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Copyrighted by SANTA CLARA COUNTY CODE & Municipal Code Corporation, 1998.

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Sec. B11-80. Subsurface leaching systems requirements.

A subsurface leaching system must consist of trenches two feet in width, a minimum of three feet and a maximum of eight feet in depth, containing four-inch perforated sewage conductor pipe and filled with washed rock three-quarters to 2 1/2 inches in size. In addition, subsurface leaching systems are subject to the following requirements:

(a) Trenches must be placed in undisturbed earth and in an accessible area.

(b) The bottom of a trench must be level (zero to two inches maximum slope/100 lineal feet).

(c) Trenches must not be excavated when the soil is so wet that smearing or compaction occurs.

(d) In clay soils when glazing occurs, the trench surfaces must be scarified to the depth of the glazing and the loose material removed.

(e) Rock material in the trench must be washed and free of fines, and must be covered with untreated building paper prior to backfilling with natural earth.

(f) The minimum design volume must be 450 gallons per day for up to a three-bedroom dwelling; for each additional bedroom or potential bedroom, 150 gallons per day must be added.

(g) Commercial sewage disposal systems must be sized based on peak flows.

(h) Trench length for each leaching system must be determined by the following:

(1) # gallons of effluent/loading rate (gallons/square foot) = sq. ft. of trench

(2) sq. ft of trench/four square feet/lineal foot of trench system = lineal feet of trench per each half of dual system

Maximum Effluent Loading Rates of Soil Absorption Systems

Percolation Rate (minutes/inch*)	Max. Loading Rate (gallons/square foot/day)
Less than 1	System prohibited
1--10	0.56
11--20	0.45
21--30	0.30
31--40	0.26
41--60	0.23
61--120	0.20**
Greater than 120	System prohibited

*Average stabilized percolation rate.

**Sufficient area must be designated for 100 percent future expansion of the required dual leachfield system.

Regardless of the above, each half of the dual leaching system must be a minimum of 200 lineal feet. When sufficient information is available, the Director may designate the length of drainfield required in lieu of a percolation test.

Each linear foot of trench equates to four square feet of effective infiltrative surface based on full credit for each sidewall below the perforated drainpipe and one-half of the bottom area.

(i) Adjacent trenches on slopes must be connected with a watertight overflow line in a manner that allows each trench to be filled with sewage effluent to the depth of the rock before the sewage flows to the next lower trench.

(j) Trenches must be constructed in accordance with the following requirements:

Length of trench	Determined by percolation rate
Width of trench, in inches*	24
Depth of trench, in feet, minimum*	3
Depth of trench, in feet, maximum	8
Minimum cover over rock, in inches*	12
Spacing of trenches, center to center, in feet, minimum	10
Rock under pipe, in inches, minimum*	18
Rock over pipe, in inches*	2
Size of rock, in inches*	3/4--2 1/2

*Other materials may be substituted for drainrock in the leaching system trenches if it is determined by the Director that the material will serve the same function as drainrock as follows: 1) support the trench sidewalls and maintain the integrity of the infiltrative surface, and 2) provide adequate storage for septic tank effluent surges. Trench width and minimum depth may be modified to accommodate materials approved as drainrock substitutes, however the length, maximum depth and spacing between trenches may not be modified. Materials approved as drainrock substitutes must provide equivalent effective infiltrative surface consistent with Section B11-80(h) of this Code but in no case may the total length of trenches be less than that required for standard trenches as defined in this section.

(Ord. No. NS-517.72, § 2, 4-15-03)

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