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Education : drilled shafts TEchniques & methodologies


This educational tutorial was co-funded by the FHWA and Florida Department of Transporation. We have taken excerpts that are most applicable to our work within the foundation drilling industry. Click here to see the original in its complete form.

 

  • Contents
  • Drilled Shaft
  • Methods
  • Excavation
  • Rebar Cage
  • Concrete
  • Glossary

What is a drilled shaft?

Drilled Shafts and their uses
A Drilled Shaft is a deep foundation that is constructed by placing fluid concrete in a drilled hole. Structures can be supported by a variety of foundations. The selection of the foundation system is generally based upon several factors, such as:

  • Loads to be imposed
  • Site subsurface materials
  • Special needs (high lateral capacity, etc.
  • Cost

Drilled shafts (also called caissons, drilled piers or bored piles) have proven to be a cost effective, excellent performing, deep foundation system, that is utilized world-wide. Typically they are used for bridges and large structures, where large loads and lateral resistance are major factors.

Advantages

  • Economics
  • Minimizes pile cap needs
  • Slightly less noise and reduced vibrations
  • Easily adaptable to varying site conditions
  • High axial and lateral loading capacity
Disadvantages
  • Extremely sensitive to construction procedures
  • Not good for contaminated sites
  • Lack of construction expertise
  • Lack of Qualified Inspectors

pier detail

End Bearing
Drilled shafts can be designed as "End Bearing" meaning the load is carried by the base or "end" of the shaft.

Friction
Shafts design for having their load dissipated throughout the materials they are formed into are called "Friction" shafts. The site subsurface soils the shaft are installed into "grab" the sides of the shaft, much like when you step in mud and try to pull your foot out.

Drilled Shaft Construction Methods

This [section] contains information on the three methods of drilled shaft construction.

Each of these methods is different and has their own potential problem areas and applicability. It is important for the Inspector to have an understanding of each of these processes to facilitate inspection of the shafts during construction.

What is a dry shaft?

A shaft excavation that can be excavated to its designed depth without the need for slurry or casing.

The dry construction method is used at sites where the ground water level and soil and rock conditions are suitable to permit construction of the shaft in a relatively dry excavation. and where the sides and bottom of the shaft may be visually inspected by the Engineer prior to placing the concrete.

The dry method is by far the least expensive method for drilled shaft construction. Given the choice of drilling methods, Contractors will try the dry method even in soil or rock of dubious quality.

Dry construction is generally defined by an amount of water accumulation permitted over a specified time period.

  • In place Soil/rock will keep the hole walls from collapsing.
  • Construction of the shaft can be in relatively dry conditions.

Dry Shaft Construction Process

The dry method consists of drilling the shaft excavation, removing accumulated water and loose material from the excavation, placing the reinforcement cage, and concreting the shaft in a relatively dry excavation.

dry shaft construction process

What What is a Wet Shaft

Often called the "slurry-method", wet shaft construction is when a slurry or water is used to keep the hole stable for the entire depth of the shaft.

When Used

  • When a "dry" excavation cannot be maintained
  • When In-place soil/rock is unstable and deforms or collapses
  • When loose material and accumulated water cannot be removed

Wet vs. Dry

  • Wet is more expensive
  • Wet requires more Contractor expertise
  • Wet requires more equipment Wet is when there is more than 12" of accumulated water in the bottom of the shaft (typically)
  • Wet precludes visual inspection of the bottom of the shaft by the Inspector

Wet Shaft Construction Process

Unlike the dry construction method, in this situation the water table may be above the shaft tip elevation or the geology consists of unstable or "caving" soils. Think of trying to dig a hole at the beach or lake near the water's edge. The hole stays open until you reach or get just below the water table or waterline. Then what happens? It collapses.

Well the same goes for drilled shafts excavated below the water table or in unstable soils. During the drilling of the hole, a slurry is introduced that "stabilizes" the sides of the hole or casing is installed and prevents the soils from collapsing into the hole.
Upon reaching the designed shaft tip elevation, the hole is cleaned, then the rebar cage placed.

wet construction process
Unlike the dry shaft method, the concrete is being placed "under the water" and therefore a tremie is lowered into the hole and the concrete placed through the tremie, which is carefully removed a little at a time to avoid "breaching" the concrete.

Types of Wet Shaft Construction
There are two types of "wet" shaft construction:

The Static Process The Circulation Process
Drilled down to the piezometric level Hole is drilled
Slurry introduced Slurry level maintained at the ground surface
Cuttings are lifted from the hole Cuttings and sand, is circulated to the surface, where it is cleaned and reintroduced down the hole.

