Cracking the Code of Hard Rock Drilling: The Game-Changing DTH Technology.
According to the geological conditions of the construction area, there are four main drilling methods commonly used by drill rigs, as shown in Figure 1.
a. Pneumatic or hydraulic-driven rotary heads combined with impact equipment transmit rotational and impact energy through the top of the drill pipe, delivering energy to the drill bit via shock waves in the drill pipe. This method is limited to small diameters and shallow depths and is generally used in quarries, construction sites, and underground mining operations.
b. Down-The-Hole (DTH) hammers are located at the bottom of the drill string. Compressed air enters the DTH through the drill string, driving a piston in a reciprocating motion that directly impacts the drill bit, transferring impact energy to the rock. This system has minimal power loss and is particularly suitable for deep holes, straight holes, and medium-hard rocks.
c. Reverse circulation (RC) drilling uses DTH to collect and transport rock samples from the face of the drill bit. Dry and uncontaminated cuttings are conveyed through the center pipe of the DTH into a sample collection device, preparing for geological analysis.
d. A gearbox driven by hydraulic or electric motors forms a rotating power head, applying sufficient feed force to the three-cone drill bit through a feed system that moves up and down on the drill rig and a thick-walled drill pipe. This method is used for softer rocks or strongly jointed hard rocks.
- Principles and Characteristics of DTH
The formations drilled with DTH are almost entirely capable of including all igneous rocks, metamorphic rocks, and sedimentary rocks that are medium-hard or harder. DTH is particularly advantageous for drilling hard rock and tough strata because hard rock tends to be brittle. Under impact loads, fractures not only occur at the direct impact site but also create a broken zone, resulting in larger rock fragments. Thus, the drilling speed is significantly faster compared to pure rotary drilling. The mechanical model in Figure 2 illustrates the various loads acting on the rock during impact rotary drilling.
In addition, DTH (Down-the-Hole) drilling is highly effective in formations prone to borehole deviation, such as strata with well-developed bedding and foliation, or rock layers with uneven hardness and many fractures. It can significantly reduce borehole deviation and also overcome difficulties in drilling through gravel layers and boulder beds.
DTH drilling was developed in the late 19th century and has a history of over a century. Although there are many types of DTH drills, they share a common feature: both the impact mechanism and the drill bit are submerged into the borehole, with rotation combined with impact to break the rock. The equipment used to generate the impact force can be categorized by their driving method into pneumatic, hydraulic, oil pressure, electric, and mechanical types.
Since impact energy experiences significant loss during transmission and can cause substantial damage to the impacted components, it is generally required in deeper drilling operations that the equipment enters the borehole with the drill tool, so that the output impact force can act directly on the drill bit or core barrel. This minimizes energy loss during transmission, improves energy efficiency, and reduces the likelihood of downhole equipment failures.
Pneumatic DTH, also known as air-driven DTH, comes in many structural types and classification methods.
- By pressure rating: high-pressure, medium-pressure, and low-pressure types.
- By overall structure: non-through and through types.
- By valve operation principle: control valve type, free valve type, and hybrid valve type.
- By piston structure: equal diameter piston, unequal diameter piston, and tandem piston types.
- By air distribution type: valved DTH and valveless DTH. Valved DTH can be subdivided into plate valve, disc valve, and cylindrical valve types, while valveless DTH can be divided into center rod exhaust type, piston air distribution type, and combined air distribution by piston, cylinder, and center rod.
- By hole washing and slag discharge method: center hole washing, front hole washing, and side hole washing.
2. Determination of Impact Mechanism Structural Schemes
2.1 Piston Self-Gas Supply Non-Valve Impact Mechanism
This type of impact mechanism primarily uses the gas passages in the piston itself for gas supply, resulting in a complex piston structure with numerous gas channels, which reduces piston strength and lifespan. However, this impact mechanism integrates the inner and outer cylinders, increasing the effective working area of the piston and enhancing the impact energy of the mechanism.
