Mid 1 soil exploration.pdf

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113
Soil Exploration
SAQ 1
(a) Explain the various purposes for which soil exploration is
carried out.
(b) Discuss the various factors on which depth of exploration depends.
6.3 OPEN EXCAVATIONS
Soil can be inspected in their natural condition and samples both disturbed or
undisturbed can be conveniently taken from open excavations. The open
excavations are considered suitable for shallow depths upto 3 m as their cost
increases rapidly with depth.
6.3.1 Test Pits
Test pits are normally either square or circular in section of about 1.2 m to 1.5 m
in width or diameter are often used for investigating all types of soils. Both
disturbed as well as undisturbed samples can be collected at various locations as
the excavation work progresses. The cost of test pits increases rapidly with depth.
Test pits are normally excavated manually however power equipment can also be
used depending upon soil conditions. Test pits are not suitable if rock strata is
encountered during excavation.
6.3.2 Test Trenches
Test trenches are normally square or rectangular in section with minimum width
of 1 m to 1.2 m. It is advisable to always use trenching machine during
excavation work. Test trenches furnish a continuous two dimensional profile. Test
trenches can be conveniently used for shallow exploration in soils requiring little
or no support for the sides of the trenches.
6.3.3 Open Borings
A open boring of about 1.2 m diameter is generally preferred. Open borings are
drilled with power machines in soil and very soft rock. A fairly stable soil
conditions are needed as open borings are incased and dry.
6.3.4 Advantages of Open Excavation
Open excavation has following advantages :
(a) It is the most suitable and reliable technique for obtaining detailed
information on soil and rock.
(b) It is possible to examine the samples and perform field tests as per the
requirement.
(c) It furnishes probable cost of proposed structure.
(d) It furnishes the difficulties that will be encountered during the
construction of the proposed structure.
(e) The samples obtained from open excavations are larger and less
disturbed as compared to other methods.
SAQ 2

114
Soil Mechanics and
Foundation Engineering
Describe the advantages of open excavation.
6.4 METHODS OF SOIL EXPLORATION BY
BORINGS
Boring are generally used for depths greater than 3 m or when difficult ground
water conditions are met. The diameter of bore hole generally varies from 50 mm
to 250 mm depending on the type of investigation, size of required samples and
the type of available equipment. The various methods of boring are as follows :
(a) Auger Boring
(b) Wash Boring
(c) Percussion Drilling
(d) Rotary Drilling
6.4.1 Auger Boring
In auger boring the hole is advance by rotations a soil auger while pressing it into
the soil. Two types of augers as shown in Figure 6.1 are normally used.
(a) Post Hole Auger (b) Helical Auger
Figure 6.1 : Augers
The normal size of hand operated auger is upto 10 cm. These augers are primarily
used in soils in which the bore hole can be kept dry and unsupported by a casing.
The auger is withdrawn when its blades have pierced the soil for their full height
and the soil is removed from the blades. Hand operated augers are most
commonly employed for subsurface exploration for highways, railways and
airfields but still they can be used upto depths of about 30 meters. Power operated
augers can be used for larger depths. If the sides of a hole cannot stand
unsupported, the soil is prevented from falling in by means of a pipe known as
casing. Casing pipe is used for drilling in sand below water table. The rate of
progress of power driven auger with casing is reduced because the auger has to be
withdrawn every time the casing pipe is driven.

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