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Transportation Curriculum Coordination Council |
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Chapter 4
Drilled Shaft Equipment and Tools
Contents
- This Chapter contains information on the various types of rigs and tools used in the construction of drilled shafts.
- A short quiz is provided at the end of the Chapter.
It is extremely important for the Drilled Shaft Inspector to be knowledgeable of the various types of rigs and tools used in drilled shaft construction. Though not responsible for accepting or rejecting equipment and tools, the Inspector must be able to identify these items for documenting on the daily report.
Drilled shaft construction equipment is typically divided into two categories Drilled Shaft Rigs and Drilled Shaft Tools. Following are some examples of each.
Drilled Shaft Rigs
Drilled shaft rig components, for the most part, are all essentially the same, regardless of the rig size as shown and described below.
1 - Power Unit- provides the power to turn the table & kelly
2 - Kelly- the rod running through the table that tools are attached to
3 - Table- connected to power unit, turns kelly
4 - Tool- bits, buckets, etc. that go down the hole
5 - Carrier (Crane)- carrier or main component
The rigs are typically classified by the carrier type and fall into these broad categories:
Truck-mounted Drill Rig
Truck-mounted rigs are typically used for smaller sized holes, generally for mast arms and sign posts.
Typically these rigs excavate holes up to 5 ft. (1500 mm) in diameter to depths on the order of 30 to 35 ft. (9 to 10 m).
Carrier-mounted Drill Rig
Carrier-mounted rigs have larger hole capabilities than the truck-mounted rigs. These generally have 2 front axles and telescoping kellys, enabling greater hole depths. Typically these rigs have the capability of drilling holes of 120 inch diameter (3000 mm) to depths ranging from 85 to 200 feet (27 to 62 m). Crane-mounted Drill Rig
For the largest and deepest holes, crane attachments are used. The attachments come as a unit which includes the diesel engine, transmission and torque converter. The unit is attached to the crane by using a "bridge", which provides for increased working and tool room under the table.
The hole diameter and depth capability is generally dependent upon the crane but holes 140 inches (3500 mm) in diameter to depths of near 300 feet (90-91 m) are common.
Crawler-mounted Drill Rig
Crawler-mounted rigs offer more maneuverability and require less overhead clearance than the other rigs, making them the rig of choice for restrictive work areas.
Typically these rigs have capabilities of holes 250 feet (78 m) with diameters of 100 inches (2500 mm).
Drilled Shaft Tools
There are a variety of tools utilized by the Drilled Shaft Contractor when constructing drilled shafts. From a wide assortment of drilling bits, for rock and soil, to casing and cleanout tools, the Drilled Shaft Contractor is equipped for whatever conditions they anticipate on the project. Regardless of how powerful the rig is, if the wrong tool or poor quality tool is used, the results are not as expected. The Inspector must be able to identify these tools for documenting on the daily report.
Drilled shaft construction tools are typically divided into the following categories:
Bits
Bits are used for drilling (excavating) the shaft (hole) and can be either Auger or Barrel. Typically the Auger bits are used for soil and rock and the Barrel types used predominately for rock.Typically, the augers are turned into the material by the rig, and upon achieving penetration equal to the bit length, the auger retracted from the hole, the material removed from the flights, and the process started again
Auger Bits
Earth Auger
Earth augers, like the one shown below, are typically used in sands and cohesive materials. Earth augers are typically constructed of lighter weight material and flat
Rock Auger
Rock augers, like the one shown below, are typically used in soft to hard rock formations. Rock augers are typically constructed of heavier material than the earth augers and typically have replaceable conical or bullet teeth for cutting, rather than the flat blades associated with the earth augers. In addition, they are generally constructed with a tapered geometry.
Single flight, single cut earth auger.
Single flight, single cut rock auger.
Single Flight/Single Cut or double Flight/double Cut Augers
Single Flight/Single Cut Earth Auger
Double Flight/Double Cut Earth Auger
Rock Bits
Quite often, when hard rock is encountered, auger bits cannot advance the hole and the Contractor must employ Rock Bits to drill the harder rock.
Cluster Rock Bit
This bit pulverizes the rock with many rolling bits and the cuttings are carried away through reverse circulation of the drilling slurry.
Step-face Roller Rock Bit
This bit grinds the rock up, first with the lead bit (makes small hole, but weakens outer rock). The other bits follow, gradually making the hole larger. Compressed air is used to remove the cutting with this type of bit.
Core Barrels
Core Barrels are different than Rock bits, in that the rock bits grind away the entire mass of rock in the hole, while barrels cut along the perimeter of the barrel, hence less rock cutting. When a joint or discontinuities are encountered, the core breaks off and be be removed. These are generally used where real hard rock is encountered and are often custom made for the project.
This is a single-wall rock core barrel. Note that it is equipped with replaceable bullet teeth.
This is a double-wall rock core barrel. The outer barrel does the cutting while the inner barrel remains stationary, holding the rock core in place.
Buckets
Buckets typically come in two types, Digging and Cleanout.
As the names imply, each has a designed use, one for advancing the hole, the other for cleaning the bottom of the hole. Typically, the buckets are turned by the rig, and the bottom configuration either digs up material or collects material.
Generally, the digging buckets are equipped with flat cutting teeth while the cleanout bucket has a single flat blade.
Digging Bucket
This is the type of digging bucket
used in cobbles, gravel and clays.Cleanout Bucket
When using this type of cleanout bucket, the rig is rotated in the normal drilling direction, picking up bottom sediments. Then while still on the bottom, rotated in the opposite direction, which closes the bucket, and removed from the hole.
Casing
Casing is used to maintain the stability of the hole and can be Temporary or Permanent, as discussed earlier in Chapter 3. In many instances, a short piece of casing is used at the surface (called surface casing) to prevent the surface material from collapsing into the hole and degradation of the top of the hole due to the in and out process of drilling.
Casing is typically made of strong steel and pieced together by welding to achieve the depths needed. It comes in a variety of diameters, such as 30, 36, 42, 48 inches , etc.
Temporary Casing- Temporary casing is just that, temporary. The casing is used to maintain an open hole for the construction process and is removed as the concrete is placed. This is the most common of the two casing types.
Permanent Casing- Permanent casing is just that, permanent. It is left in-place and becomes part of the drilled shaft. It is generally used when conditions, such as voids, preclude the construction without casing, as concrete placement could not be properly performed or maintained.
This picture shows a piece of casing being placed.
In this picture, you can see the casing extending above existing ground and the bit being lower.
Specialty Tools
The Contractors use a number of "specialty" tools during drilled shaft construction Some of these are "homemade" and some are manufactured and used widely, such as Desanders.
Desanders, such as the one pictured above are used to remove sand from drilling slurries and maintain sand content within specified limits.
Vibratory hammers, such as the one pictured above are used in some instances to vibrate casing into the ground.
A "Belling tool" (above) is used to bell out the bottom of a hole, when specified. Lowered into the hole with the cutting blades retracted, the tool is rotated and as the cutting blades cut, the blades extend outward, excavating a "bell" shape bottom.
One of the most positive methods for cleaning the hole is a down-hole pump, such as the one pictured above. These pump the bottom of the hole sediments up and out.
I have completed Chapter 4 and am ready to take the Quiz
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