City of Frisco Engineering Standards

Engineering Standards
Table 5.3: Minimum and Maximum Grades for Wastewater Mains*
Size of Pipe
(inches)
Minimum Slope in
(Percent)
Maximum Slope in
(Percent)
8 0.33 8.40
10 0.25 6.23
12 0.20 4.88
15 0.15 3.62
18 0.11 2.83
21 0.09 2.30
24 0.08 1.93
27 0.06 1.65
30 0.055 1.43
33 0.05 1.26
36 0.045 1.12
39 0.04 1.01
>39 ** **
*The slopes were calculated using Manning’s Equation and a roughness coefficient
of 0.013.
**For lines larger than 39” in diameter, the slope shall be determined using the
following equation.
where:
V = velocity of flow in wastewater main (fps)
n = roughness coefficient of the wastewater main
R = hydraulic radius of the wastewater main (ft), which is equal to
the area of the flow divided by the wetted perimeter (R=A/P)
S = Slope of the hydraulic gradient (ft/ft)
5.2.3 Separation Distance between Wastewater and Water Mains
A. The separation distance between wastewater and water mains or other appurtenances is governed
by Title 30 of the Texas Administrative Code, Part 1, Chapter 217, Subchapter C, Rule 217.53(d).
B. Wastewater mains shall be installed in trenches separate from water mains.
C. Wastewater mains shall have a minimum separation distance of 9’ in all directions from water
mains. All separation distances shall be measured from the outside surface of each of the
respective facilities.
D. If the minimum separation distances cannot be achieved for crossing water and wastewater mains,
the separation distances may be reduced under two scenarios:
1. The wastewater main has a minimum pressure rating of 150 psi.
2. The water or wastewater main is cased for a minimum of 18’ with a casing pipe having a
minimum pressure rating of 150 psi.
Section 5 – Water & Wastewater Design Requirements May 2012 Page 5-10