Drilled pier and caisson deep foundations field guide

What a drilled shaft is

A drilled shaft is a large-diameter hole bored down to a firm bearing layer, fitted with a steel rebar cage, and filled with concrete so the finished column carries the structure deep into the ground. The trade uses several names for the same thing. Drilled pier, drilled shaft, drilled caisson, and bored pile all describe a cast-in-place concrete shaft built in a hole the rig drills, and the name on your job usually comes from the region and the spec writer more than from any real difference in the work.

It is the heavyweight of deep foundations. A single shaft can carry far more load than a spread footing or a screw pile, which is why drilled shafts sit under bridge piers, high-rise columns, transmission towers, and the heavily loaded columns of big buildings. One large shaft often replaces a pile cap full of smaller driven piles, so the footprint shrinks and the cap simplifies.

The whole reliability of a drilled shaft rests on three things, and every section of this guide circles back to them. The hole has to stay open and not cave. The bottom has to be clean so the end bearing is real. And the concrete has to go in without mixing with soil, water, or slurry. Miss any one and you have a defect buried in the ground where you cannot see it. For where drilled shafts sit among spread, strip, and mat footings, see the foundation types and footings guide. For the screwed-in alternative on lighter loads and tight access, see the helical pier and screw pile guide.

How do drilled shafts work?

A drilled shaft carries load two ways at once, and the split between them depends on the soil and the depth. End bearing is the base of the shaft pressing on firm soil or rock at the tip, the same way the bottom of a footing bears on the ground beneath it. Skin friction, also called side resistance, is the load shed into the soil along the sides of the shaft through friction and adhesion over its full length. Add the two and you have the shaft's capacity.

Which one dominates is a design call, not a habit. A shaft socketed into sound rock leans on end bearing, because the rock at the tip is far stronger than the soil along the sides. A long shaft through deep firm clay or dense sand can carry most of its load in skin friction before the tip does much work at all. Most production shafts use both, and the engineer's analysis sets how much each contributes.

The load path runs from the column, into the rebar cage and concrete, down the shaft, and out into the ground through the sides and the base. That path only works if the concrete is sound the whole way down and the base is bearing on firm material instead of the loose cuttings that fall to the bottom of every hole. The mechanism is simple. Protecting it during construction is the hard part.

When are drilled shafts used?

Drilled shafts earn their place when the loads are heavy and the good ground is deep. A column carrying thousands of kips, a bridge pier in a river, a tower fighting wind and uplift, a building where the bearing stratum sits forty or eighty feet down under soft soil. In all of these a spread footing near the surface cannot reach strong enough ground, and the shaft is what drives the load down to where the soil or rock can hold it.

They also suit jobs where vibration or displacement is a problem. Drilling removes spoil instead of pushing soil aside, so a drilled shaft does not shake adjacent structures the way a driven pile can, and it can be installed close to existing foundations without the ground heave a displacement pile causes. The cost is the spoil itself, which has to be handled and hauled, and the care the cast-in-place process demands.

The shaft also carries uplift and lateral load well. A large-diameter reinforced shaft resists overturning and tension through the weight of the concrete, the skin friction working in reverse, and the bending stiffness of the cage and section, which is why towers and sign structures so often sit on a single drilled shaft. Whether a shaft beats a pile group or a mat on a given job is the geotechnical and structural engineer's call, set by the boring logs and the loads, not by what the crew ran last week.

Size: large diameter, deep, and the bearing area

The defining feature of a drilled shaft is diameter. Production shafts commonly run from about 2 ft to 12 ft, and special rigs and conditions push beyond that, into shafts large enough to drive a truck into. The diameter is what gives the shaft its end bearing area and its bending stiffness, so a column that would need a cluster of small piles can land on one big shaft instead.

Depth follows the geology. A shaft reaches whatever depth the boring logs say the firm stratum sits at, then often penetrates into it, a rock socket or an embedment into dense soil that develops the side and base resistance the design needs. Shallow shafts run twenty or thirty feet. Deep ones run well past a hundred. The engineer sets diameter and depth together to deliver the capacity with margin.

The single large shaft against many small piles is the trade-off that drives the choice. One shaft means one cap connection, less pile-cap concrete, and a smaller footprint, which matters in tight sites and constrained right-of-way. The catch is that a single shaft is a single point of dependence, so its integrity has to be verified rather than assumed. With a pile group, one weak pile shares its load with neighbors. With one shaft under a column, there is no neighbor to pick up the slack. The specifics of diameter and depth are the engineer's to set, not the crew's to round off.

