The usage of BAUER BG drilling rigs with high torque and the ability to push the tool simultaneously under high crowd pressure against the bottom of the borehole makes it possible to drill rock with high compressive strength.

The Kelly rotary drilling method is the most common way of installing bored piles around the world. For common geology such as sand, gravel or clay layers there are plenty of different rotary drilling tools available on the market. Equipped with either flat teeth or round shank chisels, these tools work perfectly even in cemented soil.

When it comes to drill into rock or boulder layers, projects face difficulties regarding drilling performance and wear and tear. The challenge is to develop a drilling tool with the capacity to overcome these problems and the possibility to combine it with the standard Kelly rotary method.

With the latest developments of BAUER Maschinen, it is now possible to use the rotary drilling method with standard tools (augers and bucket) and by just changing the tool to special rock drilling tools, rock strengths up to 300 MPa can be broken and transported.

It is the right choice of tools and drilling rig which ensures:

? Good performance in all soil conditions.
? High quality of the work.
? Maximum verticality of the bore.
? Economic and technical success.

It is essential to use heavy drilling equipment which creates sufficient crowd pressure on the tool. The usage of BAUER BG drilling rigs with high torque and the ability to push the tool simultaneously under high crowd pressure against the bottom of the borehole makes it possible to drill rock with up to 100 MPa unconfined compressive strength.

There are three main classes of tools which are suitable for breaking rock.

Conventional tools

Conventional drilling tools, mainly buckets and augers, are equipped with round shank chisels. They are positioned on the drilling tool under certain angles and cut a groove into the rock while the tool is continuously rotated.

One of the most commonly used conventional rock drilling tools is the so-called core barrel. This tool is equipped with round shank chisels and drills under high crowd pressure an annulus into the rock so the core is then released from the lateral stress. Sometimes, especially in fractured rock, the friction between core and tool increases to an extent that the core shears off horizontally and can be taken up in one piece.

In case the core stays in the borehole it can be destroyed with a progressive rock auger in dry conditions or with a Cross Cutter in wet conditions. This working step requires to change the tool first.

Tools with roller bits and air flushing

For rock strengths >100 MPa, tools are equipped with roller bits. The rollers, fitted with carbide bits, are rolled over the rock surface under high crowd pressure. While drilling, a point pressure is created, which pulverises the rock underneath the carbide bits so they can penetrate into the rock. Depending on the rock strength, the crowd pressure should be between 3 to 5 tonnes per roller bit.

One of the challenges when working with roller bit tools is the transportation of the chippings. If there is no transportation process the roller bits are rotating in a layer of chippings and thus losing their cutting characteristic.

The solution is to use an air lift system in order to remove the chippings immediately from the cutting surface. An innovative solution for bringing air to the tool without changing the concept of the Kelly bar system is the so called Down-the-hole flushing head. This tool is located between a standard Kelly bar and the drilling tool. It allows flushing air down and avoids winding the air hose around the Kelly bar due to rotating and non-rotating parts.

Compressed air (< 20 m?/min at 10 bar) runs through the flushing head and through the tool downwards and exits into one or two suction pipes. This inlet into the suction pipe is above the roller bits, so an air lift system is created which sucks up all the chippings and flushes them, together with the required fluid (either water or bentonite) into a calyx basket.

The actual drilling tool is either a core barrel or the latest development the Full Face Cutter (FFC). Big roller bits are arranged in different circular paths on a bottom plate. The arrangement of the roller bits and the continuous rotation of the tool ensures that the complete surface is cut.

The Full Face Cutter creates a planar and clean bottom of the borehole. Drilling diameters for roller bit tools range from 780 mm to 2,000 mm.

The Full Face Cutter was used for drilling rock sockets for foundation piles with a diameter of 1500 mm in February 2010 in cooperation with a Finnish customer. The pile depth varied between 6 to 25 m with a socket length up to 3 m. The rock strength was >150 MPa. At the start of the project the drilling performance was 30 cm/h which steadily increased after the crew gained experience with the new tool. Performance in rock increased up to 1 m/h during the project. When the drilling was finished an underwater camera was lowered to inspect the bottom of the borehole. It could be clearly seen that the bottom was free of chippings and the pile ready for concreting.

The most important factor when using roller bit tools is the crowd pressure. According to our experience it makes sense to cut rock up to strength of 200 MPa. For higher strength the available crowd pressure of drilling rigs becomes the limiting factor.

Hammer tools for rock and boulder breaking

Single down-the-hole-hammers are commonly used up to a diameter of about 600 mm. The air runs through a drill pipe to the bit and flushes all the chippings upwards between drill pipe and borehole wall. For bigger diameters an enormous amount of air is required.

BAUER Maschinen found a solution together with an US specialist to combine the advantages of the Kelly system with the advantage of air driven hammers. The system is called ?Multi Hammer Drill (MHD)?.

The MHD consists of multi-down-the-hole-hammers which are united in a canister. The air is supplied with a flexible air hose and connected on top of the canister just under the Kelly box. It does not require a flushing head. This means the standard Kelly bar can still be used, only the drilling tool is exchanged. The air runs the hammer, travels through the drilling bits and flushes the chippings between canister and borehole wall upwards. The chippings fall back into a calyx basket which is open at the top. After reaching about 60 cm drilling progress the calyx basket is full and the tool is lifted above ground for emptying the calyx.

The MHD system requires less compressed air compared to standard single hammers. But nevertheless, the amount of air (number of compressors) which is required for operating the tool and for flushing is a factor which has to be considered when comparing various systems.

For instance, a 1,060 mm MHD loaded with 7 hammer requires an air volume between 60 to 80 m?/min at 10 to 12 bar.
In Sochi, Russia, many piling contractors have been drilling since 2008 for the foundation works for the Olympic Winter Games 2014. Today there are still more than 60 drilling rigs working. Main part of the drilling project is to drill 1500 mm diameter piles in a former riverbed for an elevated railway line.

The subsoil consists of boulders different sizes and strength which are embedded in soft material which is a big challenge for any drilling system. Conventional drilling tools (core barrel, rock augers etc.) in combination with powerful drilling rigs are mainly used. Tests with two Multi Hammer Drills have shown very good overall drilling progress especially in combination with conventional tools attached to powerful BG drilling rigs.

An important part of the overall success on a project is certainly the selection of the right tools in advance, bearing in mind the variety of soil and rock conditions as well as the variety in the tools available. The idea of combining tools for hard rock with the standard Kelly bar results in a significant extension of the range of usage for BAUER BG drill rigs and it adds a lot of flexibility for the user when working with BG rigs in heterogeneous soil conditions.