Ranger trial landform: Particle size of surface material samples in 2009 with additional observations in 2010

Department of Sustainability, Environment, Water, Population and Communities, 2011

Internal Report 596
Supervising Scientist Division

About the report

A trial landform was constructed during 2008 and 2009 at the Ranger mine site to assist with the landform design and revegetation of the closed mine. The research design tests two types of potential surface material, waste rock and waste rock blended with approximately 30% fine-grained weathered horizon material (lateritic material), for their suitability as cap materials. The purpose of this report is to characterise the particle size statistics of the surface material of the trial landform. Bulk samples of surface material were collected at 12 sites across the trial landform with two samples collected at each site. Generally one sample was collected from between rip lines and the other sample was collected from the top of the mound formed by the rip lines. Particle size analysis by a combined hydrometer and sieve method was undertaken on the 24 samples and graphic grain size statistics calculated from the cumulative frequency distribution. A software package called 'Digital Gravelometer'™ was also used to derive particle size distributions from vertical photographs of the surface material at the same sites and the graphic grain size statistics were calculated from the cumulative frequency distribution.

The results from the sieve and hydrometer method were used for comparisons and show that there is no significant difference in graphic grain size statistics between the samples collected between the rip lines and those samples collected at the top of the mound created by the rip line. The results also show that for three of the five graphic grain size statistics there was no significant difference between the waste rock and the waste rock mixed with lateritic material. However for graphic mean size and inclusive graphic standard deviation there was a significant difference.

The graphic grain size statistics for the combined hydrometer and sieve method were significantly different to those derived from the 'Digital Gravelometer'™. The reasons for the poor correspondence in graphic grain size statistics between the two methods are that:

  • The 'Digital Gravelometer'™ is unable to determine the full range of particle sizes as provided by the sieve and hydrometer method, it is unduly influenced by the unevenness of the ground which creates shadows which are wrongly measured as individual clasts,
  • it has problems distinguishing the smaller particles and often aggregated the smaller particles into one large particle,
  • and it had problems recognising individual angular clasts of waste rock.

Particle size analysis by the combined hydrometer and sieve method provides a better estimation of the size distribution of the particles present on the trial landform surface. It does however underestimate the amount of very large particle sizes because it was not physically possible to collect a large enough sample to inclusively contain a sufficiently representative sample of these very large components.