The importance of slope in septic system installation

October 22, 2021

By Sara Heger, Ph.D.

Slope refers to the steepness of the topography. As the slope increases, the steepness increases. Slope calculations are necessary in order to evaluate constructability on the site, assess cut and fill volume calculations, and determine inverts of pipes. The slope will impact the type of equipment used to construct the system and the appropriate final treatment and dispersal components. 

Typical slope calculations involve the intersection of existing slopes to proposed slopes in cuts or fills. Slope calculations are also used when a drainage pipe buried to discharge intercepted groundwater needs to be daylighted. Additionally, slope calculations involve inverts of pipes down-gradient or up-gradient of a component. 

Slope is calculated by dividing the feet (or inches) of vertical rise by the feet (or inches) of horizontal run and multiplying by 100. Rise is the difference in elevation between two points, and the run is the horizontal distance that separates the two points. For example a roof pitch of 3:12 means there are 3 inches of vertical rise for every 12 inches of horizontal run. A 3:12 pitch would have a 25% slope ((3÷12) x 100 = 25%).  

The percent slope can be measured using devices such as a hand level or clinometer, but much of the septic system industry has moved towards using lasers. A hand level is a simple device similar to a telescopic sight. The operator looks through the eyepiece and lines the cross hairs on an object. A bubble level can be seen through the eyepiece so the operator can level the instrument. The hand level is best used by having a second person holding a survey rod. The person looking through the hand level reads off various measurements as the person holding the rod places it at all slope breaks. The difference in readings is the change in elevation. Just one person can use the hand level by sighting in various vertical objects on the site.  

A clinometer is a hand-held device used to measure angles of elevation or inclination. It is used by holding the instrument to your eyes to look through the lens and alongside the housing. A horizontal sighting line will appear. Raise or lower the clinometer (by tilting your head) to place the sighting line at eye level on an object that is upslope or downslope. Read the number closest to the sighting line. A clinometer is lightweight and can be used quickly and easily by one person. However, just as with the hand level, a second person holding a surveyor's rod is much more accurate. Although a clinometer is more accurate than a hand level, it is hard to determine accurate grades on lots that have undulating topography.

When using a laser you set up the laser level on a tripod or flat surface, which will serve as your benchmark. A vertical reference point is an object of permanent elevation, the height or surface of which cannot be easily changed. The vertical reference point may be a lot line corner stake, cornerstone of an existing building, top of a well casing, a center line of a road, or a stake placed by the soil tester in a location where it will remain undisturbed for future reference. The elevation of the vertical reference point may be arbitrarily labeled 100 feet (or any other number), as the other elevations are determined in relationship to the elevation of the vertical reference point. 

If using a manual level, you must ensure the bubble vials show it is level. Find the small screws near the vial. Adjust them until the bubbles vials show level or if it is a self-leveling model, allow it a moment to level itself. The unit emits a laser showing level. You then use a laser detector, typically attached to measuring rod, to intercept the laser. The operator adjusts the detector up and down the rod until you hear beeping. This means the detector has found the laser. You can use the change in distance from where you started (referred to as the high of the instrument) to the reading you took to determine the change in elevation. Measuring the distance between these two readings is the other critical factor needed to determine the slope.

The installer must be able to read the slope on the site, lay the system out as indicated on the plan and make the calculation of the slope and changes in elevation while installing the system.