C.1 BENT BOLT
A bolt that has a bend in it may require more torque during mixing than a
straight bolt. If the bolt scrapes the sides of the borehole wall during mixing,
the increased torque required to mix the grout can exceed the breaking torque of
the tension nut, which will result in thread take-up during mixing. Observe the
operator as the bolts are straightened and check the bolts for straightness,
then insert the bolts in the boreholes by hand without resin. If there is
excessive drag, then the bolts have not been straightened enough. In some
instances e.g., large bar diameters or higher grade material, the bolts cannot
be straightened enough to prevent thread take-up during mixing. The only
alternative in this case would be to use a torque inhibitor with a breaking
torque that is high enough to prevent premature thread take-up.
C.2 TORQUE INHIBITOR TOO STRONG/WEAK
A torque inhibitor that is too strong can prevent a support from being
properly tensioned since excessive torque must be applied to the bolt to break
the torque inhibitor. When the torque inhibitor is too weak, thread take-up
occurs prematurely resulting in a very low or a non-tensioned bolt. To check if
the torque inhibitor is functioning properly, drill a hole that is 4 or 5"
shorter than the bolt length. Next, install and rotate the bolt to mix the grout
as per the manufacturers recommendations. After the grout is mixed examine the
tension nut. The torque inhibitor should still be intact. If the torque
inhibitor has broken it is too weak. If it is still intact take a torque wrench
and turn the nut until the torque inhibitor breaks. This torque value is the
break-away torque and should not exceed 70 % of the installed torque. If after
conducting several of these checks the break-away torque is too high or too low
the torque inhibitor is not functioning properly and the tension nuts should be
replaced.
C.3 RESIDUAL TORQUE-TORQUE INHIBITOR
There should not be a significant residual torque after the torque inhibitor
breaks. To check if the torque inhibitor is interfering with the thread take-up
during tensioning, first hold the end of the bolt in a vise or with a pipe
wrench. Next, turn the tension nut until the torque inhibitor breaks. When
checking a coupler hold the coupler and turn the mechanical bolt into it. If
after a few rotations the torque inhibitor has not broken away and the
nut/coupler cannot be turned by hand, the torque inhibitor is not functioning
properly. Repeat this procedure with several nuts/couplers and if the problem is
consistent the tension nuts/couplers should be replaced. Some tension
nuts/couplers might have a low residual torque after the torque inhibitor
breaks. This residual torque should be insignificant in proportion to the
installed torque and should be consistent with each nut/coupler.
C.4 THREAD BINDING
Thread binding can interfere with thread take-up during tensioning resulting
in improperly tensioned bolts. To check for thread binding first knock out the
torque inhibitor, then thread the nut/coupler onto the bolt. The nut/coupler
should thread easily down the entire length of the bolt threads. If the
nut/coupler locks up or excessive resistance is felt, visually inspect the bolt
threads for flat threads or burrs. Check several bolts with different
nuts/couplers to determine if the problem is with the bolt or nut/coupler
threads.
If after checking the threads as described above there is no evidence of
binding and the tension nuts/couplers have shear pins the problem might be a
soft shear pin and bolts with sharp ends. Certain types of nuts/couplers have
the shear pins in the threads instead of below the threads. If the shear pin is
aluminum or soft steel and the ends of the bolts have sharp edges, the bolt will
sometimes cut through the pin instead of shearing it cleanly. As the bolt cuts
the pin the metal shards that are produced can block the threads and cause the
nut/coupler to bind up. If the bolts have sharp edges and you suspect that this
might be causing the binding, install a bolt out of pattern, then take the
nut/coupler off. Inspect the nut/coupler looking for metal shards lodged in the
threads. Also check the position of the shear pin. The pin should have sheared
and not be wedged in the threads.
C.5 LIMITED THREAD ENGAGEMENT
Check the thread engagement by threading the nut/coupler onto the bolt until
the end of the bolt is flush with the nut/coupler. Place a mark on the bolt
indicating the full engagement of the nut/coupler. Then measure the distance
from this mark to the end of the bolt. Nuts should have a tapped length at least
equal to 1 times the nominal bolt diameter with which they are to be used.
Couplers should have a tapped length at least equal to 2 times the nominal bolt
diameter.
C.6 POOR STRATA - ANCHORAGE EXCEEDED
To determine the maximum anchorage available a pull test must be conducted.
If it is determined that the anchorage available is not sufficient for the
particular application, then try a stronger grout, a longer grouted length, or
fully grouting the bolts. If it is determined that the problem is due to poor
strata conditions in the anchorage zone try longer bolts. If that does not work
or it is not feasible then the next step would be to try a different support
system.
C.7 VISCOUS GROUT
If the grout is too thick then the torque required to mix it might exceed the
breaking torque of the tension nut shear mechanism. The remedy would be to
either get a less viscous resin or nuts with a stronger shear mechanism.
C.8 PREMATURE TIGHTENING
Premature tightening by the operator can result in improperly installed
supports. It is critical that the operator waits long enough for the grout to
cure before tightening the bolts. To check for premature tightening you must
observe the operator during installation. If the operator is not waiting for the
length of time recommended by the grout manufacturer before tightening, then
instruct the operator in the correct installation procedure.
C.9 PLATE HELD TOO FAR FROM ROOF
If the operator does not hold the plate close enough to the roof there will
not be enough threads left to tighten the bolt, which could result in a
non-tensioned installation. As a check, observe the operator during
installation. If the operator is not holding the plate the correct distance from
the roof as recommended by the manufacturer, then instruct the operator in the
correct installation procedure.
C.10 PLATE GALLING
To check for galling have the bolter install some bolts out of pattern, then
remove the nuts so that a visual inspection of the nuts and plates can be made.
If galling is evident only in some instances, this could indicate poor
installation procedure. If galling occurred for each installation check for
burrs around the plate or rough spots on the nuts. Galling can also be caused by
a plate/washer mismatch (see A.18 ).
C.11 PLATE - NUT MISMATCH
If the nut flange is too large and does not conform to the plate embossment
the nut will not seat completely on the plate and excessive galling could
result.
C.12 GROUT TOO SHORT
The only way to determine if the grouted length is too short is to conduct a
pull test. In certain situations e.g., varying roof strata conditions, a grout
length that performed well in the past can become inadequate. By conducting pull
tests in areas where you suspect that the roof conditions have changed you will
be able to decide if the grout length should be increased.
WASHER BADLY DEFORMED
See Mechanical Bolt Section (include link)
WASHER CRACKS
See Mechanical Bolt Section (include link)
