Open Pit Design

Designing an open pit is an iterative process involving consideration of many design criteria, constraints and objectives. There is no single way to design a pit - every engineer does it slightly differently. Our objective in this tutorial is to show in detail how to use the various design tools in Studio to create a detailed pit design using three different methods.

The design process, naturally, starts from a geological resource or ore reserve which is defined by a block model.

The development of an ultimate pit design is often based on a series of interim pit phases that reflect successive cutbacks. This phased pit development allows the final pit wall design criteria to be based on the operational performance of each preceding phase. Lessons learned from any slope instability are applied to the design of subsequent mining phases. The understanding of geotechnical issues is progressively improved and the associated risk is reduced through the implementation of appropriate engineering designs and operating practices. To this end, observations and results from past and current experience should be applied to future mine planning.

Mine design is an interative and continual process. In the early stages of development, mine design may change frequently due to exploration drilling of the ore body. In mature operations, mine designs change as the nature of the geotechnical limitations of pit wall segments and the influence of groundwater and storm water on the pit walls is better understood. Metal prices may change mine designs but they should not change slope angles, just the wall placements.

Key Open Pit Landmarks

Your application provides many tools for designing pits, dumps and other types of surface excavations. This introduction will identify some of these tools and suggest an approach to pit design.

Geometrical Components of a Slope

Drilling down a bit further (pun intended), the main geometrical components of a slope are referenced with respect to slope definition. Whilst determining the optimum/safest face slope angles that are appropriate for each region of the pit is outside the scope of Studio OP, implementing known regional slope constraints is very much part of the pit design workflow.

(Hustrulid et al., 2013a)

The main geometrical components of a slope are:

Bench Face Angle: the angle between the horizontal plane and the bench wall. In most hard rock open pit mines, this parameter has values ranging between 55 and 80 degrees.
Bench Height: the height, which typically has to adapt to the loading equipment operating the mine.
Berm Width: the distance between the toe and crest of a bench.
Inter-ramp Angle: the angle between the toe of a slope where a ramp segment passes and the toe of a bench located immediately upwards.
Overall Slope Angle: the angle made with the horizontal of the line connecting the lowest toe to the uppermost crest.
Global Slope Height: the height projected in the vertical axis between the lowest toe and the uppermost crest.
Ramp: the road along which the trucks haul the material between a loading point and a destination.

Before considering the complex operations involved in open pit design it is useful to be familiar with some commonly-used string editing and wireframing utilities.

To gain more familiarity with Studio OP's Pit Design tools, have a look at the Open Pit Design tutorial (Help | Open Pit Tutorial).

This tutorial contains worked examples of both manual and automated pit design concepts.

Topography

Before starting a detailed pit design, it is important to understand the existing surface topography, the extent and nature of the orebody, and the economic or ultimate pit limit of the mine. The method used will depend on the information available and the preferred design method.

The diagram below explains the meaning of many of the key terms used in open pit mine design.

There are three key methods used to design open pits:

Toe + Ramp + Crest Design
Toe + Ramp Design
Contour Design

Toe + Ramp + Crest Design Method

The Toe + Ramp + Crest method is a bench by bench design method usually starting from the lowest bench and working upwards. Create the toe string, insert the ramp and finally add the crest strings. This method creates ramps that include access to the berms. It can be used either from the bottom up or from the top down.

The decision to start from the top or bottom of the deposit when designing the pit depends on the objective. Starting from the top is better for physical constraints to mining such as lease boundaries whereas starting from the bottom makes it easier to ensure that all the ore is extracted.

In this method the projection angles always relate to the face/batter angle.

Toe + Ramp Design Method

This method requires the creation of all the toe and ramp strings first and then the addition of the crest strings later. With this first step the projection angles used must be wall angles NOT FACE ANGLES. When all the ramp and toe strings have been positioned the projection angles should be changed to represent face/batter angles, and the crest strings for each bench should then be added to complete the design.

This method is quicker than the Toe + Ramp + Crest approach and it yields a continuous ramp with no offsets. However, it has the limitation that the pit design must be completed in the same session and the pit can only be designed from the bottom up.

Contour Design Method

The third technique involves inserting a ramp and crest strings into an existing set of toe strings. These toe strings are typically created by generating contours around an optimum pit shell block model.

