Installation of lattice girders with Toussaint–Heintzmann yielding elements in poor ground O Belov Oyu Tolgoi LLC, Mongolia T Roberts Jennmar, Australia KJ Ma Jennmar, USA J Ooi Rio Tinto, Australia B Baasanjav Oyu Tolgoi LLC, Mongolia Abstract In block cave mines, the permanent life of mine tunnels are required to withstand multiple changes in ground stresses and remain functional for up to several decades. In block cave mines, in general, the changes in ground stresses originate from initial induced stresses during tunnels and chamber excavations. Following this is an increase in abutment stresses due to undercut front advancement phase. Further increase in abutment stresses occurs due to cave growth and upward propagation. Then, the abutment stress reduction takes place due to the cave breakthrough to the surface. It should be noted that at some mines, the ground stresses may increase due to cave material re-compaction or due to remnant pillar loading if undercutting was not carried out properly. As a matter of good practice, the key requirement for a block cave mine permanent infrastructure, such as an ore handling system, is to have it excavated and fitted out before the start of the undercutting phase. Multiple changes in ground stresses are likely to cause large deformations in the tunnels and pose challenges to ground support designs with the choice of ground support/reinforcement types/systems/elements, particularly if the permanent infrastructure is situated in the poor ground due to the presence of large weak structures. At Oyu Tolgoi mine, the tunnel stability of the haulage level is very important for the uninterrupted movement of payload trucks delivering ore from truck chutes to crusher chambers. Affected by changes in abutment stresses and poor ground conditions in certain sections of the tunnels, excessive drive deformations are expected throughout the several decades of mine life in the haulage level requiring innovative ground support/reinforcement design involving installation of fibrecrete embedded lattice girders with yielding elements. Keywords: steel arch, lattice girders, yielding elements, poor ground, innovative design, block cave, permanent tunnelling infrastructure 1 Introduction Oyu Tolgoi is a newly built large copper/gold block cave mine situated in the South Gobi region of Mongolia. The mine utilises haulage level underneath the extraction level for moving ore by 160 t payload trucks from several truck chutes to primary crusher chambers. From the crusher chambers, the crushed ore is transported by conveyors to the loading station at production shaft SH2 and to conveyor to surface system, then the ore is transported to the surface. Drive stability in the haulage level is very important for the uninterrupted movement of 160 t payload trucks from truck chutes to crusher chambers. A large and weak lower fault intersects a section of the haulage level near Truck Chute #4. During the mining of the haulage level drive through the lower fault, a large fall-out occurred from the backs of the drive. Mine-scale geotechnical modelling indicates that potential excessive Ground Support 2023 – J Wesseloo (ed.) © 2023 Australian Centre for Geomechanics, Perth, ISBN 978-0-6450938-5-8 Ground Support 2023, Perth, Australia 667 doi:10.36487/ACG_repo/2325_46