Extensometers are devices that measure deformation including, capturing extension, compression, or shear deformation. They come in various shapes, sizes, capacities, temperature ratings, and can either contact the feature directly or take optical measurements without contact.

Proprietary extensometers or downhole inclinometers for detecting slope movements are highly accurate and commonly used at many sites, however in an emergency they can take a long time to source and, if drilling is required, may not be safe to install.

In many operating mines, "low tech" extensometers serve in an emergency as a temporary, rapid, practical and economical alternative while more advanced instrumentation are sourced and installed.

Simple and Accessible Tool

Simple extensometers can be set up without machinery using everyday materials immediately available at most work sites. Readings can be taken visually and remotely if a line of site is available from a safe location. These devices are easy to install, simple to read, and sufficient for determining if an area is stable or experiencing movement.

Examples of simple extensometers are shown below:

Reliable and Efficient Monitoring

A practical example involves placing the extensometer perpendicular to the anticipated direction of movement. If movement occurs, a relay (essentially a magnet) is pulled away from the alarm system, completing the alarm circuit and triggering the alarm. This system is powered by one or two 12V vehicle batteries. Consider integrating simple extensometers into your monitoring systems for a reliable and efficient method to detect ground movements.

A simple alarm system can be added as shown below.

The Fundão Dam failure at the Samarco Mine in Brazil and the Nerlerk berm slope failure in the Beaufort Sea were the result of liquefaction flow events brought on by the extrusion of materials within the foundation of the structure. This process is referred to as liquefaction by extrusion failure. 

In many cases, liquefaction by extrusion failure is difficult to predict because it can develop with no apparent warning (e.g. excess pore pressures will not develop prior to failure). 

While unpredictable, certain conditions make extrusion failure more likely. These include: 

• Soft material exists which can deform laterally with minimal confinement as vertical or shear load increases.
• Overlying material lacks ductility (e.g. clean sand) and cannot strain to match the soft material extrusion.
• Ductility contrast between the soft extruding material and the sand can result in reduction in lateral confinement of the sand.
• Saturated sand can collapse, and dry sand can crack.

Evaluating Extrusion Failure in Tailings Dams

In a tailings dam, the potential for extrusion failure can increase with rising water levels due to post-construction drain failure or flood storage or from foundation deformations in a soft or weak unit beneath the dam.

The following steps can be used when evaluating the potential for liquefaction by extrusion failure in a tailings dam:

Step 1:

Characterize the state parameter of the tailings using cone penetration tests (CPTs). CPTs are in situ tests where a cone on the end of a series of rods is pushed into the ground at a constant rate, and nearly continuous measurements are made of resistance to penetration of the cone and of a surface sleeve.

Step 2:

Identify the material properties of the tailings through laboratory testing, including triaxial tests on reconstituted samples.

Step 3:

Calibrate a critical state soil model to the laboratory and field behaviour, including the stress path of interest. A critical state soil model is a theoretical framework for understanding the behaviour of soil in response to changes in density and stress.

Step 4:

Prepare a 2D deformation model of the tailings dam to assess the potential response of the tailings to increasing pore pressures or foundation deformations. The model can show which conditions are more likely to result in liquefaction by extrusion failure.

Safe and Sustainable Dam Design

Because of the danger associated with these stress paths, it is common practice for upstream-constructed facilities to be designed using a precautionary approach in which the dam is designed to remain stable in the event of liquefaction. In some cases, involving pre-existing or older facilities, it is not possible to take such pre-emptive measures and an understanding of the potential for triggering liquefaction is required. 

In such situations, critical state soil models can be used to analyze the potential development of these stress paths and the potential response of the tailings.

KCB has supported tailings management at the Greens Creek Mine near Juneau, Alaska for nearly two decades. A section of the mine is located in Admiralty Island National Monument, an ecologically sensitive area that is home to one of the largest populations of brown bears in the world and various species of wildlife.

As part of an environmental upgrade project, KCB was tasked with excavating a portion of the mine’s tailings pile and collecting undisturbed soil samples that had been buried for up to 20 years.

A Modified Method to Soil Collection

Collecting undisturbed soil samples is an important component of most site investigation programs. The standard method of collecting undisturbed samples is by piston tube sampling in drill holes or block sampling in test pits.

The corner of the existing tailings pile was going to be excavated which would expose tailings that had been buried in the pile for more than 20 years. This provided an opportunity to collect undisturbed samples of these tailings for advanced geotechnical laboratory testing. The conventional method of block sampling was not practical due to the impact on construction schedule and issues with handling and transporting such samples from the site.

KCB devised a modified method and sampling device to collect undisturbed tube samples from ground surface. The sampling device could be placed directly on a prepared surface to recover in situ samples. The sampler consisted of a hydraulic ram to push modified Shelby tubes into the tailings. The tube was then retrieved by hand and trimmed and sealed. With this approach, multiple samples could be collected from an area without disrupting construction and the samples were manageable for handling and transport from the site to the testing laboratory in California.

