What is Down-the-hole Eccentric Casing Drilling Technology?

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In urban infrastructure and other construction, it is inevitable to encounter the construction of complex stratum projects such as geological disaster prevention and foundation reinforcement. The use of down-the-hole eccentric casing drilling technology can effectively avoid cavity collapse, so it has developed rapidly.

What is the down-the-hole eccentric casing drilling system?

Symmetric Casing System With Slide Blocks

The down-the-hole eccentric casing drilling system is mainly composed of the down-the-hole hammer, eccentric casing system, casing shoe, casing, and so on.

In the down-the-hole eccentric casing drilling system, a hole larger than the outer diameter of the casing is drilled through the eccentric casing system. When drilling to the predetermined formation, the casing system can be converged so that the maximum outer diameter of the casing system is smaller than the inner diameter of the casing shoe and casing so that the casing system can be removed and the casing is left in the stratum to protect the hole wall.

Working principle of down-the-hole eccentric casing drilling

Symmetric Casing System With Reamer

During ordinary drilling, the compressed air provided by the air compressor enters the down-the-hole hammer through the drill rig and drill pipe to make it work. The piston of the hammer impacts the guide of the casing system, which transmits the shock wave and drilling pressure to the eccentric bit and the center bit to break the rock at the bottom of the hole. At the same time, the drill rig drives the drill pipe to rotate, the drill pipe transmits the rotary torque to the hammer, and the hammer drives the guide of the casing system to rotate through the spline. There is an eccentric shaft on the positioner. When the positioner rotates, the eccentric drill bit opens and is limited after opening to the design position so that the center drill bit and eccentric drill bit rotate together with the positioner.

The diameter of the hole drilled by the eccentric drill bit is larger than the maximum outer diameter of the casing so that the casing can follow up without being hindered by the rock at the bottom of the hole. When the gravity of the casing is greater than the frictional resistance of the formation to its outer wall, the casing will follow up with its own weight. When the frictional resistance of the outer wall of the casing exceeds its gravity, the inner layer casing continues to break the rock forward until the convex shoulder on the guide makes contact with the convex shoulder on the casing shoe. At this time, the positioner transmits the part of the drilling pressure and the shock wave to the casing shoe, forcing the casing shoe to drive the casing and the drilling tool to follow up synchronously, protecting the drilled section hole wall. There are air holes for blowing cuttings on the surface of the positioner and air grooves for discharging cuttings at the bottom of the hole. Most of the compressed air reaches the bottom of the hole through the center hole of the positioner, the eccentric drill bit, and the center drill bit after the work of the hammer, scour the broken or loose rock at the bottom of the hole, cool the drill bit, and carries rock powder through the center drill bit, guides the powder discharge groove of the device enters the annular space of the casing, the hammer, and the drill pipe and is discharged out of the hole by the high-speed upward airflow or foam.

Application of down-the-hole eccentric casing drilling technology

Prevention and control of geological disasters

Today, with an emphasis on environmental protection and improvement of the human living environment, prestressed cable bolts are widely used in geological disaster prevention and control fields such as landslide prevention and stabilization of high slopes because of their advantages of simple structure and lightweight. Prestressed cable bolts are used for slope reinforcement in cities and towns, large and medium-sized hydropower stations, highways, important railways, etc. In the construction of prestressed cable bolts drilling work is the most important. Because many cable bolt holes will pass through slope deposits, spoils, or broken zones, it is often impossible to drill effective anchor holes by conventional methods because the strata are too fragmented and complex. The down-the-hole casing drilling provides a fast, effective, and safe method for collapsing broken rock and soil.

Hydroelectric power station and other pebble layer curtains, cofferdam high-pressure rotary spray

Down-the-hole casing drilling technology has been applied to the construction of hydropower stations, reservoirs, curtains, and cofferdams high-pressure rotary spray hole drilling. For the pebble layer and backfill curtain, weir, high-pressure rotary spray, or pendulum spray for seepage control treatment, the biggest problem encountered is the extreme difficulty of hole-making. Using other methods of drilling efficiency is extremely low, high cost, and may not even meet the project’s minimum requirements. The use of down-the-hole casing drilling technology, and hole formation efficiency of high quality and good, successfully solved the engineering problems.

Foundation treatment of urban high-rise buildings

Many high-rise buildings are built on the pebble layer, the slope support of the pebble layer pit, especially the greater depth of the pebble layer slope support, most of the combination of pile and anchor method. In some ultra-deep foundation pits, the technology of drilling holes with down-the-hole casings and placing prestressed cable bolts are adopted in the pebble layer. In addition, in the deep foundation pits of high-rise buildings, anti-floating anchor cable technology is also used, and the down-the-hole casing drilling technology has created better conditions for the technology’s implementation.

Tunnel collapse pipe shed support for highways, railways, and other tunnels

In the construction of underground projects in highways, railways, and cities, the large section tunnel construction often experiences collapse due to complex geological conditions, untimely support during construction, and vibration from cannon discharge. Generally, large-diameter steel casing over-support technology, which is stiff and strong enough to resist the huge collapse pressure, is used to deal with the problem, and good results are obtained. The method of down-the-hole casing drilling can continuously follow the steel casing into the collapsed body, cross the long collapse section of tens of meters at a time, and follow up many pipes to form a long pipe shed so that the subsequent work can proceed smoothly.

DTH eccentric casing drilling technology is also used in roadbed and bridge foundation surveys of highways and railways, dam foundation exploration work of reservoir dams and river embankments, etc.

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