 

What is Slurry


What Does the Slurry Do?
  • Maintains a Stable Borehole Prior to Concreting
  • Maintains High Effective Stresses in the Soil while the Hole is Open (Retard Softening or Loosening)
  • Facilitates Removal of Cuttings in "Circulation Drilling"

Slurry is the fluid introduced into the excavation to assist in maintaining hole stability. Generally, three basic types of "slurries", Mineral, Polymer and Water, are employed in drilled shaft construction.

In some instances, though not recommended, a blended slurry, consisting of mineral and polymer slurries is employed.

Mineral Slurry
Mineral Slurry is made from naturally occurring clay minerals.
Natural mineral clays: Bentonite, attapulgite and sepiolite

Bentonite slurries have been used commonly in drilled shaft construction in the United States since the 1960's. Other processed, powdered clay minerals, notably attapulgite and sepiolite, have been used on occasion in place of bentonite, usually in saline ground water conditions. However, Bentonite is the most common Bentonite and other clay minerals, when mixed with water in a proper manner, form suspensions of microscopic, plate-like solids within the water. This suspension, in essence, is the drilling slurry. If the fluid pressures within the slurry column in the borehole exceed the fluid ground water pressures in a permeable formation (e.g., a sand stratum), the slurry penetrates the formation and deposits the suspended clay plates on the surface of the borehole, in effect forming a membrane, or "mudcake" that assists in keeping the borehole stable.

Polymer Slurry
Polymers are semi-synthetic or totally synthetic chemical slurries.
Drilling slurries can also be made of mixtures of chemicals called polymers and potable water. Polymers have been used in preference to bentonite in well drilling for some time in soil profiles that contain considerable clay or argillaceous (clay-based) rock, because bentonite slurries have a tendency to erode clayey rocks and to produce enlargements and subsequent instabilities in the boreholes. Polymer slurries require less conditioning before reuse than bentonite slurries and can be disposed of more inexpensively than bentonite slurries.

It is also important that polymers be kept out of contact with cement as much as possible during the construction process, since cement will cause the polymer to agglomerate.

Water
Water is used in some areas as the drilling fluid, in lieu of mineral or polymer slurry. In certain geologic conditions, water when combined with the naturally occurring subsurface materials creates it own "slurry".


Generally, the use of water must be approved by the Engineer.

A misconception by many is that because water is being used, slurry testing is not necessary. However, many local specifications mandate that if water is used, it must still meet certain slurry properties and the only way to determine the specific properties values is to test.

In some instances, though not recommended, a blended slurry, consisting of mineral and polymer slurries is employed.


What is a Cased Shaft?


The casing method is often used either when shown on the plans or at sites when construction methods are inadequate to prevent hole caving or excessive deformation. In this method the casing may be either placed in a predrilled hole or advanced through the ground by twisting, driving or vibration before being cleaned out. Casings and liners play an important role in the construction of drilled shafts, and special attention must be given to their selection and use.

Casings are tubes that are relatively strong, usually made of steel, and joined, if necessary, by welding. Liners, on the other hand, are light in weight and become a permanent part of the foundation. Liners may be made of sheet metal, plastic, or pressed fibers. While their use is much less frequent than that of casings, liners can become important in some situations.

Common situations where casing is used are:

  • In generally dry soils or rocks that are stable when they are cut but which will slough soon afterwards. In such a case the borehole is drilled, and casing (a simple steel pipe) is quickly set to prevent sloughing.
  • When there is a clean sand below the water table underlain by a layer of impermeable limestone or low permeability clay into which the drilled shaft will penetrate. In this case, since the overlying sand is water bearing, it is necessary to seal the bottom of the casing into the underlying rock/soil to prevent flow of water and caving of soils into the borehole.
Types of Casing
  • Temporary Casing : Temporary casing is used to retain the sides of the borehole only long enough for the fluid concrete to be placed. The temporary casing remains in place until the concrete has been poured to a level sufficient to withstand ground and groundwater pressures. The casing is removed after the concrete is placed. Additional concrete is placed as the casing is being pulled to maintain the pressure balance. Thereafter, the fluid pressure of the concrete is assumed to provide borehole stability.
  • Permanent Casing : The use of permanent casing is implied by its name; the casing remains and becomes a permanent part of the foundation. An example of the use of permanent casing is when a drilled shaft is to be installed through water and the protruding portion of the casing is used as a form. A possible technique that has been used successfully is to set a template for positioning the drilled shaft, to set a permanent casing through the template with its top above the water and with its base set an appropriate distance below the mudline, to make the excavation with the use of drilling slurry, and to place the concrete through a tremie to the top of the casing.

