Why boreholes fail !!!

Why do so many hand drilled boreholes fail? There are multiple reasons and in the next few posts I will outline some of the main reasons there are so many failures.

We will start with an explanation of the most misunderstood cause of failure associated with direct circulation drilling. Direct circulation drilling encompasses any system of drilling whereby a pump is used to force drilling fluid down a drill stem to form a borehole for the purpose of removing debris or cuttings from the borehole. Jetting, washing and flushing are other terms used for direct circulation. In hand drilling processes the pump most commonly used is a small gasoline or petrol powered centrifugal pump capable of producing up to 150 psi at the discharge port.

In direct circulation hand drilling there is typically a pump that has a suction port connected via a suction hose to a "mud pit" and a discharge hose on the outlet port of the pump that is connected to a swivel on the drill stem. The mud pit is connected to the location of the borehole by a trench that will conduct the water from the borehole to the mud pit. The drill stem swivel allows for rotation of the drill stem. On the bottom of the drill stem is a "drill bit" that is somewhat larger than the drill stem.

In the drilling process, the pump is actuated and sucks drill fluid from the mud pit into the pump where it is pressurized and sent through the discharge hose to the swivel and then through the drill stem and out the bit. The bit is rotated by hand by turning the drill stem with a lever attached to the drill stem. As the drill stem turns, the bit is allowed to rest on the soil at the bottom of the bit and fluid from the pump is allowed to exit the bit as it turns. The turning bit under the weight of the drill stem penetrates the earth and the cuttings from the bit are forced up the borehole into the pit along with the drill fluid, The cuttings from the bit are allowed to settle out of the drill fluid before the fluid is sucked into the pump and reused to continue the drilling process. There are multiple videos on youtube or this site that one may view to better understand the process.

There are multiple problems with direct circulation drilling that make it a poor choice to use for hand drilling. The most difficult problem to understand has to do with the properties associated with natural thixotropic clays encountered in the drilling process and with thixotropic drilling additives such as bentonite clay. In the simplest terms, thixotropic drilling muds are fluid when in motion and set over time to a solid or semi solid state when no longer in motion. The impact of such muds on the hand drilling of a borehole for a water well are astounding.

During the drilling process, if clay is encountered that has thixotropic properties or if thixotropic muds are added to the drilling fluid, then complications may occur within the lower portion of the borehole that are not obvious from the surface. The driller of the well may not be aware of what is happening at the bottom of the borehole.

During the drilling of a borehole there is pressure applied through the drilling fluid to the bottom of the borehole in an effort to blow the cuttings from the bit to the surface. The fluid is usually pressurized to greater than 100 psi due to the pressure of the direct circulation pump. The high pressure of the drilling fluid in motion, filled with mud and debris, is liquid because of its thrixotropic properties. That fluid is forced under pressure into any aquifer that is penetrated. The fluid will find the path of least resistance. As a result, the high pressue of the fluid causes it to travel deep into the voids of the aquifer. Once the borehole reaches its final depth and is flushed, the pump is turned off and the drill stem is removed from the borehole. At that point, the thixotropic fluid within the aquifer, filled with debris and cuttings, is no longer in motion and therefore begins to stiffen / solidify. What from the surface of the borehole looks like a liquid, within the borehole soon stiffens and / or solidifies within the aquifer.

Typically the casing and screen are then set and the borehole is developed in an effort to clean the mud and debris from the borehole. The problem of course is that the -aquifer- is filled with mud and debris that has set because of the thrixotropic properties of the drilling fluid. Because the fluid has set deeply within the aquifer, it is difficult to impossible to cause the thixotropic mud to flow and the aquifer becomes plugged. If there is enough hydrostatic head for the mud to be caused to move, then the some water will be produced. However, the debis contaminated mud will likely remain in the aquifer and slowly be drawn into the filter around the borehole. If there is enough mud in the aquifer and if there is enough debris in the mud, then eventually the borehole will become plugged in the area before the slots in the screen or within the filter pack. In effect, over time the borehole will appear to be dry or will produce poorly, when in fact there is water in the aquifer, yet the aquifer is plugged.

This problem is only one of many problems that occur in the drilling of a borehole that lead one to believe the borehole is properly developed when in fact it is improperly installed and poorly developed. In some situations, it is nearly impossible to avoid this problem or to repair the damage once it occurs.

In my next post I will discuss one of the problems associated with auger placed boreholes.