By Claude Goguen, P.E., LEED AP
Have you ever found yourself on a project where local codes or requirements specify that vacuum testing of manholes must be done after backfill? We’ve received many calls from producers and contractors or installers faced with this same situation. Can’t really blame the owners. Testing after backfilling provides them with a degree of certainty that a watertight system has been installed. There are three major disadvantages when proceeding with this test:
- 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, system failure can occur.
So we thought we would take this opportunity to briefly explain the risks and considerations involved.
First and foremost, ASTM C1244 does not apply. It even says it right in the title: Standard Test Method for Concrete Sewer Manholes by the Negative Air Pressure (Vacuum) Test Prior to Backfill. There is no industry standard that exists for vacuum testing after backfill.
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 following 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 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 – 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 hissing sound 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 to 10 fl.oz liquid soap/gal. water (16 to 78 cm3/L ) 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.
Bottom line is, avoid vacuum testing after backfilling if at all possible. If it becomes necessary to conduct a vacuum test after backfill, take precautions and determine the impact of existing conditions on the maximum permissible vacuum to pull without damaging the system.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.