A municipal overpass project adopts bored pile foundation and octagonal pier column, and the superstructure is post-tensioned concrete box girder with a span of 25m. The pile foundation mainly passes through gravel soil (the gravel content is less than 20% and the particle size is more than 2/3 of the inner diameter of the drill pipe). The bored pile project is subcontracted to a professional construction company.

There are two situations during the construction of bored piles: (1) serious hole collapse occurred during the hole-forming process of No.7 pile; (2) Pile 21has mud inclusion quality accident. During the construction process, a safety accident occurred because the drilling construction unit did not obey the safety management of the general contractor.

Precast beams are fabricated and tensioned on site. Prestressed pipe adopts metal spiral pipe. The design does not specify the concrete strength of prestressed beam when it is tensioned, and the cement slurry used for hole grouting after tensioning is not mixed with additives. After inspection, four jacks have been used: jack 1 has been used for 7 months, but only 15 times; Jack 2 was used for 3 months and used 200 times; Jack No.3 was used for 5 months and 95 times. Jack 4 is used abnormally.

According to the scenario, answer the following questions:

1. The bored pile construction of this project should adopt the method of () hole formation.

A. positive circulation rotary drilling rig

B. Reverse circulation rotary drilling rig

C. impact drill

D. Full rotary drilling rig

Answer: b