7.5 Water Main Installation, Trenching, Repair, Pressure Testing & Disinfection (AWWA C651)

Water Main Installation, Trenching, Repair, Pressure Testing & Disinfection (AWWA C651)

Water distribution piping represents the largest capital asset of any drinking water utility. Installing new water mains and repairing broken infrastructure requires rigorous adherence to civil engineering specifications, trench safety mandates, hydrostatic testing protocols, and microbiological disinfection standards to guarantee that water delivered to the public remains pure and uncompromised.

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1. Trench Excavation, Bedding & Pipe Laying

Water main installation requires proper trench preparation and structural pipe embedment:

• OSHA Excavation Safety: Under OSHA 29 CFR 1926 Subpart P, all trenches 5 feet or deeper must utilize protective systems (sloping, benching, trench shields/boxes, or hydraulic shoring). A designated Competent Person must inspect the excavation daily and after every rain event. Ladders or ramps must be spaced within 25 feet of lateral travel for workers in trenches 4 feet or deeper.
• Pipe Bedding & Embedment (Class B Standard): Pipe must be laid on a uniform, compacted bedding cradle (minimum 4 to 6 inches of crushed stone or granular sand). Backfill material must be carefully placed and shovel-sliced into the haunch area beneath the pipe curvature, then compacted in 6-inch lifts up to at least 12 inches above the pipe crown to prevent point loading and ovality distortion.
• Pipe Jointing Mechanics: Bell and spigot ends must be wire-brushed clean. Approved NSF-61 food-grade lubricant is applied to the rubber gasket and spigot bevel. Pipe is pushed home until the insertion line matches the bell face. Over-inserting or exceeding allowable angular joint deflection ($> 3^\circ \text{ to } 5^\circ$) damages the internal gasket seal.

+-------------------------------------------------------------------------+
|                     PIPE EMBEDMENT CROSS-SECTION                        |
+-------------------------------------------------------------------------+
|      Final Backfill: Native soil compacted in lifts to grade.           |
|  ---------------------------------------------------------------------  |
|      Initial Backfill: Granular material 12 inches above pipe crown.    |
|      Haunching: Compacted crushed stone supporting lower pipe curvature.|
|      Bedding: 4 to 6 inches compacted granular cradle on trench floor.  |
+-------------------------------------------------------------------------+

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2. Hydraulic Thrust Restraint: Thrust Blocks & Restrained Joints

When water flows under pressure through a closed conduit, internal hydrostatic pressure and momentum changes exert powerful thrust forces at any point where the pipeline changes direction, changes cross-sectional area, or terminates.

$T = 2 P A \sin(\theta / 2)$ Where $T$ is thrust force (lbs), $P$ is internal pressure (psi), $A$ is cross-sectional area (sq in), and $\theta$ is the bend angle.

Thrust Locations & Restraint Methods

Thrust forces occur at: (1) Bends and elbows ($90^\circ, 45^\circ, 22.5^\circ, 11.25^\circ$), (2) Reducers, (3) Tees and crosses, (4) In-line gate valves, and (5) Dead-end caps and plugs.

1. Concrete Thrust Blocks: Mass concrete ($\ge 2,000\text{ to }3,000\text{ psi}$ compressive strength) poured between the pipe fitting and the undisturbed virgin trench wall. Concrete must never encase mechanical joint bolts, valve operating nuts, or weep holes.
2. Restrained Joint Systems: Mechanical joint restraint glands (e.g., Megalug wedging rings) or factory-welded locking gaskets that lock adjacent pipe joints together, transmitting the thrust force into soil skin friction along a calculated length of pipe.