Drilling the hole: augers, buckets, and rock tools

The hole is cut by a rotary drilling rig turning a tool on a kelly bar, and the tool changes with the ground. A flight auger is the common choice in soil. It fills with cuttings as it turns, then the rig lifts it out and spins it off to throw the spoil clear before going back down. In wet or flowing ground, a drilling bucket does the work instead. It is a cylinder with a cutting base and a hinged bottom that scoops up the cuttings and carries them out of the hole, which keeps the excavation under control where an open auger would lose material.

Rock needs heavier tooling. A rock auger fitted with hard teeth chews weathered and soft rock, and for sound rock a core barrel cuts an annular kerf and breaks out a plug, leaving a clean socket wall. Cutting a rock socket is slow and hard on tooling, and it is where schedules slip, so the boring logs that predict the rock matter as much to the bid as to the design.

Spoil handling is part of the operation, not an afterthought. The cuttings come out wet, sometimes mixed with slurry, and they have to be moved away from the hole so they do not fall back in and so the rig has room to work. On a tight urban site the spoil logistics can govern the production rate more than the drilling itself.

How do you keep the hole from caving?

A drilled hole is an open excavation with nothing holding the walls except the ground itself, and whether it stands open or collapses decides which construction method you use. There are three, and the soil and the water table pick between them. The dry method relies on the ground to stand on its own. The casing method uses a steel pipe to hold the walls. The slurry method fills the hole with a fluid that holds the walls with pressure. Many shafts use a combination, a length of casing through the upper soil and slurry below, or casing set through water with a dry hole beneath.

Get this wrong and the failure is immediate and expensive. A hole that caves before the concrete goes in fills the bottom with soil, ruins the bearing, and can trap soil in the shaft as a defect. A hole that squeezes shut around the cage stops the placement. The method is a design and means-and-methods decision the engineer and the contractor make from the boring logs and the groundwater, and a competent driller reads the ground as it comes out and changes method when the hole tells them to.

The rule under all three methods is the same. The wall has to stay where it was drilled until the concrete has replaced whatever was holding it open. Everything else in drilled-shaft construction is built around protecting that one condition.

The dry method

The dry method, also called the open-hole method, drills the shaft, sets the cage, and places the concrete in an open hole with nothing holding the walls but the soil. It is the simplest and cheapest way to build a shaft, and it works only where the ground cooperates. That means firm cohesive soil or rock that stands open on its own, above the water table, with no caving or significant inflow during the time the hole is open.

The condition people get wrong is the water. A hole that stands open dry can still take on groundwater that seeps in through the walls and pools at the bottom, and a few inches of water at the base is enough to wash the concrete and ruin the end bearing. If water comes in faster than it can be pumped and the bottom kept clean, the hole is not a dry hole anymore, whatever the plan said, and the method has to change.

When it does apply, the dry method gives the cleanest result. The driller can see the bottom, inspect it directly, clean it with confidence, and place the concrete by free fall down a clean dry hole. The verification is easy precisely because there is no slurry or water in the way.

The casing method

The casing method sets a steel pipe in the hole to hold back caving soil or water while the work goes on inside it. Temporary casing is the common case. The driller advances or drives the casing through the unstable upper soil, drills out the inside, sets the cage, and then pulls the casing back out as the concrete rises, so the concrete itself takes over holding the wall as the steel comes up. The casing can also telescope, a larger diameter set first and a smaller one nested below it to seal off successive layers.

Pulling the casing is the moment that makes or breaks the shaft, and it has to be timed against the concrete. The concrete inside the casing has to stay high enough and fluid enough that when the steel lifts, the concrete pressure pushes out against the soil and water faster than they can collapse in. Pull too early, pull too fast, or let the concrete stiffen first, and soil or water squeezes in behind the casing and you get a neck or an inclusion right where the casing was. This is a real failure mode, and it is why the concrete supply has to be continuous while the casing comes out.

Permanent casing stays in the ground. It is specified where the spec calls for it, commonly in corrosive ground, through open water, or where a cavity or very soft zone would not hold concrete against the soil. Whether the casing is temporary or permanent, and how it comes out, is set by the engineer and the spec.

What is the slurry method?

