Glossary of Technical Terms

Artesian Pressure:  a condition that exists when the water table (piezometric surface) lies above the ground surface. This condition occurs when a pervious, saturated soil is confined by an impervious soil.

Beach:   subaerial tailings slope between the dam’s crest and the pond.

Berm:  an engineered flat cut or earthfill.

Buttress:  a berm constructed at the bottom of a slope to increase its stability.

Chimney Drain:  a vertical or near vertical zone of filter sand in a dam meant to reduce the water pressure in the downstream zone of the dam.

Compacted Tailings Fill:  tailings that are mechanically compacted to increase their density.

Cone Penetration Test or CPT:  a penetration test in which a steel cone that has a 60º point is pushed into the ground at a continuous rate of 2cm/s. The resistance to penetration is measured electronically at the cone tip and cone sleeve.

Consolidation Modeling:  estimation of the reduction of air void in cohesive soils with time using a variant of Terzaghi’s consolidation equation.

Contractive Tailings:  When shearing a loose particle arrangement of tailings, the particles move together and the soil mass compresses.

Conveyor:  a mechanical device used to transport bulk materials from one location to another.

Crest:  the top of a dam or slope.

Critical State Line (CSL):  the boundary between the contractant and dilatant behavior of tailings where soils shear at constant volume. The CSL is usually determined by performing carefully controlled triaxial tests at a variety of void ratios, both drained and undrained.

Cyclic Liquefaction:  a temporary condition of zero effective stress and shear resistance under level ground caused by earthquake shaking of loose, saturated granular soils.

Cyclone Sand Placement:  placement of the underflow from a cyclone.

Cycloning:  the process of separating tailings into coarser, sandy tailings underflow and finer overflow tailings.

Deformation:  the displacement or strain of a soil mass in response to loading.

Deformation Analysis:  in the case of the Fundão dam, a computer model of the tailings which yields the deformation of the tailings in response to its self-weight.

Deviator Stress:  the difference between the major and minor principal stress in soils. Also called the shear stress.

Dilatant Tailings:  if tailings particles are in a tightly-packed arrangement, the tailings particles must first move apart in order to move past each other during shearing. This produces an increase in volume of the tailings mass.

Down-Drop Block or ‘Graben’:  a depressed block of soil or rock that is part of a landslide feature and often bordered by parallel cracks.

Downstream Slope:  the slope of a dam or dike farthest away from the tailings pond. Downstream means the direction away from the tailings pond while upstream means the direction towards the pond.

Drainage Blanket:  a layer of pervious material placed over foundation material to facilitate drainage of the foundation and/or embankment.

Drained Triaxial Compression Tests:  standard soil shear test wherein a cylindrical sample of soil encased in a membrane is subjected to an increasing vertical load while confined by cell pressure. “Drained” means the drainage valves are open so the pore pressure in the sample does not increase. “Undrained” means the drainage valves are closed so the pore pressure can change.

Dynamic Loading:  repetitive inertial loading imposed by vibrating machinery, earthquake shaking, and other sources. Cyclic loading is usually considered a case of dynamic loading imposed by an earthquake.

Dynamic Response:  the response of a structure to a dynamic load such as an earthquake shaking. That response can vary from small permanent displacements to catastrophic flow liquefaction events.

Earthfill:  compacted soil fill from a local natural source used to construct dams, including starter dams for tailings dams.

Effective Stress:  the difference between the total stress and the pore water pressure in an element of soil.

Effective Stress Analysis (ESA):  stability analysis that uses drained effective soil strengths and steady state pore water pressures.

Embankment:  Equivalent term for dam or dike. The main element of a dam (earthfill).

Engineer Of Record:  the qualified engineer responsible for assuring that a tailings dam is designed, constructed, operated, and decommissioned with appropriate concern for health and safety and the environment, and is in alignment with and meets applicable regulations, statutes, guidelines, codes, and standards.

Extrusion:  lateral movement of a soft layer in the foundation of a dam.

Extrusion Collapse Tests:  triaxial test that follows a stress path in which the confining stresses reduce.

Factor of Safety (FS or FOS):  the ratio of the available strength to the shear stress imposed by self-weight and loadings around the soil slope.

Failure Initiation Sequence:  sequence of events occurring immediately prior to the failure of the dam by flowslide.

Failure Modes:  potential causes of the dam failure evaluated by the Panel.

Fault Tree:  schematic demonstrating the process by which the panel considered the possible failure modes for the dam.

FEFLOW:  computer software used to model seepage flow in the dam in 3D.

Fines:  soil particles finer than 0.074 mm or finer than the No. 200 sieve.

Flowslide:  flowsliding or flow liquefaction occurs when the shear stresses required for static equilibrium of a soil mass exceed the shear strength of soil in its liquefied state.

Freeboard:  vertical distance between the lowest point on the crest of a dam and the pond level.

Geographic Information System (GIS):  computer modeling system used to capture and manipulate spatial and geographical data.

Hydraulically-Discharged Tailings:  Tailings that have been discharged from a slurry transport pipeline.

Impoundment:  Entire tailings deposit.

Internal Erosion:  Internal movement of fine soil particles by seepage due to filter incompatibilities in the ground.

Kananets:  flexible high-density polyethylene (HDPE) pipes used to enhance drainage of the El. 826 m blanket drain.

Kilopascal (kPa):  unit of pressure measurement equivalent to 1000 newtons per square meter.

Liquefaction:  process whereby loose susceptible materials such as sands lose strength due to pore pressure increase and behave like a liquid rather than a solid.

Loading:  the imposition of weight.

