Tailings dams are often progressively raised during mine operations to offset the start-up capital cost and to reflect changes in the operation. The potential to use mine waste – whether it is waste rock from open pit mining or the sand-fraction from cycloning of whole tailings – for construction of the raises presents the opportunity to offset costs, reduce mine waste storage footprints, and improve the safety of the dam.

The efficient use of mine waste in tailings dam construction is reliant on alignment between the overall mine plan and the tailings management plan (e.g., timing of dam raises). The ability of the tailings management plan to “speak the language” of the mine plan is a key to success.

Mine planners typically use 3-dimensional “block models” of the deposit to track the type, quantity, and timing of materials within the open pit (e.g., high-grade ore, low-grade ore, waste). A similar approach can be applied to the development of a tailings dam in support of planning alignment.

What is a Block Model?

A block model is like a series of Lego® blocks, each with a unique spatial location and extent, and associated attributes and metadata, including material type, and completion data for example. The block models can be filtered by attribute and assessed by planners for upcoming fill placement and construction sequencing. Ensuring alignment with the mine waste plan, as far as practical, can aid in ensuring the appropriate materials are available and placed in the right location at the right time.

The 3D Building Blocks

Generating a 3D block model starts with a 3D model of the dam using design and drafting software (e.g. AutoCAD or Civil3D) to create a series of wireframes, or triangulated meshes representing shapes or surfaces comprising the dam.

Each wireframe connects to adjacent wireframes to make a 3D model, without gaps or overlaps. Wireframes are developed using construction sequence records, drill hole or test control data, and design information. Former TSF models can be developed from historic aerial photography and terrain models as it was constructed.

The accuracy of a 3D block model depends on the amount and quality of available data and the minimum block size. Higher accuracies will require a greater amount of data and more computational effort. Consider that a 1 x 1 x 1 block size will generate 100 times the volume of information compared to a 10 x 10 x 1 block size.

Block Factor and Sub-Blocking

There are several methods for building block models, including block factor and sub-blocking. The block factor method generates blocks of a consistent dimension and volume and is calculated using the percentage of the block that falls within the wireframe. The sub-blocking method subdivides blocks into smaller blocks to “best fit” the wireframe.

The block factor method yields a more accurate volume of the solid wireframe, at the expense of its geometry; whereas the sub-blocking method yields a more accurate geometry of TSF components such as embankment zones, drains, or filters. The sub-blocking method also generates a far greater quantity of data than the block factor method.