2.2 Piston and Cylinder Combined Gas Supply Non-Valve Impact Mechanism
This type features a simple structure that is easy to manufacture, has a longer piston lifespan, and the gas holes are located on both the cylinder and the piston. This structure is widely used abroad.
2.3 Center Pipe Gas Supply Non-Valve Impact Mechanism
In this mechanism, the air intake passages for the upper and lower chambers are arranged on the circular pipe where the piston slides. It requires high manufacturing precision and has a relatively short lifespan for the center pipe.
2.4 Side Exhaust Impact Mechanism
The so-called side exhaust means that the exhaust gas path exits the cylinder body rather than passing through the center of the drill bit to the bottom of the hole. This type of impact mechanism typically has many intake and exhaust pathways on the cylinder body, leading to poor structural strength, potential for longitudinal fatigue cracks, and significant loss of air pressure, resulting in suboptimal debris removal and cooling effects for the drill bit.
2.5 Center Exhaust Impact Mechanism
This type of impact mechanism expels debris and gas directly from the center of the drill bit to the bottom of the hole. Direct airflow not only enhances the debris removal efficiency but also improves drilling efficiency and cooling, extending the lifespan of the drill bit. This structural type replaces the numerous longitudinal grooves in the inner cylinder of side exhaust impact mechanisms with an annular groove, greatly reducing stress concentration in the inner cylinder. It has become a widely adopted structure in recent years.
2.6 Series Piston Impact Mechanism
The series piston impact mechanism, also known as a dual-piston (head) impactor, divides the cylinder into two chambers using an isolation ring. This design allows both piston faces to work simultaneously within the same bore diameter, resulting in greater impact power and higher impact frequency. Correspondingly, there is a dual exhaust system that effectively removes rock powder from the bottom of the hole. However, its main drawback is its complex structure and the need for high precision in machining parts; for example, the piston has up to five mating surfaces with associated components, which limits its application and promotion. Therefore, this design adopts the second option, which is the non-valve impact mechanism with piston and cylinder combined gas supply. Its structure is shown in Figure 3.
3.Theoretical Analysis and Relevant Calculations for DTH
3.1 Selection of Operating Parameters**
3.1.1 Hammer Length and Weight: The preliminary design specifies a length of less than 4500 mm and a weight of less than 2500 kg.
3.1.2 Hammer Diameter: The appropriate hammer diameter is determined based on the drilling diameter, set at 540 mm.
3.1.3 Drilling Diameter: This refers to the diameter of the pile hole, generally between 550 mm and 600 mm.
3.1.4 Drilling Depth: According to pile design requirements, this is typically between several dozen meters and one hundred meters.
3.1.5 Drill Speed: DTH drilling generally operates at low rotational speeds, typically between 7 and 25 r/s.
3.1.6 Rotational Torque: The maximum torque for this design is set at 150 kN·m.
3.2 Calculation of Design Parameters
The design parameters for DTH, specifically the performance parameters of the DTH impact equipment, serve as the basis for machine design and define the performance of the manufactured equipment.
3.2.1 Design Pressure P for Impact Equipment
In China, a pressure of 0.49 MPa (approximately 5 × 10^5 Pa) is widely chosen as the design standard for pneumatic impact equipment. Given that the DTH for this design is a non-valve impact device with a large drilling diameter and heavy piston, higher air pressure will further enhance performance. Additionally, high-pressure air compressors are increasingly common, and in accordance with international standard ISO 5941-1979, a design pressure of 1.6 MPa is selected.
3.2.2 Impact Power
For DTH used to drill large diameter holes, the design impact energy can fluctuate significantly. The impact energy for this design is calculated as follows:
3.2.3 Impact Frequency
Generally, under constant impact energy, the impact frequency is proportional to the output power of the impactor. However, when the cylinder diameter is fixed, increasing the impact frequency necessitates a reduction in piston stroke, which in turn decreases the single impact power. Once the single impact power falls below a certain threshold, increasing the frequency will not yield satisfactory rock-breaking results. Thus, the selection of impact frequency is constrained by the impact power.