Cased Shaft Construction Process

  1. Drill : Hole is advanced using slurry through the caving soils.
  2. Case : Casing is then installed through the caving soils and drilling continues to desired depth.
  3. Clean : Slurry and cuttings removed from the hole.
  4. Position : Rebar cage is positioned in the hole.
  5. Place : Concrete is placed. If temporary casing, casing slowly withdrawn as concrete level in hole rises.

cased shaft construction process

 

Shaft Excavation

The Inspector has a variety of functions to perform during the shaft excavation process. From verifying the shaft is located in the proper place to verifying the shaft meets the cleanliness requirements upon completion of excavation, the Inspector needs to document construction events.

Shaft Location and Alignment

  • Is the shaft being located at the correct plan location indicated on the plans?
  • Typically there will be a plan tolerance which the Contractor must achieve.
  • Is the kelly bar plumb? This is critical as there are tolerances for axial alignment that the Contractor must achieve.

Excavation


If the Drilled Shaft Plan specified the use of casing and or slurry, the Inspector must verify and document its use.

On many projects, a "surface casing" will be temporarily installed to stabilize the surface soils during the construction process. The constant in and out of the hole with drilling tools can quickly degrade the surface soils conditions if not protected.

The Inspector needs to be concerned with, in general, the following.

  • Documenting the type of drilling tool and its diameter, and condition. Also remember to record its length, as the Inspector needs this to add to the kelly bar to determine depths.
  • Documenting the length, diameter and type of any casing used.
  • If slurry is used, verifying and documenting that the required sampling and testing is performed.
  • Maintaining, in the required format, a log of the material excavated. Typically, there will be forms for Rock Coring, Soil/Rock Excavation and possibly others.
  • Document, Document, Document. Job site photographs are a very valuable form of documentation.

Slurry Testing


Slurry needs to be maintained properly, as discussed earlier in, if it is to be effective. Typically, the specifications for a project will specify the type and number of tests to be performed on the slurry. The most common tests are:

  • Viscosity- also know as Marsh Funnel Test, is the test used to measure the flow rate or consistency of slurry.
  • Mud Balance Test- also known as the Mud Density Test, is used to measure the density of the slurry.
  • pH Test- used to determine the alkalinity and acidity of the slurry.
  • Sand Content Test- used to determine the sand content of the slurry. Generally the specifications have a maximum allowable percentage of sand permitted.

The lower cap is lower to the desired depth and the tube then lower on the cable to that depth. The top cap is allowed to slide down thereby trapping slurry at the sample depth.

Shown here is the sand washed and collected from slurry at the final step of the Sand Content Test. The percentage of sand is read from the graduated glass vial.


Shaft Cleaning

Depth Verification and Cleaning
During shaft excavation, the Inspector estimates the bottom of shaft depth by noting the depth marks on the kelly and adding the length of the particular tool to it, the sum of which provides the total depth. Upon achieving the desired shaft tip elevation and following cleaning of the shaft bottom, the Inspector needs to verify the depth and cleanliness.

Generally, cleanliness requirements will be specified and are typically based upon the amount, or thickness, of sediment permitted on the bottom of the shaft.

In making this determination, the Inspector uses a weight tape and takes "soundings" at numerous locations (normally 5) around and in the center of the shaft. These are recorded on the specified form. This should be done as soon as possible, as the longer the hole is open, the greater the potential for problems.

Rebar Cage Fabrication and Positioning

Drilled shaft foundations are constructed with a rebar cage inside to provide for strength and stability. The rebar cages are constructed to meet the needs of the design, both in rebar size and number required.

The Inspector must verify that the cages are fabricated, lifted and positioned properly and are within the allowable tolerances for "top of cage elevation" after positioning.

Quite often, post installation integrity testing will be specified and the access tubes for performing these test are part of the cage assembly.

Remember, it is imperative that the hole be clean and this should have been verified by the Inspector before the rebar cage is installed.

rebar pier cage

Cage Fabrication

The Inspector must verify that the cages are constructed in accordance with the plans and specifications, which includes verification of:

  • Bar size
  • Number of bars and condition
  • Type and percentage of ties
  • Diameter and length
  • Couplers/splices
  • Spacers and Standoff

Cage Lifting and Positioning

Following fabrication of the cage, the Contractor will then lift the cage and lower it into the shaft.

Remember that prior to cage placement the Inspector verified the shaft depth and cleanliness.

It is important that the Contractor properly support the cage during lifting to avoid bending the cage so much that it is permanently distorted. If distorted to much, it won't fit down the shaft without damaging the shaft walls.

Typically the cage will have standoffs on the bottom to maintain a certain clearance from the bottom of the hole and spacers on the outer edges to maintain a specified distance from the shaft walls.

This space between the shaft walls and the cage is to provide for the specified "concrete coverage".

Once positioned in the shaft, the top of the cage is to be within a specified tolerance of the elevations shown in the plans.

