing protection from both surface and subsurface water coming into the site. This could include the installation of swales or berms or even a curtain drain upslope from the site.

Two notes here: The soil descriptions should indicate if there is a wetness problem, so this in itself should affect the depth and type of system to be installed. Installers should check the soil report and see if it agrees with their assessment of the site. If not, this should be resolved before the installation proceeds.

Although the descriptions of hillslope and landscape were developed in the Midwest, the concepts apply everywhere – even in areas with drier climates such as the Southwest. We’ve seen examples where systems have been installed in landscape positions where water will collect. Everything goes fine for a number of years in these areas and then it finally rains. Rain in the summer during the monsoon period comes fast, hard and in large amounts. If the system is installed in an area with heavy water accumulation, it not only gets flooded but often gets taken right out of the ground!

KEEP IT DRY

Other aspects of the installation process can also be impacted by landscape characteristics. If wet soils are a problem, the installer may have to wait until the soil is dry enough. Remember the guiding principle of KIDD: Keep It Dry! Different kinds of equipment may be needed; routes into and away from the site may need to be changed, as well as where spoil piles or materials are stockpiled. Now almost everyone also needs a plan to control erosion and sediment, so there may be a need for silt fencing or other measures just due to the location of the system.

In upcoming articles, we will explore other conditions and problems relative to the landscape a system is installed on.



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