Exercise set
Tailings Mine Waste Closure, Reclamation, and Bonding Exercises
Solved mine closure exercises for cover quantities, PAG routing, encapsulation, acid-base accounting, water treatment, bonding, escalation and handover gates.
These exercises focus on mine waste, closure, reclamation and financial assurance: closure cover quantities, haulage, topsoil stockpile, PAG routing, encapsulation capacity, acid-base accounting, water-treatment liability, bonding, escalation, monitoring, progressive reclamation credit, handover records and closure release gates. Active TSF water balance and seepage control are handled in a separate specialist exercise set.
Use these calculations as closure-planning screens. Real closure work requires approved closure criteria, geochemical testing, landform design, water treatment basis, construction tolerances, regulatory conditions, stakeholder obligations, long-term monitoring and responsible sign-off.
How to use these exercises
Use the set as a closure liability and handover review. Exercises 1 to 4 establish reclamation quantities, cover material, haulage and topsoil sufficiency. Exercises 5 to 9 screen PAG routing, encapsulation capacity and acid-base accounting. Exercises 10 to 14 convert treatment, earthworks, contingency, escalation and approved credits into a financial assurance basis. Exercises 15 to 18 test handover records, schedule, residual risk and release gates.
Before calculating, name the closure domain, regulatory criterion, design drawing, survey source, unit-rate basis, material source, monitoring obligation and accountable owner. A cover volume, PAG cell capacity or bond amount is release evidence only when it is tied to the approved closure plan and the condition being closed. The engineering comment below each exercise identifies the missing design, geochemical, regulatory or financial control that must be closed before handover.
Release Evidence Notes
Closure evidence should connect each quantity and cost to a closure domain, design assumption, unit-rate basis, material source, survey record, water-quality obligation, monitoring duration, contingency policy, escalation basis and accountable owner decision.
The evidence package should keep physical closure, chemical closure, financial assurance and post-closure handover separate. Physical closure covers grading, cover, topsoil, erosion control and revegetation. Chemical closure covers PAG routing, acid-base accounting, seepage, water treatment and long-term water-quality trends. Financial assurance covers bond, contingency, escalation, credits and residual liabilities. Handover covers records, monitoring acceptance and residual-risk ownership.
Closure evidence must also preserve timing. A bond value without escalation is not the same as closure-year cost, and a monitoring result before seasonal recovery may not prove final landform success. Where uncertainty remains, the record should identify whether it is covered by contingency, protected reserve, additional monitoring or a regulatory hold point.
Engineering Boundary Notes
The exercises below simplify closure costing and geochemistry. They do not replace acid-rock drainage studies, closure design, regulator-approved financial assurance, long-term water treatment studies or independent review. Treat pass results as closure-planning screens and failed results as hold points for redesign, additional characterization, bond update, monitoring extension or regulator review.
The main boundary is site specificity. Cover thickness, topsoil needs, PAG classification, encapsulation performance, water treatment duration and revegetation success depend on climate, geology, hydrology, land use and closure criteria. The second boundary is accountability: a calculation may show enough material or money, but closure cannot be released if ownership, monitoring records or residual high-risk items are unresolved.
Common Release Mistakes
Common mistakes include underestimating cover borrow, treating PAG waste as benign, ignoring water treatment duration, removing contingency too early, claiming progressive reclamation credit without evidence and closing handover with missing monitoring records.
Another common mistake is presenting a single closure cost without showing which part is quantity, unit rate, contingency, escalation, long-term treatment or credit. That makes it impossible to audit whether the bond protects the regulator and community against the real closure liability.
Do not treat progressive reclamation as automatic bond reduction. Credit should depend on accepted performance criteria, stable landform evidence, vegetation or erosion monitoring, water-quality behavior and confirmation that the completed work has not created hidden maintenance or treatment obligations.
Scenario Map
| Scenario | Exercises | Primary check | Engineering decision |
|---|---|---|---|
| Closure quantities | 1, 2, 3, 4, 16 | Cover, haulage, topsoil and schedule quantities | Confirm construction resources. |
| Mine-waste routing and chemistry | 5, 6, 7, 8, 9 | PAG volume, encapsulation, acid potential and NPR | Route, stockpile, encapsulate or hold. |
| Bonding and handover | 10, 11, 12, 13, 14, 15, 17, 18 | Liability, contingency, escalation, records and release gates | Update bond, accept closure or keep open. |
Exercise 1: Closure Cover Volume
A waste-rock landform covers 42 ha and needs a 0.75 m cover. Estimate cover volume.
Solution
Engineering Comment
Borrow suitability, compaction, erosion allowance and placement tolerance can change the required volume.
Plausibility Check
One metre over 42 ha would be 420,000 m3, so 0.75 m gives 315,000 m3.
Exercise 2: Cover Borrow Mass
The cover material dry density is 1.65 t/m3. Find mass for 315,000 m3.
Solution
Engineering Comment
Closure plans should verify that borrow source volume and material quality are both adequate.
Plausibility Check
Mass is larger than volume because density exceeds 1 t/m3.
Exercise 3: Haulage Requirement
Truck payload is 90 t. Estimate trips for 519,750 t of cover material.
Solution
Engineering Comment
Trip count affects fuel, road maintenance, dust, traffic safety and closure schedule.
Plausibility Check
About 520,000 t divided by about 100 t/trip gives a little over 5000 trips.
Exercise 4: Topsoil Stockpile Sufficiency
Reclamation needs 86,000 m3 of topsoil. Surveyed stockpile volume is 78,000 m3 with 5 percent swell loss. Find usable shortfall.
Solution
Engineering Comment
Topsoil shortages should be resolved before final landform release.
