Session
Poster Presentation, Arizona Foyer
Abstract Summary
SRK was commissioned by Terrafame Oy to conduct an overall geotechnical assessment of the slope design parameters of the Kuusilampi deposit area as an input into Terrafame’s mine planning processes for 2020/21. The existing Kuusilampi open pit is 200m deep, with pushback and deepening planned down to 400m and mostly concentrated on the North East and north areas. This paper presents the design work that was carried out under challenging geotechnical and hydrogeological constraints owing to the adverse and persistent structures in an otherwise strong rock typical of Nordic environments.
The Kuusilampi (KL) pit operation is within a mostly highly competent crystalline rock mass with slopes constructed in good compliance to design. The main geotechnical domains are controlled by the main lithology types and the dominant fabric (foliation and jointing) within each. The rock mass of the deposit area is typical to Nordic blocky to massive hard rock environments with well-developed structural fabric related to the foliation fabric and jointing. The study comprised slope reconciliation, reviewing and updating geotechnical and hydrogeological data sets, compilation of the data, defining geotechnical domains and pit design sectors, bench scale kinematics and IRA/Overall slope analysis.
The phase 6 to 8 cutbacks of the North and North east slopes are conditioned by a 1-5m wide sheared Talc-Tremolite Schist (TTS) separating Mica Schist (MS), and Black Schist (BS) units. The importance of the TTS to slope design is its adverse orientation, its relative lower strength and its variable fabric—from intact rock to intense mineral alignment through to crushed and clay-dominant content. A suite of analyses was thus needed to cater for the variable rock mass and hydrogeological conditions and their impact on slope design.
In the west wall, slope orientation to the structural fabric is the main control on bench stability and informs the bench configuration and slope design. As a result, fabric, jointing and strength anisotropy were explicitly modelled in the finite element and limit equilibrium models that were constructed to analyse inter-ramp and overall stability. Bench scale analyses also presented a challenge in applying wedge software to jointed anisotropic rock mass. Examination of the current slope behaviour and of operational constraints was key in interpreting results for practical pit design.
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