100-Ton (890 kN) Rock Anchor Pullout Test

Abstract

The paper recounts the writers’ experience in conducting an 890 kN (100 t) rock bolt pullout test, which was done to confirm the data for the design of the foundation to support a 54.860 m (180 ft) tall steel lattice self-supported communications tower to be located on South Mountain, Phoenix, Ariz. This was the writers’ first experience with a pullout test; a test of 890 kN (100 t) seemed too great a task, even when the design calculations indicated a factor of safety of 2.6. The tower foundation is to be designed with three independent reinforced concrete blocks, each of which must be tied down to the underlying rock by four rock bolts designed to withstand an uplift of 320 kN (36 t). During construction, the bolts will be proof tested to 445 kN (50 t) and then tied off at 355 kN (40 t), 110% of the design load. A 10.67 m (35 ft) rock bolt was placed in a hole with a depth of 9.45 m (31 ft) and a 75 mm (3 in.) diameter and grouted in place. The test was stopped at 890 kN (100 t) to prevent any possibility of a catastrophic failure. The conduct of the test is described, the calculations are explained, and the test equipment is shown. The paper discusses the procedures followed in conducting the test and the difficulty the writers encountered in placing complete faith in the engineering calculations, which were not supported by any prior pullout test experience. The writers recommend that any structural engineer who has a choice begin with a test smaller than 890 kN (100 t) for a first experience.