Overburden Drilling Systems

The Symmetrix System

Symmetrix is a patented system comprised of an ingeniously simple concentric method of drilling through overburden with casing. Symmetrix drills straight holes, in any type of ground conditions, at any angle and to depths beyond 328 feet.

The three main components of the system are made up of:

A pilot bit that drills away the center part of the hole and guides the drill string. The pilot bit is attached to any common DTH hammer shank or top hammer rod thread.

A casing shoe welded to the casing pipe, which is pulled down by the impact of the hammer and pilot bit.

A symmetrical ring bit that is locked onto the pilot bit drills the void for the casing to advance down the hole. There are three different types of ring bit and casing shoe assemblies:

• A solitary ring bit with no connection to the casing shoe.
• A welding ring that holds the ring bit and casing shoe together.
• A factory-assembled ring bit set with integrated ring bit and casing shoe.

Drilling in Overburden

Casings are installed into the overburden for basically two different reasons:

1. To create a conduit through the overburden, for example:
   • gas, oil, water, and geothermal wells
   • utility lines
   • starter casings for exploration
   • grouting casings
2. To create foundation and support structures, such as:
   • piles
   • forepoling, pipe roofs
   • ground stabilization
   • anchors

Depending on the use of the casing, it is either left in the hole permanently or retrieved by pulling out. Symmetrix range includes systems for drilling both permanent and retrievable casings.

In the case of permanent casing the ring bit is sacrificed and left in the bottom of the hole, while drilling retrievable casings and all system components are retrieved for reuse.

End Bearing Piles - A very important factor that determines the choice of foundation and its design is formed by the geotechnical properties of the ground.

In may cases piles can be footed in a firm stratum, such as dense till or bedrock, in which case they will get most of their bearing capacity from the base and consequently be classified as end bearing piles. Solitary ring bits can be used in this type of drilling. In case there is a special need to tie the casing shoe and ring bit together (like cavities or soft clay in ground or where drilling starts over water surface) the connection can be made economically by using a separate welding ring. Ring bit and casing shoe can be integrated in the factory. These factory-assembled ring bit sets are easy to use.

The systems are available through the whole product range, the smallest being for 4 1/2" casing and largest size available at the moment is for 48" casing.

1. Casing is installed until bedrock.
2. Drill string is pulled out from the hole.
3. Reinforcement element is installed and concrete poured into the hole.

Skin Friction Piles - Where there is no particular dense underlying stratum, the bearing capacity will depend on the friction forces that are mobilized along the length of the pile. These piles are called skin friction piles.

Casings are often retrieved after reinforcement structure has been installed. Skin friction piles can be drilled with the solitary ring bit as above, pulling out the casing after the concrete is poured in, but more common is to use a system where the ring bit is also retrieved. Such systems always have large ring bit inner diameters to allow maximal inside diameter usage for reinforcement structure.

The same method can also be applied to well drilling for applications where casings are retrieved. Retrievable systems are available for casing sizes from 3 1/2" to 48".

1. Casing is installed.
2. Drill string is pulled out from the hole.
3. Reinforcement element is installed and concrete poured in. Finally, the casing is
   pulled out with the ring bit set.

Drilling with Rotary and DTH in the same hole

Due to Symmetrix's unique locking system it is possible to drill with a tricone bit or drag bit and with a DTH hammer in the same hole. A special ring adapter is used when drilling with the tricone or drag bit in order to rotate the ring bit. When DTH drilling is required, the drill string is lifted out and the DTH hammer with Symmetrix pilot bit is locked into the ring bit.

Torque Requirement

The Symmetrix method needs less torque compared with eccentric drilling systems. A drill rig should have, however, the minimum torque shown for various casing sizes in the table.

Rotation Speed

Rotation speed depends on the size of the Symmetrix bit and also on the formation being drilled. The rotation speed may have to be varied as the formation changes. The harder the ground or the larger the bit, the slower the rotation speed.

Air Pressure

Recommended air pressure in overburden is 174 - 203 psi.

