In recent years, large-diameter rotary drilling methods have been widely promoted in pile foundation construction. In large-diameter rotary drilling, drilling in hard strata is one of the main difficulties encountered. The hard strata typically encountered in rotary drilling mainly include riverbeds (former channels), gravel- and boulder-bearing strata and weathered rock formations in mountainous and hilly areas. Short spiral augers are mainly used to drill these relatively hard strata. The selection and use of short spiral augers is an important factor affecting construction efficiency.

　　1 Structure and classification of short spiral augers

　　The structure of a rock-embedded short spiral auger for drilling hard strata is shown in Figure 1; it consists of a flange, a core shaft tube, main spiral blades (guide flights) and pilot spiral blades (cone flights). Commonly used rock-embedded short spiral augers can be further classified by structural form into single-cone single-flight short spiral augers, double-cone single-flight short spiral augers, and double-cone double-flight short spiral augers.

　　A single-cone single-flight short spiral auger refers to a spiral auger composed of one set of main spiral blades (guide flights) and pilot spiral blades (cone flights) (Figure 2); double-cone double-flight refers to one composed of two sets of main spiral blades (guide flights) and two sets of pilot spiral blades (cone flights) (Figure 3). A double-cone single-flight short spiral auger is composed of one set of main spiral blades (guide flights) and two sets of pilot spiral blades (cone flights). There are also other non-full-flight, non-full-cone auger forms, which are generally less commonly used. The blade spacing of the single-cone single-flight type equals the lead, and the blade spacing of the double-cone double-flight type equals 1/2 the lead. The upper part of the pilot spiral blades carries cutting tools, which perform the tasks of breaking rock and discharging soil (stone), while the flat spiral blades serve to guide and discharge soil (stone). The cutting tools are generally cutting picks with tungsten-cobalt hard alloy brazed on the heads.

　　2 Working principle of short spiral augers

　　During drilling, under drill pressure, the center tooth at the bottom end of the core shaft tube first "cuts a groove" at the center of the hole bottom, forming a free face for breaking; the cutting tools on the spiral cone flights then follow, forming a conical borehole. The trajectories formed by the drill teeth during drilling project onto the hole bottom as a set of concentric circles. Rock cuttings, soil, stone, etc. rise in the reverse direction along the spiral blades; after filling the spaces between the spiral blades, they are carried out of the hole when the auger is lifted, or after falling into the hole, they are salvaged with a sand-grab bucket.

　　The main parameters of a short spiral auger include pitch, spiral line length, cone angle of the spiral auger, cutting tool spacing (as shown in Figure 4), the rotation angle of the spiral line from the cutting tool to the diameter (as shown in Figure 5), the arrangement angle and length of the drill teeth, and the size of the alloy head, etc.

　　The single-cone single-flight short spiral auger has relatively few cutting tools and is generally used to drill loosely cemented gravel layers, especially strata with many and large gravels, and weathered rock formations. Because the blade spacing of the single-cone single-flight auger is relatively large, rock cuttings, soil blocks and crushed stones easily fall into the hole during lifting, so a sand-grab bucket is needed for cleaning the bottom; generally, one pass with the spiral auger is followed by one pass with the sand-grab bucket, alternating. In many cases, the spiral auger serves to break up or loosen the weathered layer or cemented gravel layer at the hole bottom.

　　The double-cone double-flight short spiral auger has cutting tools arranged on double cone flights, with 1.5 to 2 times the number of cutting tools of the single-cone single-flight short spiral auger. It is generally suitable for drilling weathered rock formations and gravel layers with relatively small gravels. During drilling, two drill teeth cut simultaneously, giving high drilling efficiency and relatively stable drilling.

　　If the two sets of pilot spiral blades have cutting tools arranged alternately, then the drilling trajectories of the double-cone double-flight short spiral auger are relatively denser than those of the single-cone single-flight short spiral auger, making it suitable for drilling under high drill pressure. The double-cone double-flight auger has somewhat better guiding performance and better capability to carry rock and soil than the double-cone single-flight auger; because the blade spacing is smaller, it is easily compacted by rock and soil, but this is unfavorable for extracting larger gravels.

　　The cone angle of the spiral auger is an important indicator. Generally, a larger cone angle means a smaller rotation angle of the spiral line, relatively fewer cutting tools, and lower manufacturing cost, and it can meet the requirements for drilling in strongly weathered and gravel layers. However, for hard strata such as moderately weathered and slightly weathered rock, an auger with a small cone angle should be used with high drill pressure.

　　The cutting tool arrangement technique also affects the drilling performance of the auger. Generally, when drilling relatively hard weathered rock strata, the downward deflection angle of the drill teeth (the angle between the tooth and the horizontal direction) and the protrusion (extension) amount should be small; while when drilling loosely cemented gravel layers, the downward deflection angle and protrusion (extension) amount of the drill teeth can be larger to improve drilling efficiency.

　　3 Application examples of short spiral augers

　　3.1 In July 2004, during construction at a site in Tonghua, Jilin, there was a gravel layer in the strata that was very difficult to drill. Short spiral augers were used for drilling, and a sand-grab bucket was used to grab cuttings and clean the bottom. However, during construction it was found that the spiral auger had low drilling efficiency and was difficult to advance, and the sand-grab bucket had difficulty grabbing cuttings. Forcibly applying pressure and increasing torque caused the spiral auger blades to tear, the drill teeth and seats to wear or be lost, the bottom plate of the sand-grab bucket to tear, and even accidents in which the lower bottom plate was twisted off and fell into the hole. Based on the construction phenomena and analysis of the grabbed cuttings, we concluded that this section was mainly a loosely cemented muddy gravel layer containing many large pebbles (boulders). These could not enter between the spiral blades or pass through the openings in the bottom plate of the sand-grab bucket into the bucket, causing drilling difficulties and damage to the auger (bucket). After switching to a single-cone single-flight short spiral auger, the larger pebbles (boulders) were first carried out of the borehole by the spiral auger (Figure 5), and the drilling effect improved markedly.

　　3.2 In September 2004, during construction at a site in Nanyang, Henan, the owner used an ED5500 rotary drilling rig manufactured in Japan to drill 1500 mm diameter pile holes. When drilling into gravel layers and mudstone-shale strata, a rock-embedded short spiral auger was used. Because the rig power was too low and pressure could not be applied, drilling efficiency was low. At the site, we improved the auger structure, changing the auger cone angle from 120° to 82°, thereby increasing the specific pressure of drilling. After the structural improvement, the drilling rate increased from almost no penetration to 0.6 m/h, and the gravel layer and mudstone-shale strata were successfully drilled.