Problem

2021 ARMA Student Design Competition

Problem Statement

Ore is being mined from an open pit. For the purposes of this problem, it can be assumed that the orebody is infinite in length and extends vertically downward from the ground surface. Determine maximum mining depth (to the nearest 10 m) such that total income from the ore equals the total cost.


Design Acceptance Criteria:

  • Factor of Safety ≥1.3 or Probability of Failure ≤5%

  • The final pit geometry may be made up of more than one slope segment.

  • Mined slopes do not need to have benches, but benches may be added to improve overall stability, if desired.

Orebody Characteristics:

  • Width = 100 m

  • Ore value = $200/m3

Rock Mass Mean Properties:

Rock Mass Strength Variability:

  • Friction Coefficient of Variation = 0.1

  • Cohesion Coefficient of Variation = 0.4

  • Tension Coefficient of Variation = 0.4

Costs:

  • Excavation = $20/m3 of rock

  • Dewatering = $500/m3 of water



Ground Water Assumptions:

    • Initial groundwater is at the ground surface.

    • Assume all water is contained in porosity.

    • There is no recharge or groundwater flow.

    • All water from mined ore must be removed.

    • There cannot be any water inside the pit (i.e., no pit lake)

    • Saturation above phreatic surface is zero.

    • Saturation below phreatic surface is 100%.

    • Water pressure below phreatic surface is hydrostatic.

Software

Any software may be used.

FLAC/Slope is available for free:

https://www.itascacg.com/contact-itasca/request-free-flacslope-software

Guidance

1. No groundwater flow analysis is required.

2. Water tables (phreatic surfaces) should be assumed.

3. The dewatering cost is based on the amount of water removed above the phreatic surface after excavation.


Report

The submitted report should address the following topics:

  • Problem Definition

  • Figure representing the geological/geotechnical model.

  • Overall approach, including assumptions.

  • Define and justify the analysis methodology.

  • How is variability of rock mass strength addressed?

  • Slope Design Procedure and Results

  • Financial Analysis

  • Final Design Selection – specifying maximum mining depth.

  • What are the key risks to the proposed solution?

  • How could risk be mitigated?


Ideally, the report should be less than 8 pages, but there is no formal limit.