Pit Phases and Pushbacks

Pit phases divide an open pit into volumes that can be developed and extracted as the mine advances. A pushback, also called a cutback, is an increment of excavation that expands a previous pit configuration. These divisions connect the final excavation geometry with the sequence of working fronts, access development and production. They provide manageable mining units, while retaining dependencies between earlier and later excavation.^[1]

Phases, Shells and Time Periods

A pit shell is a modeled boundary; the material between successive boundaries defines an increment. An engineered phase also needs a practicable route into that increment and space for the intended work. A calendar period is different again: it describes when work occurs. One phase may extend across several periods, and more than one phase may be active during the same period.

In the MineLib formulations, scheduling determines when blocks are extracted and incorporates operational resource limits. The authors also explain that material in a later-started pushback can be extracted before every block in an earlier pushback is finished. This is a useful distinction when translating phase geometry into a production plan: the phase label alone does not establish the detailed precedence or resource availability.^[2]

Define the convention used in drawings and schedules. In particular, distinguish a cumulative pit surface from the incremental volume assigned to a phase. Otherwise, quantities can be double counted or a drawing can be mistaken for the excavation available at a particular date.

Establishing the Purpose of a Phase

Start by identifying what the phase must achieve. It might expose an ore zone, maintain a supply of suitable plant feed, create room for a deeper ramp or provide another working area while a different sector is unavailable. These purposes can lead to different boundaries even within the same final pit.

Evaluate the phase as an excavation sequence. Which work enables its first production? Where is the initial entry? What material is encountered before the target feed becomes available? Which earlier excavation must be completed, and which facilities must be ready? This turns a geometric proposal into a set of dependencies that can be checked against the schedule.

The information required includes the current pit surface, geological and material models, geotechnical design domains, proposed access, equipment operating requirements and destination capacities. Keep material categories consistent with the production plan. A phase containing substantial total tonnage may still provide little feed of the type needed during the next operating interval.

Generating and Testing Candidate Boundaries

Parametric optimization can produce nested shells by varying economic parameters. Bai and colleagues describe using these shells as candidates that are selected and modified to meet practical and resource requirements. Their work emphasizes minimum mining width, boundary smoothness and continuity; sufficient width depends on the mining system and equipment. A shell series supplies alternatives, but its geometry still needs engineering checks.^[3]

Inspect the increments as well as their outer boundaries. Two acceptable cumulative shells can leave a narrow intervening slice, an awkward isolated area or an entry that conflicts with the next cutback. Examine sections through critical locations and the planned direction of advance rather than relying on a single plan view.

The gap problem arises when a parametric shell series jumps between pit sizes, leaving large or uneven incremental volumes for pushback design. Meagher and colleagues explain how a relatively small parameter change can make a large ore-and-waste package enter the solution together. Adding more parameter steps does not necessarily produce a suitable intermediate phase. A design response needs to address the required volume and geometry, then reassess the resulting schedule and value.^[1]

Candidate selection should therefore be tied to the intended mining programme. A small initial phase may offer early access but leave demanding expansion work behind it. A larger phase may provide more working space while requiring additional advance stripping. These are alternatives to quantify under consistent assumptions; neither phase count nor equal tonnage is an objective that applies to every mine.

Working Space and Operational Access

Width is meaningful only when its measurement and purpose are stated. Specify the working area available at a given elevation and stage of excavation, including space occupied by access, drainage or other required features. The distance between two theoretical shell lines is insufficient if part of that space is unavailable to the operation.

Araya and colleagues examine the relationship between pushback width, equipment performance and project value using a hypothetical case study. Their results illustrate that the width producing high equipment productivity need not produce the highest project value, because widening also changes the timing of stripping and access to ore. This supports evaluating operating performance and cash-flow timing together; the study's selected widths and productivity adjustments are not general design criteria.^[4]

For each candidate, ask whether the equipment can reach and leave the work area in the required configuration. Check entry cuts, ramp connections, turning and loading arrangements, and the space needed as a face advances. A layout may be workable at the start of a bench but become constrained near its end. The review should identify that change explicitly rather than applying one productivity assumption to the entire phase.

Also distinguish temporary access from the permanent route needed for later work. If excavation removes an existing ramp, identify the replacement connection and the point at which it becomes available. A phase sequence that requires continued use of a ramp after that ramp has been excavated away contains an unresolved access dependency.

Building a Schedule from the Geometry

Break the phase into scheduling units that represent the intended operation, and identify their enabling work. MineLib notes that blocks are modeling constructs: satisfying relationships between vertically stacked blocks can still leave a block inaccessible from the side. Tactical scheduling may therefore require aggregation and more representative sequencing rules. The model's unit size and time resolution must match the decision being tested.^[2]

For review, organize the dependencies in a simple sequence:

1. Establish the entrance, working area and services needed to begin the phase.
2. Advance stripping sufficiently to make the planned production area available.
3. Coordinate ore and waste extraction with the available haul routes and destinations.
4. Complete the connections and excavation needed for subsequent phases.

This is a framework for checking a plan, not a requirement to finish one phase before starting another. Where phases overlap, identify their shared equipment, roads, work areas and facilities. Sum the simultaneous demands on those resources. Separate phase schedules can each appear attainable while their combined demand is infeasible.

Check the sequence at its difficult interfaces: opening a phase, transferring equipment, replacing access and completing the bottom. Annual totals may conceal a feed shortfall during a transition. Conversely, having a large exposed inventory does not prove that the planned material can reach the plant in the required sequence.

Material Variability and Alternative Designs

Phase performance depends on the material delivered to each destination, not solely on total excavation. Grade, material type and processing route can vary together. Goodfellow and Dimitrakopoulos present a method that modifies an initial pushback design using geological simulations to reduce variability in destination tonnages while retaining similar average quantities. Their formulation demonstrates why two designs with comparable total size can have different delivery risks.^[5]

A practical comparison can examine the following questions without assuming that a single average model resolves them:

Keep the scope of optimization clear. A formulation that manages destination variability does not automatically design ramps, verify slopes or resolve every equipment interaction. Check those interfaces alongside the numerical comparison. A high-ranking candidate becomes useful when its operational assumptions are explicit enough to test.

Updating Phases During Operation

Use surveyed excavation, material reconciliation and operating observations to identify where actual progress differs from the phase plan. Distinguish a quantity variance from a location variance: extracting the planned tonnage elsewhere does not necessarily expose the next scheduled area or complete an essential access connection.

For example, if a planned entry is delayed, inspect the consequences for stripping, feed availability, equipment allocation and the next phase's start. An alternative working area helps only if it is accessible, its material is suitable and the supporting capacity is available. Revisions should carry those dependencies through the schedule, together with any changed design quantities.

Retain the revised boundaries, the associated inventory, the access assumptions and the schedule as a consistent set. This lets planners explain what a phase is intended to deliver, what must occur first and what evidence would justify changing it.