PERCUSSION DRILLING:
INTRODUCTION
The percussion drilling method is used for making holes in rocks, boulders
and other hard strata. In this methods, a heavy chisel is alternatively lifted
and dropped in vertical holes. The material gets pulverized. If the point
where chisel strikes is above the water table, water is added to the hole, the
water forms a slurry with the pulverized material, which is removed by the
sand pump or a bailer at intervals, Percussion drilling may require casing. It
is also used for drilling of tubes.
The main advantage of the percussion drilling method is that it can be used
for all type of materials. It is particularly useful for drilling holes in glacial
tills containing boulders. One of the major disadvantage of this method is
that the material at the bottom of the hole is disturbed by heavy blows of
chisel. it is not possible to get good quality of undisturbed soil samples.
Further , the method is generally more expensive than other methods,
moreover its difficult to detect minor changes in properties of strata
penetrated.
This system is used for medium-large diameter and low-medium depth
drilling works.
The action is carried out by cable percussion tools of three main types:
- Buckets, used in the absence of water;
- Chisels, used in compact soils.
-Probes or dippers, used in loose or medium compact soils and in the
presence of water
Optimal depth: from 0 up to 100 m
Optimal diameter: from 400 up to 1.200 mm
EQUIPMENT AND METHOD - Driven Wells
Whenever the water table lies at shallow depths (23 feet or 7 meters), a well
screen equipped with a drive point may be driven through the overlying soil
and into the water-bearing formation. This method employs a drive hammer.
Three basic types of drive hammers are in common use: (1) the hand driver,
consisting of a sliding weight and an attached pipe that fits over the riser
pipe, (2) an internal driving bar, which strikes directly upon the driving
point, or (3) a sliding weight and drive stem or guide that attaches to the
uppermost riser pipe coupling.
The basic equipment required for a driving rig ranges from a 4-foot (1.2
meter) section of oversized pipe (used as a sliding hand driver) to more
elaborate systems requiring a tripod, pulley, rope, and driving bar or drive
stem and sliding hammer. The driving rig will also require two or three pipe
wrenches and a shallow well hand pump to develop and remove soil debris
from the well screen.
Geological Applications
Driven wells are generally one of the most efficient methods of drilling
whenever the water table is within 23 feet (7 meters) of the surface and the soil
consists principally of sand with minor quantities of silt and clay. Under ideal
soil conditions, a small diameter well point may be driven to a depth of 25 feet
(7.6 meters) in 15 minutes. In heavy soils such as stiff clay, or soils that contain
numerous boulders, drilling with an auger or percussion bit is faster than
driving with a well point.
Hand-driven well points of 1 1/4 to 2 inches (3 to 5 centimeters) in diameter
can be driven up to 25 feet (7.6 meters). If heavy, 100-300 lb. (45 to 135 kg)
drive hammer assemblies are used, 4-inch (10 centimeters) well points and
casings can be driven to depths of 33 to 49 feet (10 to 15 meters).
Advantages
Unlike any other drilling method, percussion can remove boulders and
break harder formations, effectively and quickly through most types
of earth.
Percussion drilling can in principle deal with most ground conditions.
Can drill hundreds of feet (one well hand-drilled in China in 1923 was
over 4000 feet deep).
Can drill further into the water table than dug wells, even drilling past
one water table to reach another.
Disadvantages
The equipment can be very heavy and relatively expensive.
Especially in harder rock the method is slow (weeks, rather than
days).
When temporary casing has to be used, the time taken driving and
removing it can significantly increase drilling time.
Equipment costs are high and the method is slow (resulting in high
cost / drilled meter).
Labor Requirements
Given the proper soil and water table conditions, small diameter driven wells
may be completed by one to two unskilled persons. Large diameter driven
wells require a heavy-duty drive hammer and a tripod assembly. The crew
necessary for operation of this equipment consists of six people for manual
methods, and two or three for a motorized cathead system.
Fabrication Skills
All well point driving equipment can be constructed easily from locally
available scrap pipe or steel bars and standard pipe fittings. The fabrication
of simple drive hammers requires basic metal working and blacksmith
abilities. Construction of heavy-duty drive hammers, which weigh in excess
of 50 lbs. (22.5 kg), will require the aid of an electric arc welder or basic
metal casting techniques.
Cost of Equipment
Excluding the initial cost of a well screen, drive point, and riser pipe, a
locally constructed hand drive system that requires no tripod will be
relatively inexpensive. Heavy-duty systems may cost more for the
fabrication of both drive hammer and drive stem, depending on the type of
tripod used. If a driving rig incorporates a motorized cathead, the system
price could increase again. A hub-driven cathead would cost considerably
less but would require a support vehicle.
Source: https://bestengineeringprojects.com/percussion-drilling/
Fig: Percussion drilling
Source: https://www.sciencedirect.com/topics/earth-and-planetary-sciences/percussion
