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Vacuum testing is a quick, safe and practical way to validate manhole system integrity. Manhole sections can be tested at the precast concrete plant prior to delivery or on site prior to backfilling. Following prescriptive guidelines in the ASTM Manual is essential for successful vacuum testing.
When to vacuum test
Many codes and specifications require that a vacuum test be performed after the manhole has been installed and backfilled. Testing after backfilling provides a degree of certainty that a watertight system has been installed. The major disadvantage is the fact that no industry standards exist for vacuum testing after the structure has been backfilled. Secondly, it is often difficult to determine the cause of or locate and repair a system breech once the manhole has been backfilled.
The resolution to this problem is to perform a vacuum test prior to backfilling and, if necessary, again after backfilling.
ASTM C1244
ASTM C1244, Standard Test Method for Concrete Sewer Manholes by the Negative Air Pressure (Vacuum) Test Prior to Backfill, has been developed to govern the proper vacuum testing procedure for testing concrete manholes. ASTM C1244 clearly states that a vacuum test “is intended to be used as a preliminary test to enable the installer to demonstrate the condition of the concrete manhole prior to backfill.”
When following the strict guidelines within ASTM C1244, a vacuum of 10 inches of mercury (10” Hg) is drawn on the manhole after all lift holes are plugged and pipes entering the manhole are temporarily plugged and securely braced. The time is measured for the vacuum to drop to 9” Hg. The manhole is acceptable if the measured time meets or exceeds the values presented in Table 1 of ASTM C1244. If the manhole fails the initial test, it may be repaired by an approved method until a satisfactory test is obtained.
Note: The latest edition of ASTM C1244 should be used for proper testing procedures and criteria.
Vacuum testing in the presence of ground water
Vacuum testing after backfilling should be performed only after a successful nonbackfill test has been completed in accordance with ASTM C1244.
Many people do not fully understand the effects of vacuum testing backfilled manhole systems in the presence of ground water. Vacuum testing backfilled manhole systems is not recommended, especially in the presence of ground water. Vacuum testing a manhole system that is already subjected to hydrostatic pressure may exceed the design limits of critical flexible connectors, leading to a system failure.
If ground water is present, use the following information to determine if a reduction in vacuum pressure is warranted:
Note: For simplicity, the effects of soil pressure are not taken into account in the following information and examples. In the field, the in-place loads may be greater when in-place soil conditions (effective stress) are taken into consideration. To determine the actual loads induced on a backfilled structure, use the following information in addition to the actual in-place soil properties to properly calculate the effective stress at the critical location.
1. Depth to water table
2. Pressure rating for flexible connector
3. Depth to bottom-most critical connector
With this information, you can determine that the theoretical in-place loads experienced by the deepest connector as outlined in the following example. If the combined pressure differential (vacuum and hydrostatic) between the interior and exterior of the manhole exceeds the connector’s pressure rating, appropriate adjustments must be made.
XYZ County specifications require that all manhole systems be vacuum tested in accordance with ASTM C1244 requirements after backfilling.
Manhole Diameter: 48 inches
Manhole Depth: 30 feet
Depth to Water Table: 8 feet
Acceptance Test: ASTM C1244*
Resilient Connector: ASTM C923**
Depth to Connector: 28 feet
* ASTM C1244 requires a vacuum between 10” Hg and 9” Hg (-5 psig to -4.5 psig) be maintained for 74 seconds.
** ASTM C923 requires resilient connectors to withstand a hydrostatic pressure of 13 psig when installed in a straight alignment and 10 psig when axial deflected 7 degrees.
External Pressure at Critical Depth (Hydrostatic pressure)
Internal Pressure at Critical Depth (Vacuum pressure)
Does the Pressure Differential Exceed the ASTM C923 Pressure Rating?
The combined vacuum test and in-place hydrostatic loads exceed the connector’s pressure rating, so appropriate adjustments must be made.
Adjusting vacuum pressure
Most flexible connectors have a pressure rating of only 10 psi when deflected, which is fairly common in a field installation; therefore we will use the more conservative 10 psi pressure rating as a base point. Drawing a vacuum of 10” Hg creates a pressure differential of 5 psi between the interior and exterior of a manhole system. A water column of 11.5 feet creates an additional 5 psi of external pressure. This ultimately creates a pressure differential of 10 psi at a connector located 11.5 feet under water when drawing a vacuum of 10” Hg.