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3. Colorado Sanitary Separation Distance Mandates

To prevent contamination of potable water lines by leaking sanitary or storm sewers, the CDPHE Water Quality Control Division enforces strict minimum horizontal and vertical separation distances:

+-------------------------------------------------------------------------+
|                  COLORADO SANITARY SEPARATION CRITERIA                  |
+-------------------------------------------------------------------------+
| 1. Horizontal Separation:                                               |
|    - Minimum 10 FEET horizontal clear distance between the outside of   |
|      the potable water main and the outside of any sewer line.          |
|                                                                         |
| 2. Vertical Crossing Separation:                                        |
|    - Water main must cross ABOVE the sewer line whenever possible.      |
|    - Minimum 18 INCHES vertical clearance between the bottom of the     |
|      water main and the top of the sewer line.                          |
|                                                                         |
| 3. Substandard Crossing Protections (When 18 in / 10 ft is impossible): |
|    - Construct the sewer line using pressure-rated pipe (C900 PVC or    |
|      Ductile Iron) pressure-tested to 150 psi.                          |
|    - Center a full 20-foot joint of water pipe directly over the sewer   |
|      crossing so that joints are as far from the sewer as possible.     |
|    - Encase either line in a continuous steel or concrete casing pipe   |
|      extending at least 10 feet on each side of the crossing.           |
+-------------------------------------------------------------------------+

4. Hydrostatic Pressure and Leakage Testing (AWWA C600 / C605)

Before disinfection and backfilling completion, newly installed water mains must pass a rigorous Hydrostatic Pressure and Leakage Test:

• Test Pressure: At least 1.5 times the anticipated maximum working pressure at the lowest point of the pipeline profile, or 150 psi, whichever is greater.
• Test Duration: Minimum continuous duration of 2 hours (120 minutes).
• Procedure: The pipeline is slowly filled with potable water, venting all entrapped air through high-point taps. A hydrostatic pressure pump elevates pressure to the test setpoint. The volume of make-up water required to maintain pressure within $\pm 5\text{ psi}$ of the test pressure is measured.

AWWA Allowable Leakage Formula

The maximum allowable make-up water leakage is calculated using the AWWA standard formula:

$Q = \frac{S \times D \times \sqrt{P}}{148,000}$

Where:

• $Q$ = Maximum allowable leakage (gallons per hour, gph)
• $S$ = Length of pipeline tested (feet)
• $D$ = Nominal internal pipe diameter (inches)
• $P$ = Average test pressure during the hydrostatic test (psig)

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5. Water Main Disinfection Protocols (AWWA Standard C651)

Newly laid water mains, repaired mains, and re-lined pipes must be disinfected in strict accordance with AWWA Standard C651 and CDPHE rules:

1. Continuous-Feed Method (Standard Method for New Mains)

• Procedure: Water entering the main is continuously dosed with chlorine solution (via chemical feed pump through a corporation stop) as the main is filled.
• Initial Dose: Dosed to maintain a minimum of $25\text{ mg/L}$ free available chlorine throughout the entire length of the pipe.
• Contact Retention Time: Retained undisturbed in the pipe for at least 24 hours.
• Clearance Residual: After 24 hours, the free chlorine residual must be at least $10\text{ mg/L}$ at every sampling point. If residual is $< 10\text{ mg/L}$, the main must be re-chlorinated.

2. Slug Method (Large Transmission Mains)

• Procedure: A concentrated slug of chlorinated water is moved slowly through the pipeline so that all interior surfaces contact the slug.
• Initial Dose: Minimum chlorine concentration of $100\text{ mg/L}$ free chlorine.
• Contact Retention Time: Every section of pipe must contact the slug for at least 3 hours, maintaining at least $25\text{ mg/L}$ residual at the end of the contact period.

3. Spray / Swab Method (Emergency Repairs & Cut-Ins)

• Procedure: When repairing an active main break where trench water is kept away from pipe interiors, replacement pipe spools, couplings, and tapping sleeves must be directly sprayed or swabbed with a concentrated $200\text{ mg/L}$ to $1,000\text{ mg/L}$ (1% sodium hypochlorite) chlorine solution immediately prior to installation.