Large Pass-Through Inner Diameter

There are casing drilling applications where the passage through the ring bit is essential. Such requirement is typical in both piling as well as in well drilling. Symmetrix product range includes models taking this requirement into consideration.

In piling a large pass-through ring bit inner diameter is normally required to drill a hole for a rock socket without the casing with a DTH bit. On the other hand, the structure of pile reinforcement placed into the casing requires a certain space, and when casings are retrieved ring bit inner diameter needs to be large enough.

In well drilling the casing is drilled through overburden into the bedrock and then the drilling of rock well is continued with a DTH bit. For that reason the ring bit inner diameter needs to be large enough.

1. Casing is installed until bedrock.
2. Drill string is pulled out from the hole.
3. Drilling of the well continues with a DTH bit, which passes through the ring bit.

Systems with Drill-Through Pilot Bit

Sometimes it is economically reasonable to drill the hole for the rock socket with the actual Symmetrix pilot bit. This applies to all cases where the rock socket length is short compared to the casing length. By utilizing the Symmetrix pilot bit for drilling the rock socket time is saved since there is no need to replace the Symmetrix pilot bit with a DTH bit between work phases.

The Symmetrix product family has these drill-through systems for both permanent and retrievable casings.

Drill-through systems are available for most casing sizes. In the product designation code not only the ring bit inner diameter but also the outer diameter of the pilot bit is given since the pilot bit makes a larger hole than the ring bit's inner diameter.

1. Casing is installed until surface of bedrock.
2. Drill-through pilot bit is unlocked from ring bit and after that pilot bit drills a rock socket. Drill string is pulled out from the hole.
3. Reinforcement element is installed and concrete poured into the hole.

Heavy Duty Systems

The Symmetrix product family includes a heavy duty system that has very strong striking and pulling surfaces. In horizontal drilling the friction between soil and casing can grow very high, and Symmetrix heavy duty drill bits are designed to withstand the abrasion in such conditions.

These systems are intended for break-through drilling where the ring bit is recovered for reuse. The system can also be used for drilling deep vertical holes, if there is no requirement on a large ring bit inner diameter.

Heavy duty systems are available for casings from 5 1/2" to 48".

1. Casing is installed through overburden layer.
2. Ring bit and casing shoe (with starter casing) are recovered from receiving pit. Drill string is withdrawn to starter pit.

Top Hammer Systems

While most casing drilling is done with DTH hammers, there are also requirements to drill casings with top hammers. The Symmetrix product range naturally consists of models for drilling both permanent and retrievable casings with top hammer.

1. Casing is installed.
2. Drill string is pulled out from the hole.
3. After grouting, installation is finished.

Special System Designs

Clay Face Design

Sometimes clay makes casing advancement very problematic. Clay cannot be drilled with impact as its elastic structure does not break or produce any cuttings to be flushed out, making penetration very slow. Hard rubber-type clays can be drilled with cutting only.

For that reason Symmetrix clay face has been designed. Symmetrix clay version (which is optional for all models) has, despite its large clay cutting face area, enough surface with tungsten carbide inserts to also allow drilling into solid rock.

Sensitive Conditions

When facing sensitive drilling conditions, such as underpinning and construction close to existing structures, pilot bits with extra deep face flushing grooves can be of great use.

These Symmetrix pilot bits are used with long ring bit assembly to keep flushing within the face area and collect the cuttings back into the casing. This means less disturbance for surrounding structures. This method can also be utilized in marine construction, where this assembly ensures controlled drilling and cuttings removal in sea bottom conditions. This system not only keeps the flushing under control, but also allows accurate and straight penetration in steeply sloped bedrock face.

Symmetric Features

Each Symmetrix system is made from high quality alloy steel and has been precision-machined to produce a perfect bit body, heat-treated to the required hardness and fitted with durable tungsten carbide buttons.

Ballistic Buttons for Specific Conditions

Symmetrix drill bits are manufactured with hemispherical buttons, but also can be dressed with ballistic buttons for use in soft and medium hard formations to achieve a higher penetration rate. Ballistic buttons are efficient in cutting clay and are also able to penetrate into wood when drilled within old wooden foundation.