The slurry method, also called the wet method, keeps the hole full of a fluid while drilling so the fluid pressure holds the walls open against caving and groundwater. The driller fills the hole with slurry, drills through it, sets the cage down through it, and then places the concrete by tremie from the bottom up, displacing the slurry out the top as the concrete rises. It is the method for caving, water-bearing soil where neither a dry hole nor casing alone will hold, which covers a large share of real ground.

Three slurry types are used, and the difference matters in the field. Mineral slurry is bentonite clay mixed with water, the original drilling fluid in use since the 1960s, which builds a filter cake on the wall and holds by its density and that cake. Polymer slurry is a water-based synthetic that holds the wall differently, needs less processing to reuse, and is often cheaper to dispose of, which is why it has spread across many soil types. Plain water serves only where the formation is stable enough to hold with head alone.

The slurry only works if the head is kept up. The fluid level in the hole has to stay above the groundwater table so the net pressure pushes out on the wall, not in, and if the level drops the wall can collapse on the cage or the placement. Slurry also has to be tested and cleaned, because spent slurry heavy with sand will not hold the wall and will not let clean concrete displace it. Density, viscosity, pH, and sand content all have limits in the spec, and they are checked, not assumed.

Cleaning the bottom

A soft bottom kills end bearing, and there is no fixing it after the concrete is in. Every drilled hole collects loose cuttings, sloughed soil, and settled sediment at the base, and if that muck stays there, the shaft bears on a cushion of debris instead of the firm material the design counted on. The shaft then settles more than it should, the load sheds onto the sides in ways the analysis did not assume, and the problem is buried.

So the bottom gets cleaned before the cage goes in. In a dry hole the driller can muck it out and inspect it directly, sometimes by lowering an inspector or a camera. Under slurry or water you cannot see the bottom, so cleaning is done with a cleanout bucket, an airlift, or a pump, and verified indirectly by sounding the base, checking the slurry sand content, and measuring against the design depth. ACI 336.1 gives tolerances for how clean and how level the base has to be.

This is the step that gets rushed when the schedule is tight, and it is exactly the wrong one to rush. A hole that took a day to drill can have its end bearing thrown away in the last twenty minutes by a crew eager to set the cage and pour. Clean the bottom, sound it, and confirm it against the spec before anything else goes down the hole.

The belled or under-reamed base

A bell is an enlarged base at the bottom of the shaft, cut wider than the shaft itself to spread the load over more bearing area. The driller swaps the auger for a belling tool, an under-reamer with arms that fold out as the tool turns and carve a flared cone at the base. A typical bell flares out to two or three times the shaft diameter, so a 3 ft shaft might end on a 7 or 8 ft base, multiplying the end bearing without drilling a wider hole the whole way down.

Bells only work where the soil holds the shape. They need a firm cohesive soil or soft rock that will stand as a cut overhang long enough to set the cage and place the concrete, which rules out sand, soft clay, and any caving or water-bearing ground. You cannot bell a hole that needs slurry to stand, because the overhang will not hold and you cannot inspect or clean the flared base under fluid.

When the soil suits a bell, it can save shaft length and rebar by getting the bearing area from a wider base instead of a deeper socket. Whether to bell, and how big, is the engineer's call from the soil and the loads. It is not a default.

The rebar cage

The rebar cage is the steel that gives the shaft its tension, bending, and shear capacity, since concrete alone handles compression but little else. It is built as a cylinder of longitudinal bars running the length of the shaft, wrapped or tied with spiral reinforcement or hoops that hold the longitudinals in place and confine the core. The cage is usually assembled on the ground and lifted into the hole as one piece, or spliced in sections as it goes down on deep shafts.

Concrete cover is the detail that protects the steel, and it has to be held all the way around. Centralizers, often plastic or concrete wheels or skids fastened to the outside of the cage, keep it centered in the hole so the concrete forms a continuous cover over the bars instead of letting the cage drift against the wall, where soil could contaminate the steel and corrosion could start. Drilled shafts call for more cover than a beam or a column, because the concrete is placed against soil, not formwork.

Lifting the cage in without racking it is its own skill. A long slender cage wants to bend and the spiral can distort if it is picked wrong, so the crew uses a pick plan and stiffening to keep it true going down the hole. Splices have to develop the bar, and the splice length and detail come from the structural drawings. The cage also has to land at the right elevation and stay there, held off the bottom and tied so the rising concrete does not float it or push it down. If integrity testing by crosshole sonic logging is required, the access tubes get tied to the inside of the cage before it goes in the hole, because there is no adding them later.

