Sub surface exploration (part-1)

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Subsurface Exploration
IntroductionIntroductionIntroductionIntroduction
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Definition
The process of determining the layers of
natural soil deposits that will underlie a
proposed structure and their physical
properties is generally referred to as
subsurface exploration/Sub Soil
Exploration/Investigation

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The purpose of the soil – exploration
progrm
To obtain information that will help the
geotechnical engineer in the following:
1.Selection of the type and the depth of
foundation suitable for a given structure
2.Evaluation of the load bearing capacity of the
foundation
3.Estimation of the probable settlement of a
structure

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4. Determination of potential foundation
problems (expansive soil, collapsible soil,
sanitary landfill, and so on)
5. Establishment of ground water table
6. Prediction of lateral earth pressure for
structures like retaining walls, sheet pile,
bulk heads, and braced cuts
7. Establishment of construction methods for
changing subsoil conditions

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Scope of Soil Investigation
The scope of a soil investigation depends on the
• the type, size, and importance of the structure
• the client and the engineer’s familiarity with
the soils at the site, and
• local building codes.

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Phase I. Collection of available information
phaseII. Reconnaissance survey of a
proposed site.
Phase III. Preliminary soil exploration.
Phase IV. Detailed soil exploration
Phase V. Writing a report
Phases of a Soil Investigation
A site investigation must be developed in phases.

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Phase I: Collection of available information such as
site plan,
type, size, and importance of the structure,
loading conditions,
previous geotechnical reports,
topographic maps,
airphotogrphs,
geological maps,
hydrological information
Existing Infrastructure and
Highway department manuals

8GEOLOGICAL MAP

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TOPOGRAPHICAL MAP

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GEOLOGICAL CROSS-SECTION

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Existing Data Sources
Various online sources like Google Earth
Geological survey of Pakistan
National/Local Geological Survey maps, reports,
and publications.
Flood zone maps prepared by National or local
Departments (Department of Irrigation, National
Disaster Management Authority of Pakistan)

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Site Plans showing locations of ditches,
driveways, culverts, utilities, and pipelines.
Maps of streams, rivers and other water bodies
to be crossed by bridges, culverts, etc.,
Earthquake data, seismic hazards maps, fault
maps, and related information

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Phase II : Preliminary reconnaissance or a site
visit to provide a general picture of the
geotechnical topography and geology of the
site.
Information collected during this phase:
Design and construction plans
General site conditions
Access restrictions for equipment

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Traffic control requirements during field
investigations
Location of underground and overhead
utilities
Type and condition of existing facilities (i.e.
pavements, bridges, etc.)
Adjacent land use (schools, churches,
research facilities, etc.)
Restrictions on working hours
Right-of-way constraints

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Environmental issues
Erosion features, and surface settlement
Flood levels
Water traffic and access to water boring sites
Benchmarks and other reference points to aid
in the location of boreholes
Equipment storage areas/security

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Phase III: Detailed soil exploration.
The objectives of a detailed soil exploration are:
1.To determine the geological structure, which
should include the thickness, sequence, and
extent of the soil strata.
2.To determine the ground water conditions.
3.To obtain disturbed and undisturbed samples
for laboratory tests.
4.To conduct in situ tests.

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• Phase IV : Writing a Report
The report must contain a clear description of
the soils at the site, methods of exploration, soil
profile, test methods and results, and the
location of the ground water.

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• Phase IV Preliminary site investigation
In addition to desk study and
reconnaissance, a few confirmatory
bore holes are sunk or probing is
done.

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Site Exploration Plan
Once the site has been selected, a detailed
investigation has to be conducted.
Since the cost of such an investigation is
enormous, it is important to prepare a
definitive plan for the investigation, especially
in terms of the
bore-hole layout and spacing between bores
the depth of boring at each location.

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Depth of Bore holes or Test pit
Approximate required minimum depth of bore hole shall be
predetermined
Depth can be changed during drilling depending upon
subsurface conditions
There is no hard and fast rule….

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Approximate method to determine
depth of boring holes or test pit
Net Increase in stress
due to structure
Vertical effective stress

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No of
stories
Boring depth
1 3.5 m
2 6 m
3 10 m
4 16 m
5 24 m
∆σ′
σo′
D

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(a)Min Depth of boring is at which the
net increase of stress = 1/10 estimated net stress (q)
on the foundation
(b) Min Depth of boring is at which the
05.0'
0
=
∆
σ
σ
Approximate Min Depth shall be minimum of (a) and (b)

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Method suggested by Sowers and Sowers (1970)

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How Deep?

