White Paper1:
Grouting Variables and their Contribution to the
Diameter of Ground Source Heat Pump Loop Wells
As a benefit to members of the National Ground Water Association and others, this document
provides the ground source heat pump systems professional with basic knowledge of grouting
practices related to loop wells. This document provides a basic understanding of the issues.
Definitions
ground source heat pump:
A heat pump that uses the earth itself as a heat source and heat sink. It is coupled to the ground
by means of a closed-loop heat exchanger (ground coil) installed horizontally or vertically
underground. Sometimes referred to as a geothermal heat pump.
grout:
A bentonite material or fluid mixture introduced into a borehole to seal between materials.
grouting:
The operation by which grout is placed, from bottom to top, in a borehole.
pumpability:
The quality, state, or degree of being pumpable.
tremie method:
Method whereby filter pack is emplaced or bentonite/cement slurries are pumped uniformly
into the annular space of the borehole through the use of a tremie pipe.
tremie:
A pipe or hose of sufficient diameter to allow for the pumping of thermal grouts to the bottom
of the loop well.
There are many variables contributing to the interior diameter (ID) of a loop well borehole.
They are all related to striving for groundwater protection and optimal geothermal heat pump
system performance.
Some 12 variables interplay to lead to the pumpability of the grout through a tremie to result
in proper grout emplacement that (a) creates an effective barrier from surface water intrusion
and potential contamination of groundwater; (b) prevents intermingling of groundwater zones in
the subsurface; and (c) serves as an effective medium for heat transfer, and thus, optimal system
performance.
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“White Paper – An authoritative report or guide written for industry audiences that addresses issues and
possibly options as to how to solve them. White papers are used to educate readers and help people make
decisions for themselves.” From NGWA Policy on Official Document Types, June 5, 2013.
NGWA White Paper
Compiled: 3/14/2014
Each of the variables and their relationship to tremie ID are:
1. Viscosity of the grout fluid
Viscosity is defined as the resistance to flow. The higher the viscosity the more resistance to
flow; as a result higher pumping pressures will be realized. Higher pressures may cause some
grouts to set up prematurely. Higher pressures may also exceed the pressure limitations of the
grout emplacement equipment. In either case, a plugged tremie or plugged grout pumping
equipment and grout placement failure could result.
2. Interaction between the constituents of the fluid
The components of the grout may react with formation fluids or residual drilling fluid left in
the borehole, which may alter the yield characteristics of the grout and affect the ability to
place the grout.
3. Concentration, size, and shape of the solids in the fluid
The concentration, size, and shape of the solids in the fluid all affect the ability of the fluid to
flow. Care must be used to ensure any solids introduced into the grout conform to the recommendations of the grout manufacturer.
4. Consideration of all restrictions in the grouting system
The geometry of the grouting system has a direct impact on the ability to place the grout.
Every change in direction of flow or in the inside diameter of the lines causes additional resistance to flow. Every effort should be made to minimize any changes in direction of flow or
reductions in the inside diameter of the tremie.
5. Water quality in the grout mix, such as the hardness of the water
Grouts are susceptible to the quality of the fluid used to mix the grout. Hard water, water with
any salt (Ca, K, Mg, Mn, etc.), low pH, high pH, or excessive chlorine can have a detrimental
effect on the initial yield and the stability of the grout.
6. Temperature of the water used in mixing the grout
Grout material yield time is affected by the water temperature. High temperature accelerates
the yield and reduces the working time while low temperature retards the yield and increases
the working time.
7. Temperature changes over time in the grout mix
Temperatures can vary during the course of a grouting project and alter the behavior of each
batch of grout.
8. The volume of the water added to each grout batch
Grout behavior is dependent on the ratio of grout material to water. The grout manufacturer’s
instructions must be followed.
9. Working time of the grout
Understand the grout manufacturer’s estimated working time of the grout and the factors
impacting upon working time.
10. Grout pump output
Grouting equipment limitations must be understood. The grouting program should provide
a margin of accommodation.
11. Grout pump efficiency
The efficiency of a grout pump is affected by many factors, including geographic elevation, the
length and depth of the suction, and the grout pump’s age and condition.
12. The grout manufacturer’s product composition
Every grout product has its own unique behavior. Understand the characteristics of the grout
product in use.
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Theoretically, using these 12 parameters, the specific tremie ID could be calculated. Once the
ID is calculated, the outside diameter (OD) of the tremie can be determined. But since these
parameters vary from loop well to loop well and some parameters even within a loop well,
contractors rely on their loop well grouting experiences to determine the optimum tremie size.
Caution should be exercised to assure a tremie is being used within its rated pressure range.
To determine what will be an adequate diameter of a loop well’s borehole, the effective cross
sectional diameter of all loop tubes plus the tremie must be determined. The placement of the
tremie with respect to the loop tubes defines the geometric structure. The effective cross sectional
diameter is the diameter of this geometric structure.
Loop tube frequently arrives at a job site in a coiled form. When uncoiled, it usually will seek
to return to its coiled condition. To emplace the loop tubes to the bottom of the borehole, a competent drilling contractor will construct a borehole with a diameter that is greater in diameter than the
effective cross sectional diameter of the loop tubes and tremie. This minimum borehole diameter is
essential for proper grouting to assure groundwater protection and optimal geothermal heat pump
system performance.
This document was prepared as the result of regular meetings of a task group composed of representatives from among the NGWA membership and duly affected individuals. These included:
• Thomas Christopherson
• David Henrich, CWD/PI
• Todd Hunter, CWD/PI
• Stewart Krause
• Dan Meyer, MGWC
• Ron Peterson
• John Pitz, CPI
Disclaimer:
This publication is a collaborative effort to try to set forth guidance on this topic but individual
situations and local conditions may vary, so members and others utilizing this publication are
free to adopt differing standards and approaches as they see fit. The Association assumes no
liability or responsibility for the contents of the publication.
The National Ground Water Association is a not-for-profit professional society and trade
association for the groundwater industry. Our members from all 50 states include some of
the country’s leading public and private sector groundwater scientists, engineers, water well
contractors, manufacturers, and suppliers of groundwater related products and services.
The Association’s vision is to be the leading groundwater association that advocates the
responsible development, management, and use of water.
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Copyright © 2014 by National Ground Water Association Press
ISBN 1-56034-024-X
Published by: NGWA Press
National Ground Water Association
601 Dempsey Rd.
Westerville, OH 43081-8978
Phone/ 614 898.7791
Fax/ 614 898.7786
Email/ customerservice@ngwa.org