Introduction to Mine Backfill

Mine backfill properties of importance in the design of a backfill system include:

mineralogy

specific gravity

moisture content

percent solids

void ratio

porosity

rheology

grain size distribution

unconfined compressive strength

shear strength

Mineralogy

It is important to understand mineralogy because

• some minerals (particularly quartz) are very abrasive and will cause a lot of wear in the backfill lines

• some minerals cause break down of the cement over time

• flat minerals settle slower than rounded particles

Specific Gravity (S.G.) is the ratio of mass of fill particles to mass of an equal volume of water.  An engineer at a mine site can measure the S.G. of tailings in the mine's lab using the following technique.

Apparatus required:  

250 ml volumetric flask, digital scale accurate to 0.01 grams

Step 1:  Measure weight of flask + 250 ml water (W1)

Step 2: Empty flask and add sample of fill material.  W2 = weight of dry sample

Step 3:  Partly fill flask and stir continuously for at least 5 minutes to ensure complete  wetting of all particles.  Then fill flask to 250 ml line with water and weigh.   W3 = flask+sample+water

Then the S.G. of the sample is given by:

Specific Gravity of Various Rock Types

Moisture content is reported either on a dry weight basis or total weight basis. This is an important distinction to make.  Mineral processors define moisture content on  a total weight basis, i.e.

Geotechnical engineers use dry weight basis, i.e. 

To avoid confusion, it is recommended that for mine backfill work that the geotechnical engineer's definition of moisture content be used and that the term " percent solids"  be used  when discussing the water content of the backfill.

Void Ratio

Void Ratio is defined as the volume of voids (Vv) to the volume of solids (Vs) and is a very sensitive measure of the packing density of a fill.

Void Ratio can be calculated if you know the S.G., bulk density, and moisture content of the backfill

where r _s is the bulk density of the soil

Porosity

Porosity is the ratio of the volume of voids to the total volume of the soil

Percent solids = mass of solids/(mass of solids+water).

Small changes in pulp density can result in a dramatic increase in the line pressure particularly for paste backfills.

The grain size distribution will determine the permeability of the fill. The higher the "slimes" content, the lower the percolation rate.

Grain size distribution affects the pumpability of fill. In the case of hydraulic fill, the larger the grain size the faster the terminal settling velocity. The velocity of the slurry in the pipe must exceed this settling velocity.

Grain size distribution affects void ratio and ultimately the strength of the fill. The lower the void ratio, the higher the strength. A more uniformly graded fill has a lower void ratio.

Unconfined compressive strength is measured on cemented fill by casting cylinders and compressing them much like a concrete cylinder test. Sample diameters are 50-75 mm in the case of sandfill and tailings backfill. Samples sizes up to 300 mm or more are desirable for consolidated rockfill.

The elastic modulus can also be calculated from this test.

The unconfined compressive strength value is usually used as an indirect measurement of the shear strength of the fill.

-measured by direct shear tests or triaxial testing

-not normally performed

Rheology is the study of the shear strength behavior of liquids. Viscosity is a measure of the shear strength or the resistance to movement between different layers in a fluid or mixture. In concrete terminology this is also known as the workability. The viscosity of a paste mixture is difficult to predict and is influenced by many factors including: pulp density, grain size, mineralogy, and grain shape. The concrete slump test has generally been used as a measure of the viscosity of paste mixtures. Paste mixtures commonly exhibit slumps of 15-20 cm (6-10") on a standard (12") cone.

Paste mixtures behave as non-Newtonian fluids, that is, they do not exhibit constant viscosity with variation in flow rate. The yield stress of a paste is greater than zero before flow commences. Research and experience to date, indicates that paste backfill can be considered to be a Bingham plastic fluid, exhibiting constant viscosity with increased velocity, once the yield stress has been overcome. It can also be a pseudo-plastic fluid, exhibiting decreasing velocity as velocity increases