Auger Drilling Through the Soft Stuff

Augers come in many different types for drilling. Hollow stem are used for sampling and concrete injection and Kelly bar systems can drill holes up to 14’ (4.2m ) diameter. A very useful type is the solid stem auger. They traditionally have a solid hexagonal coupling from 1 1/8” hex to 2 ½” hex with a cross hole for pinning them together. Applications range from water well, geothermal or foundation piling. The 1 1/8” hex couplings due to their size are limited to smaller bore sizes from about 3” up to 6”. The most popular size uses the 1 5/8” Hex couplings and go up to 12” bores. One word of warning, on the 1 5/8” size you can get a metric size that looks similar. They are not interchangeable. You might push them together in the rig however pulling a metric 40mm hex box apart from an imperial1 5/8” hex pin can be very frustrating..

An important factor in auger drilling is torque. The rotary head is providing the drill cutting action and also is lifting the spoil. In air or mud rotary drilling the lifting is provided by the air compressor or the water pump which have a considerable number of horse power behind them. This extra horse power now has to come to 80-120’ (25-35m) which five times more than you need to air or mud drill. Also make sure the torque on you rig will not over stress the couplings. If you snap a hexagon it will always happen down the hole and you will lose your auger string below the break

What rotation speed does auger drilling require? You need to a reasonable rotary speed more than down the hole hammers, a little slower than mud rotary in the region of  50 -120 rpm. If the rpm is too slow then drilling will be slow. If it is too fast the augers will chatter in the hole with vibration. Slow down about 10 rpm below the chatter speed. The maximum speed is determined by the ground conditions, bores diameter and auger pitch. The pitch is the distance between the flights. The larger the pitch the higher the torque needed to turn the auger in the ground. The lower the pitch the slower you drill. The pitch is normal all worked out for you when you buy your auger.

The pitch also contributes to the throat size of the auger. This is the area created between the pitches of the flight, the wall of the bore and the central stem of the auger. A small throat means that only small pieces of spoil can be lifted. A larger throat, then larger lumps come up. Larger lumps means less chewing up by the drill bit. So this explains why a smaller auger can sometimes be harder to drill with than one with a larger throat size. In soft material this is irrelevant but in gravels it can make quite a difference to the smooth operation of the auger.

Next feed rate verse rotation speed. To drill deep don’t cork screw the auger into the ground. Reduce the feed rate so that the rotation of the auger flighting gives the visual effect that it is moving up. This allows the auger bit to cut the soil and then feed it into the auger flighting which lifts it to the surface. If you cork screw the auger in the force required to break the spoil away increases dramatically, to the point where you do not have enough torque to turn the auger. The only advantage of cork screwing is to cut out a relatively undisturbed soil sample for testing.

Joining the augers. Each auger has a hexagonal male and female coupling. These fit into the next auger and are held together by a cross pin. A nut and bolt can be used but is time consuming; a better way is to use a U-Pin or C-Pin it the same part just different names. These can be tapped into place with the ‘C’ part of the pin wrapping around the central stem. Caution is needed if the pin comes out down the hole, the auger can slip apart dropping valuable parts

 down the hole, not good! So use pins that clip solidly into place and use them the right way round. Which is the right way? Insert the pin with the clip end pointing up the flighting. This stops any spoil from wedging under the clip and forcing the pin out of the cross hole. To remove the pin a hammer and punch or a pointed hammer is used

As you drill down spoil will be lifted out of the hole. Clear this away being careful not to touch the rotating auger. Entanglement with augers is a common cause of accidents on drill rigs. Don’t touch any rotating or moving part of the rig or drill string…….ever!!! To increase spoil clearance lift the auger one sectional  length out of the ground and then reinsert. This will assist in lifting the spoil to the surface and allow you to drill deeper. Do this up and down movement just before you have to add another auger section. Once the next auger has been connected lift the drill string 6” (150mm) before stating to rotate again. This reduces the start up torque loads and will increase the life of you drill rig and auger components.

Once the desired depth has been reached extract the augers. There are two ways to do this with or without rotation. Rotation makes extraction easier but roughens up the inside bore. As a rule of thumb, if I have enough pull back force on the rig I extract without rotation. If the rig is struggling near its maximum pressure I will reduce friction on the side wall of the auger by adding rotation.

When the auger section is clear of the ground insert the slotted auger plate on the lower auger section. Allow enough flighting above the plate to stop the auger falling back down the hole if it rotates through the plate. This is a common occurrence when drilling with large heavy augers in wet conditions. A useful way to stop this is by drilling a small hole on the auger flighting and welding a pin to the top of the auger plate. Lower the auger onto the pin to stop it rotating and slipping down the hole. The position of the pin must allow you to extract the U-Pin from the auger coupling.

The sections of auger are removed one at a time until the bore is clear of drill string. Each auger will be loaded with spoil and heavy. So even if you have a 10 or 20’ stroke rig I recommend 5’ auger section for ease of handling. The bore will slowly collapse if it is not filled quickly so the faster the augers are removed the better the hole. Give auger drilling a try. It’s fast in the right conditions when other systems seem to get bogged down. Keep safe.

Written by Simon Duck for the World Wide Drilling Resource Magazine

May 2009