In summary, the Inspector needs to verify and document:

  • Lifting of the cage
  • Positioning of the cage
  • Top of cage elevation
  • Couplers/splices

The photograph above shows the Inspector observing the lifting of the cage and the photo to the right, a cage lifted and ready to be placed in the hole.

This photograph shows the cage being lowered into the hole. Notice that the standoffs and the side spacers are used to maintain the proper "coverage".

Here is a photograph of the cage after being positioned in the hole. The Inspector needs to verify and document the "top of cage" elevation and if it is within the specified allowable construction tolerance.

Access Tubes


Post installation integrity testing of drilled shafts has become very popular throughout the country. More economical than conventional load tests, some of the methods used provide a "picture" so to speak of the shaft in the ground.

To perform these types of test, access tubes, which permit lowering of instrumentation down into the shaft, must be installed on the cage prior to placing the cage in the hole.

The Inspector must verify and document that the tubes are of the length, diameter, and material specified, together with verifying they are secured to the cage and straight in accordance with the the project plans.

Shown here is an access tube inside the cage. Normally, they are installed on the inside of the cage, which helps protect them from damage.

Note the cap on the tube- this prevents debris or concrete from getting into the tube, which can prevent the instrumentation from going down the tubes.

Determining Circumferences


Determining circumferences is one of the math computations the Inspector must be proficient in performing.

Typically the number of side spacers that help maintain the proper coverage distance, as discussed earlier in this Chapter, are generally determined by the cage circumference. The plans or specifications will typically indicate a certain number of spacers, based upon inches of circumference of the cage, be placed per level.

pier formulas

Circumference is the length of the outer boundary (perimeter) of a circular object.

 

Concreting Operations


Concreting of the shaft is the final step in the construction process itself. Up until this time, the Contractor has been willing to spend time with the Inspector but often this changes once the concrete is on the way. There are generally time limits, slump requirements, etc., a whole host of issues or potential problems that can occur during this phase. Remember, if concreting goes bad, the shaft is lost and everything the Contractor has done up until this point is essentially lost.

The Inspector needs to perform their duties promptly and efficiently. Speed is of the essence, but do not sacrifice quality and thoroughness.These duties may, depending upon the specifications, include performing standard field concrete tests, monitoring concrete placement and development of the placement curves.

Concrete Type and Slump

When the concrete arrives on-site, the Inspector may be required to verify the proper mix design is being delivered, that it meets the slump requirements and perform standard field tests. Typically there will be a time limit imposed by the specifications relating to the length of time for concrete placement.

Remember, it is imperative that the hole be clean and this should have been verified by the Inspector before the rebar cage was installed.

Typical concrete field tests the Inspector may be required to perform include:

  • Slump
  • Air Content
  • Temperature
  • Test Specimens
Below is the typical slumps specified for drilled shaft concrete. slump image

Concrete Placement Methods

A variety of methods or techniques are used by the Contractor to place the concrete. This selection generally depends upon the type of shaft construction being used and the most common one.


Tremie Placement - Gravity-fed tremie placement is generally used for wet shaft construction. In this method, the concrete is introduced into the hole, starting at the bottom, using a water tight tremie (tube). The concrete is fed by pump or bucket into the tremie and falls by gravity and continuously placed until the shaft is full.

Pump-line Placement - This method is similar to the tremie method except that the concrete is "pumped" into the hole, rather than gravity fed. (A pump-line can be used to feed concrete to a tremie in tremie placement).

Free-Fall - In this method, the concrete is placed by free-falling from the top of the shaft to the bottom and is typically used for dry shaft or dry cased shaft construction only. Of importance with this method is that the concrete must be directed to free-fall down the center of the cage and not make contact with the cage or shaft walls. The specifications will often specify the maximum distance concrete may free-fall.

Concrete Placement Process

The goal of concrete placement is to get the shaft filled with the specified concrete and have no voids or sediment/debris inclusions that effect the structural integrity of the shaft.

During placement by tremie or pump-line, the discharge end is placed near the bottom of the hole and concrete flow started. The concrete, as it rises and fills the shaft, displaces the sediments.

During the pour, whether by tremie or pump-line, the concrete flow must be continuous and the discharge end of tremie or pump-line must be immersed in the concrete a specified distance, typically 5 ft. (1.5 m). If not, and the discharge end breaches (raised above the concrete flow level) the shaft is rejected. The tremie is raised as the concrete level rises, but the required immersion distance maintained.

The placement continues until fresh concrete overflows the top of the shaft.