Mass Balance:  a calculation in which the mass or weight of tailings delivered to a tailings dam by a slurry pipeline is used to calculate the density of the deposited tailings whose volume is known through successive topographic surveys.

Mean Effective Stress:  the average of principle effective stresses.

Mobilized Instability Ratio (MIR):  the ratio of the deviator stress and mean effective stress to the ratio at the onset of collapse.

Mohr-Coulomb Relationship:  the loci of the available resistance of a frictional material as shown on a plot of shear stress versus confining stress. This is the common way of illustrating shear strength at different confining stresses, but one starting density.

Moment Magnitude (Mw):  magnitude is a number that characterizes the relative size of an earthquake. Magnitude is based on measurement of the maximum motion recorded by a seismograph. Several scales have been defined. The moment magnitude (Mw) scale, based on the concept of the seismic moment, is uniformly applicable to all sizes of earthquakes.

Newmark-Type Analysis:  an analytical technique used to calculate the permanent displacement of soil slopes due to earthquake shaking in the absence of liquefaction.

Norsand Model:  a constitutive relationship for sands developed by Jefferies and Been (2016) based on critical state soil mechanics.

One Dimensional-Consolidation Test:  a compression test conducted by adding vertical weights to a soil sample in a ring called an oedometer. The vertical deformation of the sample is measured with time after each loading increment is added.

Overflow Channel:  the channel constructed between the Dike 2 and Dike 1 reservoirs to decant water to the Secondary Gallery while the Main Gallery was being repaired/decommissioned.

Overtopping:  a freeboard of zero whereby the pond water flows over the crest of dam. If the dam is erodible, overtopping usually fails the dam.

Peak Shear Strength:  the maximum shear strength that a soil can perform at a given density, confining stress, stress path, and loading rate.

Pellet:  the product of iron ore powder, water, and clay, which is baked to form a hard shell used in the production of steel.

Pellet Plant:  the facility where iron ore is transformed into pellets used in the production of steel.

Phreatic Surface:  water table defined as zero pore pressure.

Piezometer:  a device for measuring groundwater pressure.

Piping:  internal erosion that develops into an open “pipe” through the soil

Pore Pressure:  the water pressure within the voids of a soil mass.

Reinforcement (Equilibrium) Berm:  a berm of earthfill or tailings, either compacted or not, put at the toe of a dam or slope to increase the factor of safety by loading the toe.

Reservoir:  water pond retained by a dam.

Residual Strength:  minimum shear strength of a cohesive soil after displacement along a shear plane or plane of weakness. Also used to describe the minimum undrained strength of a cohesionless soil during a flowslide.

Sand Tailings:  defined as the coarser fraction of tailings at Fundão but still contains particles passing the No. 200 sieve.

Saprolite:  a soil that retains the fabric or rock from which it has been weathered but has the engineering properties of a soil. In the weathering profile, saprolite is above the transition zone from rock to soil.

Saturated:  a condition in which all drainable voids between soil particles are filled with water.

Seepage Flow:  groundwater flow after a change in water table.

Seismic Deformation:  permanent displacement of sloping ground caused by earthquake shaking. Usually calculated using a variant of the Newmark analysis but only if liquefaction is not involved.

Setback:  the upstream relocation of the dam crest alignment on the left abutment of Fundão.

Slimes Tailings:  finer fraction of tailings at Fundão which originated at the concentrator.

Slurry:  mixture of fine particles of a solid material and liquid which behaves like a relatively uniform liquid when pumped through a pipeline.

Solution Feature:  cavities in rock formed as a result of dissolution of the rock by groundwater with time.

Spigot:  a pipe connected to a tailings header (or delivery line) used to discharge sand tailings at specific locations to build up the beach. Spigot points at Fundão were routinely moved to raise the crest of the dam uniformly.

Stability Analysis:  an analytical or numerical method that compares the available resistance of a soil slope to the imposed shear stresses of that same slope.

Starter Dam:  the initial dam in a tailings dam usually built of earthfill to create tailings storage before the mill starts producing tailings.

Static Liquefaction:  A type of liquefaction that occurs when the effective stress of soil is reduced essentially to zero, corresponding to complete loss of shear strength, resulting from a single sudden occurrence of change in stress (“monotonic loading”), rather than a cyclic event (such as an earthquake or vibration). A soil in a loose state, and one which may generate significant pore water pressure on a change in load, are most likely to liquefy. This is because loose soil has the tendency to compress when sheared, generating large excess pore water pressure as load is transferred from the soil skeleton to adjacent pore water during undrained loading. As pore water pressure rises, a progressive loss of strength of the soil occurs as effective stress is reduced. It is more likely to occur in sandy or non-plastic silty soils, but may in rare cases occur in gravels and clays.

Static Load:  a constant load calculated according to Newton’s Second Law.

Stratigraphy:  the layering of soil and rock.

Tailings:  finely ground rock particles remaining after the iron ore extraction process.

Tailings Beach:  see “Beach”.

Toe:  the intersection of dam slope with the natural ground.

Undrained Strength:  the strength of a soil when loaded sufficiently fast that pore pressures cannot dissipate. For a given soil, undrained strength is a function of density, stress path, and rate of loading, among many other factors.

Undrained Strength Ratio:  the ratio of undrained strength to effective stress.

Unsaturated:  a condition in which all drainable voids between soil particles are filled only with air.

Void Ratio (e):  the ratio between the volume of the voids and the volume of the solids in a soil.

Water Balance:  an accounting of the water inputs and losses to determine the pond level in a tailings dam.