Geothermal Drilling Basics
Part One — Overburden Drilling
Brandon Wronski Sep 04, 2012
Editor’s Note: No two geothermal loop systems are the same, which is why constant education is a requirement for geothermal drillers. In this new department, we will explore the variety of loop systems, installation methods, drilling approaches and unique product offerings in the residential geothermal heat pump market. Take it away, Brandon.
In this series for Compact Equipment’s Geothermal Journal, we will look at the various drilling methods for installing vertical geothermal systems, starting from the ground down. So first question: What exactly is overburden? Well, overburden is aptly named. It is the material lying on top of or “over” the bedrock, and it can be challenging or a “burden” to drill through efficiently.
Overburden can be anywhere from 20 ft deep and beyond. Quite often overburden is unconsolidated soil which has the tendency to collapse. For this reason, it is common for casing, 4 to 6 in. in diameter, to be inserted into the open bore. There are several types of steel casing methods used for water well drilling that have been adopted by some geothermal drillers, however PVC casing is cost-effective and efficient and therefore more commonly used for geothermal applications.
PVC casings can be inserted in conjunction with any drilling method, yet auger drilling through the overburden is much tidier than mud or air drilling thus saving time and money in the long run. It is also very efficient if you have the key ingredient: high torque. Torque is king when drilling with augers. A drill rig equipped with high torque (6,000 to 10,000 ft-lbs for this application) is the most efficient method of drilling through overburden.
A Note on Augers: There are usually three basic types of augers for compact geothermal drilling rigs:
- Standard continuous flight augers, used for regular open-hole drilling.
- Continuous flight augers with hollow couplings, which allow water, grout or cement to be pumped into the hole.
- And hollow stem augers, used for soil sampling, with removable center pilot bits.
The method discussed in this article refers to the use of standard continuous flight augers. Hexagon couplings at the end of each auger section allow new lengths to be added quickly. These are locked together with locking pins and allow reverse rotation under full torque. An auger bit is attached to the leading auger and cuts a hole slightly larger than the auger diameter which provides adequate clearance for the auger flights. An auger plate (also called a foot plate) is used when extracting augers. It is a simple tool/fork that slides in between the auger flights to prevent them from falling back down the bore hole.
A big advantage of overburden drilling with augers is that the mechanical action of the auger flighting packs the drilled soil into the sides of the bore, creating a stronger side wall than air or mud rotary drilling. This reduces the chances of the side walls collapsing and allows the bore to stay open for a longer period of time compared to other rotary drilling methods. In some soil, the bore will stay open long enough to be able to drill the rest of the hole with either mud or air rotary, insert the HDPE loop and grout the hole.
In less stable soil conditions, PVC casing can be inserted by hand or by using the drill rig’s down force into the open bore. To reach the required depth, sections of PVC casing can be threaded together or bonded using PVC cement. Once the casing is in place, the down-the-hole hammer or mud rotary bit (air and mud drilling will be explained in detail in the following articles) can be inserted and drilled into the bedrock to complete the remainder of the bore hole.
The PVC casing stops side wall erosion of the soft overburden and keeps the bore open at a constant diameter. This helps maintain the up-hole velocity of the flushing air or mud and keeps drilling penetration rates high. Once the HDPE loop is installed, and while the hole is being filled with grout, the PVC casing can be removed by hand or using the drill rig’s winch line. PVC pipe is far less expensive than commonly used steel casing and a thicker PVC pipe such as Schedule 80 that is also threaded can be used over and over again.
Casings, auger drilling and overburden are all important parts of the geothermal loop installation process. These issues are complex, yet drilling does not have to be burdensome if you use the right method, tooling and high torque drill rig.
Brandon Wronski is an equipment specialist with RigKits LLC, based in Charlotte, N.C.