For pneumatic DTH operating at a design pressure of 0.5 MPa, the frequency should not exceed 16.8 Hz. Since the DTH operates at design pressures between 0.5 and 2.5 MPa, the impact frequency can vary significantly. The initial selection of the impactor's frequency can be calculated as follows:
f = 10.4 + 7.6P
(2) where P is the system supply pressure. For this design, P = 1.6 MPa, therefore:
f = 10.4 + 7.6 × 1.6 = 22.5 Hz.
3.3 Structural Parameter Design
The main structural parameters of DTH include cylinder bore diameter, piston stroke, and piston dimensions. Increasing the cylinder diameter can enhance both impact power and frequency, so the diameter should be maximized within the limits of structural size. Typically, the difference between the outer diameter of the DTH and the bore diameter should not be less than 15 to 20 mm, and the outer casing of the DTH should not be too thin. Therefore, the ratio of DTH cylinder diameter to drilling diameter is generally above 0.5.
3.3.1 Working Diameter of the Cylinder and Structural Stroke
The working diameter D of the cylinder can be calculated as follows:
D = K × D(bore) = (0.57 - 0.68) × D(bore) (3)
For this design, D(bore) = 600 mm, so D is taken as 360 mm. The structural stroke S is empirically taken as S = 500 mm.
3.3.2 Piston Mass
The radial dimensions of the piston are constrained by the size and structure of the cylinder, allowing for either equal or different diameter pistons. The linear dimensions depend on the weight of the piston, which also relates to the velocity it has when striking the drill bit. Therefore, determining the structural dimensions of the piston is a complex aspect of DTH design. The mass of the DTH piston can be estimated as follows:
m = 0.0205D^2.84 (4)
where m is the piston mass in kg; D is the working diameter of the cylinder in cm.
Substituting D = 36 yields: m = 540 kg.
The DTH mainly consists of a torque transmission structure and a pneumatic impact mechanism. The torque transmission structure connects the drill rod and DTH, transmitting rotational cutting and pulling forces; the pneumatic impact mechanism generates impact, providing axial power to the impact drill bit. Specific structure details are shown in Figure 3.
The torque transmission structure connects the drill rod and the impactor. The upper joint connects with the drill rod and the impactor through pipe threads, primarily to ensure gas tightness while also transmitting torque and pulling force. The check valve prevents mud and water from entering the impactor and drill rod, controlled by a spring. The air intake seat with the gas supply rod functions to introduce compressed air into the cylinder, facilitating the gas supply action together with the cylinder and piston, thereby achieving combined gas supply. The spring safety ring prevents the piston from sliding out of the cylinder when replacing the drill bit.
3.4 Finite Element Analysis of DTH Drill Bit
The DTH drill bit is subjected to impact forces from the piston and torque from the drive head. The force exerted on the piston by the compressed air is given by:
F = P × S (5)
where P is the system pressure in Pa; S is the force area of the piston in m².
Thus, F = 1.6 × 10^6 × 0.084 = 134400 N.
Therefore, the impact force on the drill bit is:
F' = kF (6)
where k is the impact coefficient; F is the force exerted on the drill bit by the compressed air in N. Thus, F' = 20 × 134400 = 2688 kN.
The torque applied to the DTH drill bit by the drive head is N = 150 kN. Applying F' = 2688 kN and N = 150 kN to the drill bit while fixing the lower end face of the drill bit, and using QT500-7 material with a yield strength of 320 MPa, finite element analysis is performed. The specific loading and constraint conditions and mesh division are shown in Figures 4 and 5.
The results of the finite element analysis are shown in Figure 6, indicating that the maximum stress is 144.355 MPa, which is less than the yield strength of QT500-7 (320 MPa), thus meeting the requirements.