Geotechnical investigations for resumption of mining

Introduction

SLR was appointed to provide geotechnical support for the recommissioning of a mine in Malawi. The work involved comprehensive site investigations and technical assessments to support safe and sustainable mine redevelopment. Activities were conducted in phases beginning in 2022, with follow-up work in 2024 and onwards as construction activities progressed. The geotechnical investigations undertaken were in support of the design to resume mining. These entailed investigations for the remediation of a palaeo-landslip that had severely damaged parts of the processing plant, investigations of alternative sites for the development of tailings storage facilities, stability analyses of the open pit side-slopes and the assessment of founding conditions for new plant structures.

Challenge

SLR were appointed to assess a creep failure at the plant site, which was moving at approximately 40mm/year and had impacted on the structural integrity of the plant site’s foundations. An initial assessment was carried out and it was evident that a major rotational paleo-landslide (MPL) had occurred, with several slumps evident behind the MPL. The failure was interpreted as a possible multiple retrogressive slide that has eroded and developed into a succession of slips, with the mechanism driving slope movements being due to the external disturbances in the form of cut and fill operations for the plant construction, as well as loading in the upper slope due to the construction of Waste Rock Dumps. These have induced an altered balance between the driving and resisting forces, which have led to renewed movements in a slope that previously prevailed in a delicate state of equilibrium.

A rotary core drilling program was developed to evaluate the geometry and position of the large strain surfaces causing ground movement. Drilling also allowed for detailed logging and the collection of core samples for soil and rock testing. Structural orientations were measured by means of a downhole televiewer. Based on the drilling results three geotechnical domains were identified. A pile design was carried out comparing pile diameters, pile spacing, pile length and required stiffness versus the ultimate loads, using PyPile a software program based on p-y curve method and finite element method, which calculates bending moment and shear force can be assessed with different load cases and multiple pile types.

2D limit equilibrium and numerical modelling was carried out using SLIDE2 and RS2, on three sections developed from the Leapfrog model. 3D numerical modelling was carried out using FLAC3D, incorporating the 3D Leapfrog mode. The conclusion of the investigation was that ground movements could potentially be reduced by removing the load imposed by the waste rock dumps in the upper slope, combined with a system to manage the driving forces associated with the slope movements and the installation of drainage.

Solution

In 2022 three alternative sites were investigated for possible TSF construction, one being the existing TSF and two greenfield sites located to the north of the mine. The investigations comprised a combination of test pitting, rotary core drilling, in-situ testing, sampling and laboratory testing. The decision was ultimately made to extend and raise the existing TSF and additional investigations were conducted in 2024 compirsing ntailing infill test pitting and drilling for the expansion and raising of the existing TSF, detailed mapping of a faulted gneiss / sedimentary contact that runs through the eastern side of the TSF and investigations to identify suitable construction material sources.

SLR undertook geotechnical monitoring and construction quality assurance (CQA) for the development of a pioneer rockfill platform along the toe of the west flank of the TSF embankment. The platform, constructed directly over tailings, was designed to provide toe support and stabilisation for subsequent embankment raises, while also acting as a countermeasure against excessive settlement and potential rotational failure. Construction was executed across variable ground conditions, including dry tailings, saturated tailings, and open water areas. The scope of work included assessment and verification of founding conditions, identification and approval of suitable rockfill sources, specification and oversight of placement and compaction of rockfill layers, and stability assessments of the adjoining west flank natural hillslope.

Processing Plant Foundations
Geotechnical input was provided for construction of new plant infrastructure, with a particular focus on the foundations for settlement-sensitive and load-bearing structures. The site investigation programme comprised core drilling, test pitting, in-situ testing, and laboratory analysis to establish the stratigraphy, geotechnical parameters, and founding conditions across the development footprint.

Based on the outcomes of the investigations, engineered foundation solutions were recommended. This included soil rafts enhanced with geosynthetics to improve load distribution and to act as a separator layer, preventing the migration of fines into the rockfill. This was combined with engineered fill and selected rockfill placement to achieve the required bearing capacity and long-term foundation stability. Such measures were implemented to mitigate risks of differential settlement, which can adversely impact structural performance. Particular attention was paid to critical plant components such as tanks, which is usually intolerant to movement.

The combined application of ground improvement measures, together with the controlled selection, placement, and compaction of engineered fill materials, provided a stable and competent foundation for the proposed infrastructure, ensuring adequate bearing capacity and minimizing the risk of differential settlement.

Impact

The project demonstrates SLR’s expertise in complex geotechnical problem-solving, including slope remediation, tailings facility design, foundation engineering, and construction support in challenging terrain. Construction is currently underway, with SLR providing ongoing geotechnical input to ensure safe and effective implementation.