The kind of ground conditions a driller encounters will play a huge role in how the project goes and the types of decisions he will need to make. Ground conditions include more than just the hardness of the ground and choosing the right matrix for a particular hardness. While important, other decisions will need to be made.
Fractured ground is common in drilling and should lead to the following considerations:
Potential water loss
Poor water circulation or loss of water return because of a void or fault in the ground may lead to poor bit performance, less than optimal core recovery and the risk of washing away the core sample. The use of drilling fluid additives can help improve water circulation.
The returning flow of water from the bore hole has a cushioning effect on the drill rods. If you encounter fractured ground at some point in your hole, the driller can lose return water. This means that the upper part of the drill string may not experience this cushioning effect and vibration may be increased at that point, which leads to the next consideration.
Potential vibration in the hole
Fractured ground, especially in deep holes, can also lead to the hole diameter increasing slightly. This allows more room for your rods to move. The bore hole annulus should not be so large as to allow your drill rods to bang around as this will cause vibration. Read more about how to avoid vibration in a hole.
Hitting a void
When drilling in highly fractured or broken ground, you run the risk of hitting a void or large fracture and having your drill water escape through that fault. You find yourself with a dry hole and dry bore holes are a pain to deal with. You end up drilling blind because you are not getting any water return and you will not get an indication that the core is blocking. No water return greatly increases your drilling cycle time and, if you are in an area with limited water resources, you will not be able to recycle your water. One way to remedy this is to mix up a pill and inject it into the area where you believe the void is located.
Another possibility is having to cross a zone with high pressure and high flow ground water. Core recovery can be greatly reduced when you are faced with water spouting out a hole and drill rods. There can also be a risk of the inner tube exiting the drill hole due to excessive water pressure. Changing or adding a rod can take much longer and can result in water pouring into the drill station This can slow down or stop drilling.
Products are on the market that were developed for ground that is fractured and has voids.
DD X-Pand is a good example of a product that fills in fissures, cracks and holes easily and efficiently . Read this blog to learn more about how to control water loss in a bore hole or this blog post to learn more about the top five drilling additives.
The AquaGuard system is an example of a product that was developed to limit water flow coming into the bore hole, so that drilling and core recovery can continue in artisian flow zones.