Unlike most water dams, which are typically constructed in a single campaign, a tailings storage facility (TSF) is most often constructed in a number of campaigns, over the life of the mine (or the life of the TSF). Why? A water dam is constructed to build up a certain amount of water over a predicted duration of time, say over the course of a few years, while it is more convenient to construct a TSF in stages to correspond to mining and mill production forecasts. Thus, a TSF may be constructed over a period of decades, in campaigns called “stages”, and it follows that these campaigns are termed “staged construction”. This also allows for monetary expenditures to be deferred into the future, which is always a consideration. As I like to say, the three most important economic considerations for TSF construction are: defer, defer, and defer. You heard it here first (unless somebody said it before me)!
The individual stages of TSF construction include the starter facility which will usually receive tailings for at least the first two years of the mining operation. The additional raises are constructed in time to match mill production forecasts, and adjusted as necessary as mine production schedules change over time.
The TSF is most often designed by a engineer, a group of engineers, or an engineering design firm, and again, most often, the engineer(s) is a specialist in mine waste management. The TSF designer is often termed the “design engineer of record”. In recent years, the term “engineer of record (EOR)” has been taking on meanings that were never before intended, and as an industry, several publications, regulations and policies have recently been put forward. I’ll leave some of that for a future blog, but for now, let’s just say that for an engineer to consider themselves an EOR for a TSF, they must be involved in more that just the design. They also need to be involved in the construction, operation (to some degree), monitoring, inspections and be informed of any changes to the systems that may alter any aspect of the TSF operation. I gave a presentation to the New York City chapter of the SME a few years ago on this topic; I’ll save some of that content for a future blog.
So, the TSF is constructed in stages, and the TSF designer has completed geotechnical work, including geotechnical analyses, that are based on a number of assumptions, including material properties, compaction, moisture content, presence of drains, or other internal features, such as clay core zones, transition zones, filter zones, rip-rap, etc. During the construction process, the engineer, and his team of technicians and field engineers use observations and testing to assess whether or not the various components of the TSF are being constructed in compliance with the design, and its technical specifications (a detailed instruction manual containing information on how the TSF and its various components are to be constructed). If the construction is found to be out of compliance, measures are taken to regain compliance. This work is on par with a building inspector, except for a TSF, the inspections, observations and testing are almost always completed on a full-time basis. These tasks are called Quality Assurance (QA), and sometimes Construction Quality Assurance (CQA). And sometimes Quality Control (QC)…and sometimes QA/QC.
One example of testing is the Proctor compaction test. The Proctor compaction test is a laboratory method of establishing the optimal moisture content at which a given soil type will become its most dense, given a specific amount of energy going into the compaction. The test is named for Ralph Proctor, who developed this test in 1933. His original test is most commonly referred to as the standard Proctor compaction test; his test was later appended to create the modified Proctor compaction test. The modified Proctor test generally applies to the larger earthmoving equipment that was developed during and after WWII. Sometimes the standard Proctor test is used for clayey, non-structural zones of a dam (such as the clay core) where low permeability is desired, as opposed to structural strength.
By knowing the soil’s maximum density and optimal moisture content, the engineer can specify what percentage of this maximum density (and usually the variance from optimal moisture content) will satisfy the design. That’s step one. The engineer’s field team then takes in-place density and moisture tests to assess whether the field conditions adhere to the design. There are other such methods that are also used by the engineer and his field team. More of that (maybe) in future blogs. This team is pretty important to the successful completion of the TSF construction work.
I mentioned that observations are an important aspect of this work. Why? Well, the field in-place density test represents a sample of soil roughly equivalent to the size of a gallon of milk. It is the observations on the engineer and his field team that allow the reasonable assumption that the relatively small sample is (or isn’t) representative of a larger area. An owner seldom saves money by employing “inexpensive” CQA services, although some projects require smaller CQA teams than others do (each project has its own unique CQA requirements).
Since the TSF is constructed in stages, it is often desirable to have the same engineering team involved for the life of operation of the TSF. This is even more desirable if representatives of the mining company are not continual over the life of the TSF. The value of continuity and institutional knowledge can sometimes never be replaced. There I go again lecturing on the need to have an EOR.