The concrete

Drilled-shaft concrete has to flow and consolidate without vibration, because you cannot get a vibrator down a deep hole around a cage and through slurry. So the mix runs high slump, often in the 7 to 9 in range, or it is a self-consolidating concrete that flows and fills under its own weight. Compressive strengths commonly fall in the 4,000 to 6,000 psi range for structural shafts, but the mix design belongs to the spec and the engineer.

The mix has to stay workable for the whole pour. A shaft is placed in one continuous operation that can run an hour or more on a deep hole, and the concrete placed first has to still flow when the last truck arrives, so the set time and the slump retention are designed for the placement, not just the strength. A mix that stiffens early traps slurry, leaves the cage half-covered, or refuses to displace cleanly.

How the concrete goes in depends on the method. A clean dry hole can take free-fall placement down the center, dropped so it does not hit the cage or the wall on the way down and segregate. A hole with slurry or water at the bottom cannot be poured into open air, because the concrete would wash and mix with the fluid. That hole gets the tremie.

Tremie placement under slurry or water

The tremie is how concrete goes into a wet hole without contaminating it. A tremie is a smooth steel pipe, commonly 10 to 12 in in diameter, with a hopper on top, run down to the bottom of the hole. The first concrete goes in behind a plug or a closed end that keeps it separated from the slurry, then the concrete builds up from the bottom and pushes the slurry up and out ahead of it. The fresh concrete displaces the fluid from the bottom up instead of falling through it.

The one rule that governs tremie work is keeping the tip embedded. The bottom of the tremie pipe has to stay buried in the fresh concrete the whole time, usually by several feet, so the concrete always exits inside the mass already placed and never falls through slurry or water. As the concrete rises the crew lifts the tremie and removes sections to keep the embedment in the right window, deep enough to stay sealed but not so deep the concrete cannot flow out.

Pull the tip out of the concrete, even once, and slurry or water rushes up the pipe and into the next concrete that comes down, leaving a contaminated layer across the full section of the shaft. That is a defect that cuts the shaft in two, and it is unforgiving. Keep the tip embedded, keep the concrete coming, and never let the tremie run empty or break the seal.

Contamination, necks, and soft inclusions

The signature failure of a drilled shaft is concrete that did not stay clean and continuous, and it takes a few forms. Contamination is concrete mixed with slurry, water, or soil, leaving a weak zone or a layer of laitance and trapped fluid across the section. A soft inclusion is a pocket of soil or slurry caught in the concrete, often because the tremie lost its seal or the slurry was too dirty to displace. A neck is a constriction where soil squeezed in and reduced the concrete cross-section, often from pulling casing wrong or losing slurry head.

What ties them together is that they all happen during placement and they are all invisible once the concrete sets. The shaft looks finished from the top. The defect is forty feet down where nobody can see it, and the structure above is counting on a section that is not there. This is why the placement discipline matters more than almost anything else in the work. The concrete is the structure, and the few hours of the pour are when the structure is made or ruined.

The defenses are the ones already named. Keep the hole open so soil does not fall in. Keep the bottom clean so the base is sound. Keep the tremie embedded and the concrete continuous so no fluid gets trapped. Then test the finished shaft, because no amount of care during the pour is proof on its own.

Placing the concrete continuously

A drilled shaft is placed in one continuous pour from the bottom to the top, with no planned stop. A delay lets the concrete already in the hole begin to set, and the next concrete then will not bond to it or displace cleanly, leaving a cold joint or a contaminated band across the shaft. So the concrete supply is lined up before the pour starts, with enough trucks staged that the placement never has to wait on the plant.

The cage has to stay put through the pour. Rising concrete can float a light cage up or, if the tremie or free-fall stream hits it wrong, push it down, so the cage is tied and held at elevation and watched as the concrete comes up. A cage that moves during the pour ends up with the wrong cover or the wrong embedment, and on a slurry shaft a shifted cage can foul the tremie.

This is why a drilled-shaft pour is a committed operation. Once the concrete starts, it does not stop until the shaft is full and overpoured at the top to push the weakest, most contaminated concrete up out of the structural length. Plan the pour so nothing interrupts it, because the interruption is the defect.

How are drilled shafts tested?

