Craig's Soil Mechanics 7th Edition
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Craig's Soil Mechanics 7th Edition

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of water content.

Soil compaction 27


1.1 The results of particle size analyses and, where appropriate, limit tests on sam-
ples of four soils are given in Table 1.5. Allot group symbols and give main and
qualifying terms appropriate for each soil.

1.2 A soil has a bulk density of 1.91Mg/m3 and a water content of 9.5%. The value
of Gs is 2.70. Calculate the void ratio and degree of saturation of the soil. What
would be the values of density and water content if the soil were fully saturated
at the same void ratio?

Figure 1.14 Moisture condition test.

Table 1.5

BS sieve Particle size Percentage smaller

Soil E Soil F Soil G Soil H

20mm 100
6.3mm 94 100
2mm 69 98
600mm 32 88 100
212mm 13 67 95 100
63mm 2 37 73 99

0.020mm 22 46 88
0.006mm 11 25 71
0.002mm 4 13 58

Liquid limit Non-plastic 32 78
Plastic limit 24 31

28 Basic characteristics of soils

1.3 Calculate the dry unit weight, the saturated unit weight and the buoyant unit
weight of a soil having a void ratio of 0.70 and a value of Gs of 2.72. Calculate
also the unit weight and water content at a degree of saturation of 75%.

1.4 A soil specimen is 38mm in diameter and 76mm long and in its natural con-
dition weighs 168.0 g. When dried completely in an oven the specimen weighs
130.5 g. The value of Gs is 2.73. What is the degree of saturation of the specimen?

1.5 Soil has been compacted in an embankment at a bulk density of 2.15Mg/m3 and
a water content of 12%. The value of Gs is 2.65. Calculate the dry density, void
ratio, degree of saturation and air content. Would it be possible to compact the
above soil at a water content of 13.5% to a dry density of 2.00Mg/m3?

1.6 The following results were obtained from a standard compaction test on a soil:

Mass (g) 2010 2092 2114 2100 2055
Water content (%) 12.8 14.5 15.6 16.8 19.2

The value of Gs is 2.67. Plot the dry density–water content curve and give the
optimum water content and maximum dry density. Plot also the curves of zero,
5 and 10% air content and give the value of air content at maximum dry density.
The volume of the mould is 1000 cm3.

1.7 The in-situ dry density of a sand is 1.72Mg/m3. The maximum and minimum dry
densities, determined by standard laboratory tests, are 1.81 and 1.54Mg/m3,
respectively. Determine the density index of the sand.


1 American Society for Testing and Materials Annual Book of ASTM Standards, Vol. 04/08,

Hitchin, Herts.

2 British Standard 1377 (1990) Methods of Test for Soils for Civil Engineering Purposes,

British Standards Institution, London.

3 British Standard 5930 (1999) Code of Practice for Site Investigations, British Standards

Institution, London.

4 British Standard 6031 (1981) Code of Practice for Earthworks, British Standards Institu-

tion, London.

5 Collins, K. and McGown, A. (1974) The form and function of microfabric features in a

variety of natural soils, Geotechnique, 24, 223–54.

6 Department of Transport (1993) Earthworks, in Specification for Highway Works, HMSO,

Series 600, London.

7 Grim, R.E. (1962) Clay Mineralogy, McGraw-Hill, New York.

8 Parsons, A.W. and Boden, J.B. (1979) The Moisture Condition Test and its Potential

Applications in Earthworks, TRRL Report 522, Crowthorne, Berks.

9 Rowe, P.W. (1972) The relevance of soil fabric to site investigation practice, Geotechnique,

22, 193–300.

10 Wagner, A.A. (1957) The use of the unified soil classification system by the bureau of

reclamation, in Proceedings of the 4th International Conference of SMFE, London, Vol. 1,

Butterworths, London, pp. 125–34.

References 29

Chapter 2



All soils are permeable materials, water being free to flow through the interconnected
pores between the solid particles. The pressure of the pore water is measured relative to
atmospheric pressure and the level at which the pressure is atmospheric (i.e. zero) is
defined as the water table (WT) or the phreatic surface. Below the water table the soil is
assumed to be fully saturated, although it is likely that, due to the presence of small
volumes of entrapped air, the degree of saturation will be marginally below 100%. The
level of the water table changes according to climatic conditions but the level can
change also as a consequence of constructional operations. A perched water table can
occur locally, contained by soil of low permeability, above the normal water table
level. Artesian conditions can exist if an inclined soil layer of high permeability is
confined locally by an overlying layer of low permeability; the pressure in the artesian
layer is governed not by the local water table level but by a higher water table level at
a distant location where the layer is unconfined.
Below the water table the pore water may be static, the hydrostatic pressure

depending on the depth below the water table, or may be seeping through the soil
under hydraulic gradient: this chapter is concerned with the second case. Bernoulli’s
theorem applies to the pore water but seepage velocities in soils are normally so small
that velocity head can be neglected. Thus

h ¼ u

þ z ð2:1Þ

where h is the total head, u the pore water pressure, �w the unit weight of water
(9.8 kN/m3) and z the elevation head above a chosen datum.
Above the water table, water can be held at negative pressure by capillary tension;

the smaller the size of the pores the higher the water can rise above the water table. The
capillary rise tends to be irregular due to the random pore sizes occurring in a soil. The
soil can be almost completely saturated in the lower part of the capillary zone but in
general the degree of saturation decreases with height. When water percolates through
the soil from the surface towards the water table some of this water can be held by
surface tension around the points of contact between particles. The negative pressure
of water held above the water table results in attractive forces between the particles: this
attraction is referred to as soil suction and is a function of pore size and water content.


In one dimension, water flows through a fully saturated soil in accordance with
Darcy’s empirical law:

q ¼ Aki ð2:2Þ


v ¼ q
¼ ki

where q is the volume of water flowing per unit time, A the cross-sectional area of soil
corresponding to the flow q, k the coefficient of permeability, i the hydraulic gradient
and v the discharge velocity. The units of the coefficient of permeability are those of
velocity (m/s).
The coefficient of permeability depends primarily on the average size of the

pores, which in turn is related to the distribution of particle sizes, particle shape
and soil structure. In general, the smaller the particles the smaller is the average size
of the pores and the lower is the coefficient of permeability. The presence of a small
percentage of fines in a coarse-grained soil results in a value of k significantly lower
than the value for the same soil without fines. For a given soil the coefficient of
permeability is a function of void ratio. If a soil deposit is stratified the permeability
for flow parallel to the direction of stratification is higher than that for flow
perpendicular to the direction of stratification. The presence of fissures in a clay
results in a much higher value of permeability compared with that of the unfissured
The coefficient of permeability also varies with temperature, upon which the viscos-

ity of the water depends. If the value of k measured at 20 �C is taken as 100% then
the values at 10 and 0 �C are 77 and 56%, respectively. The coefficient of permeability
can also be represented by the equation:

k ¼ �w

where �w is the unit weight of water, 	 the viscosity of water and K (units m
2) an

absolute coefficient depending only on the characteristics of the soil