The Delft Sand, Clay & Rock Cutting Model, 2019a

2.4. Soil Mechanical Parameters.
Basic Soil Mechanics.
2.4.1. Grain Size Distribution/Particle Size Distribution.
Soils consist of a mixture of particles of different size, shape and mineralogy. Because the size of the particles
obviously has a significant effect on the soil behavior, the grain size and grain size distribution are used to classify
soils. The grain size distribution describes the relative proportions of particles of various sizes. The grain size is
often visualized in a cumulative distribution graph which, for example, plots the percentage of particles finer than
a given size as a function of size. The median grain size, d 50, is the size for which 50% of the particle mass consists
of finer particles. Soil behavior, especially the hydraulic conductivity, tends to be dominated by the smaller
particles; hence, the term "effective size", denoted by d 10, is defined as the size for which 10% of the particle mass
consists of finer particles.
Sands and gravels that possess a wide range of particle sizes with a smooth distribution of particle sizes are called
well graded soils. If the soil particles in a sample are predominantly in a relatively narrow range of sizes, the soil
is called uniformly graded soils. If there are distinct gaps in the gradation curve, e.g., a mixture of gravel and fine
sand, with no coarse sand, the soils may be called gap graded. Uniformly graded and gap graded soils are both
considered to be poorly graded. There are many methods for measuring particle size distribution. The two
traditional methods used in geotechnical engineering are sieve analysis and hydrometer analysis.
% Finer by Weight
Cumulative Grain Size Distribution, Data Roberts et al. (1998)
100
90
80
70
60
50
40
30
20
10
0
10 -4 10 -3 10 -2 10 -1 10 0 10 1
2.4.2. Atterberg Limits.
Grain Size in mm
d 50
=0.0057 mm d 50
=0.0148 mm d 50
=0.0183 mm d 50
=0.0480 mm d 50
=0.0750 mm
d 50
=0.0125 mm d 50
=0.2220 mm d 50
=0.4320 mm d 50
=1.0200 mm d 50
=1.3500 mm
d 50
=0.0057 mm d 50
=0.0148 mm d 50
=0.0183 mm d 50
=0.0480 mm d 50
=0.0750 mm
d 50
=0.125 mm d 50
=0.2220 mm d 50
=0.4320 mm d 50
=1.0200 mm d 50
=1.3500 mm
Figure 2-18: The particle size distributions of the sands used by
Roberts et al. (1998).
The Atterberg limits are a basic measure of the nature of a fine-grained soil. Depending on the water content of
the soil, it may appear in four states: solid, semi-solid, plastic and liquid. In each state the consistency and behavior
of a soil is different and thus so are its engineering properties. Thus, the boundary between each state can be
defined based on a change in the soil's behavior. The Atterberg limits can be used to distinguish between silt and
clay, and it can distinguish between different types of silts and clays. These limits were created by Albert Atterberg,
a Swedish chemist. They were later refined by Arthur Casagrande. These distinctions in soil are used in picking
the soils to build structures on top of. These tests are mainly used on clayey or silty soils since these are the soils
that expand and shrink due to moisture content. Clays and silts react with the water and thus change sizes and have
varying shear strengths. Thus these tests are used widely in the preliminary stages of building any structure to
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