Because the shaft is cast in the ground where nobody can see it, integrity testing after the pour is how you confirm the concrete is sound, and several methods do it. Crosshole sonic logging, CSL, is the common one on important shafts. Access tubes are tied to the cage before it goes in, and after the concrete cures, probes are lowered down pairs of tubes to send sound between them at many depths. Where the concrete is sound the signal travels fast and strong. A slow or weak signal between tubes, commonly a wave-speed reduction beyond about 10 percent, flags a possible defect at that depth for further investigation.

Thermal integrity profiling, TIP, reads the heat the curing concrete gives off, measuring temperature down tubes or on wires cast into the shaft. A full, well-formed section cures warm and even. A neck, an inclusion, or a thin cover spot runs cooler, and the temperature profile shows it, with the added benefit that TIP sees the concrete outside the cage where CSL cannot. Sonic echo and pulse echo testing, PIT, tap the shaft head and read the reflected stress wave to estimate length and find gross defects, a quicker but coarser check often used to screen shafts that have no access tubes.

No single method catches everything, and they can disagree, so the spec sets which tests are required and how the results are judged. The point of all of it is the same. The shaft is buried and unrepeatable, so it gets verified rather than trusted, and the engineer decides what the data means and whether the shaft is accepted, repaired, or replaced.

Load testing

Where the design or the AHJ calls for it, a load test confirms the shaft actually carries what the analysis says it will, by loading a real shaft and measuring how it responds. A static load test pushes against a reaction frame or kentledge and reads the settlement under increasing load, the direct way following ASTM D1143 for axial compression. It is the reference test, and it is also the most expensive and slowest to set up, so it shows up on large or critical projects more than on routine ones.

The Osterberg cell, the O-cell, gets around the reaction problem by casting a hydraulic jack into the shaft itself. Pressurized, it pushes the base down and the shaft up at the same time, so the two halves react against each other and you read the end bearing and the skin friction separately, without a frame overhead. Bidirectional testing makes very high capacities testable that a top-down frame could never reach. Dynamic testing reads the shaft's response to a high-energy impact and back-calculates capacity faster and cheaper, with more interpretation involved.

Load testing is verification of capacity, where integrity testing is verification of the concrete. A shaft can be sound and still not carry the design load if the soil is weaker than the boring suggested, and a shaft can hit the load and still hide a defect that shortens its life. Which tests a job needs, and how many, is the engineer's call against the spec and the AHJ.

Who designs the shaft

The diameter, the depth, the reinforcement, the bearing capacity, and the construction method are designed by the geotechnical and structural engineer, and they are not numbers a crew picks in the field. The geotechnical engineer reads the borings, identifies the bearing stratum, and sets the side and base resistance the soil and rock can supply. The structural engineer sizes the shaft and the cage to carry the column load down that path with margin. The two work from the same boring logs, and the design is only as good as those borings.

This matters on drilled shafts more than on most foundations because the loads are large and the shaft is buried and singular. There is no spread footing you can over-build cheaply and no pile group where a neighbor covers a weak member. The capacity rests on soil the engineer characterized from a few holes across the site, so the borings, the design, and the field verification are one chain, and the crew's job is to build exactly what was designed and to flag the moment the ground does not match the logs.

When the ground that comes out of the hole does not match the boring, the work stops and the engineer is told, not worked around. A bearing stratum that shows up deeper than expected, a cavity, a seam of soft material, water where the design assumed none. Each of those changes the design, and the call belongs to the engineer, who may extend the shaft, change the method, or revise the cage. Building past a surprise to keep the schedule is how a shaft ends up under-designed for the ground it actually sits in.

Drilled shafts vs helical and driven piles

All three are deep foundations, and the difference is how the load gets into the ground and how the element is built. A drilled shaft is cast in place: a hole drilled, a cage set, and concrete poured, with no vibration and high capacity but spoil to handle and a buried pour to verify. A driven pile is a steel, concrete, or timber section hammered or vibrated into the ground, which displaces the soil and develops capacity as it drives, fast and proven by driving resistance but noisy, with vibration that can disturb neighbors and structures. A helical pier is a steel shaft with helical plates screwed into the ground, lighter capacity, installed with small equipment in tight access and verified by installation torque, with no spoil and no concrete cure.

