Hydraulic Structures: Fourth Edition

The above relationship applies only if the soil is at or near full satu-

ration and if laminar flow conditions prevail, and may be rewritten in the

form of the familiar Darcy equation:

QkiAs (2.10)

whereQis the flow, and Asis the gross cross-sectional area subject to flow.

Soil permeabilities are markedly anisotropic, with kh, the coefficient

of horizontal permeability, several times larger than kv, the coefficient of

vertical permeability. In compacted earthfills the ratio kh/kvmay exceed

20. The coefficient of horizontal permeability is most reliably determined
    in situ, e.g. via field pumping tests in boreholes. Direct and indirect labora-
    tory techniques are also available, but reproducibility of the results is poor
    and they are best regarded as indicative of relative orders of magnitude for
    permeability rather than as absolute values.
    The permeability of non-saturated soils, e.g. embankment earthfills
    prior to impounding and saturation, is very much more complex (see
    Section 2.3.7). It is not considered in depth in this text, but reference may
    be made to Das (1997).

2.3.5 Compaction

Compactionis the process of densification by expulsion of air from the soil

void space, and results in closer particle packing, improved strength,

reduced permeability and reduced settlement. (The process must not be

confused with consolidation, in which volume decrease is a result of the

gradual expulsion of water under applied load – Section 2.3.3.) Field com-

paction of embankment fills is normally achieved by rolling the earthfill in

thin layers, often assisted by vibratory excitation of the plant. The process

may also be applied to in situsoils, and it is the most common and cheap-

est of large-scale ground improvement techniques.

The degree of compaction of a soil is measured in terms of dry

density, (^) d(or dry unit weight (^) d), i.e. the mass (or weight) of solids per
unit volume of soil exclusive of moisture:
(^) d /(1w) (2.11)
where is the bulk or in situdensity, and wis the water content.
The dry density achieved during compaction varies with the water
content of a soil and the compactive effort applied. The effects of
these variables are apparent in the plots of dry density–moisture content
relationships shown in Fig. 2.7. The application of a specified compactive
effort to samples of soil prepared at different water contents yields curves

54 EMBANKMENT DAM ENGINEERING