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Types of Drilling Rig

On this page we will make a review of the types of drilling rigs that can be used to take soil samples for geotechnical of geo-environmental purposes. I have ordered the drilling rigs in ascending cost. The first too options are not drilling rigs but I have included them to show that sometimes the simplest options are effective, when investigating sites Southwest Environmental Limited always start with the cheapest option, we do not want to waste your money. All methods have limitations.

Spade

In some cases particularly for geo-environmental sampling soil need only be gathered from the top 1 meter depth. It is quite possible provided that you have a flask of tea and a good packed lunch to dig 10 x 1 meter deep holes in a day using a small spade, a digging bar and scissor shovels. This presumes a clay or sand soil. In made ground comprised of brick, and concrete chunks, 1 person may dig perhaps 5 holes.

Most holes that are drilled with a "proper" drilling rig will have to be dug down to 1.2m with hand tools. This is part of risk reduction, in avoiding hitting cables and pipes and other underground services.

A big advantage of hand digging is that it can be taken anywhere. Back gardens of terrace houses for example. It is also very neat and leaves little mess. It is also quiet.

Digger

In clays, sands, and loose fill an excavator of 2.5 tons or more can excavate a useful trial pit to take samples, or install standpipes (although these generally end up a bit wonky, and use far more filter media). A large digger or excavator in clays can reach perhaps 3.5 meters and dig out large chunks of intact soil, which can then be tested for strength using a hand vein or pocket penetrometer.

An excavator makes a great deal of mess, they are heavy and can damage landscaping of surfaces. The holes required can also be rather large. One of the main benefits of a digger is when dealing with cobbles (stone of approx. 10cm or larger) which are the bane of all the below drilling methods. With an excavator you can dig through very course materials, which can be useful when all other methods fail.

Handheld Window Sampling

This is the most basic form of drilling or boring, a hollow tube with "windows" in the side is bashed in to the ground using a pneumatic or hydraulic breaker. It is then jacked out using a hand powered, or machine powered jack.

Handheld Window (or Windowless) Sampling is great for hard to reach places. In confined spaces where you would not want fumes, it is easy to swap your hydraulic breaker, for an electric one and avoid the fume problem. The equipment can be carried anywhere and can reach depths of 5 meters in Clays, and about 3 meters in sands and gravels.

We have not witnessed casing being used with handheld window sample gear and this limits depth in granular soils, where the hole is likely to collapse, when the sample tube is removed.

Window Sample Rig

These are a very popular option for soil sampling.

It is hard to measure the in-situ strength of granular soils with all of the above methods. Because none of the above methods are capable of carrying a Standard Penetration Test (SPT). This uses a large weight of (45kg from memory) to bash a steel rods in the ground, you count how many bashes (blows) it takes to hit the rods in to the ground, and then you can work out strength, shear angle and allsorts of other things. Shell and Auger Drilling and Rotary Drilling can both undertake SPTs.

A windows sample rig also has the ability to case the hole. Casing is a best though of the bricks that you would see inside a traditional water well, the bricks hold the well open, and stop soil falling in. Instead of bricks we would use a steel tube that is bashed down as we drill or bore downwards so as to keep the hole open. It is difficult to case deeper than 3 meters with a window sampler as the weight is not heavy enough to drive the large diameter tubing deeper than 3 meters.

A window sample rig cannot sample rocks, perhaps very very weak rocks could be sampled in short sections, but it would be slow going. A windowless sample rig can drill 20 x 1 meter deep holes in one day, or perhaps 2 x 5 meter holes. All rods and casing are generally extracted using a hydraulic ram, and as such progress is slow on deeper holes.

Shell and Auger

A shell and auger rig is a very old fashioned, but effective way of obtaining soil samples the sample recovery in soils is excellent.  The soils are chopped out piece by piece and a driller or geologist can watch these pieces coming out and get a very good idea of the soil profile.

Shell and auger rigs have a number of tools which attach to the end of a very heavy weight (approx. 80kg) and dropped down a hole, the tool is then winched back out of the hole, the soil wedged in the end of the tool, is then poked out with a stick, and the tool dropped down the hole again and the process repeated.  Tools include the shell (hollow tube), bailer (tube with flap on end for "ladelling" through sand and gravel), a stubber for chopping clay, and a chisel; a solid metal blade used to chisel through rock.

