Vacuum Testing of Manholes
Understanding the limits of the
methodology
By Michael R. Miller
Michael R. Miller is
Systems Director and Quality Manager for Press-Seal
Gasket Corp.
During the past five years,
the precast concrete industry has seen a dramatic
change in testing of installed products, especially
manholes. The use of vacuum (negative air pressure)
testing has gained ground almost everywhere
and is now the primary standard by which manholes
are tested and accepted. This broad adoption
means that more inspections are being performed
in a wider variety of conditions. As these conditions
vary and include more extremes, both installers
and inspectors find that vacuum testing, like
all test methods, has its limitations and cautions.
Water pressure effects
in hydrostatic manhole testing
The relationship between
the height of a water column and the pressure
it exerts is direct and linear. If you have
a column of water 23 feet tall, you have a pressure
of 10 psi at the bottom, 5 psi at the midpoint
and 0 psi at the surface. If the column is 46
feet tall, you have a pressure of 20 psi at
the bottom, 10 psi at the mid-point, and 0 psi
at the surface. Thus, when a 15-foot-deep manhole
is filled with water for an exfiltration (leakage)
test, the bottom of the manhole is pressurized
to 6.5 psi, the midpoint to 3.25 psi and the
surface to 0 psi.
Based on this linear relationship,
it is easy to see how water testing provides
unequal testing pressures throughout the manhole
structure. Because of the increased pressure,
water leaks have a greater impact as they occur
lower in the manhole. Conversely, because the
water pressure is zero, it is impossible to
test the top end of the manhole without pressurizing
it. However, pressure inside a sealed manhole
can create problems quickly. By sealing a standard
48-inch-ID manhole and pressurizing it to 5
psi, 9,050 pounds of uplift is generated. Such
uplift is usually sufficient to separate a section
joint, releasing the pressure but severely damaging
the manhole assembly. If a section joint doesn’t
release, then the pressure buildup can create
a significant safety hazard to workers in the
area.
Vacuum effects in
manhole testing
Vacuum testing establishes
a differential pressure between the inside and
outside of the manhole. By pumping atmospheric
air out of the manhole (thereby reducing internal
pressure), you create exactly the same set of
differential conditions as if we had increased
pressure outside the structure. Neither the
manhole sections nor the joints can tell any
difference.
Because this test is performed
with a gas whose pressure does not vary with
height, the applied pressure differential is
uniform throughout the manhole structure. If
there is a 5-psi differential at the bottom
of the manhole, there is also a 5-psi differential
at the top. This results in equal testing of
the entire manhole stack with joints at the
top tested at the same differential as those
at the bottom.
Dynamic effects on the manhole
components are more dramatic with vacuum testing
than with water exfiltration testing. Because
the assembly is sealed, the test force is distributed
throughout the manhole. Each joint is pulled
tighter together as the 9,050 pounds of compressive
force is added to the weight of the stack. Rubber
and sealant materials are compressed further
in joints. Rubber connectors are stretched inward.
Plugged pipes are strained into the manhole.
As such, an 8-inch (ID) PVC pipe is pulled inward
with at least 280 pounds of force, requiring
substantial blocking to resist this force.
Water, air and soil are drawn
into minute cracks and pores in the concrete.
Some of these seal as they fill with fines.
Leaks can be easily identified by damp areas
inside the manhole. If the manhole is not backfilled,
repairs can be made on the exterior by applying
slurry to areas where leaks are detected or
suspected. The slurry then can be pulled into
the wall by the pressure differential and then
it seals the leak. Other patching or repair
techniques can be easily used as well.
Soil pressure load
effects in manhole testing
While it is easy to illustrate
differential external pressures using a water
column, soil pressure loads are not as easily
determined or calculated, and are greatly dependent
upon the composition of backfill materials,
their placement and compaction techniques. The
variation in these materials and methods means
that the actual loads on the manhole are often
difficult to determine accurately, even with
the best engineering resources. Therefore, it
is necessary to acknowledge this and to understand
how to adjust test requirements to prevent overloading
of the structure during the test.
Accumulated loads
during vacuum testing
From the above we see
that the accumulated loads (existing and applied)
on the manhole during the test vary greatly,
depending upon the conditions of the individual
manhole. Knowing what these loads are –
and compensating for them – is critical
to ensuring that the test conditions do not
inadvertently exceed the installed limits of
the manhole.
When these limits are
exceeded, failure of any part of the assembly
can occur, sometimes with disastrous consequences.
Excessive accumulated loads during vacuum testing
are capable of compromising a well-made, properly
installed manhole and can affect the long-term,
watertight performance of the structure. Needless
to say, repairing a backfilled manhole is difficult
and often requires that the structure be re-excavated.
Compensating for existing
external pressures and loads
This problem can be solved
in either of two ways. The best method is to
conduct manhole vacuum testing prior to backfilling.
Deep manholes can be tested in graduated lifts,
as the structure is assembled and before each
lift is backfilled. Testing before backfill
effectively prevents buildup of external pressure
and makes repairs to the manhole stack simpler
and more effective. It is important that proper
equipment and techniques be available to permit
the rapid and safe testing of open manholes.
Then installation and testing crews need not
be required to choose between proper or safe
vacuum test practices.
An alternative practice is
to reduce the amount of vacuum used so that
the total differential pressure at its greatest
point (the manhole base) does not exceed the
capacity of the structure or its components.
It is best if both hydrostatic head (water column
height) and soil pressure loads are known. Measuring
water column height is easily accomplished by
using a standpipe next to the manhole. Estimating
soil loads is much more difficult, and this
information is usually not available in the
field at the time of testing. Figure
1 offers guidelines.
Using these guidelines will
ensure that the loads applied to the manhole
by the vacuum test itself remain within suitable
limits, in this case not exceeding 5-psi differential
pressure at the manhole base regardless of test
condition.
There is no doubt on the
direction the market is headed – there
will be continued growth in vacuum testing.
Areas that have water testing likely will continue
to switch, and areas with no testing likely
will begin to adopt vacuum testing. Customers
will insist on proof of their installed systems.
It is critical that specifiers and installers
have an understanding of the important technical
issues involved in manhole testing so that they
are prepared to respond and provide leadership
to project owners.
Figure
1
|
Condition |
External Pressure |
Test Vacuum Reduction |
Effect on Test |
| Unbackfilled |
N/A |
None |
None |
|
Backfilled |
Known water column height
(WCH) |
1" Hg per foot WCH |
Test at MH base unaffected |
|
Backfilled |
Known soil pressure load
(SPL) |
1” Hg per 60 psf
SPL |
Test at MH base unaffected |
|
Backfilled |
Known WCH and SPL |
The greater of the two
adjustments |
Test at MH base unaffected |
|
Backfilled |
Unknown WCH and SPL |
1” Hg per foot manhole
depth |
Cannot be determined |
Back
to top