Adhesion The property of a substance (in our case, cohesive soil) to "stick", "cling", or "adhere" to a solid structural element such as a concrete pier or pile, and thus establish a resistance to shearing movement between the soil mass and the structural element.
ADSC Association of Drilled Shaft Contractors (The International Association of Foundation Drilling Contractors),
P. O. Box 75228, Dallas, TX 75228.
Aggregate The stone used in making concrete. "Fine aggregate" is sand; "coarse aggregate", gravel or gravel-size crushed stone.
Air Lift A device used to clean material from the bottom of a fluid-filled shaft, usually constructed using an open-ended steel pipe into which compressed air is injected near the bottom in an upward direction.
Allowable Load The load which cannot be exceeded without incurring (in the opinion of the designer) risk of damaging structural movement.
Anchor Pier A pier designed to resist uplift or lateral forces
Artesian Water Subsurface water underlying a confining bed which has sufficient pressure to rise above existing ground (or water surface) when encountered in cased holes during drilling.
Attapulgite A clay mineral consisting of complex magnesium aluminum silicates. It occurs naturally near Attapulgus, Georgia where it is mined as Fuller's earth. Also made into commercial drilling mud useful in salt or brackish water environments.
ASTM American Society for Testing and Materials
Auger A helical rotary tool for drilling a cylindrical hole in soil and/or rock.
Axial Load That portion of the load on a pier or pile which is in the direction of its axis.
Backfill A bucket-like tool for removing water from the hole during drilling or in preparation for concrete placement.
Bailing Bucket A bucket-like tool for removing water from the hole during drilling or in preparation for concrete placement.
Batter Angle with the vertical, normally expressed as a ratio of horizontal to vertical (i.e., 1:4= 1 horizontal to 4 vertical).
Bearing Stratum A soil or rock stratum that is expected to carry the drilled shaft load (either by end bearing or by sidewall friction, or by a combination of the two).
Bell Enlargement of the lower end of a shaft excavation, to increase the bearing area of the drilled shaft (Also called "underream").
Belling Bucket
Underreaming Bucket
  