The choice follows the load and the site. Drilled shafts own the heaviest loads and the cases where vibration is unacceptable or the bearing is in deep rock. Driven piles suit large pile groups where production speed wins and vibration is tolerable. Helical piers win on lighter structures, underpinning, fast load-on, and constrained sites where a rig cannot fit. They overlap in the middle, and the engineer picks from the borings and the loads. For the screwed-in option in detail, see the helical pier and screw pile guide.

One framing helps on the jobsite. The drilled shaft trades the speed and the no-spoil of the other two for raw capacity and a vibration-free install, and pays for it with a cast-in-place process that has to be protected and verified. That trade is exactly why it sits under the heaviest columns and why its quality control is so strict.

Groundwater and the method

Groundwater is the single condition that most often decides the construction method, because water in the hole undermines both the wall and the bottom. Water above the base means the dry method is off the table unless the inflow is slow enough to pump and keep the bottom clean, which it rarely is for long. Water that flows into the excavation carries soil with it, caving the wall, and it pools at the base where it washes the concrete.

The answer is casing or slurry. Casing set through the water-bearing zone and sealed into firmer material below can give a dry working hole inside it. Slurry held above the groundwater table holds the wall by pressure and lets the concrete go in by tremie under the fluid. Which one fits depends on the soil, the depth of the water, and the spec, and the call sits with the engineer and the contractor. On some sites dewatering the area is part of the plan, and the interaction between dewatering, the slurry head, and the surrounding ground has to be thought through, since dropping the water table can settle nearby soil. For controlling water in an excavation more broadly, see the dewatering guidance in the related concrete guides.

What does not work is pretending the water is not there. A hole logged as dry that takes on water during the pour will ruin the bottom and the concrete unless the method already accounts for it, so the water is planned for before the rig moves in, not discovered at the bottom of the hole.

What the inspector checks

Cast-in-place deep foundations get special inspection, and the inspector watches the steps you cannot verify after the fact. The hole comes first: the diameter, the depth against the design, the bearing material at the bottom matching the boring logs, and the bottom cleaned and sounded. Then the cage: size, length, spacing, splices, cover and centralizers, and that it is set at the right elevation and held there. Under slurry the inspector also checks the slurry properties, density, viscosity, pH, and sand content, against the spec limits before the concrete goes in.

Concrete volume is the inspector's best window into a buried defect. The volume actually placed gets logged against the theoretical volume of the hole, and the comparison tells a story. A serious underrun, less concrete than the hole should hold, points to a neck or a void where soil squeezed in and took up space the concrete should fill. A large overrun points to caving or overbreak, the hole eating more concrete than its design diameter, which is its own warning about wall stability. The volume curve plotted against depth as the pour rises is one of the few real-time signals that something went wrong down the hole.

IBC Chapter 17 addresses special inspection of cast-in-place deep foundation elements, and the project's statement of special inspections sets exactly what is observed and recorded on a given job. The exact provisions and which standard is referenced shift between code editions, so confirm them against the adopted edition and the AHJ. The inspector is the second set of eyes on a process that cannot be redone, and on a buried, unrepeatable element that role carries real weight.

The shaft log and the records

Every drilled shaft should leave a record complete enough to defend the foundation years later, because the shaft itself is buried and the log is all anyone can check. The shaft log captures the as-built reality of one hole: the location and the date, the diameter and the final depth, the bearing material found at the bottom, the construction method and any casing or slurry used, the cage size and elevation, the concrete mix and the volume placed against the theoretical, and the test results that accepted the shaft.

The volume record deserves its own discipline. Logging concrete placed against theoretical at intervals as the pour rises is the field's running check for a neck or a void, and that data only protects you if it is captured truck by truck while the pour happens, not reconstructed afterward from delivery tickets. The same goes for the slurry test readings and the bottom soundings: they are worth recording the moment they are taken, with the depth and the time, because they are the evidence the bottom was clean and the wall was held when the concrete went in.

A field tool like FieldOS is built for exactly this kind of as-built capture, logging each shaft with its depths, soundings, volumes, photos, and the engineer's sign-off attached to the shaft, time-stamped and in one place instead of scattered across notebooks and phones. The engineer's acceptance and the integrity and load test results belong in the same record, so when a question comes up about a column that settled or a foundation under load, the proof that the shaft was built and verified as designed is there to pull, not gone.

Where drilled shafts go wrong

The failures that put a drilled shaft at risk are a short, repeating list, and they all trace back to the three conditions that make a shaft sound. They are worth naming together because most of them are preventable with discipline rather than equipment.

