É«É«À²

Author: Zanjar Abubakir 

Supervisor: Prof. Lauri Uotinen

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

Funding: Helsingin Kaupunki and Suomen kalliomekaniikkayhdistys ry

Abstract:

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.

Author: Joonatan Seppänen

Supervisor: Prof. Jussi Leveinen

Advisor(s): DI Antti Larkela ja Insinööri (YAMK) Sanna Anttila

Funding: Ramboll Finland ja Helsingin kaupunki

Abstract: 

The usage of crushed concrete made from concrete waste is increasing in Finland. It is replacing virgin natural stone material used in earthworks, thus advancing circular economy.  Despite the growing use of the material, there is no information on whether driven piles or sheet piles can be installed through crushed concrete layers.  This was investigated in a test piling carried out in autumn 2025 in Jätkäsaari’s Tritoninpuisto.

The test piling was carried out in a test structure that had previously been used to study the suitability of concrete crushed aggregate as a sea fill material. The test structure contains, across four test areas, three crushed concrete aggregates of different grain sizes, 0/90, 0/150 and # 0/300, and pre-crushed blasted rock of size # 0/300 as a control material.  The five-metre-deep structure is partially below the groundwater table, and a full-scale dynamic compaction test was performed on it.  The test piling examined three pile types: 300 × 300 mm2 reinforced concrete piles, and driven and bored 170/12.5 mm steel tube piles, as well as profile PU12 steel sheet pile walls. Four of each pile type were installed in each of the four test areas, except for the sheet piles, for which five per area were installed. The number of blows required for penetration was recorded for the driven piles, from which the blow resistance of the layers was assessed. PDA measurements were also carried out on those piles. For the bored piles and sheet piles the installation time was monitored. Lastly, the installed piles were pulled up and checked if they were damaged.

The test results showed that the concrete crushed aggregate formed very hard layers in places and, in general, it became denser the smaller the grain size used. All the tested piles penetrated the crushed concrete intact, but the number of blows climbed to high numbers for some of the reinforced concrete. On the densest, 0/90 aggregate area, installation of the sheet piles was not successful, but in the other areas they it was possible. Both the driven and the bored steel tube piles penetrated the layers relatively easily and can be regarded as well suited for this type of structure.