A drilling bucket tool with expanding cutters that can enlarge the bottom of the drilled hole, to form a bell or underream. See Bucket Auger, Drilling Bucket.
Bentonite The mineral, sodium montmorillonite, a highly expansive colloidal clay; the basis for a type of commercial
Boulder A rock, usually rounded by weathering and abrasion, greater than 200 mm in size.
Bucket Auger (or Drilling Bucket) A cylindrical rotary drilling tool with a hinged bottom containing a soil cutting blade; spoil enters the "bucket" and is lifted out of the hole, swung aside, and dumped by releasing the latch on the hinged bottom.
Cage Reinforcing bars preassembled for quick placing in a drilled shaft.
Cake (Filter Cake) A layer of clay or clayey soil, built up on the wall of a boring drilled with slurry (drilling mud, bentonite, etc.), having the effect of forming an impermeable lining to prevent (or diminish) loss of water from the hole, and maintain slurry pressure against the wall of the hole.
Calcarenite Mechanically deposited carbonate rocks consisting of sand size carbonate grains (1/16 to 2 mm diameter)
Calcilutite Refers to a rock composed of more than 50% silt and clay size carbonate particles.
Calyx (or Shot) Barrel A core barrel without hard-metal cutting teeth, with which the rock is cut (or ground up) by chilled steel shot which roll and are ground up under the rotating steel edge of the barrel.
Capillarity The upward movement of water, due to effects of wetting and surface tension, that occurs through the very small void spaces that exist in a soil mass.
Carbonate Rocks Rocks composed of more than 50% by weight, of carbonate minerals.
Casing An open-end steel pipe installed by drilling, driving or vibrating; to support the wall of a hole; to seal out groundwater; or to protect the concrete of the shaft from contamination by sloughing of the sides of the hole.
Caving (or Sloughing) A soil that tends to fall into an uncased hole, during or after the drilling. Usually a cohesionless soil.
Changed Conditions Job conditions, which differ, substantially from conditions as represented in the plans and specifications, and/or the contract documents.
Chert A hard, dense microcrystaline sedimentary rock, consisting chiefly of interlocking crystals of quartz. It may contain amorphous silica (opal). Chert occurs principally as nodular or concretionary segregations, or nodules, in limestone and dolomite, and less commonly as layered deposits, or bedded chert. The term flint is equally synonymous.
Clay A mineral particle of any composition having a diameter less than 0.002 mm.
Cleanout Bucket A cylindrical tool used for removing "cuttings" from the shaft bottom. The bucket typically has a bottom that opens up when turned clockwise and closes when turned counterclockwise.
Coarse-Grained Soil The soil types which have particles large enough to be seen without magnification. The coarse-grained soils include the sand and gravel (or larger) soil particles.
Cohesion The bonding or attraction between particles of certain fine-grained soils that enhances shear strength and is independent of confining pressure.
Cold Joint Surface where concrete placement was interrupted then later resumed.
Concrete Pump A truck mounted pump specially designed to transfer fluid concrete through lines (hoses and pipes) to deliver ready mix to locations not readily accessible otherwise.
Continuous Flight Auger A string of helical augers and a cutting head, used to bore a hole in the earth, into which a pile section may be set, concrete cast in place, or tieback grouted.
Coquina A soft, porous limestone made up largely of shells, coral, and fossils cemented together.
Core Barrel A cylindrical rock-drilling tool, designed to cut an annular space around a central cylindrical core of rock, which can then be removed to classify the material or in the case of a drilled shaft removed to deepen the hole.
Crane Carrier A specially built truck for mounting a drill rig or for carrying a crane.
Crowd The soil types which have particles large enough to be seen without magnification. The coarse-grained soils include the sand and gravel (or larger) soil particles.
Cuttings Particles of soil or rock resulting from the cutting action of drilling or augering a hole. See also Spoil.
Dense Compact
Desander A specially designed piece of equipment consisting of a series of screens and hydrocyclones which remove sand and silt particles from the slurry used in constructing a fluid-filled excavation.
Dewatering (1) The removal of water from a construction area, as by pumping from an excavation or location where water covers the planned working surface. (2) Lowering of the groundwater table in order to obtain a "dry" area in the vicinity of an excavation which would otherwise extend below water.
Diatomaceous Earths Silts containing large amounts of diatoms-the siliceous skeletons of minute marine or freshwater organisms
Dolomite A carbonate rock composed of more than 50% by weight, of the mineral dolomite.
Drawdown Lowering of the level of groundwater; for example, when a work area is dewatered for construction.
Downdrag A downward force exerted on a drilled shaft, pile, or other structural element by settling soil. Sometimes called "negative skin friction".
Drilled Pier/Drilled Shaft A reinforced or unreinforced concrete foundation element formed by drilling a hole in the earth and filling it with concrete. Also called a "caisson", or a "large-diameter bored pile".
Drilling Bucket A closed rotary boring tool with its cutting edge at its base. Spoil is removed from the bucket by lifting it out, swinging it to one side of the hole, and releasing the hinged bottom of the bucket.
Drilling Mud, Mud,
or Slurry
A fluid mixture of water and clayey soil, or commercial "driller's mud" which may be bentonite or attapulgite.
Elastic Movement Movement under load which is recoverable when the load is removed.
"Elephant's Trunk" A collapsible conduit of fabric or plastic which, when coupled to the bottom of a concrete hopper, directs the concrete to a point near the center of the reinforcing cage to prevent concrete from striking the cage or the sides of the shaft.
End Bearing The portion of load carrying capacity a shaft or pile has due to the end area bearing on the material below.
Extractor A device for pulling piles or casings out of the ground. It may be an inverted steam or air hammer with yoke so equipped as to transmit upward blows to the pile body, or a specially built extractor utilizing this principle. Vibratory hammers/extractors may be especially effective.