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How Deep (Bridges)?
Boring depth is governed by various factors,
including:
– Foundation type
– Foundation load
– Lowering of grade line at underpass?
– Channel relocation, widening, dredging?
– Scour?
Rules of Thumb
– Generally speaking, 50’- 80’ is reasonable
– Local experience is helpful
– Look at nearby structures if available
– If no experience or other info available, plan for
long first hole, then adjust.

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Boring depth is governed by various factors,
including:
– Wall type (Fill vs. Cut)
– Lowering of grade line at wall?
– Scour?
• Rules of Thumb:
– Fill Walls: Depth = Wall Height +/-
– Soil Nailed Walls: Depth = Through Nailed Area,
plus 10’
– Drilled Shaft Walls: Depth = Exposed Wall Height plus
150% of Wall Height
How Deep (Retaining Walls)?

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A BB
L P B
D =1 ½ B, when A P4B
L
(L>B)
B B BA A
D =1 1/2 L, when A <2B
(A)
ISOLATED SPREAD OR MAT FOOTINGS
(B)
ADJACENT FOOTINGS

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W
AB B BA
BBA
L=W
D = 4 ½ B, when A P 2B
= 3B, when A > 2B
= 1 ½ B, when A > 4B
(c)
ADJACENT ROWS OF FOOTINGS

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B
H
D = 1 ½ B or 1 ½ H
which ever is greater
(D)
RETAINING WALLS
D = 25′ to 100′,
confirm
competent
strata
Pile
(E)
PILES

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Water Surface
B
H
D = 10′ minimum *
= B when B ≤ H
= H when B > H
d
(A) DEEP CUT AND FILL SECTIONS ON SIDE HILLS

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D = 10′ minimum *
(B)
NORMAL CANAL SECTIONS

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H
D = H1 minimum
(C)
HIGH EMBANKEMNTS

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Points to note
The above approximate methods are not useful for
bedrocks
If the foundation load be transferred to bedrock
Min depth of boring in bed rock is 3 m
For irregular or weathered rock , it may be deeper
Further…when deep excavations are required, the min
depth of broing be 1.5 times the depth of excavations,

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How Many Borings & How Deep?
“No hard-and-fast” rule exists for determining the
number of borings or the depth to which borings are
to be advanced.”
But guidelines exist in –
• Textbooks
• Design manuals

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Spacing of Bore holes or Test pit
The spacing can be decreased or increased
depending on sub soil conditions
If subsoil is uniform and predictable less
number of bores may be needed
If subsoil is non uniform and un predictable
more number of bores are needed

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The exploration cost generally should be 0.1% to 0.5% of
the cost of structure

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Conventional Wisdom
– The number (density) of borings will increase:
As soil variability increases
As the loads increase
For more critical/significant structures
Rules of Thumb:
– Soft soils, critical structures – 50'
– Soft Soils - Space 100' to 200'
– As soils become harder, spacing may be increased up
to 500’
How Many Borings?

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How Many Borings?
Source: Sowers 1979
Structure or
Project
Subsurface
Variability
Spacing of Borings (ft)
Highway
Subgrade
Irregular 100-1000 (200, typical)
Average 200-2000 (500, typical)
Uniform 400-4000 (1000, typical)
Multistory
Building
Irregular 25-75
Average 50-150
Uniform 100-300

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How Many Borings?

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Economics
The
exploration
cost generally
should be 0.1
to 0.5% of
the cost of the
structure

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A soil exploration program usually involves test
pits and /or soil borings.
A detailed soil exploration consists of:
1.Preliminary location of each bore hole and /or
test pits.
2.Numbering of the bore holes or test pits.
3.Planned depth of each bore hole or test pit.
4.Methods and procedures for advancing the bore
holes.
5.Number of samplings and their frequency.
6.Requirements for ground water observations.

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Methods of Subsurface
Exploration
Destructive Methods
Non- Destructive Methods

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Methods of Subsurface
Exploration
The following are the methods of subsurface
exploration to determine the stratification and
engineering characteristics of sub-surface soils.
Trial pits or trenches,
Hand Auger Borings (post hole, helical or
spiral , dutch auger, gravel auger; barrel
auger)
Mechanical Auger Borings

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Shell and Auger Borings
Percussion Boring
Wash Boring
Rotary Boring
Diamond core drilling

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Trial Pits and Trenches
Size - 1.5m * 1.5m
Depth – normal 3.0m,
Backfilled with proper compaction.
Cheapest method for shallow depth
Can be excavated either by labors or
mechanical excavator.
Any weak lenses or pockets can be
seen.
Expensive when depth is above 6m
or below water table specially when
the subsoil consists of sandy soil

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the excavated material should be placed
on surface
separate stacks to for the materials
obtained from different depths
Various measurements should be
recorded such as…
orientation,
depth of the pit,
depths and the thickness of each stratum

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Augers
PpPost Hole
Auger
Helical Auger
Extension
Rod Post Hole
Auger
Helical
Auger
Gravel
Auger
Dutch
Auger
Open and closed
Spiral Augers
Flat Spiral Shoe
Barrel Auger

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Screw Auger( spiral auger)
Used in very cohesive, soft or
hard soils.
cannot be used in very dry or
sandy soils since these soil
types will not adhere to the bit.
Good for boring holes quickly,
but more difficult to remove
from the hole.

