water at an increasing rate during the pressure test, the operation becomes
p r e s s u r e washing.
(2) Pressure washing a grout hole should be continued as long as an in-
crease in the rate of injection can be observed. If the wash water vents from
surface fractures or from nearby grout holes, the washing should be con-
tinued as long as the venting water is muddy. If two or more holes are in-
terconnected, it is often advantageous to reverse- the flow of water in the
subsurface openings by changing the pump line from one hole to another. If
a large, partially filled cavity is encountered, removal of the filling by min-
ing is indicated, since a large volume of water would be required for effec-
tive washing. On occasion grout holes on anticipated final spacings have
been drilled ahead in a section of grout curtain to facilitate. the washing of
nearby horizontal openings. After the washing is completed, all the split-
spacing holes are filled with sand to prevent entry of grout from the primary
h o l e s . The intermediate holes are reopened for grouting by washing out the
sand. This procedure is not recommended except for very unusual condi-
tions or as an emergency expedient, because sand from the filled holes may
enter groutable openings and make them ungroutable.
j . M i x e s . Water -cement ratios of portland-cement grout can be indi-
cated by either weight or volume. The volume basis is more convenient for
field work and is commonly used. In field mixes a sack of cement is con-
sidered equal to 1 cu ft. The mixes most frequently used range from 4:1 to
0.75:1, by volume. These mixes may also be expressed as 4.0 and 0.75.
Mixes as thin as 20:1 and as thick as 0.5:1 have been used, but mixes thinner
than 6:1 and thicker than 0.6:1 are rare. In general, grouting is started with
a thin mix. Thicker mixes are used as the behavior of the hole during grout-
ing indicates its capacity to accept them. Admixtures and fillers may be
added to portland-cement grout to change setting time, increase the strength,
or impart other characteristics to the grout. Sand is often used to provide
additional strength for the contact grouting of tunnels.
--.
k
Pressures.
(1) The control of grouting pressures is vital to the success of any
grouting operation. This control is maintained by gages on the pump and at
the collar of the hole. The grouting inspector must determine that the gage
at the collar of the grout hole is accurate. Most grouting is done at pres-
sures approaching the maximum safe pressure. An inaccurate gage, espe-
cially one that registers low, could result in the spread of grout into areas
beyond any possible usefulness, or in wasteful surface breakouts, or in dam-
age to a structure by displacing rock in its foundation. In such instances,
grout is not only wasted, but the quantities injected may make tight ground
seem open and require intermediate holes to check the adequacy of the work.
A new gage is not necessarily accurate. A new gage or any gage in use
should be checked frequently against a master gage of known accuracy or
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