Several of the Contour Design commands such as Road Between Contours (rbc) are also commonly used when modifying existing pit designs.

Interactive Open Pit Design Commands

In addition to the managed tasks on offer, Studio OP includes a series of other design commands that can be used to fine-tune existing open pit data, although you should note that this is not an exhaustive list.

Menu Command	Quick Key	Description
Road Segment	rseg	Creates a road (Ramp) segment between two RLs.
Road Contour	rcon	Builds a contour string by projecting a toe string a set distance.
Road Berm	rbe	Creates a berm string offset a fixed distance from a crest contour.
Set Face Angle	fng	Set a fixed face angle that is used by the Road Segment, Road Contour and Project String commands.
Set Berm Width	sbw	Set a fixed berm width which is used by the Road Berm command.
New Rosette	nr	Creates a new rosette.
Delete Rosette	dro	Deletes a rosette.
Use Rosettes	tur	Use rosettes to define projection angles instead of the hard coded values.
String to Road	stro	Builds a road outline string from a single road design string.
Road Between Contours	rbc	Builds a single road design string between two contour strings.
New Road String	crs	Creates a single ramp string based on user defined azimuth, gradient and distance criteria.

Automated Pit Design in Studio OP

Studio OP provides a wide range of options for pit design, and a combination of automatic and interactive commands. The purpose of Studio OP's interactive pit modelling functions is to enable engineers to rapidly produce and evaluate alternative open pit mine designs based on strategic plans and to refine the designs as mining progresses.

Studio OP's workflow-based solution will enable even a relatively inexperienced engineer to design a set of pits based on the pushbacks that define a strategic plan. Using an appropriate starting point (planning model, ultimate pit shell, pit design strings and so on), you choose what happens next.

Some of the core functions on offer include;

A pit shell formed from isosurface generation based on input planning model attributes.
Construction of an "auto-project" design: an unconstrained pit using a top-down or bottom-up approach, using a pit rim or base string as a starting point.
Manual construction of pit elements using interactive face, berm and road editing commands, supported by a raft of generic CAD-style design tools.
Definition of slope regions and face angles using rosettes, using a hierarchical approach to setting up azimuth-based berm width and face angle parameters.
Automatically produce toe strings around each pushback bench using a variety of techniques, generating data that can subsequently constrain the ultimate pit design. Contours can be generated from either the planning model or an existing pit phase DTM.
Automatically generate the road, bench crests and berms according to a set of design parameters.
Bringing data such as rosettes, road rules, constraints and other design elements into a single dynamic pit design console, allowing you to test various pit generation options with instant feedback.

Following initial setup, many managed tasks within Studio OP require selection of a pit/phase context before you can start to work.

For example, you can generate contour strings (as potential constraints) for a pit/phase, and the same is true for generating ramp designs in the Ramp Layout task.

Studio OP will always try to persist your current pit/phase selection so that the same are automatically selected in subsequent tasks.

Sometimes it isn't possible to do this (e.g. a pit/phase combination is missing important entry criteria for a task, in which case the first qualifying pit/phase combination is selected instead).

Pit Design functionality is linked tightly with Studio OP's Reserves Workflow - you define your pit, model and bench parameters once and these project elements are automatically made available to the interactive design facilities available on the Design ribbon. Working from a single underlying database ensures there is only ever one version of the truth. There are stacks of options available to test different design paradigms and react quickly to newly-discovered mining requirements/constraints.

How you progress your pit design is up to you. The following diagram shows just one example of how a strategic input model feeds into downstream reserves and pit design processes within Studio OP:

Visualizing Task Data

Studio OP's Pit Design tasks are visualized using the Task window. The data objects each task creates are deliberately segregated from the usual 3D window data in order to reduce lengthy lists of objects.

You can, however, access managed task data using the dedicated Pit Data control bar. This provides a useful 'map' of the data objects associated with reserves mining block generation and sequencing, plus data objects used as an input to automated pit design.

The Pit Data Control Bar showing access buttons for shell wireframes and generated contours.

	Related Topics
	Define Slopes Pit Contours Task Ramp Layout Task Auto Design Task Pit Design Tutorial