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Bucket Auger
They are made of a cylinder or
barrel to hold the soil, which is
forced into the barrel by the cutting
lips.
Bucket augers work well in most soil
conditions. Therefore, it is
considered the most universal
auger.
These augers are available with
different tips designed for specific
soil types, such as mud and sand.

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1. The sand auger tips are formed to touch in order
to hold the very dry and sandy soils.
2. Tips of the mud auger are spaced further apart
than the regular soil bucket to allow for easier
removal of heavy, wet soil and clay.
These also have an opening in the cylinder for
removal of the cohesive soils.
Sand Auger Mud Auger

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Dutch Auger: (Edelman auger)
designed for wet, clay, high fibrous, heavily
rooted swampy areas, and extremely wet
boggy soil.
A sand version is available where the blades
are much wider and closer together to help
capture the loose material, but a bucket
auger can retain this material with greater
ease
Bucket augers bore more slowly than the screw
and Dutch but are easier to remove from the hole
and can provide a semi undisturbed sample.

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Planer Auger
Similar to the bucket auger with its
cylinder shape,
but designed to flatten and clean out
the bottom of the predrilled hole in
preparation for core sampler to obtain
a quality undisturbed sample.

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Stony Soil Auger
Used in stony soils,
gravely soils containing
small stones and
asphalt.

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Augers may be classified as either
bucket augers or
flight augers

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Flight augers may be classified as
short-flight augers or continuous
augers.

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Continuous augers can be classed as:
(i)solid stem continuous-flight augers
or
(ii)hollow stem continuous-flight augers
May be operated by hand or power
Cheap method to determine the type
of soil.

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HOLLOW STEM AUGER

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Casing with outer spiral
Inner rod with plug/or pilot
assembly
For sampling, remove pilot
assembly and insert sampler
Typically 5ft sections, keyed,
box & pin connections
Maximum depth 60-150ft
Hollow Stem Auger

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This method may be used in all types of soil
including sandy soils below the water table
but is not suitable if the soil is mixed with
gravel, cobbles etc.
A hollow stem is sometimes preferred since
standard penetration tests or sampling may be
done through the stem without lifting the auger
from its position in the hole.

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Besides, the flight of augers serves the purpose
of casing the hole.
In case of hollow stem, the hollow stem can be
plugged while advancing the bore and the plug
can be removed while taking samples or
conducting standard penetration tests (to be
described later on).
Due to plugging, hollow stem functions like solid
stem

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The drilling rig can be mounted on a truck or a
tractor.
Holes may be drilled by this method rapidly to
depths of 60 m or more.

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Advantages and Disadvantages of
CFA
soil moving up from the base of the hole is
free to mix with the soil at higher levels on the
edge of the borehole
Hollow-stem auger drilling is often fast and
reliable method of boring.

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CFA
in coarse gravels the continuous-flight auger is
unusable because it must be removed each
time a sample or in-situ test is to be carried
out. At this stage the hole will collapse.

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Cleaning of Auger Flights
• In case of boring in very cohesive soils, like
clay, the cleaning of auger flights is
cumbersome.
CFA

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AUGER BITS

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Hollow Stem Continuous
Flight Auger Drilling Systems:
(a) Comparison with solid
stem auger;
(b) Typical drilling
configuration;
(c) Sizes of hollow stem
auger flights;
(d) Stepwise center bit;
(e) Outer bits;
(f) Outer and inner
assembly.

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Boring bits/dia 7.5 cm to 30 cm.
Applicable to where soil can stand without
casing or bore hole stands
unlined/unsupported due to cohesion.
Not suitable in a deposits containing large
cobbles or boulders.

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Drilling:
Continuous Flight Auger

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DRILLING RIG AND HALLOW STEM AUGER SYSTEM

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Hollow Stem Auger

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BUCKET AUGER
Bucket auger consists of an open-topped cylinder
which has a base plate with one or two slots
reinforced with cutting teeth, which break up the
soil and allow it to enter the bucket as it is
rotated.

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BUCKET AUGER
The top of the bucket is connected to a rod which
transmits the torque and downward pressure
from the rig at ground level to the base of the
hole: this rod is termed a ‘Kelly’.
They are used for subsurface exploration in the
USA, but are rarely used in the other parts of the
world.
This is probably because they require a rotary
table rig, or crane-mounted auger piling rig for
operation, and this is usually expensive to run.

