# Impact Characteristics of a Bidirectional Pneumatic DTH Hammer for Unconsolidated Formations
## 5 Field Experiment of Pneumatic Bidirectional DTH Hammer  
A set of DTH hamsters was manufactured and field experiments were conducted in a gravel formation of Sichuan Province, so as to verify its working performance. The pneumatic bidirectional DTH hammer drilling experiments were conducted in a deep excavation of Chengdu Haifu Commercial Center, Kehua Middle Road, Chengdu, Sichuan Province, China. The strata in the area where the foundation pit is located from top to bottom is the Quaternary Holocene artificial fill layer (Q\({}_{4}\)\({}^{4}\) + mL), the ice water accumulation cohesive soil layer of Quaternary middle lower Pleistocene (Q\({}_{1}\) + \({}_{2}\)\({}^{[\mathrm{g}]}\)), and mudstone of Cretaceous Guankou formation (K\({}_{2}\)\({}^{\mathrm{g}}\)). The Quaternary artificial fill layer is a miscellaneous fill with a thickness of about 0.8 m to 3.5 m at the upper strata. The lower part of the Quaternary artificial fill layer is a plain fill and mainly composed of cohesive soil with a loose structure, whose thickness is about 1.4 m to 3.8 m. For the ice water accumulation cohesive soil layer of the Quaternary middle lower Pleistocene, the upper strata is clay with a layer thickness of about 12.8 m to 14.4 m, in which a small amount of pebbles is contained. The lower part of the Quaternary middle lower Pleistocene is a pebble-containing cohesive soil with a layer thickness of about 0.9 m to 2.8 m, which is mainly composed of granite and sandstone. The pebble particle size is about 50 mm to 150 mm, with some being even more than 200 mm, with a medium roundness and pebble content of about 50% to 60%. The mudstone of the Cretaceous Guankou formation is an extremely soft rock and argillaceous texture structure, which is mainly composed of clay minerals.  
Figure 16: Impact energy at different inner tube assembly lifting distance.  
Figure 15: Piston displacement at different inner tube assembly lifting distance.  
The drilling rig used in the field experiments was a YG-100A hydraulic rig for anchoring borehole construction, which was manufactured by Chengdu Jindi Prospecting Machinery Co., Ltd. (Chengdu, China) All movements of this drilling rig are hydraulically driven, with a drilling depth of 50 m to 150 m, a drilling diameter of 89 mm to 250 mm, and a motor power of 25 kW, as shown in Figure 17. The bidirectional pneumatic DTH hammer has an outer diameter of 130 mm. During the experiments, the bidirectional DTH hammer was in good working condition, achieving a high drilling rate. In fact, the maximum drilling rate can reach up to 1.5 m/min, and the actual bit on the bottom time was much shorter than the auxiliary time of adding drill pipes. By lifting the DTH hammer away from the bottom of the borehole and pumping compressed air, the DTH hammer piston can achieve a high frequency backward impact. To verify the function of releasing sticking drilling tools using a backward impact, an appropriate amount of rock cuttings was added to the borehole to simulate a drilling tools' sticking accident. Rock cuttings above the DTH hammer could be cleared away and the drill pipes and DTH hammer could be lifted out of the borehole smoothly. There are no drilling tools' sticking or burying accidents. The above results indicated that the bidirectional pneumatic DTH hammer can effectively form boreholes in loose formations and deal with drilling tools' sticking or burying accidents.