2.

2 Soil in Natural State

Using engineering aspects, wide practically unlimited range of soils can be found in nature, from hard
pieces of rocks through gravel, sand and clay to organic sediments of compressible peat. Therefore, soils
in pits can have highly variable properties and the estimation of these properties highly depend on the
soil genesis of that particular area.

The following can be found

residual soil that originated from weathering in a place and have not been transported

Sediments- soils that have been transported from one place of their origin to another via transport
media.

⚫ Man-made sediments - sediments created by activity of man as a result of deposition of different

extracted soils or waste material on the earth's surface.

2.2.1Soil Genesis

In principle, soil make up the top layer of the earths' crust from orders of meters to tens of meters and
exceptionally even to hundreds of meters in some place. The remaining part of the earths' crust is made
up of rocks with thickness of about 25-50km. The earths' surface is not constant, huge changes are going
on with time. Geology describes these changes and classifies them with respect to time in the form of
names, geological groups and eras. Atkinson (1993) states in brief, that the materials of Cenozoic age
are generally regarded as soils for engineering purposes; materials of the Mesozoic age are generally
regarded as soft rocks and materials of the paleozoicage are regarded as hard rocks.

Geological evolution on the earth is still going on and it is possible to describe it using a closed geological
cycle: denudation deposition- sediment formation-crustal movements. Individual processes can be
described as follows. Vanicek (1982a). Denudation covers all processes that contribute to the removal of
top layer of the earths' crust.

The most important process is weathering. It's a process which includes all destructive mechanical,
chemical and biological processes that disturb the existing composition of the earths' surface. The
weathering process is connected with erosion and transport of weathered products by different means
(gravity, water, wind) from one area to the other. Deposition describes the process of accumulation of
transported mass. Sediment formation describes the processes by the influence of which accumulated
sediments are hardened. Crustal movements include slow (epirogenetic) movements, generated by
unloading of areas (uplift) or by loading from new sediments (downthrown) as well as rapid d
movements (tectonic movements). From the above mentioned it is obvious that the character of soils
and their behavior will be influenced mainly by weathering, type of transport and sedimentation rate.

2.2.2 Soil Mass Structure

The orientation of particles in a mass depends on the size and shape of the grains as well as upon the
minerals of which the grains are formed. The structure of soils that is formed by natural deposition can
be altered by external forces. Figure 2.1 gives the various types of structures of soil. Fig. 2.1 is a single
grained structure which is formed by the settlement of coarse grained soils in suspension in water.
Figure 2.1(a) is a dispersed structure formed by the deposition of the fine soil fraction in water. Figure
2.1(b) is a flocculated structure particles are oriented in a flocculent structure will have edge-to-face
contact. Book house structure which is formed by the disintegration of a flocculent structure under a
superimposed load as shown in Figure 2.1(c) whereas in a honeycomb structure, the particles will have
face-to-face contact as shown in Figure 2.1(d). Natural clay sediments will have more or less flocculated
particle orientations as shown in figure 2.1(e).

Figure 2.1: Single Grain Structure

2.3 Soil Description and Classification

It is essential that a standard language should exist for the description of soils. A Comprehensive
description should include the characteristics of both the soil material and the in-situ soil mass. Material
characteristics can be determined from disturbed samples of the soil, i.e. samples having the same
particle size distribution as the in-situ soil but in which the in-situ structure has not been preserved. The
principal material characteristics are particle size distribution or (grading) and plasticity, from which the
soil name can be deduced. Particle size distribution and plasticity properties can be determined either
by standard laboratory tests or by simple visual and manual procedures. Secondary material
characteristics are the color of the soil and the shape, texture and composition of the particles.