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6. Environmental Dechlorination & Bacteriological Clearance Sampling

Dechlorination

Highly chlorinated water used for main disinfection ($10\text{ to }100+\text{ mg/L}$) cannot be discharged to storm sewers or open terrain. The water must be treated through a chemical neutralization diffuser using sodium thiosulfate ($\text{Na}_2\text{S}_2\text{O}_3$), sodium bisulfite ($\text{NaHSO}_3$), or ascorbic acid (Vitamin C) until the chlorine residual is reduced to $0.00\text{ mg/L}$ before entering the environment.

Bacteriological Clearance Sampling

After flushing out super-chlorinated water and refilling the main with standard treated water containing a normal distribution residual ($0.2\text{ to }1.5\text{ mg/L}$):

1. Sample Collection Protocol: Dedicated sampling taps (copper whips) must be used. Fire hydrants and garden hoses are strictly prohibited for regulatory coliform clearance sampling.
2. Sampling Schedule: Under standard CDPHE and AWWA C651 rules, two (2) consecutive sets of bacteriological samples collected at least 24 hours apart (or two sets collected 16 hours apart after standing under pressure) must be analyzed for Total Coliform and E. coli.
3. Passing Standard: Both sets must show Total Coliform ABSENT (Negative). If any sample tests coliform positive, the main must be re-flushed, re-sampled, or re-chlorinated before placing into service.

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7. Worked Testing & Disinfection Calculations

Worked Example 6.5.1: Hydrostatic Allowable Leakage Calculation

A contractor installs 2,000 feet of 12-inch ductile iron water main. The line undergoes a 2-hour hydrostatic pressure test at an average pressure of 150 psi. Calculate the maximum allowable leakage in gallons for the entire 2-hour test duration.

Step 1: Calculate allowable hourly leakage ($Q$ in gph): $Q = \frac{S \times D \times \sqrt{P}}{148,000} = \frac{2,000\text{ ft} \times 12\text{ in} \times \sqrt{150\text{ psi}}}{148,000}$ $\sqrt{150} = 12.2474$ $Q = \frac{2,000 \times 12 \times 12.2474}{148,000} = \frac{293,938.7}{148,000} = 1.986\text{ gallons per hour (gph)}$

Step 2: Calculate total allowable leakage for the 2-hour test: $\text{Total 2-Hr Allowable Leakage} = 1.986\text{ gph} \times 2.0\text{ hours} = 3.97\text{ gallons}$

Conclusion: If the make-up water pumped into the main over 2 hours is $\le 3.97\text{ gallons}$, the pipeline passes the hydrostatic pressure test.

Worked Example 6.5.2: Disinfection Chemical Feed Calculation

A new 1,500-foot-long, 16-inch diameter water main must be disinfected using the Continuous-Feed Method with an initial free chlorine dose of 25 mg/L. Calculate the volume of water in the pipe and the pounds of 100% pure chlorine gas required to treat the pipeline volume.

Step 1: Calculate pipe volume in gallons ($V = 0.0408 \times D^2 \times L$): $V = 0.0408 \times (16)^2 \times 1,500 = 0.0408 \times 256 \times 1,500 = 15,667.2\text{ gallons}$ $\text{Convert to MGD: } \frac{15,667.2\text{ gal}}{1,000,000} = 0.015667\text{ MG}$

Step 2: Calculate chlorine required using the standard Pounds Formula: $\text{Chemical (lbs)} = \text{Volume (MG)} \times \text{Dose (mg/L)} \times 8.34\text{ lb/gal}$ $\text{Chemical (lbs)} = 0.015667\text{ MG} \times 25\text{ mg/L} \times 8.34\text{ lb/gal} = 3.27\text{ lbs of pure } \text{Cl}_2$

Conclusion: The operator must inject 3.27 lbs of pure chlorine gas (or approximately 26.2 lbs of 12.5% sodium hypochlorite solution) as the pipeline is filled.