Exploratory Drilling

MODULE -1
DRILLING AND BLASTING
Manas Kumar Mallick
Assistant Professor
Department of Mining Engineering
BIT Sindri, Dhanbad

What is Exploration?
 The process of collection of rock sample data
for the assessment of rock properties at a site
through series of laboratory and field
investigation is collectively called Exploration
 Enables the engineers to draw rock profile
indicating the sequence of strata and the
properties of rock involved.

Main Objectives
To find out
 Nature of Deposit
 Depth and Thickness of Strata
 Horizontal extent of Deposit
 Depth of GWT and its fluctuations
 Engineering properties of Rock
 Insitu Rock Properties
 Collection of Rock Sample

Direct Method: Test Pits
 Depth upto 3m
 Uneconomical at greater depths.
 Supports are required at greater depths. Especially
in case of weak strata.
 Problems with GWT and the same should be lowered.
 Open type Exploration.
 Soils are investigated in natural condition.
 Soil samples are collected for determining strength
and Engineering properties.

Semi-Direct Method: Drilling/Boring
Drilling a hole into the soil or rock strata up to specified depth
is known as Boring. The Exploratory Drilling methods are
classified are as follows:
1. Auger Drilling
2. Wash Boring
3. Percussion Drilling or Churn Drilling or Cable tool
4. Rotary Drilling
5. ODEX

Auger Boring
 Drilling is made using a device
called Soil Auger
 Power driven (upto 3 to 5m)
and Hand operated (Greater
than 5m)
 Advancement is made by drilling the
auger by simultaneous rotating and
pressing it into the soil
 Dry and unsupported bore holes
 When the auger gets filled with soil
same, it is taken out and the soil
sample collected

Auger Drilling
 Casing is provided in case of weak strata
 First the casing is driven and then the auger
 Boring rig is used for power driving (hand rig for depth
up to 25 m)
 Soft rocks are broken using chisel bits.
 Sand pumps are used in the case of sandy soils.
Disadvantage:
 Whenever the casing is to be extended, the auger has
to be withdrawn which hinders the quick progress of
the work.

Wash Boring
 Below GWT, It may not be used for soils mixed with gravel
and boulders.
 Initially, the hole is advanced for a short depth by using an auger.
 Then a casing pipe is pushed in and driven with a drop weight. The
driving may be with the aid of power.
 A hollow drill bit is screwed to a hollow drill rod connected to a rope
passing over a pulley and supported by a tripod.
 Water jet under pressure is forced through the rod and the bit into the
hole.
 This loosens the soil at the lower end and forces the soil-water
suspension upwards along the annular space between the inner wall of
the drill hole and outer wall of the hollow drill rod.

 This suspension is collected in a settling tank.
 Soil particles are allowed to settle down and water is
allowed to overflow into a sump which is then
recirculated
 Very disturbed sample is obtained. Hence cannot be
used for determining engineering properties.
 whenever a soil sample is required, the chopping bit is
to be replaced by a sampler.
 The change of the rate of progress and change of
colour of wash water indicate changes in soil strata.
Wash Boring

Typical set up for Wash boring

Percussion Drilling or Churn Drilling
 A heavy drill bit called ‘churn bit’ is
suspended from a cable and is
driven by repeated blows.
 Water is added to facilitate the
breaking of stiff soil or rock.
 The slurry of the pulverised material is
bailed out at intervals.
Disadvantages:
 Cannot be used in loose sand and
is slow in plastic clay.
 The formation gets badly disturbed by
impact.

Percussion Drilling or Churn Drilling
In case of hard soils or soft rock,
auger boring or wash boring
cannot be employed. For such
strata, percussion drilling or cable
tool drilling method is usually
adopted
Here advancement of hole is
done by alternatively lifting and
dropping a heavy drilling bit which
is attached to the lower end of the
cable.
At intervals, the bit is removed
from the hole and cuttings are
withdrawn with the help of bailer.

