Soil absorption trench.
El Paso City Code § 18.21.230
in effect18.21.230 - Soil absorption trench.
A soil absorption trench may be used if the proposed site provides sufficient room and is of suitable soil. An experienced soils scientist, registered professional engineer, or registered professional sanitarian should be consulted to determine if the site qualifies for trenches.
A. Absorption Trench Field for Level Terrain. Where the topography or ground slope is not too steep, a flat or level system of gravel-filled trenches or percolation beds is recommended. The use of a looped trench system will avoid dead ends and assure maximum effective utilization of all portions of the system. No individual trench shall exceed seventy-five feet in length.
1. The field bottom must be at least twelve inches lower than the flowline of the treatment tank. The capacity of any particular absorption system is determined by the total area of trench or bed bottom built into the system. The amount of this required minimum area will depend upon the expected sewage load, the average soil percolation rate, and the site's soil evaluation results. The soil percolation rate may be determined by performing a percolation test as described in Section 18.21.110 of this chapter. The trench dimensions for single family residential units may then be estimated from Table 18.21.230. For sewage flows of less than five thousand gallons per day from commercial or institutional establishments, A, the absorption trench bottom required, is determined by formula A=1.25 Q/Ra, where Ra, the allowable application rate, is found in Table 18.21.230 and Q is the daily flow.
2. All parts of the trench or bed bottom shall be at the same elevation. Trenches should be constructed as shallow as possible with a minimum depth of eighteen inches and a maximum depth of thirty-six inches. Deeper trenches should be used where snow may saturate the upper portion of the trench. For trench depths greater than twenty-four inches (except where snow exists), sand shall be used to fill the trench up to the top soil over as shown in Figure 18.21.230(A). The trench width shall not exceed thirty-six inches, as narrow trenches (twelve to eighteen inches) are recommended. Although trench length is based on bottom area only, sidewall area is important since much of the wastewater is absorbed through the sidewalls and is eventually evaportranspirated. Maximum allowable spacing between adjacent edges of parallel trenches is three trench widths.
3. Liquid from the sewerage unit is conducted to the absorption system via a watertight line similar to the house sewer. The liquid is distributed uniformly through the gravel-filled trenches by the use of three-inch to ten-inch perforated plastic pipe of any one size, or equivalent pipe materials. It is important that the distribution piping be laid reasonably level (four inches per one hundred feet maximum allowable grade) in the trenches, with a minimum of six inches gravel depth under the pipe. A total gravel depth of approximately twelve inches is required. The trench media must be clean graded gravel, broken vitrified brick, washed rock, crushed stone, or similar aggregate that is generally one uniform size (from 0.75 inches to 2.0 inches). Oyster shell, other types of shell, and soft limestone are not allowed for trench media because the cementitious properties of this type of material often result in early trench failure. The distribution pipe shall consist of plastic perforated pipe, four-inch corrugated polyethylene, or equivalent materials with an SDR ratio (ratio of pipe diameter to wall thickness) not numerically greater than forty-one. Jointed tile is not recommended for use because of the difficulty in maintaining joint spacing and keeping the line level. Covering of several layers of newspapers, a layer of butcher paper, or geotextile filter fabric material over the top of the gravel is required to prevent the sandy loam or sand backfill from invading the gravel until the backfill becomes stabilized. When only sandy loam is placed over gravel, only geotextile filter fabric shall be allowed. Tar paper or other impervious material shall not be used under any circumstance. The pipe selected for drainfield construction shall have sufficient strength to resist crushing from external loadings such as earth fill, garden tractors, riding mowers, and similar yard equipment. Bituminous fiberboard or paper pipe shall not be used anywhere in the sewerage system. Poor construction practices will cause serious damage to the soil absorption system. Prior to issuance of a permit, notice shall be given to the licensing authority of the types of piping proposed.
4. It is extremely important that care be taken to avoid sealing the surface of the bottom and sides of the absorption trenches through smearing. Trenches or beds shall not be excavated when the soil is sufficiently wet so as to smear or compact easily. All smeared or compacted surfaces occurring during construction shall be raked to a depth of one inch and loose material removed just before the gravel or other media is placed. The absorptive areas should not be walked on unnecessarily. The completed surface of the disposal area must not be paved, used for parking of vehicles, or covered with impermeable materials.
Table 18.21.230
Absorption Trench and Bed Sizing for Single-Family Residential Dwellings
Average Percolation Rate (Minutes/ Inch) | Sewage Application Rate, R a | Soil Texture (See Table 18.21.160 (A), Soil Textural Classifications) | Minimum Bottom Area (Sq. Ft.) for a One or Two Bedroom House: | Minimum Bottom Area for Each Additional Bedroom* (Sq. Ft./ Bedroom):
Minutes Per Inch | Inches Per Hour | Gallons Per Sq. Ft. Per Day | Trench | Bed | Trench | Bed
Less than 5 | More than 12 | Too great for consideration | Sand/ gravel | Sections on alternative systems
5—15 | 4—12 | 0.6 | Sandy loam | 380 | 750 | 200 | 250
15—30 | 2—4 | 0.5 | Sandy clay | 500 | 900 | 250 | 300
30—45 | 1.3—2 | 0.4 | Silty clay | 625 | 1125 | 300 | 400
45—60 | 1.0—1.3 | 0.3 | Clay loam | 800 | 1500 | 400 | 500
More than 60 | Less than 1.0 | Less than 0.3 | Clay | See sections on alternative systems
_____
Minimum trench bottom area is calculated to include capacity for washing machine wastewater, organic material from garbage grinders, and infiltration of rainfall.
Required minimum spacing between parallel conventional absorption trenches is 3 trench widths.
* When dwellings consist of a large living area relative to the number of designated bedrooms, the following guidelines should be used to approximate the trench area:
Less than 1,500 sq. ft.—Use trench area for two-bedroom house. 1,500 sq. ft. to 1,900 sq. ft.—Use trench area for three-bedroom house. For each additional 800 sq. ft.—Add trench area equal to one bedroom.
This criteria is valid for normal water consumption by an average household occupancy and is not applicable to collective sewage system design.
[image omitted; see the official code]
B. Absorption Trench Field for Irregular Terrain. Where the topography or ground slope is too steep for feasible construction of a closed-looped trench system, the following alternate layout may be used. There shall be a minimum sixteen-inch drop from the bottom of the outlet pipe to the bottom of the first trench when trenches are installed in this configuration.
1. A single level trench, constructed like the closed-looped trench, is built along a contour. The overflow from this trench is conducted via a watertight pipe to the next lower level where a second trench can be built along a contour similar to the upper trench. The pattern can be repeated until the required minimum trench bottom area has been provided. It is required that no individual trench exceed seventy-five feet in length. This technique is graphically illustrated in Figure 18.21.230(B). Other details of trench construction described in subsection (A)(1) of this section, and shown in Figure 18.21.230(A), should be followed. The crossover trench must be as shallow as possible to prevent effluent from draining from upper to lower levels through disturbed soil.
2. Table 18.21.160(C) should be reviewed if the irregular terrain has sharp slopes and breaks.
(Ord. 9448 (part), 1988)
Source: City of El Paso, City Code and City Charter (Municode Library, library.municode.com)URL: https://library.municode.com/tx/el_paso/codes/code_of_ordinances?nodeId=TIT18BUCO_CH18.21PRSEFA_ARTVIDESTEFDISY_18.21.230SOABTRFetched: 10/28/2026