Fill Any man-made soil deposit. Fills may consist of soils that are free of organic matter and that are carefully compacted to form an extremely dense, incompressible mass, or they may be heterogeneous accumulations of rubbish and debris.
Fine-Grained Refers to silt and clay-sized particles which exist in a soil.
Fixed-Head Pier A pier whose top, when deflected laterally with application of lateral force, is so restrained that the pier axis at the top must remain vertical during such movement.
Friction/End-bearing Pier A pier that achieves support from the combination of side friction and tip (end) bearing.
Friction Shaft A pier that derives its resistance to load by the friction or bond developed between the side surface of the pier and the soil or rock through which it is placed.
Fuller's Earth Soils having the ability to absorb fats or dyes. They are usually highly plastic, sedimentary clays.
Full-Scale Load Test A load test made on a full-scale shaft or other structural element, with the load carried at least to the structural design load, and preferable to twice (or more) the design load.
Geomaterial Material (soils, rock, clays, silts, etc.) underlying the surface.
Geotechnical Engineer An engineer with specialized training and knowledge of structural behavior of soil and rocks, employed to do soil investigations, to do design of structure foundations, and to provide field observation of foundation investigation and foundation construction.
Grains Discrete particles larger than 0.074 mm. They may form the rock framework, similar to sand grains in a sandstone, or they may be subordinate to smaller particles in the rock.
Grain Size A term relating to the size of grains. (See above)
Gravel Small stones or fragments of stone or very small pebbles larger than the particles of sand, but often mixed with them. Generally 4.76 to 75mm in size. (Stones 75 to 300 mm are usually called "cobbles".
Ground Loss Subsidence of surface of ground adjacent or close to a shaft excavation, caused by soil moving into the excavation laterally during drilling, or during dewatering after drilling is complete. Common in soft organic soils or clays, and cohesionless soils below the water table.
Groundwater Level A shallow pit, excavated adjacent to a boring location, used to contain drilling mud (slurry) during drilling.
Hardpan A term that should be avoided by the engineer. Originally, it was applied only to a soil horizon that had become rocklike because of the accumulation of cementing minerals. The name implies a condition rather than a type of soil.
Head Shortened form of the phrase "pressure head", referring to the pressure resulting from a column of water or elevated supply of water.
Hollow-Stem Auger An earth auger with an end bit on a hollow center shaft.
Hydraulic Pump The hydraulic pump is the same and performs the same functions as the electric submersible pump except it is hydraulic.
Impervious Impervious soil is soil in which the spacing of the soil particles is so close as to allow only very slow passage of water. For example, movement of water through a typical clay (an "impervious" soil) may be only 1/1,000,000 as fast as through a typical sand.
Kelly bar (or Kelly) The kelly bar transfers the rotary and pull-down force to the drilling tools. The kelly bar is also used to raise and lower the tools in the shaft. It may be solid or hollow with two or more bars telescoping inside each other. The ability of the bar to telescope, allows excavation to greater depths than the boom height would otherwise allow.
Laitance A fluid mixture of water, cement, and fine sand that appears at the top of concrete soon after pouring
Lateral Load That portion of load that is horizontal, or at 90E to the axis of a pier or pile, or of the supported structure.
Limestone A carbonate rock composed of more than 50%, by weight, of the mineral calcite.
Load Cell A device for measuring the pressure exerted between the soil (or rock) and a structural element (e.g., the bottom or side of a pier); used with a hydraulic or electrical indicating or recording instrument at ground surface.
Matrix The natural material in which any fossil, pebble, crystal, etc., is embedded.
Micrograined A grain-size term pertaining to carbonate particles smaller than 0.0625 mm and larger than .004 mm diameter.
Mud See Drilling Mud
Mud Pit A shallow pit, excavated adjacent to a boring location, used to contain drilling mud (slurry) during drilling.
Mudding-In The technique of stirring soil and water by and auger; sometimes with the addition of commercial "driller's mud", to form a slurry as the hole is advanced by auger drilling.
MultipleUnderreams Additional underream cut in a bearing soil, at elevations above the bottom underream, to force shearing resistance in the soil into a larger peripheral surface.
Moisture Content The reduction in diameter in a section of a drilled shaft.
Natural Moisture Content Moisture content in-situ, at the time of measurement or investigation. May be subject to seasonal variation
Necking The reduction in diameter in a section of a drilled shaft.
Negative Skin Friction Effect of settling soil that grips a pile or pier by friction and adds its weight to the structure load. Also called Downdrag.
NX Core Rock core taken with an "NX" core barrel, which cuts a core 60mm in diameter.
Oolite Small spherical or subspherical carbonate accretionary grain generally less than 2.0 mm in diameter.
Over Reaming Enlarging the diameter of the shaft to remove any slurry cake build up
Piezometric Head (See Artesian Pressure)
Plasticity Term applied to fine-grained soils (such as slays) which when moist can be remolded without raveling or breaking apart.
Rebar A bar of reinforcing steel.
Reverse Circulation A counterflow method of circulating drilling fluid and spoil in a drill hole. In the direct circulation method, drilling fluid is pumped down a hollow drill pipe, through the drill bit, and back to the surface in the annular space around the drill pipe; and the cuttings are carried to the surface by the flow. In the reverse-circulation or counterflow system, drilling fluid is pumped out of the drill stem at the top circulated through a pit where cuttings are removed, and returned to the annular space around the drill stem. Circulation is upward inside the drill stem and downward outside it.
Rig, Drilling Rig A machine for drilling holes in earth or rock.
Rock A naturally occurring mineral substance cohesively bound by chemical bonds and forming the basic structure of the earth's crust.
Rock Auger An auger-type drilling tool, equipped with hard-metal teeth to enable it to drill in soft or weathered rock.
Rock Socket That portion of a shaft, which penetrates into a rock formation beneath less competent overburden.
