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°Õ±ð°ì¾±Âáä: Zanjar Abubakir 

Valvoja: Prof. Lauri Uotinen

Ohjaajat: D.Sc. Mateusz Janiszewski MSc.  Jussi Haiko (Helsingin kaupunki)

Rahoitus: Helsingin Kaupunki ja Suomen kalliomekaniikkayhdistys ry

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The second generation of Eurocode 7 has been introduced in Finland, with a major addition in the design of rock bolts, which now requires analytical verification of both ultimate and serviceability limit states. This requires grouted rock bolts to be verified at two interfaces: the bolt-grout and the grout-rock. Verification calculations of these interfaces can only be relied upon by applying the newest Eurocode 2. Applying this to rock bolt design carries the risk that the calculated results may lead to significantly longer anchorage length requirements that are necessary based on experience.

The anchorage length of grouted rock bolts and the shear stress at the interfaces were investigated through a literature review, analytical calculations, and experimental pull-out tests. The pull-out tests were carried out in the underground research laboratory of Aalto University, where 20 mm diameter rebar bolts with strain gauges installed were grouted to the tunnel wall with different bond lengths. After the experiments, the results were compared with the Eurocode calculations and with other similar pull-out tests.

The results showed that bond failure occurred only at the bolt-grout interface and only in cases where the bond length was 200 mm, whereas at higher bond lengths, the failure occurred in the bolt. Based on this, it could be estimated that the critical anchorage length is approximately 250 mm. In addition, based on the strain gauges, the effective anchorage length was approximately 450 mm, beyond which no further increase in bond strength was observed. Whereas, according to the Eurocode calculations, the design anchorage length would be 920 mm with the same parameters.

The new Eurocode 2 calculations give an anchorage length that is almost twice as large as suggested by the pull-out tests and are therefore too conservative. These findings emphasize the need to critically evaluate the applicability of Eurocode 2 to rock bolt anchorage length design. A similar kind of pull-out testing in-situ was proposed to verify the interface strength and anchorage length. 

°Õ±ð°ì¾±Âáä: Joonatan Seppänen

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