Going is rock is slow, and not really appropriate for this type of rig, although there are some add ons that can be used to drill better in rock. You can drill very deep holes with a shell and auger rig, in clays! You are only really limited by the length of the cable. Sands and gravel are a little more difficult, and perhaps the most annoying are "blowing" sands, where sands and gravels solifluct (become an amorphous liquid) when agitated and "blows" back up inside casing. Very annoying.

Rotary Drilling

Rotary Drilling is perhaps the most fail safe method of obtaining soil and rock samples. A rotary drilling rigs will sample anything (except cobbles). If you arrive on site with a Rotary Drilling rig and the right tootling you can be sure that you will make a good depth. Below are the different drilling methods that can be used on a Rotary Drilling Rig:

Flush

An important aid to most of the below drilling methods. Some of the below method require use of a "flush" a flush is air or water that is forced down the hole whilst drilling to flush out the bit of loose stone or soil. Even for a small rig will need a large compressor will be required, and for large rigs very large compressors can be inadequate, this is where a water flush may be used, it is very messy but allows high rates of removal. A drilling mud is sometimes used with polymers added to increase removal rate of debris.

Augers

Augers are the most basic form of tooling for a rotary rig, they require very high torque to twist in the ground, and can only be used in soils or very weak rocks. They resemble a large cork screw, which is wound in to the soil, and then pulled out, with soil clinging to the auger "flights" the soil is scrapped of, and then the process repeated.

Hollow Stem Augers

Again augers, but these have a hollow stem, a tube running down the middle of the augers. This enable samples to be taken, and testing to carried out without removing the augers. The hollow stem augers also act like a casing. Augers do not require a "flush".

Down Hole Hammer

This is pneumatic drill on the end of the drilling rods, and will go through pretty much anything, you do not get any samples, but if you are in tough ground, it is the one sure fire way of getting past the tough bits and on to something a bit better.

Rock Rollers

These classic tri-cone bits are not used much in site investigation work. They are more useful in softer to moderately strong rock and when sample recovery is not high on the agenda.

 Coring

Coring is the method for recovery rock samples, it can also be used in very stiff clays.

In this method a core barrel is spun around, and gently lowered in to the hole, around the base of the core barrel is a cutting shoe. This cutting shoe can have tungsten teeth similar to a saw, or it can have a diamond cutting action. The cutting shoe munches through the rock allow a solid core of rock to be captured in the core barrel. Air, Water or Both are used to flush the core barrel. The flush is important when coring as it keep the cutting shoe cool, and lubricates the core barrel, which aids with good recovery.

Generally the strong the rock is the better the recovery will be. Weaker rocks are prone to breaking in to pieces. There is a lot of energy involved in drilling through rocks, and weak rocks have to be very carefully drilled to get good recovery.

Coring can be problematic is a tough rock that is highly fractured, in this instance chunk sof rock can become lodged in the base of the core barrel, and this prevents downward drilling. Instead of the cutting teeth coming ion to contact with the rock, they are held back by the chunk of rick jammed in the cutting shoe, and all of the energy goes in to rubbing two rocks together rather then applied to the cutting teeth.

Dynamic Sampling

This is similar to window sampling. A sample tube is driven in to the ground using a hydraulic hammer. OK in soils.

Piston Sampling

In very soft ground such as peats or silts, piston sampling a very quick way of taking samples. This uses a hydraulic ram to push a sample tube in to the ground. This method give very high quality samples in the right conditions.

Dynamic Cone Penetrometer

This is perhaps the most technological method of "drilling" available.  It involves pushing a rod in to the ground using hydraulic rams, the rams are mounted inside a very heavy truck or crawler so as to provide reaction weight (something to push against).

On the end of the rods is a special type of cone packed full of sensors, they measure friction, and more recently environmental / chemical parameters from the soil as they go. There is no sample recovery, but an awful lots of holes can be undertaken in not much time. It is best suited to clays, sands and silts, it cannot go through rock, cobbles or boulders.