Rotary Boring A method of boring using rotary (as opposed to percussive) means of excavation.
Rotary Drill Rig A rotary drilling machine powered hydraulically, pneumatically, electrically or mechanically to bore exploratory holes or for installation of drilled shafts, caissons, or in-situ piles. The equipment may use a continuous-flight auger or a rotary table and Kelly bar with various attachments and tools to perform the work.
Sand Cohesionless soil whose particle sizes range between 0.074 and 4.76 mm in diameter.
Seepage Small quantities of water percolating through a soil deposit or soil structure.
Segregation Separation of poured concrete into zones of coarse aggregate without fines, and sand-water-cement without coarse aggregate.
Settlement (1) The amount of downward movement of the foundation of a structure or a part of a structure, under conditions of applied loading. (2) The downward vertical movement experienced by structures or soil surface as the underlying supporting earth compresses.
Shaft Inspection Device (S.I.D.) The shaft inspection device is an instrument that allows the inspector to see the bottom of the drilled shaft. It has a video camera that is lowered to the bottom of the drilled shaft. It can also measure the thickness of sediment on the bottom of the shaft and sample sidewall soils.
Sidewall Grooving The cutting of circular or spiral grooves in the walls of a drilled shaft hole in rock or soil, with the objective of improving sidewall support.
Sidewall Shear Frictional resistance to axial movement of a pier or pile, developed between the soils surrounding the shaft and the peripheral surface of the shaft. (Does not include resistance to movement of an enlarged base, due to development of shearing strains within the soil below the base).
Silt A fine-grained nonplastic soil; often mistaken for clay, but quite different in its behavior. (Particle sizes ranging from 0.002 to 0.074 mm).
Skin Friction Resistance to shearing motion between the concrete of the shaft and the soil or rock in contact with it.
Slurry See Drilling Mud
Soil Auger The soil auger is used for cutting and removing the soil from the shaft volume. It typically has several flights of 30 degrees or less.
Sonotube A cylindrical form of treated cardboard, for forming round columns of concrete; a commercial product
Spacers Spacers are used to keep the steel cage centered in the drilled shaft and insure proper concrete cover. The spacers should be concrete wheels o other approved non-corrosive spacing.
Spoil Soil or rock removed from an excavation; to be wasted or used elsewhere as fill.
Squeezing Ground A soil formation, usually of clay, silt, or organic material, which tends to bulge or squeeze into the hole during drilling, or afterward if the hole is left uncased.
Standard Penetration Test (SPT) (N) The number of blows required to drive a 2-inch O.D., 1-3/8 inch I.D., 24-inch long, split soil sampling "spoon" 1 foot with a 140 pound weight freely falling 30 inches. The count is recorded for each of three 6-inch increments. The sum of the second and third increments is taken as the N value in blows per foot. (This is ASTM Designation D 1586).
Strain Gauge An instrument or device for measuring relative motion (compression, elongation, or shear) between two points in a mechanism or in a structural member such as a drilled shaft
Swelling Soil A soil subject to volume increase caused by wetting, oxidation, buildup of crystals, or relaxation after load removal.
Telltale A strain indicator, usually comprised of a sleeved free-standing rod cast in place in a drilled pier or pile to measure relative movement between the anchored (embedded) tips of two or more rods or between the rod anchor and the top of the pier or pile.
Template A fixed template is required during all excavation and concreting operations when drilling from a barge. This is to maintain shaft position and alignment. A template is not required on land if the contractor can satisfactorily show that he can maintain proper position and alignment without it.
Temporary Casing Casing left in place until concrete has been placed, or casing placed as protection for workmen or inspector.
Test Hole With the test hole, the contractor must demonstrate that his construction methods will work. A test hole is typically the same size as the shafts to be constructed.
Tremie (1) (verb)To place concrete below water level though a pile, the lower end of which is kept immersed in fresh concrete so that the rising concrete from the bottom displaces the water without washing out the cement content. (2) (noun) The hopper and drop pipe used to place the concrete underwater.
Tremie Pipe The tremie pipe is used to place concrete in the drilled shaft. In shafts constructed by the wet method, the tremie pipe must extend to the bottom of the drilled shaft. In shafts constructed by the dry method, the tremie pipe must extend to within five feet of the shaft bottom. The tremie pipe serves several purposes. It transports the concrete through the slurry. It keeps the concrete from segregating during placement. Also, it helps keep the concrete from mixing with the drilling slurry at the slurry/concrete interface.
Twisting Bar A tool attached to the kelly, used for "screwing" down casing through caving or squeezing soil. Sometimes used for pulling casing.
Underream Enlargement of the lower end of an augered or drilled pier hole to increase its bearing area. Also called "bell".
Underreamer, Belling Tool See Belling Bucket.
Unit Weight The weight per unit volume of a material such as soil, water, concrete, and so on. Typically expressed as pounds per cubic foot, rams per cubic centimeter, or kilograms per cubic meter.
Uplift An upward force exerted on a pier, pile, or other structural elements, by expanding soil or rock, hydraulic pressure, or structural loading.
Vibratory Driver/Extractor A pile-driving and extracting machine which is mechanically connected to a pile or casing and loosens it while driving or pulling by oscillating it through the soil. Power source may be either electric or hydraulic.
Vug A small cavity in a vein or in rock.
"Walking Off" Tendency for a rotating bit to deflect laterally when encountering harder, deflecting layer of rock or irregular surface.
Water Content The ratio of the quantity (by weight) of water in a given volume of soil mass to the weight of the soil solids, typically expressed as a percentage.
Water Table The subsurface elevation at which free water will usually be present. Also called "groundwater".

 

 

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