A conservative rule of thumb is to reduce the vacuum by 1” Hg for every 1 foot of hydrostatic head between 12 feet and 21 feet. A vacuum test should not be performed when the hydrostatic head exceeds 22 feet.
Using the recommendation above, the testing agency should draw only 2” Hg to prevent overloading the flexible connecto.
External Pressure at Critical Depth (Hydrostatic pressure)
Internal Pressure at Critical Depth (Adjusted Vacuum pressure)
For more information on precast concrete manholes, please contact NPCA at (800) 366-7731 or visit www.precast.org.
Disclaimer
The above information is intended solely for use by professional personnel who can evaluate the significance and limitations of this information and accept total responsibility for its application. The National Precast Concrete Association (NPCA) disclaims any and all responsibility and liability for the accuracy and application of the information.
I’ve educated myself in vacuum testing concrete precast manholes, but not quite sure I understand the table that calculates the amount of time each manhole needs to be tested.
I’m hope you will answer my question for me without having to purchase an 8 page PDF file for $50.00.
i.e. 4′ wide X 14′ 7 1/2″ deep would you test for 40 seconds?
Thank you for the comment Taylor. Kayla Hanson, one of our technical engineers, has provided the following response:
“The document Vacuum Testing Precast Concrete Manholes will be quite useful in supplementing your current understanding of the test procedure without having to pay for the ASTM itself.
Looking at the second paragraph on Page 3. I want to point out the second paragraph on Page 3. It states, “When utilizing ASTM C1244, a vacuum of 10 inches Hg is drawn on the manhole after all lift holes are plugged, and pipes entering the manhole are temporarily plugged and securely braced. The time is measured for the vacuum to drop to 9 inches Hg. The manhole is accepted if the measured time meets or exceeds the values presented in Table 1 of ASTM C1244. If the manhole fails the initial test, it may be repaired by an approved method until a satisfactory test is obtained.”
Going back to the example you listed in your inquiry – 48” diameter manhole buried at 14’ 7.5”:
The table in the ASTM lists the depths of bury in increments of two feet. In your example, if we round down to a 14-foot depth of bury, we would need the mercury to hold for at least 35 seconds. If we were to round up to a 16-foot depth of bury, we would need the mercury to hold for at least 40 seconds. Therefore, to be more conservative, it is recommended that you round your depth of bury up to the nearest measurement on the table. So in your example, we would round the depth of bury up to 16 feet, meaning we would need the mercury to hold for at least 40 seconds. Therefore, when the manhole is being vacuum tested, we would create a vacuum measuring 10 inches Hg and we would measure the amount of time it takes for the vacuum to drop from 10 inches Hg down to 9 inches Hg. For your example, this length of time must be greater than or equal to 40 seconds for the manhole to be accepted.
In short, it sounds like you are reading and interpreting the table correctly. I hope this explanation helps clarify the procedure a bit.”
Is this test required for ALL manholes? What is the frequency requirement?
Thank you for the comment, Jim. The following response was provided by Eric Carleton, P.E., director of codes and standards.
“The answer to your question is the frequency depends on a few factors. If the vacuum testing is occurring within the precast producer’s plant as part of their QA/QC program, the frequency of testing for manhole products used within sanitary sewer applications or other installations where measurable infiltration/exfiltration requirements exist would be established within the plants written quality control manual. There are no specific code requirements stating what that frequency shall be.
Similarly, installed manhole structures per the ASTM standard do not have a testing frequency specified within the standard. ASTM C1244 is primarily a testing method standard. For application on a project, it requires the testing frequency to be stated within the project specification. Additionally, many states have included this testing within their specific sanitary sewer regulation along with the testing frequency expectation. I know in Indiana the regulation reads every manhole shall be tested. It needs to be noted these state regulations also permit the approval testing per an infiltration/exfiltration type of hydrostatic test, but many prefer the ease a vacuum test provides rather than the water-type test setups. Check with your specific environmental sewer regulations for Oregon or other states of interest requirements.
The article which you referenced was from 2010 and I am here to report since that time the issue regarding the testing installed manholes before or after backfilling has still not been resolved. There is an ASTM task group developing a specific installed and backfilled manhole vacuum test standard, but it has not been approved. If there is high groundwater on the outside of the installed manhole to be vacuum tested analysis and care needs to be taken to ensure the combination of internal vacuum and external hydrostatic pressure, do not exceed the recommended maximum forces on the resilient rubber pipe/manhole connector leading to unintended leakage.”