Rotary Drilling
 Suitable for rock formations.
 A drill bit, fixed to the lower end of a drill rod, is rotated
by power while being kept in firm contact with the hole.
 Drilling fluid or bentonite slurry is used under pressure
which brings up the cuttings to the surface.
 Even rock cores may be obtained by using suitable
diamond drill bits.
Disadvantage:
 Not used in porous deposits as the consumption of drilling
fluid would be high.

Rotary Drilling
Rotary drilling method of boring is
useful in case of highly resistant
strata.
It is related to finding out the rock
strata and also to access the quality
of rocks from cracks, fissures and
joints.
Here, the bore holes are
advanced in depth by rotary
percussion method which is similar
to wash boring technique.
A heavystring of the drill rod is
used for choking action.

Rotary Drilling
The broken rock or soil fragments are removed by
circulating water or drilling mud pumped through the drill
rods and bit up through the bore hole from which it is
collected in a settling tank for recirculation.
If the depth is small and the soil stable, water alone can be
used
However, drilling fluids are useful as they serve to stabilize
the bore hole. Drilling mud is slurry of bentonite in water.
The drilling fluid causes stabilizing effect to the bore hole
partly due to higher specific gravity as compared with water
and partly due to formation of mud cake on the sides of the
hole.
As the stabilizing effect is imparted by these drilling fluids
no casing is required if drilling fluid is used

ODEX Drilling
90 % of the land surface of the earth is covered with loose,
unconsolidated material such as soil,clay,silt,sand,gravel and
boulders which varies in depth from a few centimetres to
hundred meters
Drilling through this so-called overburden is often
problematic, due to the tendency of the earth to cave in behind
the drill bit. This makes it difficult to retrieve the drill strings
after the hole has been drilled. In practice the bore hole has
been lost before a casing tube can be inserted to support it.
Other problems are caused by cavities or porous ground,
which interfere with the circulation of the flushing medium and
prevent the drill cuttings from being flushed out of the hole.

ODEX Drilling
In places where overburden strata are
mixed or when their drillabilty is unknown,
it is difficult for the driller to decide what
tools to use in order to get the best overall
results without risking the loss of
equipment in the hole.
ODEX equipment enables you to drill
and case deep holes simultaneously in all
types of formation even those in large
boulders.

ODEX Drilling
The word is a short for Overburden Drilling
Excentric.
The drilling is used casing to line the hole
when passing through the soil overburden
before entering into the sound rock.
Casing diameters from 89 mm (ODEX 76) to
273 mm (ODEX 240) can be used.
The method is based on a pilot bit and
eccentric reamer, which together drill a hole
slightly larger than the external diameter of the
casing tube. This enables a casing tube to follow
the drill bit down the hole.
The most frequently used diameter of hole 127
mm with casing having an inner diameter 115
mm.

ODEX equipment enables to drill and case deep holes simultaneously in
most of the geological formations even those in large boulders.
The method is based on a pilot bit, eccentric reamer and guide device,
which together drill a hole slightly larger than the external diameter of the
casing tube.
It enables the casing tube to follow the drill bit down the hole.
Part of the impact energy meant for the drill bit is diverted to the casing
tube via the guide device to the casing shoe at the lower end of the
casing.
The impact energy can be supplied either by top hammer or DTH
hammer. In both cases the casing is driven down into the hole without
rotation.
The top hammer cover smaller hole dia 96 to 152 mm and DTH 123 to
306 mm.
As the casing enters into the bed rock, drilling is temporarily stopped and
reverse rotation is applied, which causes the reamer to turn in thus
reducing the overall diameter of the drill bit assembly.
ODEX Drilling

Flushing media is normally air for ODEX drilling.
The drill cuttings are carried by the return flow between the rod and
the steel casing
ODEX Drilling

Core Barrels
To collect the core of rock drilled, a device is
known as core barrel.
Core barrel retains rock samples from drilling
operation.
The length varies from 0.5 – 3 m generally.
There are three types of core barrel used in
exploration purposes.
1. Single tube core barrel
2. Double tube core barrel
3. Triple tube core barrel

Single Tube Core Barrel
Most rugged and least expensive.
It consists of head section, core recovery tube, reamer
shell, and cutting bit.
Often used as starter when beginning core operations.
Coring in homogeneous hard rock (where the core doesn’t
wash away or crumble easily) and penetrating rock layers of
the strata (where high core recovery is not essential)
Single tube barrel are often used as starter barrel during
the beginning of core operations.
A single tube core barrel is suitable for homogenous
formations where the core is not eroded by flushing water
and a solid can be taken without risk of blockage in the
barrel.

