Applications Manual Bosch Geothermal Heat Pumps
| 75
Bosch Thermotechnology Corp.
Data subject to change
Fig. 69
Courtesy of International Ground Source Heat Pump Association/Oklahoma
State University
Bentonite sealing materials typically include high solids
(powdered or granular) bentonite. High solids bentonite
can be pumped before its viscosity is lowered. These
grouts will usually require higher pumping pressures than
cement grouts.
In several states, pure bentonite grout with approximately
20 percent solids content has become the standard for
sealing vertical loops in borings. This type of grout is
composed of at least 20 percent solids content by weight
of bentonite when mixed with water.
To determine the percentage, the weight of bentonite is
divided by the weight of the water plus the weight of the
bentonite. For example, if 75 lbs. of powdered bentonite
and 250 lbs. of granular bentonite were mixed in 150
gallons of water (at 8.33 lbs. per gallon), the percentage
of high-solids bentonite is approximately 20 percent [325 ÷
(1,251 + 325) x 100].
Volume required to back ll a U-tube borehole is typically
expressed in Gallons per 100 Feet of Borehole as indicated
in this typical chart (Table 32).
Gallons per 100 Feet of Bore Hole Required to Back ll
Diameter of Bore
U-tube Dia.
3.5
"
4.0
"
4.5
"
5.0
"
5.5
"
6.0
"
6.5
"
7.0
"
¾"
41
56
74
93
114
136
163
191
1"
-
51
69
88
109
133
154
186
1¼"
-
-
60
80
101
124
150
177
1½"
-
-
-
73
94
117
143
170
Tab. 32
10.11 Grouting Procedures
Grouting is a highly variable operation in which many
things can go wrong. Many factors can affect the behavior
of the grout including components, ratios, temperature,
chemical compositions of the water and bentonite, pH,
etc. For example, inappropriate mixing ratios can lead to
ineffective seals. Also, improper emplacement techniques
may lead to gaps in the seal. For each type of bentonite,
the manufacturer's instructions must be followed. In
addition, it is critical for the contractor to be experienced
and prepared for the grouting operation. Different
bentonite types and mixing ratios can produce much
different viscosities and setup times.
“Closed-Loop” vertical bore holes should be grouted in
one continuous operation from the bottom to the top using
a “Tremie” or conductor pipe (Fig. 70). For high solids
bentonite grout, the type of pumps used includes positive
displacement pumps such as piston, gear and moyno
(progressive cavity) pumps. A paddle mixer is typically
used to mix the grout.
Fig. 70 Tremie / Conductor Pipe
Pelletized and coarse-grade bentonite also can form
good seals, but problems exist in the placement of this
type of bentonite. Because pelletized and coarse-grade
bentonite is poured down the borehole, extreme care must
be taken so that the particles do not bridge above the
bottom of the hole. At least 2 inches of space should exist
around the heat exchanger pipe if this method is used.
When groundwater is not in the borehole, water must
be added often to hydrate the bentonite. Because of the
potential for gaps in the seal, this method is generally not
suitable for the deep borings that are required for the heat
exchanger pipes. However, this method is excellent for
sealing abandoned borings. Emplacement of dry bentonite
granules by air injection into a saturated borehole can be
accomplished. Quartz sand can be added to increase the
viscosity and conductivity of the grout. Air injection of
bentonite can produce high percentage solids content of
bentonite.
