Large-grained materials such as cobbles and boulders are
sometimes considered to be soil. The differentiation of cobbles and boulders
depends somewhat on local practice, but boulders are generally taken to be
particles larger than 200 to 300 mm or 9 to 12 in.
The Unified Soil Classification System suggests that
boulders be defined as particles that will not pass a 12-in. (300 mm) opening.
Cobbles are smaller than boulders and range down to particles that are retained
on a 3-inch (75 mm) sieve.
Gravels and sands are classified as coarse-grained soils;
silts and clays are fine-grained soils. For engineering purposes, gravel is
defined as soil that passes a 3-inch (75 mm) sieve and is retained by a No. 4
sieve (4.75 mm or 0.187 in.) or No. 10 sieve (2.00 mm or 0.078 in.), depending on
the classification system.
Sand is defined as soil particles smaller than gravel but
retained on a No. 200 sieve (0.075 mm or about 0.003 in.). Soils passing the
No. 200 sieve may be silt or clay. Although grain-size criteria were used in
some older classification systems to differentiate silt from clay, the two
systems described herein make this differentiation based on plasticity rather
than grain size.
Opening sizes of commonly used sieves are shown in Table
15.1.
A sample of dry soil is poured onto the top sieve, the nest
is covered, and it is then shaken by hand or mechanical shaker until each
particle has dropped to a sieve with openings too small to pass, and the
particle is retained.
The cumulative weight of all material larger than each sieve
size is determined and divided by the total sample weight to obtain the percent
retained for that sieve size, and this value is subtracted from 100% to obtain
the percent passing that sieve size.
Results are displayed by plotting the percent passing (on a
linear scale) against the sieve opening size (on a log scale) and connecting
the plotted points with a smooth curve referred to as a grain-size distribution
curve.
0 comments:
Post a Comment