Single Tube Core Barrel
Limitations:
Wrong interpretation of stratification.
Probability of loss of information during core recovery.

Double Tube Core Barrel
Double tube core barrel is used for standard practice.
Outer barrel rotates with cutting bits.
Inner barrel is either fixed or swivel type (without bearing)
that retains core sample.
Core diameters generally ranges from 21- 85 mm.
The standard diameter of NX core size is 54 mm.
Double Tube Core Barrels are swivel head i.e. the inner
tube is attached with the core barrel head through bearings
which makes the rotation of the inner tube independent of
the core barrel
.

Double Tube Core Barrel
It offers higher percentage of core
recovery and little erosion of core.
Water in the double tube core barrel
passes in between the outer tube and inner
tube.
Perforation at the lower end of the inner
tube permits the flow of water

Rigid and Swivel type Double tube core Barrel

Triple Tube Core Barrel
Good for obtaining core samples in fractured rock and
highly weathered rocks.
Outer core barrel for initial cut and second barrel to
cut finer size. Third barrel to retain cored samples.
Reduces frictional heat that may damage Samples.
It consists of stainless steel inner tube split
lengthwise, inside a inner tube.
The middle tube is mounted on an assembly that
de-couples it from the rotating outer tube & isolates
the core from the drilling water.
A latching mechanism retains the middle tube in
place during drilling operation.

Consists of a stainless steel inner tube split
lengthwise, inside a middle tube
The middle tube is mounted on an assembly that
de-couples it from the rotating outer tube & isolates
the core from the drilling water
A latching mechanism retains the middle tube in
place during drilling.
When the inner split tube is full, an “overshot” is
run through the drill string on a wire rope to retrieve
core.
Once retrieved, the split inner tube is hydraulically
separated from the middle tube.

By tugging, the core in the inner tube snaps off just
below a core catcher.
Middle & inner tubes are released by a latch from
the outer tube, then pulled to the surface through
the drill string.
The drilling string remains in the borehole unless
the bit needs replacement
Outer Core Barrel Inner Core Barrel

Wire line Core Barrel
Wire line core drilling is a special type of core drilling,
most commonly used for exploration drilling.
While drilling in medium hard and soft formations as are
usually encountered in coal areas (sandstone, shale and
coal). It is seen that in deeper holes more time is spent in
hoisting and lowering of drill string, for taking out core
sample after every 3 m run in comparison to the time spend
in cutting the 3 m depth.
With the objective of cutting the down time spend in
hoisting and lowering of the drill string, the wire line system
of drilling was developed.

In drilling with conventional equipment, the entire drill
string with core barrel is required to be hoisted out of the
borehole after every 3 m run to take the core sample out,
and again lowered back with the drill string, to start the next
run.
With wire line drilling a barrel of core can be removed
from bottom of the hole without removing the drill rod string
assembly.
When the driller wants to remove the core, an overshot is
lowered on the end of a wire line.
The overshot attaches to the back of the core barrel inner
tube and wire line is pulled back and the inner tube
disengages itself from the barrel.

For wire line drill the rods are made of fine high-tensile
steel. This makes them thin, so that the core can be as
large as possible.
In this process most of the time is available for drilling and
so speedier progress can be made with the same time
spent in actual drilling.
Hoisting of the drill string is required to be done only
when the bit has to be changed.
Wire line drilling equipment is specially suited for drilling
in coal formations, where the drilling depth involved are
over about 200 m.
Normally core barrels of 0.5 – 3 m length are employed.