Benefits of a Hybrid Sampling Method

While the hybrid sampling method proved to be simple, efficient, and cost effective, the sampling device was heavy and awkward to maneuver, and relies on the weight of people to provide the reaction force for the jack to drive the tube into the ground. In stiff soils, several people are needed to collect samples. However, these challenges could be overcome without impacting sample quality and allowed the team to collect more samples that would have been able using conventional methods.

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.

Some geotechnical site investigations face challenging conditions such as poor site access, restrictions on the operation of heavy equipment, and limited budget and time to complete the program. In these situations, the use of portable light percussion drilling systems can be a practical and efficient method for obtaining soil samples. In KCB’s project work at the Fruta Del Norte mine in Los Encuentros, Ecuador, these systems have been an invaluable tool because of their mobility and ease of use.

The Fruta Del Norte mine is located in the province of Zamora, in the jungle region of Ecuador. KCB has been working at the mine since 2009 undertaking site investigations, geotechnical assessments, and feasibility studies at the tailings storage facility (TSF) and Plant Site. The mine is in a densely vegetated jungle where the presence of thick residual soil horizons and high yearly precipitation (3000 mm per year) make the logistics for field programs difficult.

To manage some of the challenges in recovering soil samples at the project site, the KCB team adopted a portable light percussion drilling system. The system is a portable gas-powered percussion drilling apparatus with a core sampler. It operates by advancing steel gouges and/or core samplers into the ground by a telescopic drilling method, where progressively smaller diameter gouges are driven into the soil. The soils contained in each gouge is sampled through a window in the side of the tube. Depending on the drilling apparatus’s specifications, some have drilling depths of up to 10 m.

Advantages

• Low cost compared to conventional drilling rigs.
• Easy transportation and operation.
• High penetration rates (up to 3 holes of 6 m per day).
• Good recovery up to 6 m depth.
• Very good recovery in ‘cohesive’ fine-grained soils.

Disadvantages

• Disturbance of each sample is unavoidable.
• Requires a few people to manoeuvre the equipment.
• Poor recovery of wet coarse-grained soils with the supplied core sampler, and at depths below 6 m.

Decisions are a part of our everyday life. They can include a low-stake decision like buying a cup of coffee before your drive into the office, or a high-stake decision like buying a house. As engineering professionals, your project work requires a multitude of decisions and the opinions of stakeholders throughout the process. Decisions made in a project setting often require a more structured approach to produce a rational and auditable methodology for determining a choice between competing options.

A decision analysis process can help build consensus among stakeholders, consider a wide range of options, identify potential risks, and develop a plan with specific actions. There are several that are commonly used in project management including the Kepner-Tregoe method and Multiple Accounts Analysis. The advantage of a decision analysis process is it can bring together informed people in many fields (fields of interest are called accounts) and can include social, environmental, technical, and cost aspects of a project. These fields of interest often have competing requirements and different risk profiles which the process describes in plain language and evaluates from different stakeholder viewpoints, often in a workshop setting.

Define the Problem

The first step when conducting a decision analysis is defining the problem. During this step, you will develop a thorough description of the situation, its purpose, and identify a core team of stakeholders from a variety of fields (geotechnical, environmental, operations, finance, etc.) who can contribute. The core team should determine the purpose of the decision and any constraints that will guide the scope of the process. It is during this step that you will consider multiple criteria, identify the account structure (social, environmental, technical, cost, etc.), and define assumptions.

Establish the Objective

Once you have developed an understanding of the problem, you can establish the objectives of the decision. Start by compiling a list of objectives and divide them into “musts” and “wants”. "Must” objectives are criteria that must be met for an alternative to be successful (e.g. regulatory criteria). “Want” objectives provide the means of differentiating between options (e.g. maximize opportunity to reach passive care) and do not need to be met for an alternative to succeed.

Identify Alternatives

In this step, you will identify alternatives to meet the decision. These can be achieved through a matrix of elements to develop various options which are discussed throughout the process. If any alternatives do not fulfill all the “must” objectives, screen them out.

Engage Stakeholders

Now, compare the alternatives to the defined objectives. (This is typically undertaken in a workshop setting with key stakeholders and a capable facilitator guiding the process.) Rank each alternative based on its ability to achieve the objectives. For this step, alternatives must meet all the “must” objectives and are evaluated against the “want” objectives. Assess your results by conducting sensitivity analyses and a risk assessment to reduce the overall risk to as minimal as possible.

Make your Decision

Decide on an alternative based on how it ranks above other alternatives, its costs, and risks. Once a decision has been made, develop an action plan, documenting the approach and rationale of the decision, to move the project forward. This step usually includes a forward high-level work plan for the project.

Tips when conducting a decision analysis:

• Be clear on the situation appraisal and problem analysis prior to undertaking the decision analysis.
• Have the right people in the room to make decisions. Identify and include key stakeholders to increase the success of the decision analysis process.
• Achieve consensus at each step.
• An objective framing workshop is a useful way to engage decision makers and identify policy, strategic and tactical objectives.
• Characterize each alternative with a supporting body of knowledge enough to compare each option without prior judgement.
• It can be as simple or complicated as it needs to be. But do not over-complicate. Not all the answers are needed to make an informed decision.