One of the 6 factors (Rule 420-3-1-.71)Certain limits for OSS on steeper slopes

There are 4 soil related factors And 2 site related factors

All are to be evaluated on each site

What is the slope?% Slope = (Rise ÷ Run) x 100

Rise

Run

40 ft

100 ft% Slope =

(40/100) x 100 = 40%

• Requires Engineer Design• Start with Table 4 in Rules• Rules require

Deeper trenches More space between trenchesMore cover over trenches

MinimumTrench

Depth**

MinimumCover

Percent Slope

Trench Spacing (Side Wall to Side Wall)*

26-30 6 feet 28” 16”31-40 7 feet 33” 21”

Table 4SlopeSpacing of TrenchesBased on Width of Trenchand Slope

* The distances between trenches are measured between adjacent sidewalls.

** If the minimum trench depth is measured on the downhill side of the trench, the minimum cover as listed in the table should be assured.

Some general guidelines are:• Make a detailed evaluation of the site to

determine optionsGenerally, the larger the lot, the more optionsThe steeper the slope, the more difficult to overcome

• Select the best location and landform• Keep it as simple as possible• Use a pump (effluent) if needed• Renew your liability insurance

Then:• If it doesn’t pass the “feel good” test; walk away

• Gravity • Erosion • Soil Stability • Variable Soil Depth• Shallow Rock• Surface Water• Spring Activity

Certain Landforms• Headslopes• Toeslopes• Depressions• (These are all concave

shapes)

• Also:• Drains• Rock Outcrops• Complex Slopes

Focus On The Better Landforms Where Waters Diverge• Convex Ridges,

Sideslopes

Presenter

Presentation Notes

Another view of slope names.

Requires deep soils

Must meet setbacks• Uphill • Downhill• Over Rock

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Presentation Notes

The steeper the slope, the deeper the cut must be to have enough space to install. We can’t just bench line by line (Unless there is a large area to be used), but must bench out for the entire system. 10’ cut compared on different slopes.

.67(2)c Requires geologist’s report & PSC map

(subject to LHD)

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Presentation Notes

Conventional Situation – Cut has no restriction per .67 (or .95) – 15’ setback due to treatment used Three lines 1 and 2 deep enough – no cover Line 3 – Entire trench is in ground but needs cover. Pushover can be used if it meets rules.

.67(2)f Does not allow downhill pushover fill for EDF for >25% slope

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Presentation Notes

Conventional Situation - No restriction – Treatment to reduce setback Lines 1-3 – same as previous Line 4 – In pushover, NOT ALLOWED 9>67, (.95))

The amount of wastewater that can be applied each day over an indefinite period of time to a square foot of soil such that the effluent is absorbed and properly treated.

Horizontal Flow

1

2

3

If Benching is proposed• Setbacks may be difficult to meet• 25’ setback from uphill cut unless pretreatment• 25’ setback from downhill slope over 40%

Consider Drip technology

Try to read and interpret • Landforms• Soils• Water movement & concentration

Use these to best advantage – avoid bad locationsDesign for the long termCall in other professionals if needed

PermeabilityDepth To ASHES (ave seasonal high extended saturation)Depth To Rock and Other RestrictionsSlope and Landform LimitationsFlooding FrequencyHydric Soils

ABILITY OF SOIL TO ALLOW WATER TO PASS THRU IT

Permeability is affected by anything that affects PORES (pathways for water to travel)

TextureStructureConsistenceBulk DensityClay MineralogyDissimilar Layers

From Surface To Gray Colors Chroma 2 or less2% or more of the volume6” or more thick

>2%

6”Thick

OBVIOUS COLOR CONTRAST LESS OBVIOUS

HARD VS. SOFT ROCK

THEY SAID DIDN’T FLOODWATER FRONT LOTS FOR SALE

SOUTH AL HYDRIC SOIL NORTH AL HYDRIC SOIL

If perc test are used check results against what the soil says

Some signs are obviousSome are more subtle

A 10 min/Inch perc rate during the dry season in a sticky, plastic clay in an area dominated by cedars and red buds is a...

Non-Representative Perc Result.

A 10 Min/Inch Perc Rate In A Soil With A Sandy Loam Texture

But Has A High Bulk Density Layer at ~ 24” (Fragipan)

Is Also Non-Representative

A 10 min/inch perc rate at 12” in this sandy topsoil is likely

System may actually be underdesigned for the long term

Because of the clay texture just below.

Consider all 6 factors or limitations

Interpret the soil and site characteristics

Compare test results w/ what’s present

If questions arise – ask for assistance

If we can a avoid a problem down the road - we all win

Filling a drainage channel for an EDF is not recommendedMost undisturbed natural soils are stable• They can be trusted to stay in place and retain

their integrityFill requires time to develop stability• Or use stabilizing practices

Establish vegetationRetaining wallsExtended shoulders (aprons)

SYSTEM CONVENTIONAL ENGINEERED

LIMITATION SLIGHT MODERATE SEVERE EXTREME

AT Required1. Percolation(Min/In) (1) 5-30 31-90 91-120 1-<5 121-240 >240 1-<5<1

1a MVS (2) fromRedox(3) 24 18 18 36 18 12

24w/AT

1b MVS fromHard Rock 18 18 18 18 12

1c MVS fromOther R/L (4) 12 12 12 12 6

Min trench depthfor above 12” below NGS Natural Ground Surface (3) May vary but either 6” above or below(not

exactly at) the fill/soil interface

2% Slope (%) 0-15 16-25 26-40 >40 (7)

3. Flooding Frequency Chance/Year None Rare

<5%Occasional

5-50%Frequent (7)

>50%

4. Landform(Slope Position)

Summit Shoulder Back & Other

Linear or Convex

Lower Back Foot & Other

Slightly Concave

ToeHead

Depression& Other Concave

Swamp, Wetlands, FloodplainDrain, Gully, Mine Spoil, Hydric

Soil Area (7)

Table 15

Conv Conv Conv Eng Eng Eng*

Perc Rate

5-60 61-90 91-120

120-240

>240 w/AT

1-<51-<5

MVS /2

24” 18” 18” 18” 12” 36”24”

Hard Rock

18” 18” 18” 18” 12” 24”

Soft Rock

12” 12” 12” 12” 6” 24”

Other 12” 12” 12” 12” 6” 24”

Slope 0-6 7-15 16-25 26-40 > 40