Core size less than BX is not possible with wire line
coring equipment which is there for used for drilling holes of
NX and BX size only.
The speed of the drilling with this equipment is nearly 80
m per shift in the types of rock met within the coal fields.
All the drill rods need to be withdrawn to the surface only
when the bit has to be changed.
Wire line drilling is possible up to the depth of 1000 m.
As, stated earlier the rods used for wire line drilling have
specifications as laid down in ‘Q’ series decided by
DCDMA.
Ordinary drilling equipment can be adopted to wire line
drilling and hoisting with suitable modifications.

Wire line cable: A wire rope 4.8 - 6.4 mm in diameter is
used to handle the inner tube of a wire line core barrel.
At the end of the core run, the drill string is broken at the
top joint so that an overshot latching device can be lowered
on the cable through the drill string. When it reaches the
core barrel, the overshot latches onto the retractable inner
tube assembly, which is locked in the core barrel during the
core run.
The upward pull of overshot releases the inner tube and
permits it to be hoisted to the surface through the drill rod. It
is then emptied, serviced and lowered back into the hole,
where it relocks itself in the core barrel at the bottom.

Wire line Drill rod: Drill rod having
coupling that are nearly flush in the
inside and designed so that the inner
tube of wire line core barrel and
overshot assembly can be run inside
the rod.

Core Logging
No details should be
ignored, as this log may be
the only record obtained for
the particular site & will be
relied upon in future years
Detailed descriptions of
core logging are presented
by the Geological Society
(1970) & ISRM (1978)

Core Logging
Important points to be noted down during the core
logging period, those are given below:
Dates of drilling
Drilling method
Drilling machine make & model
Drilling fluid type & amount
Depth and type of casing
Zones of severe core loss
Problems that necessitated casing
Fluid losses
Bad drilling zones
Zones of severe bit wear
Groundwater levels & dates measured

Storage of Cores
The fundamental objective of core drilling is to collect sub
surface samples in the shape of core and accompanying
sludge material in order to study their mineral assemblage,
chemical composition, rock structure and physical strength
for various purposes.
After removing the core from core barrel, it should be
placed in the core box in the exact order in which it is taken
from the ground.
The core box is a shallow tray 1 m long with partition
running length wise between which the core fits in to. Each
compartment is 1 – 1/16” to 1/8” wider than the core
diameter.

Storage of Cores
The core box may be wood with hinged or screwed cover
or with mild steel with rounded groove or rectangular core
compartments.
As the core from each run is placed in the box, a wooden
block upon which the depth of the hole is written should be
placed after the last piece of core of each run.
Each run is thus separated and thus shows the exact
depth at which it was made. The arrangement of core in
core box is done in different ways.
Book Fashion: It is like reading a book starting from left
and proceeding to right.
Snake fashion: It starts from left to right in the first groove.

Storage of Cores
Once the first groove is completed the cores are placed
from right to left in the second groove and so on and so
forth.
Book fashion is preferred the most as it speeds up core
logging, identification of footage at any point in the hole and
less chance of error.

Core Recovery
Cores should be stored in either wooden boxes or
corrugated cardboard box.
Box marked with boring number, depth of core
run, type core, bit type, core recovery (CR), rock
type, RQD, and other notes.
Core operations should be well documented i.e.
Loss of fluid, sudden drop of rods, poor recovery,
loss of core etc.
Core Runs taken in either 5-10 foot sections (1.5-
3 m sections).
Log the amount of material recovered.
Core Recovery is percentage retained which is
expressed in percentage.

Core Recovery
Core recovery depends on quality of the rock mass,
choice of core barrel, skill of the operator, stability of the drill
rig and lack of vibration.

Rock Quality Designation (RQD)
The RQD is a modified core recovery
Measure of the degree of fractures, joints, and
discontinuities of rock mass.
RQD = sum of pieces > = 100 mm (4 inches)
divided by total core run.
Generally performed on NX-size core.
Mathematically RQD is expressed as:

Rock Quality Designation (RQD)
Core Recovery Rock Quality Designation (RQD)

Exploratory Drilling

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