Clay Pipe Engineering Manual

Chapter 9, Inspection and Testing

Inspectors are an important link between the engineer's design and the completed project. They assist all of the parties involved during the actual construction.

Qualifications

Inspectors should be able to transfer levels from reference stakes to trench bottoms. It is important that they have a knowledge of how to set up and check laser alignment equipment.

Inspectors must also be thoroughly familiar with good practice in sewer construction, have the ability to read construction drawings and make such computations necessary to interpret the

drawings. They should have knowledge of safety regulations for construction sites and see that all regulations and proper procedures are followed.

On the Job

The resident engineer has the overall responsibility for inspection, but the immediate responsibility rests with the onsite inspectors. It is therefore neces-sary that they be fully acquainted with the construction contract. They must be sufficiently experienced to note serious differences between actual construction procedures and those methods contained in the drawings and specifications.

Duties

The inspectors have many duties included in their work. They should make a complete record of all occurrences incident to the construction.

These records are most important especially if changes must be made in the original construction plans. Such changes may involve extra work and payment for this can be computed only after the work is done. Any important deviation between design work and actual construction should be noted.

The day-to-day records of all satisfacto-rily completed work is the basis for interim payments to the contractor.

Among the important items noted by inspectors are the precise locations of all sewer connections by station and depth. These locations are transferred to the permanent records of the project.

The long-term efficiency of sewer sys-tems depends upon the combined efforts of the engineers, the inspectors, the contractors and the material suppliers.

The inspector must be familiar with the job specifications in order to assure that the requirements include proper trench bottom preparation, proper control of design trench widths, bedding material selection and placement, line and grade transfer, pipe installation, initial back-filling, compaction, trench restoration and witnessing of acceptance testing.

ACCEPTANCE TESTING

Testing is done to determine the integri-ty of the completed sewer line. Good practice is to test each section from manhole to manhole after it is back-filled. The first section of any sewer pro-ject should be tested immediately upon completion to insure that the installation procedure will produce the results required by the specifications. Experience dictates that continual testing as a job progresses will improve procedures and keep the job under proper control. When many lines are involved, the project should never be completed before testing begins.

Test Methods

The generally accepted methods of test-ing are the infiltration test or the low pressure air test.

Infiltration Testing

Where the measured water table is 2 ft. or greater above the pipe barrel at the midpoint of the test section, infiltration testing is the preferred and least expen-sive method of testing. The infiltration test measures the ground water enter-ing the pipeline and manholes. The allowable amount of infiltration is specified by the design engineer and is commonly expressed in gallons per inch of pipe diameter per mile per day. ASTM C 1091 Standard Test Method for Hydrostatic Infiltration and Exfiltration Testing of Vitrified Clay Pipe Lines describes the procedure for Infiltration Testing.

To get an accurate infiltration reading, it is necessary to isolate the section of pipeline being tested from the upstream side. All pumping of ground water should be discontinued 24 hours prior to testing.

The critical measurement for determin-ing the infiltration in a sewer system is the rate of flow at the furthest down-stream point of the section being tested.

It has been customary to use a direct reading V-notch weir to determine the flow in the pipeline. Experience has shown, however, that the direct reading V-notch weir is not sufficiently accurate to measure small infiltration amounts. A more accurate method is to actually collect and measure the flow over a specified time period. This can be done with flow-through plugs, dams or troughs. These quantities, usually measured in ounces per minute or another suitable measure of volume per time unit can be converted to gallons per day or to gal-lons per inch of diameter per mile per day.

The set up in the diagram is recommended to achieve this result. After the leakage for the pipe is determined, the lower plug in the upstream manhole can be removed and the combined infiltration from the pipeline and the manhole can be measured. The manhole infiltration is calculated by simply subtracting the pipeline infiltration from the combined pipeline and manhole infiltration. Other procedures for infiltration testing may be equally satisfactory.

Calculation of Infiltration Rate

Pipe Size 8 inch

Quantity Collected 1.1 gals.

Length of Test Section 485 ft.

Elapsed Time 1 hour

Solution

Infiltration Rate in Gallons/Inch Dia/ Mile/Day

AIR TESTING

The low pressure air test is an accurate method of testing a sewer line. This test is used either for line acceptance or leak location.

The line acceptance test is generally per-formed to establish the tightness of a section of newly laid sewer pipe. A spec-cific drop in air pressure within a pipe section over a specified length of time determines acceptance or failure of the line in question.

The test time and the acceptable pres-sure loss are determined by the engineer having the responsibility for the partic-ular job. All acceptance tests should be performed with authorized personnel present to observe the results.

Clean the sewer line by flushing before testing to wet the pipe surface and clean out any debris. A wetted interior pipe surface will produce more consistent results. Plug all pipe outlets to resist the test pressure. All stoppers in laterals should be braced.

Summary of Method

The section of sewer to be tested is plugged. Low pressure air is introduced into this section of line. The rate of air loss is used to determine the acceptabil-ity of the section being tested.

Preparation of the Sewer Line

ASTM C 828 Standard Test Method for Low-Pressure Air Test of Vitrified Clay Pipe Lines describes the procedure for air testing sewer lines. Air test tables derived from C 828 are available from NCPI.

Procedures

The pressure-holding time is based on an average holding pressure of 3 psi gauge or a drop from 3.5 psi to 2.5 psi.

Add air until the internal air pressure of the sewer line is approximately 4.0 psi gauge. After an internal pressure of approximately 4.0 psi is obtained, allow time for the air pressure to stabilize. The pressure will normally show some drop until the temperature of the air in the test section stabilizes.

When the pressure has stabilized above the 3.5 psi gauge reading, reduce the pressure to 3.5 psi gauge and start test. Record the drop in pressure for the test period. If the pressure has dropped not more than 1.0 psi gauge during the test period, the line is presumed to have passed.

The test procedure can be used as a presumptive test which enables the installer to determine the acceptability of the line before backfill and subsequent con-struction activities.

Safety

The air test can be dangerous if a line is improperly prepared due to a lack of understanding or carelessness.

Before attempting to plug any sewer pipe, calculate the amount of back pressure the plug must withstand and be certain the plug being used is designed to withstand this pressure. Always use a pressure gauge and regulator when inflating a sewer plug. Underinflated plugs will not be able to withstand the required back pressure. Overinflated plugs can rupture causing possible damage and injury.

It is extremely important that the various plugs be installed and braced in such a way as to prevent blowouts. In as-much as a force of 250 lbf is exerted on an 8-inch plug by an internal pipe pressure of 5 psi, it should be realized that sudden expulsion of a poorly installed plug, or of a plug that is partially deflated before the pipe pressure is released, can be dangerous.

As a safety precaution, pressurizing equipment should include a regulator or relief valve set at perhaps 10 psi to avoid overpressurizing and damaging an otherwise acceptable line. No one shall be allowed in the manholes during testing.

NCPI Engineering Manual

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