A hole left to cave with no casing or slurry fills the bottom and traps soil in the shaft. A dirty bottom that nobody cleaned throws away the end bearing the shaft was designed around. A tremie tip pulled out of the concrete lets slurry or water cut a contaminated band across the section. A concrete underrun against theoretical that nobody investigated hides a neck or a void. A shaft placed with no integrity test that nobody required goes into service unverified. And a shaft built without the engineer designing it, or built past a surprise in the ground without telling the engineer, sits under a heavy column on assumptions the soil never confirmed. Each of these is buried the moment the concrete sets, which is why the work is built to prevent them, not to find them later.

Common mistakes

• Drilling in caving or water-bearing soil with no casing or slurry, so the hole collapses before or during placement.
• Setting the cage and pouring on a dirty bottom, throwing away the end bearing the shaft was designed for.
• Pulling the tremie tip out of the fresh concrete, letting slurry or water contaminate the section.
• Stopping the pour partway and creating a cold joint, or letting the concrete stiffen before the casing is pulled.
• Logging a concrete underrun against theoretical and pouring on without investigating the neck or void it signals.
• Skipping integrity testing on a shaft the spec required it on, or treating a flagged anomaly as noise.
• Building from a sketch with no geotechnical and structural design, or working past a surprise in the ground without telling the engineer.

Field checklist

What to document

The shaft log is the foundation's defense, so it captures the as-built facts of each hole while the work is in front of you. The table below is the working set most specs ask for, with why each item earns its place in the record.

Standards and references

The design starts with the geotechnical and structural engineer working from the site borings, and no standard replaces that. The borings set the bearing stratum and the soil resistance, and the engineer sets the diameter, depth, reinforcement, capacity, and method. Treat the numbers in this guide as how the work is commonly done, and let the engineer's design, the project spec, and the AHJ control the specifics on your job.

For construction practice, ACI 336.1, the specification for the construction of drilled piers, covers the hole, the cleaning tolerances, the slurry, the cage, and the concrete placement, and ACI 318 governs the structural concrete and the reinforcement. The ADSC, the association of drilled shaft contractors, publishes drilled-shaft practice widely used in the trade, and the FHWA drilled-shaft manuals are the reference on highway and bridge work. These describe means and methods. They do not override the project spec when it is stricter.

For inspection and testing, IBC Chapter 17 addresses special inspection of cast-in-place deep foundation elements, and the project statement of special inspections sets what is observed and recorded. Integrity and load test methods follow their own standards, including ASTM D1143 for static axial compression load testing. Code editions change and they are adopted and amended by jurisdiction, so confirm the adopted edition, the referenced standards, and any local amendments with the AHJ before you cite them. The constants under all of it are three: keep the hole open and the bottom clean, place the concrete continuously by tremie under slurry, and integrity-test the shaft while the engineer designs it.

Units and terms

Drilled-shaft work carries several names for the same things, and the terms shift between the soils report, the structural drawings, and the spec. The definitions below are the ones that come up on the jobsite.

Drilled pier / drilled shaft / caisson / bored pile

All names for a large-diameter cast-in-place concrete shaft drilled to a firm bearing layer and reinforced with a steel cage

End bearing

Load carried by the base of the shaft pressing on firm soil or rock at the tip

Skin friction (side resistance)

Load shed into the soil along the sides of the shaft through friction and adhesion over its length

Dry, cased, and slurry methods

The three ways to hold the hole open: an open hole in stable ground, a steel casing, or a fluid that holds the wall by pressure

Slurry (mineral / polymer)

Drilling fluid that holds the wall, either bentonite clay (mineral) or a water-based synthetic (polymer)

Tremie

A smooth steel pipe, commonly 10 to 12 in, used to place concrete from the bottom up under slurry or water without contamination

Belled / under-reamed base

An enlarged base cut wider than the shaft to spread end bearing over more area, where the soil holds the shape

Integrity testing / CSL

Post-pour tests for buried defects, including crosshole sonic logging (CSL), thermal integrity profiling, and sonic or pulse echo

Rebar cage

The cylinder of longitudinal bars and spiral reinforcement that gives the shaft tension, shear, and bending capacity

Related tools

Calculators and readiness checks for this work

Compare your options

FAQ

People also ask

• What is a helical pier?
• What is a footing?
• What is a driven pile?
• Why do you evacuate an AC system?
• What is Type X drywall?