# Impact Characteristics of a Bidirectional Pneumatic DTH Hammer for Unconsolidated Formations
## 1 Introduction  
In drilling projects, unconsolidated formations like the sand layer, gravel bed, and cobles formation are regarded as one of the most complex and challenging formations to deal with [1; 2; 3], since it is prone to causing drilling accidents such as drilling tools sticking, drilling tools burying, and borehole collapse. Once drilling tools are stuck or buried in unconsolidated formations, retrieving them from boreholes is almost impossible, which will incur additional time and cost consumption [4; 5; 6]. In order to drill boreholessuccessfully in unconsolidated formations, casing while drilling finds increasing utilizations recently, including long spiral rotary drilling with casing and percussive rotary drilling with casing [7; 8]. In the literature [9], a risk analysis to Casing-while-Drilling (CwD) programs was carried out to highlight its technical benefits and a modeling approach was presented for applying risk analysis to CwD to achieve successful drilling programs. [PERSON] and colleagues utilized an orthogonal experimental method to study a drill bit for CwD and investigated the influence of bit parameters on reverse circulation performance [10]. [PERSON] et al. presented a fractographic analysis and failure analysis of K55 seamless casings used for drilling an oil well with the method of CwD and recommended large-grained tube steels in conjunction with high-quality premium connections to enhance the durability of the casings for withstanding repeated torque and compression stresses [11]. In the literature [12], drilling-with-casing (DwC) equipment for a water well was introduced and newly developed Down-the-Hole (DTH) hammers for DwC were studied, which exhibit higher impact energy. In the literature [13], CwD technology was proven to be the most effective solution against borehole instability in the Sultanate of Oman, where Fiqa formation was present. [PERSON] et al. [14] designed a drill bit for reverse circulation DTH hammer drilling with casing to pass through complicated formation, and impact of drill bit gas channel structural parameters on reverse circulation performance was then studied using a combination of numerical simulations and experimental analysis. Compared with conventional drilling methods, CwD can effectively reduce drilling cost and time consumption. However, this method has its flaws. Firstly, the process of CwD is complex, and on the other hand, due to the friction between the casings and borehole wall, drilling depth by this method is limited. With advantages of high efficiency, low cost, safety, and reliability, DTH hammer drilling has found extensive application in oil and gas drilling, mineral exploration, geological exploration, and foundational engineering construction [15; 16; 17]. According to the actuating medium, DTH hammers can be divided into two classes: hydraulic and pneumatic [18; 19; 20]. Since air is used as the circulating medium, there is no environmental pollution in pneumatic DTH hammer drilling and drilling fluids leakage can be avoided in loose and unconsolidated formation. [PERSON] et al. [21] formulated a dynamic model for a DTH hammer and analyzed its operational characteristics, so as to enhance the impact efficiency of the DTH hammer by optimizing design parameters and suggesting modifications. In the literature [22], computational fluid dynamics (CFD) were adopted to evaluate the performance of RC-DTH air hammers and the effects of parameters such as rebound coefficient, input air pressure, and piston mass were thoroughly examined. In the literature [23], a pneumatic dynamic model for a DTH hammer and a motion model for a drill bit were established, and then these two models were combined to assess impact efficiency under varying rock conditions. Aiming at breaking into rocks with reduced weight on the bit and lower power, and to also remove rock cuttings effectively, [PERSON] et al.