Plausibility Check
Usable stockpile is less than surveyed volume, so the shortfall is larger than 8000 m3.
Exercise 5: PAG Waste Volume
A waste schedule contains 1.8 Mt of material. PAG fraction is 14 percent and dry density is 1.9 t/m3. Estimate PAG volume.
Solution
Engineering Comment
PAG material needs controlled placement, cover, drainage and monitoring.
Plausibility Check
Fourteen percent of 1.8 Mt is 252,000 t, and density near 2 gives about 130,000 m3.
Exercise 6: Encapsulation Capacity Gate
The designed encapsulation cell has 145,000 m3 capacity. Survey uncertainty is 8,000 m3. Compare guarded capacity with PAG volume of 132,632 m3.
Solution
Engineering Comment
The guarded capacity passes but with little margin for construction tolerance or additional PAG discovery.
Plausibility Check
The margin is positive but small relative to the cell size.
Exercise 7: Acid Potential
A waste sample has 1.2 percent sulphur. Use acid potential AP=31.25S kg CaCO3/t. Find AP.
Solution
Engineering Comment
Sulphur form and mineral texture matter; AP alone is not a complete drainage prediction.
Plausibility Check
At about 1 percent sulphur, AP should be a few tens of kg CaCO3/t.
Exercise 8: Net Neutralization Potential
Neutralization potential is 55 kg CaCO3/t and acid potential is 37.5 kg CaCO3/t. Find net neutralization potential.
Solution
Engineering Comment
Positive NNP is useful but should be checked against site classification criteria and kinetic tests.
Plausibility Check
Neutralization potential exceeds acid potential, so NNP is positive.
Exercise 9: Neutralization Potential Ratio
Using the same values, find NPR.
Solution
Engineering Comment
An intermediate NPR may require uncertainty review rather than unconditional benign routing.
Plausibility Check
NP is only moderately larger than AP, so the ratio is below 2.
Exercise 10: Water Treatment Liability
Post-closure water treatment costs 420,000 USD/year for 12 years. Ignore discounting. Estimate liability.
Solution
Engineering Comment
Treatment duration is often one of the largest closure uncertainty drivers.
Plausibility Check
Ten years would cost 4.2 million USD, so twelve years costs just over 5 million USD.
Exercise 11: Base Closure Bond
Earthworks cost is 8.4 million USD, water treatment liability is 5.04 million USD and monitoring allowance is 1.2 million USD. Find base bond.
Solution
Engineering Comment
Base bond should not omit indirect costs, contractor mobilization or long-term care.
Plausibility Check
The sum is between 14 and 15 million USD.
Exercise 12: Contingency Allowance
Apply 25 percent contingency to a base bond of 14.64 million USD.
Solution
Engineering Comment
Contingency should reflect design maturity, quantity uncertainty and residual risk.
Plausibility Check
Twenty-five percent of 14.64 is 3.66, so total is 18.30.
Exercise 13: Escalation to Closure Year
Escalate 18.30 million USD for 4 years at 3 percent per year.
Solution
Engineering Comment
Financial assurance should match the expected timing of closure work, not only current prices.
Plausibility Check
Four years of 3 percent escalation adds a little over 12 percent.
Exercise 14: Progressive Reclamation Credit
Regulator-approved completed reclamation credit is 2.4 million USD. Escalated bond is 20.59 million USD. Find net bond.
Solution
Engineering Comment
Credit should require evidence that the reclaimed area meets criteria and no hidden liability remains.
Plausibility Check
Subtracting a 2.4 million credit from about 20.6 million gives about 18.2 million.
Exercise 15: Monitoring Record Completion
Closure handover requires 60 monitoring records. Fifty-six are submitted and 3 are rejected. Find usable completion.
Solution
Engineering Comment
Handover should remain open if missing records affect trend interpretation or compliance proof.
Plausibility Check
Usable records are 53 out of 60, slightly below 90 percent.
Exercise 16: Closure Schedule Critical Path
Final grading takes 8 weeks, cover placement 10 weeks, revegetation 6 weeks and acceptance monitoring 12 weeks in sequence. Find total duration.
Solution
Engineering Comment
Seasonal constraints can make the calendar duration longer than the arithmetic duration.
Plausibility Check
The sum of four positive sequential tasks must exceed any single task.
Exercise 17: Residual Risk Count Gate
Closure criteria allow zero high-risk residual items. The register has 12 residual items, 1 high, 4 medium and 7 low. Decide.
Solution
The gate fails.
Engineering Comment
Low and medium items may be manageable, but any unresolved high-risk item blocks closure.
Plausibility Check
The criterion is zero high-risk items and the count is one.
Exercise 18: Closure Release Gate
Closure release requires net bond updated, monitoring completion at least 95 percent and zero high-risk residual items. Results are updated, 88.3 percent and one high-risk item. Decide.
Solution
The release fails because monitoring records and residual risk do not pass.
Engineering Comment
Financial assurance alone cannot close a site without evidence and residual-risk control.
Plausibility Check
Two of three criteria fail, so the hold decision is required.
Validation Package Checklist
- closure domains, material quantities and source assumptions are stated;
- PAG routing and encapsulation capacity include uncertainty;
- acid-base accounting is tied to site classification criteria;
- water treatment, monitoring and maintenance durations are explicit;
- contingency, escalation and reclamation credit are justified;
- survey records, as-built drawings and material source controls are traceable;
- financial assurance states base cost, contingency, escalation, credits and owner;
- residual-risk register identifies high, medium and low items with closure action;
- handover evidence and residual-risk register support the release decision.
A complete validation package should make the closure decision auditable after personnel changes. Another reviewer should be able to see what was built, what was monitored, which risks remain, what money is protected and which authority can accept, condition, defer or reject final release.