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BUCKET AUGER AND DRILLING RIG

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Shell and Auger Borings/Percussion
boring/percussion drilling
The auger consists of a cylinder with bit at
the bottom.
Shell is also similar as a cylinder with
cutting edge and hinged flap at the
bottom.
Used for drilling a deeper bore hole.

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LIGHT
PERCUSSION
DRILLING RIG

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Shell and Auger Borings
Shell & auger rigs are simple and cheap to
operate, they are excellent for drilling sand &
gravel and soft clays and chalk.

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Percussion Drilling
The drilling rig (Fig) consists of a collapsible ‘A’
frame, with a pulley at its top,
a diesel engine connected via a hand-
operated friction clutch (based on a brake
drum system) to a winch drum which provides
pulling power to the rig rope and can be held
still with a friction brake which is foot-
operated used to raise and lower a series of
weighted tools on to the soil being drilled.

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LIGHT
PERCUSSION
DRILLING RIG

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LIGHT PERCUSSION
DRILLING TOOLS

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The rig is very light and can be
readily towed with a four-wheel
drive vehicle. It is also very easy
to erect, and on a level site can
be ready to drill in about 15
minutes.
In clays, progress is made by
dropping a steel tube known as a
‘clay cutter’ into the soil (see
Fig.).

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This is slowly pulled out of the borehole and is
then generally found to have soil trapped
inside it
When the claycutter is withdrawn from the
top of the hole, the soil is removed with a
metal bar which is driven into it through the
open slot in the claycutter side

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In granular materials, such as sands or
gravels, a shell is used.
At least 2 m of water is put in the bottom of
the borehole, and the shell is then surged,
moving about 300mm up and down every
second or so.
Surging the shell upwards causes water to be
drawn into the bottom of the hole, and this
water loosens the soil at the base of the hole
and forces it to go into suspension

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As the shell is dropped on the bottom of the
hole the mixture of soil and water passes up
the tube of the shell, past the simple non-
return valve (sometimes called a ‘clack’). As the
shell is raised, the clack closes and retains the
soil
By repeatedly surging the shell up and down at
the base of the hole, soil can be collected and
removed from the hole. By this method the
boulder or rock formation are pulverized .
Highly disturbed samples are collected.

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WASH BORING
Wash boring is a relatively old method of boring
small-diameter exploratory holes in fine-
grained soil.
Soil exploration below the ground water table is
usually very difficult to perform by means of
pits or auger-holes. Wash boring in such cases is
a very convenient method provided the soil is
either sand, silt or clay.
The method is not suitable if the soil is mixed
with gravel or boulders.

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The purpose of wash boring is to drill holes only
and not to make use of the disturbed washed
materials for analysis.
Whenever an undisturbed sample is required at a
particular depth, the boring is stopped, and the
chopping bit is replaced by a sampler. The
sampler is pushed into the soil at the bottom of
the hole and the sample is withdrawn.

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Figure shows the assembly for a wash boring.
To start with, the hole is advanced a short
depth by auger and then a casing pipe is
pushed to prevent the sides from caving in.
The hole is then continued by the use of a
chopping bit fixed at the end of a string of hollow
drill rods.
A stream of water under pressure is forced
through the rod and the bit into the hole, which
loosens the soil as the water flows up around the
pipe. The loosened soil in suspension in water is
discharged into a tub.

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Wash boring

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The soil in suspension settles down in the tub
and the clean water flows into a sump which
is reused for circulation.
The motive power for a wash boring is either
mechanical or man power. The bit which is
hollow is screwed to a string of hollow drill
rods supported on a tripod by a rope or steel
cable passing over a pulley and operated by a
winch fixed on one of the legs of the tripod.

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DRILLING
Layout for small-scale
rotary core drilling

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Rotary Wash Drilling
System:
(a)Typical drilling
configuration;
(b) Casing and
driving shoe;
(c) Diamond, drag,
and roller bits;
(d) Drill fluid
discharge;
(e) Fluid cuttings
catch screen;
(f) Settling basin
(mud tank).

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Coring Bits
Three basic categories of bits are in use: diamond,
carbide insert, and saw tooth.
Diamond coring bits may be of the surface set or
diamond impregnated type.
• Diamond coring bits are the most versatile of all the
coring bits since they produce high quality cores in rock
materials ranging from soft to extremely hard.

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Carbide insert bits
Carbide insert bits use tungsten carbide
in lieu of diamonds.
Bits of this type are used to core soft to
medium hard rock.
They are less expensive than diamond
bits
but the rate of drilling is slower than
with diamond bits.

Sub surface exploration (part-1)

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