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Alluvium - sediment deposited
by flowing water, as in a riverbed, flood plain, or delta. Also
called alluvion.
Alluvial gravel - gravel with sediment.
Aquifer - An underground bed or layer
of earth, gravel, or porous stone that yields water.
Auger - A machine having a rotating helical
shaft for boring into the earth.
Cable Tool Drilling (Percussion/Drilling
and Driving)
Percussion drilling is still used effectively in many parts of the
world. Drilling is accomplished using a weighted string of drilling
tools attached to hoisting cable that is repeatedly lifted and dropped.
The force of the impact crushes the material at the bottom of the
borehole. The crushed material is periodically cleared using a bailer.
Advantages: low capital investment, maintenance and operating costs,
minimal cross-contamination, water is the only media required for
cuttings removal, large diameter holes can be drilled. Disadvantages:
drilling is slow in hard formations, boulders are difficult to drive
casing through, often requiring the use of dynamite, casing penetration
rates decrease with depth in a given formation, noise and vibration
can be significant and of special concern when drilling in populated
areas or near sensitive structures.
CWD - persons identified as a "Certified
Well Driller" (CWD) and/or "Certified Pump Installer"
(CPI or CWD/PI) have earned these designations by passing exams
testing their expertise generally and in specific categories. To
maintain certification, they must take and report continuing education
courses, adhere to high professional standards and have no legal
action pending against them pertinent to well construction or water
systems installation.
Dual Rotary
Drilling (DR Drilling)
The distinguishing feature of the Dual Rotary rig is a lower rotary
drive which is used to independently advance the casing. The rotary
top drive handles the inner drill string, which can be tooled with
a down-the-hole (DTH) hammer, drag bit, or rolling cone bit. Cuttings
are typically removed using air from either the on-board compressor
or an auxiliary compressor. Rotary drilling method, employed in
conjunction with a cyclone collection system, ensures an accurate
sample of the formation being drilled. A key benefit of casing rotation
is a straight hole, even when drilling in cobbles and boulders.
A straight hole helps to minimize sidewall friction, reduce stress
on casing joints and welds and enable greater casing depths.
Gravel - An unconsolidated mixture of
rock fragments or pebbles.
Piezometer - A device for measuring pore
water pressure (i.e. measuring the location of the water table).
Some types of piezometers can also be used for collecting water
samples. As a result, wells designed specifically for collecting
water samples are often referred to, incorrectly, as piezometers.
Rotary Drilling with Casing Hammer
A casing hammer is a well-established alternative for advancing
casing in situations where the hole cannot otherwise be kept open.
A rotary top drive rig may be equipped with a casing hammer powered
by compressed air, hydraulic or mechanical power that drives the
casing as drilling progresses. Typically, a DTH hammer or tri-cone
bit is used to open a hole ahead of the casing and is then retracted
inside the casing while the casing is driven with the casing driver.
The striking action of the casing hammer can be reversed to aid
casing removal.
Rotary Drilling (Open-Hole Method with 'Mud')
It is possible, in some situations, to keep the hole open while
drilling without the benefit of casing. Bentonites and synthetic
stabilizers are mixed with water and circulated in the borehole.
The resultant fluid, commonly referred to as mud or drilling mud,
is used to cake and stabilize the borehole wall. The mass of the
fluid also provides pressure in the hole, which helps to keep it
open. The drilling fluid is circulated down the hole through the
drill pipe, where it exits through ports in the bit. The fluid (mud)
flushes the cuttings away from the face of the bit and carries them
up the annulus to the surface. Reverse circulation with mud is also
possible. In either case, once the mud reaches the surface, it feeds
into a settling tank where the cuttings are separated from the mud
before it is circulated down the hole again.
Turbid- having sediment or foreign particles
stirred up or suspended; muddy: turbid water.
Turbidity - the amount of sediment or
particles suspended in liquid.
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