Mass characteristics should ideally be determined in the field but in many cases they can be detected in
undisturbed samples. i.e. samples in which the in-situ soil structure has been essentially preserved. A
description of mass characteristics should include an assessment of in- situ compactive state (coarse
soils) or stiffness (fine soils) and details of any bedding. discontinuities and weathering. The
arrangement of minor geological details, referred to as the soil macro-fabric, should be carefully
described, as this can influence the engineering behavior of the in-situ soil to a considerable extent.
Examples of macro-fabric features are thin layers of fine sand and silt in clay, silt-filled fissures in clay,
small lenses of clay in sand, organic inclusions and root holes. The name of the geological formation, if
definitely known, should be included in the description; in addition, the type of deposit may be stated
(e.g. till, alluvium, river terrace), as this can indicate, in a general way, the likely behavior of the soil. It is
important to distinguish between soil description and soil classification.
Soil description includes details of both material and mass characteristics, and therefore it is unlikely
that any two soils will have identical descriptions. In soil classification, on the other hand, a soil is
allocated to one of a limited number of groups on the basis of material characteristics only.

Soil classification is thus independent of the in-situ condition of the soil mass. If the soil is to be
employed in its undisturbed condition, for example to support a foundation, a full soil description will be
adequate and the addition of the soil classification is discretionary. However, classification is particularly
useful if the soil in question is to be used as a construction material, for example in an embankment.
Engineers can also draw on past experience of the behavior of soils of similar classification.

2.3.1 Soil Description Details

A detailed guide to soil description is given in BS 5930 [3]. According to this standard the basic soil types
are boulders, cobbles, gravel, sand, silt and clay, added to these are organic clay, silt or sand, and peat.
These names are always written in capital letters in a soil description. A soil is of basic type sand or
gravel termed as coarse soils if, after the removal of any cobbles or boulders, over 65% of the material is
of sand and gravel sizes.

A soil is of basic type silt or clay termed as fine soils if, after the removal of any cobbles or boulders, over
35% of the material is of silt and clay sizes. However, these percentages should be considered as
approximate guidelines, not forming a rigid boundary. Sand and gravel may each be subdivided into
coarse, medium and fine fractions. The state of sand and gravel can be described as well graded, poorly
graded, and uniform or gap graded. In the case of gravels, particle shape (angular, sub-angular, sub-
rounded, rounded, flat, elongated) and surface texture (rough, smooth, polished) can be described if
necessary. Particle composition can also be stated. Gravel particles are usually rock fragments such as
sandstone and schist. Sand particles usually consist of individual mineral grains such as quartz and
feldspar.

Fine soils should be described as either silt or clay: terms such as silty clay should not be used. Fine soils
containing 35-65% coarse material are described as sandy and/or gravelly silt or clay. Deposits
containing over 50% of boulders and cobbles are referred to as very coarse and normally can be
described only in excavations and exposures. Mixes of very coarse material with finer soils can be
described by combining the descriptions of the two components, e.g. cobbles with some finer material
(sand); gravelly sand with occasional boulders.

2.3.2 Soil Classification

According to the texture or the "feel," two different soil types can be identified. The coarse- grained soils
include gravel and sand and fine-grained soils silt and clay. While the engineering properties primarily
strength and compressibility of coarse-grained soils depend on the size of individual soil particles, the
properties of fine-grained soils are mostly governed by the moisture content. Hence, it is important to
identify the type of soil at a given construction site since effective construction procedures depend on
the soil type. Geotechnical engineers use a universal format called the unified soil classification system
(USCS) to identify and label different types of soils.
The system is based on the results of common laboratory tests of mechanical analysis which is
conducted in two stages:

⚫ Sieve analysis for the coarse fraction (gravel and sand)

⚫ Hydrometer analysis for the fine fraction (silt and clay).

2.3.2.1 Sieve Analysis

Conducted according to American Society for Testing and Materials (ASTM) D421 and D422 procedures,
using a set of U.S. standard sieves. During the test, the percentage by weight of the soil sample retained
on each sieve is recorded, from which the percentage of soil passing through a given sieve size is
determined. On the other hand, if a substantial portion of the soil sample consists of fine-grained soils
(D<0.075mm), then sieve analysis has to be followed by hydrometer analysis.

Figure 2.2: ASTM Sieve Stack and Mechanical Shaker