Section 320. Design details

A. Sizing. For the purpose of this chapter the gravity sewer design details as described herein represent the best available standards of practice. Hydraulic computations and other design data should clearly establish the capacity of proposed sewers that do not conform to the minimum standards included in this section.

1. Sewer size shall not be less than eight inches in diameter, except under the following conditions:

a. Laterals serving six connections or fewer on cul de sacs or as sidewalk collector lines may be six inches in diameter.

b. Sewer lines carrying settled sewage, such as septic tank effluent, may be as small as 1-1/2 inches in diameter.

2. Engineering calculations and justifications indicating that reduced line sizes are adequate shall be included with the submission.

B. Placement. Gravity sewers shall be of suitable material and placed such that their design capacity is maintained and leakage into and out of the pipelines is within allowable values.

1. Sewers shall be installed at a sufficient depth to prevent ice formation due to cooling of the wastewater flows, resulting in blockage of the flow channel. Sewers carrying nonsettled sewage and sewers carrying settled sewage shall be designed and constructed to give mean velocities, when flowing full, of not less than two feet per second and 1.3 feet per second, respectively, based on Manning's formula using a pipe material roughness coefficient ("n") value of 0.014. Use of other "n" values and slopes less than those specified herein shall be justified on the basis of pipe material specifications, research, or field data, presented with the submission for approval.

2. The following list represents the minimum slopes, which should be provided for gravity sewers; however, slopes greater than those listed are desirable:

	Sewer Size	Minimum Slope in Feet per 100 Feet
		Nonsettled Sewage	Settled Sewage
	3 inch	Not Allowed	0.53
	4 inch	Not Allowed	0.47
	6 inch	0.49	0.21
	8 inch	0.40	0.15
	10 inch	0.28	0.12
	12 inch	0.22	0.086
	14 inch	0.17	0.068
	15 inch	0.15	0.063
	16 inch	0.14	0.058
	18 inch	0.12	0.050
	21 inch	0.10	0.040
	24 inch	0.08	0.034
	27 inch	0.067	0.029
	30 inch	0.058	0.025
	36 inch	0.046	0.020

3. Decreased slopes may be provided where the depth of flow will be 0.3 of the diameter or greater for design average flow. Whenever such decreased slopes are selected, design consultants must furnish, with their report, computations of the depth of flow in such pipes at minimum, average, and peak daily or hourly rates of flow. Otherwise, it must be recognized that decreased slopes may require available resources for additional sewer maintenance.

4. Sewers shall be installed with uniform slope between manholes.

5. Sewers constructed on 20% slope or greater shall be anchored securely with concrete anchors or equal. Suggested minimum anchorage is as follows:

a. Not over 36 feet center-to-center on grades 20% and up to 35%.

b. Not over 24 feet center-to-center on grades 35% and up to 50%.

c. Not over 16 feet center-to-center on grades 50% and over.

6. Gravity sewers shall normally be installed with a straight alignment between manholes. Curved sewers should be installed only on curved streets, where the curve of the street and the curve of the sewer are concentric. The use of curved alignment for sewers may be considered, with the following restrictions:

a. Justification shall be provided by the design consultant to verify that the curved alignment is more advantageous for that installation.

b. The use of curved sewers shall be limited to conveyance of settled sewage unless the owners can document that the specialized equipment necessary to clean the sewers will be obtained and used as necessary.

c. The minimum radius of the curve shall be based on the maximum allowable joint deflection in accordance with the appropriate ASTM standard or other appropriate standard.

d. The sewers shall be installed with smooth radius curves.

7. Gravity sewer size shall normally remain constant between manholes. Where a smaller sewer joins a larger one, the relative elevations of the inverts of the sewers shall be arranged to maintain approximately the same energy gradient. An approximate method for securing these results, which may be used, is to align the 80% capacity flow level, or to align the internal pipe crown or top invert, of both sewers, at the same elevation.

8. Where velocities greater than 15 feet per second are expected, special provisions shall be made to protect against internal erosion by high velocity. The pipe shall conform to applicable ASTM, AWWA, ANSI, or other appropriate standards or specifications, which provide protection against internal erosion.

9. Any generally accepted material for sewers will be given consideration, but the material selected shall be adapted to local conditions such as character of industrial waste, possibility of septicity, soil characteristics, exceptionally heavy internal-external loadings, abrasions, and similar problems. The pipe material shall conform to applicable ASTM, AWWA, ANSI, or other appropriate standards and the pipe is to be marked with an approved identification such as the specifications standard.

10. All sewers shall be designed to prevent damage from superimposed loads. Proper allowance shall be made for loads on the sewer as a result of the width and depth of the trench.

Historical Notes

Former 12VAC5-581-380 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-320, Virginia Register Volume 20, Issue 09, eff. February 12, 2004; Errata 20:12 VA.R. 1526 February 23, 2004.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.