Sediment Grain Size Analysis
Grain size distribution is
one of the most important characteristics of sediment. This is true because grain size is a powerful
tool for describing a site’s geomorphic setting, interpreting the geomorphic
significance of fluid dynamics in the natural environment, and distinguishing
local versus regional sediment transport mechanisms as well as because grain
size is a dominant controlling factor in sediment geochemistry. Cations derived
from mineral weathering and pollution sources are preferentially adsorbed onto
clay, which has the highest surface area to volume ratio of any particle size
class.
The grain size distribution
of a sediment sample is determined by any one of a number of techniques or
combination of techniques depending on the range of sizes present in a
sample. For this procedure, “fines” are
any particle smaller than 63.5 microns.
Sand is 63.5 microns to 2 mm in diameter. Gravel is larger than 2 mm.
Characterizing
the physical properties of sediment is important in determining its suitability
for various uses as well as studying sedimentary environments and geologic
history.
The physical properties of sediment can be described by several parameters. Grain size is the most important of these and is the main way in which sediment (and clastic sedimentary rocks) is classified. Other commonly used properties of sediment are sorting and shape (roundness and sphericity). All of these properties are important in describing sediment and determining its suitability for various uses, such as a construction aggregate or fill for a beach volleyball court.
Standard grain size terms of sedimentary
particles and the arbitrary boundaries between them are as follows:
Grain size analysis procedure:
1.) Label two plastic trays with
the sample number and time of sampling.
2.) Open a ziplock bags we
collected on the boat and use a large table spoon to transfer two
representative, similar size and samples onto the two plastic trays.
3.) Weigh both trays and put one
tray into the oven @ 50oC. Enter weights on spreadsheet.
4.) Assemble the two sieves with
the coarse one up over a sink in the lab.
5.) Transfer the sediment from
the 2nd tray onto the top sieve and use tap water to flush the finer
particles through the sieves. Shake the sieves during this process.
6.) The fraction <63µm
will go through both sieves and will be flushed down the drain. The fraction
>63µm, but <2mm will remain on the bottom
sieve, and the fraction >2mm on the top sieve.
7.) Label 2 additional trays
with sample number and ‘coarse’ and ‘medium’, respectively.
8.) Use a brush and perhaps
water to transfer those coarse and medium fractions onto the trays.
9.) Clean sieves etc.
10.) Put samples into oven and
let dry overnight.
11.) Weigh all fractions the next
days and enter data on spreadsheet.
12.) Clean the trays – so that
they can be reused.
13.) Determine relative weight
fractions of clays and silt (<63um, fine), sand (<2mm, >63um, medium),
and gravel (>2mm, coarse). Calculate fine, medium and coarse fractions from
the spreadsheet. Express the results in %.