By Claude Goguen, P.E., LEED AP
Whenever you have something inspected, there’s a natural tendency to want the inspection to occur at the latest stage of the process in order to measure performance in its final position. For example, wouldn’t you feel better having a plumber test a new water heater in your home once installed rather than test it while it’s still in the delivery truck?
Likewise, when a manhole is installed, it’s understandable when the owner, inspector or specifier wants a performance test once it is completely installed and in use. When it comes to vacuum testing, this presents an issue.
Vacuum testing, according to ASTM C1244-05, “Standard Test Method for Concrete Sewer Manholes by the Negative Air Pressure (Vacuum) Test Prior to Backfill,” is a great way to measure watertightness; however, as the standard title states, it should only be performed prior to backfill. When faced with trying to test manholes with vacuum after backfilling, there are three major disadvantages:
- There are no existing industry standards for vacuum testing the structure after backfill.
- It is often difficult to determine the cause or locate and repair a system breech once the manhole has been backfilled.
- If done without taking into account the additional pressures involved, such as soil and ground water pressures, system failure can occur.
So bucking the natural tendency to test the performance after installation requires some special considerations.
How vacuum testing works
The whole idea behind vacuum testing is to suck air from within an enclosed space (manhole structure) and create a pressure differential between the interior and the atmospheric pressure on the outside of the manhole. Air pressure will seek an equilibrium state and therefore a vacuum is created. Atmospheric pressure is about 15 psi absolute
(103 kPa). When we pull a vacuum of 10 in. (25 cm) Hg, an internal pressure of roughly -5 psi gauge (-35 kPa), or 10 psi absolute (69 kPa), is created. The atmospheric pressure on the exterior of the manhole, 15 psi absolute, will thus exert 5 psi (35 kPa) of differential pressure on all surfaces, joints and connectors as seen in the above diagram.
Why vacuum testing after installation is not recommended
Now let’s consider a backfilled manhole. We no longer have just atmospheric pressure on the outside, we now must consider soil pressure, and depending on the height of the water table, we may also have hydrostatic pressure to take into account. Many people do not fully understand the effects of vacuum testing backfilled manhole systems 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. That is why vacuum testing backfilled manhole systems is not recommended, especially in the presence of ground water.
If you are faced with having to pull vacuum on a backfilled manhole, you must use extreme caution. It is paramount that hydrostatic head is determined for the lowest section elevation of each manhole and the test should be vacuum adjusted accordingly to prevent overloading system components during the test. It is also important to consult the test equipment manufacturer’s recommendations.
Let’s say, for example, you have a manhole that’s 30-ft (9.1-m) deep and the depth from grade to the water table is 8 ft (2.4 m). The pressure on the bottom connector is already 8.67 psi (60 kPa) due to the hydrostatic load. If you were to incorrectly apply ASTM C1244 and pull an additional 5 psi (35 kPa) or 10 in. (25 cm) Hg, you will have placed 13.67 psi (94 kPa) of pressure on the manhole’s connections, which exceeds the prescribed pressure for a straight aligned pipe according to ASTM C923-08 “Standard Specification for Resilient Connectors Between Reinforced Concrete Manhole Structures, Pipes and Laterals”.
Detecting leaks is much harder after backfilling
Prior to backfill, one simply has to listen for a hissing sound during the pressure test as air enters the manhole to find the leak’s source. Leaks can also be detected by spraying water on the exposed wall surface – infiltrating air will produce a dry spot. After backfilling, the best way to detect leaks is to spray a soapy solution on the interior of the manhole. Solutions of 2 fl.oz/gal. to 10 fl.oz/gal. (16 cm3/L to 78 cm3/L) of liquid soap have been recommended for ambient temperatures below 80 F (27 C). A similar solution with a few ounces of corn syrup is recommended for temperatures above 80 F. Bubbles will form at leaks.
The bottom line is to avoid specifying or requiring vacuum testing after backfilling if at all possible. If it becomes necessary to conduct a vacuum test after backfill, make sure the contractor takes precautions and determines the impact of existing conditions on the maximum permissible vacuum to pull without damaging the system.
For more information on vacuum testing before and after backfilling, call the NPCA Technical Department at (800) 366-7731.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.
I just had my first experience with testing a manhole via 10in vacuum, and it did not go so well. When I informed my office we had to test with a vacuum they looked into it and sent me the link to your site. Since we have not had to do this before and did not have the equipment to perform the test we hired a local contractor that does this daily. My first structure was a double drop manhole 16′ tall with 10′ of backfill. Surprisingly this one passed with flying colors. However we were not so lucky on the second structure. This one is 12′ tall with 12′ of backfill. On the outlet side of the manhole I had a 2′ 4″ piece of pipe that connected into the boot placed into the manhole. The ballon plug was placed in this piece but not past the joint connecting it to the next full length of 12″ SDR pipe. Testing began. I was told at 6in it failed. Removed the lid to look inside, and to all involved surprise, that 2′ 4″ piece had been sucked out of the hub, and pulled completely into the manhole. Having pulled these joints apart in the past I know it takes a good amount of force to make this happen. So today I am digging this up and will reinforce this piece so this does not happen a second time. What is the formula for figuring the necessary vacuum to be pulled to equal a true 10in vacuum? Better yet, can you give me that number along with the math it takes to get it? The water table is not an issue therefore there is no hydrostatic pressure to factor. Maybe then I could convince my local inspector to accept a lesser vacuum. I believe after reading your article and our experience in the field we will not be bidding any future work involving vacuum testing.
Thank you for the comment Chris. Eric Carleton, director of codes and standards, provided the following response:
“It my hope this initial problem experience will not keep you from further work requiring vacuum testing. The reason for conducting this test is similar to air testing for pipelines. The owners are looking to obtain the most leak resistant sewer system, and a means to verify that.
The test standard you are likely being tested to is ASTM C1244, “Standard Test Method for Concrete Sewer Manholes by Negative Air Pressure (Vacuum) Test Prior to Backfill.” The original intent of this standard was to test structures prior to backfill. As noted within the article you referenced, the test criterion for certain installed conditions can exceed the range of the resilient booted connection. However, that is little comfort to sewer owners who want to test a complete system to ensure the backfilling process didn’t create a defect. Currently, the ASTM C13 Committee is developing a new standard to address vacuum testing of manholes after installation.
The problem you encountered with a pipe implosion is strictly a pressure issue. Air, whether compressed for positive pressure or sucked down to create a vacuum for negative pressure, will exert a force against an object adjacent to it with a different pressure zone. In your case, the manhole interior was at 10 inch ± Hg which equates to roughly 5 psi of reduced differential pressure to the outside of the manhole. (1 inch Hg = 0.49 psi at 0.5 psi) Therefore, when the (2 feet, 4 inch) 30-inch diameter pipe was plugged it had a differential force of 5 psi trying to push it into the manhole. And that is measured to the pipe outside diameter. If this 30-inch diameter pipe is a B-wall reinforced concrete pipe, the minimum force is 5,400 pounds. Consequently, if this pipe is not adequately blocked off with struts it will likely move as you have proven. A similar situation can occur with a positive air test pipe which is sealed on both ends and pressurized, thus pushing the pipes apart and into the manhole structure if not adequately blocked. Look for more on this subject by clicking here.”
Here is another reason to vacuum test a manhole prior to backfilling. I test manholes on a daily basis as my company, TruTest is a full service sewer testing company. I have seen several times a manhole pass a vacuum test with water actually leaking in. We believe the water doesn’t allow the air to be pulled in, instead water comes in and the vacuum doesn’t drop very fast. I have seen this on several tests and we checked to make sure we weren’t doing anything wrong.