Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
Pré-visualização50 páginas
Differentiation and Textural Classification
The two most common types of soil classification for engineering purposes 
are particle-size differentiation and textural. Particle-size or grain-size differen-
tiation of soils classifies or identifies soils on the basis of ranges of limiting 
sizes. In the left-hand bar chart shown in Figure 1.3, the distinction between 
gravel, sand, silt, and clay (called separates) is made on the basis of a limiting 
maximum or minimum size. Thus, for example, the limiting maximum size 
for clay, first proposed by Atterberg (1908, 1911), is 2 \u3bcm; or one could say that 
the limiting minimum size for silts is 2 \u3bcm. Sands and silts are also differen-
tiated in terms of fine, medium, and coarse sizes. Fine sands are greater than 
0.06 mm and less than 0.2 mm in particle size. Sand grains with sizes greater 
than 0.2 mm and less than 0.6 mm are classed as medium sands, and coarse 
sands are particles with sizes ranging from 0.6 to 2 mm. Particles greater 
than 2 mm are called gravels. We should point out that whilst the limiting 
In
cr
ea
sin
g p
ar
tic
le 
siz
e, 
m
m
2 mm
0.002 mm, (2\u3bcm)
0.06
0.006
0.02
0.2
0.6
Si
lt
Sa
nd
Coarse
Medium
Fine
Coarse
Medium
Fine
Clay
<< 0.002 mm, (2\u3bcm) 
>> 2 mm
Gravel
Stone
100
100
100
20
20
20
40
40
40
60
60
60
80
80
80
0
0
0
Percent sand
Pe
rce
nt
 cl
ay
Percent silt
Clay
Silty
clay
Siltyclay
loamClay loam
Silt loam
Loam
Sandy
clay
Silt
Sandy clay
loam
Sandy loamLoamy sand
Sand
FIguRE 1.3
The left-hand bar chart shows the grain-size (particle-size) differentiation for classification of 
soils. The right-hand chart shows the textural classification of soils based on proportions of 
sand, silt, and clay in a soil.
13Origin and Function of Soils
size boundaries for these separates are considered by some as arbitrary, they 
have nevertheless been accepted in common practice.
The triangular chart shown in the right-hand side of Figure 1.3 is a textural 
chart that identifies soil type according to the proportions of sand, silt, and 
clay in the soil. As with the particle-size classification scheme shown on the 
left-hand side, textural charts classify soils based on particle sizes, meaning 
that these kinds of charts and classification schemes are more applicable for 
granular soils (gravels, sands, and silts). It is important to understand that 
the clay classification used in both the particle-size classification scheme and 
the textural chart refers to clay-sized particles. Some confusion may arise if 
one does not distinguish this from clay minerals as a particular type of soil. 
We will be discussing this in greater detail in the next chapter (Chapter 2).
1.3.3.2 Particle-Size Distribution Curves
A particle-size distribution curve is useful as an indicator of the kind of soil 
under consideration. This does not require an arbitrary division of particles 
into separate sizes (separates). Figure 1.4 shows some typical particle-size dis-
tribution curves in a semilog plot. The ordinate shows the weight, in terms of 
percentages, of particles finer than a particular size, and the abscissa shows 
the effective particle diameter.
0
20
40
60
80
Effective Particle Diameter, mm
Pe
rc
en
t F
in
er
, b
y W
eig
ht
0.0001 0.001 0.10.01 1.0
100
10
Silt GravelClay Sand
A B
C
FIguRE 1.4
Particle-size distribution curves. Curve A represents a clay. Curve B is a clayey silt, and Curve 
C is a silty sand. Note that identification of soil types is based solely on proportions of separates 
in the soil sample and not on any particular behaviour characteristic.
14 Environmental Soil Properties and Behaviour
To obtain a particle-size distribution curve, it is important to obtain a rep-
resentative soil sample that is completely dispersed; that is, all the particles 
must be able to act individually and separately. Dispersion of a soil sample 
requires removal of cementing or bonding materials responsible for aggre-
gating the individual particles, that is, deflocculating the soil sample. Typical 
cementing materials in surficial soil samples include carbonates, iron and 
aluminium oxides, and organic matter. Note that after removal of cementing 
material, the soil sample must be deflocculated or peptized. This is especially 
critical for clays since the forces of attraction that are always present between 
particles (see Chapter 2) will cause flocculation of the clay particles. Since 
the forces of attraction cannot be easily decreased, they need to be overcome 
by increasing the forces of repulsion between the clay particles (Chapter 2), 
generally accomplished by having a monovalent exchangeable ion on the 
clay and a low salt concentration in the solution. A common procedure is to 
add sodium ions (sodium metaphosphate) and washing the soil suspension 
free of soluble salt. The advantage of using sodium metaphosphate as the 
source of sodium ions is that the metaphosphate will form complexes with 
any remaining calcium or magnesium ions.
1.3.3.3 Atterberg Limits Classification
The initial work in agricultural science undertaken by Atterberg (1911) on 
classification of soils, designed to provide an assessment of the state of a soil 
in relation to its water content, serves as the basis for classification of soils in 
respect to its state of consistency. The consistency limits devised by Atterberg, 
shown in Figure 1.5, provide an indication of the plasticity or plastic behav-
iour of soils (mainly clays) in relation to this water content. These limits are 
most often referred to as the Atterberg limits, and the tests are part of the 
group of index tests that include determination of particle-size distribution.
The shrinkage limit (SL) shown in Figure 1.5 represents the water content 
boundary between a solid soil and a semisolid soil. Using the measured vol-
ume of the test sample as a guide, the shrinkage limit is the water content 
at which a further reduction in water content will not result in any observ-
able decrease in volume of the test soil sample. In terms of a water content-
volume change definition, the shrinkage limit is the only limit that can be 
rigorously defined. Using the relationship shown in Figure 1.5, the SL can be 
defined as the point where volume change is no longer linearly proportional 
to the change in water content. The SL is sometimes referred to as the lower 
plastic limit. The total shrinkage limit shown in Figure 1.5 is the point where 
the volume of the test soil sample is defined by its oven-dry volume.
The plastic limit (PL) is the water content at which any increase in water 
content beyond this limit would result in a plastic state for the test soil. A 
semisolid state of the test soil exists at water contents between the SL and PL. 
Increasing the water content of the test soil above the PL would produce a more 
plastic soil, to the point where the soil would exhibit liquid-like behaviour; 
15Origin and Function of Soils
that is, a liquid-like state of the soil would be obtained. At this point, the 
water content defines the liquid limit (LL) of the soil. The range of water con-
tents between the plastic limit (PL) and the liquid limit (LL) is defined as the 
plasticity index (PI). Laboratory test procedures have been devised and stan-
dardized for determination of the various consistency limits.
1.3.3.4 Unified Soil Classification System
The Unified Soil Classification System (Waterways Experiment Station, 1953) 
is a soil classification scheme that uses field identification procedures and 
laboratory classification criteria to organize the soils under consideration 
into specific groups. Particle-size distribution information allows for