Geotechnical Engineering

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436 GEOTECHNICAL ENGINEERING


zone in the soil and then works its way towards the surface. A cylindrical vibrator weighing
about 20 kN (2 t) and approximately 400 mm in diameter and 2 m long, called the ‘Vibroflot’, is
suspended from a crane and is jetted to the depth where compaction is to start.


The jetting consists of a water jet under pressure directed into the earth from the tip of
the vibroflot; as the sand gets displaced, the vibroflot sinks into the soil. Depths up to 12 m can
be reached. After the vibroflot is sunk to the desired depth, the vibrator is activated. The
compaction of the soil occurs in the horizontal direction up to as much as 1.5 m outward from
the vibroflot. Vibration continues as the vibroflot is slowly raised toward the surface. As this
process goes on, additional sand is continually dropped into the space around the vibroflot to
fill the void created. To densify the soil in a given site, locations at approximately 3-m spacings
are chosen and treated with vibroflotation.


12.6.2 Control of Compaction in the Field


Control of compaction in the field consists of checking the water content in relation to the
laboratory optimum moisture content and the dry unit weight achieved in-situ in relation to
the laboratory maximum dry unit weight from a standard compaction test. Typically, each
layer is tested at several random locations after it has been compacted.


Several methods are available for the determination of in-situ unit weight and moisture
content and these have been considered in some detail in Chapter 3. The common approaches
for the determination of unit weight are the core-cutter method and sand-replacement method.
A faster method is what is known as the Proctor needle method, which may be used for the
determination of in-situ unit weight as well as in-situ moisture content.


The required density can be specified either by ‘relative compaction’ (also called ‘degree
of compaction’) or by the final air-void content. Relative compaction means the ratio of the in-
situ dry unit weight achieved by compaction to the maximum dry unit weight obtained from
an appropriate standard compaction test in the laboratory. Usually, the relative compaction of
90 to 100% (depending upon the maximum laboratory value), corresponding to about 5 to 10%
air content, is specified and sought to be achieved. Typical values of dry unit weights achieved
may be as high as 22.5 kN/m^3 (2250 kg/m^3 ) for well-graded gravel and may be as low as 14.4
kN/m^3 (1440 kg/m^3 ) for clays. Approximate ranges of optimum moisture content may be 6 to
10% for sands, 8 to 12% for sand-silt mixtures, 11 to 15% for silts and 13 to 21% for clays (as got
from modified AASHO tests).


A variation of 5 to 10% is allowed in the field specification of dry unit weight at random
locations, provided the average is about the specified value.


Proctor Needle


The Proctor needle approach given here, is an efficient and fast one for the simultane-
ous determination of in-situ unit weight and in-situ moisture content, it is also called ‘penetra-
tion needle’. The Proctor needle apparatus is shown in Fig. 12.8.


The apparatus basically consists of a needle attached to a spring-loaded plunger through
a shank. An array of interchangeable needle tips is available, ranging from 6.45 to 645 mm^2 , to
facilitate the measurement of a wide range of penetration resistance values. A calibration of
penetration against dry unit weight and water content is obtained by pushing the needle into
specially prepared samples for which these values are known and noting the penetration. The

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