Helical pier and screw pile foundations field guide

What a helical pier is

A helical pier is a steel shaft fitted with one or more helical plates that is screwed into the ground until the plates reach firm bearing soil, where they carry the structure in compression or tension. The trade also calls it a screw pile, a helical pile, or a helical anchor when it works in pure tension. The shape is the whole idea. The plates act like a screw thread biting into the earth, so the machine turns the shaft and the pier pulls itself down to the depth where the soil is strong enough to hold the load.

What sets it apart from a footing or a driven pile is that you learn the capacity as you install. The harder the soil fights the plates, the more torque it takes to keep turning, and that torque correlates to how much load the finished pier will hold. So the crew gets a running read on capacity instead of waiting on concrete to cure or a pile-driving rig to reach refusal. That correlation is real, but it belongs to the engineer and the manufacturer, not to a rule someone carries in their head.

This guide covers how the pier carries load, how the torque proves the capacity, when the system earns its place, and what has to be designed, monitored, and recorded for it to stand. For where helical piers sit among spread footings, strip footings, and other deep foundations, see the foundation types and footings guide. For keeping the structure they support dry below grade, see the below-grade waterproofing guide.

How do helical piers work?

A helical pier carries load two ways, and on most piers both are working at once. The helical plates bear on the soil beneath them, the same way the base of a footing or the tip of a driven pile presses on bearing soil, so each plate transfers a share of the load into firm strata. The shaft also picks up some load in skin friction along its length, though on a slender shaft that share is small and the plates do the real work.

Turn it over and the same pier resists tension. Pull up on the shaft and the plates push against the soil above them instead of below, so the pier fights uplift with the bearing strength of the ground over the helices. That is why the same hardware underpins a settling house one week and anchors a guyed tower or a solar array against wind uplift the next.

The mechanism is bearing on firm soil through the plates. Everything else, the shaft size, the plate count, the depth, follows from getting enough plate area onto strong enough soil to carry the design load with margin. Miss the bearing stratum and no amount of torque on a shaft spinning in soft ground gives you a foundation.

What is torque-to-capacity?

Torque-to-capacity is the relationship that lets a crew verify a helical pier's capacity from how hard it was to install. As the plates advance into stronger soil, the torque needed to keep the shaft turning rises, and that final installation torque correlates to the ultimate load the pier will hold. The common form is Qult equals Kt times T: ultimate capacity equals a torque correlation factor, Kt, multiplied by the average final installation torque.

The Kt factor is not a number you guess. It depends on the shaft type and size, the helix configuration, and whether the load is tension or compression, and the manufacturer publishes a value for each product, backed by load testing in its ICC-ES evaluation report. For a 1.5 to 1.75 in solid square shaft the default in ICC-ES Acceptance Criteria 358 is around 10 per foot, while larger round shafts run lower, roughly 7 to 9 per foot, because the factor falls as the shaft grows. Use the value for the exact pier you are installing, from the engineer and the report, and treat the rule of thumb as triage only.

The correlation is empirical, not a law of physics. With a calibrated in-line torque transducer and correct technique it commonly lands within about 10 to 15 percent of a load test, and a hydraulic pressure gauge can be off 20 percent or more. The published Kt can also underread capacity at low torque and overread it at high torque. So the torque proves the pier against the engineer's design, and where the design or the AHJ calls for it, a load test confirms the relationship on the actual site.

The helical plates, lead section, and extensions

The helical plates are the part that carries the load, so the diameter and number of plates set the bearing area, which sets the capacity for a given soil. Plates commonly run 6 to 16 in in diameter and are welded to the shaft at a true helix pitch so the pier advances one pitch per turn without augering and disturbing the soil it needs to bear on.

A pier is built from a lead section and extensions. The lead section is the bottom piece that carries the helices, single-helix for some jobs and multi-helix when the design needs more bearing area, with plates stepped in diameter, smaller at the tip and larger above, so each one cuts into ground the one below already loosened. Extensions are plain shaft sections coupled on above the lead to push the helices deeper until they reach bearing. A 10/12/14 lead, for instance, carries a 10 in, a 12 in, and a 14 in plate up the shaft.

Plate count and diameter are an engineering decision, not a field swap. More plate area raises capacity in a given soil but also raises the installation torque, and a multi-helix pier needs the plates spaced far enough apart, commonly about three helix diameters, so they act as separate bearing plates instead of one big plug. The engineer picks the configuration off the soil profile and the load.

Depth to bearing: the soil decides, not the tape

You do not install a helical pier to a fixed depth. You install it to bearing, which is the depth where the plates reach soil firm enough to produce the design torque, and the soil decides where that is. On one pier of a row it might be 12 ft. On the next it might be 22 ft because the bearing layer dips. Both are right if both hit the torque.

This is the habit that separates a foundation from a hole. A crew that screws every pier to a set depth off the drawing, then stops, has proven nothing about capacity. The drawing's depth is an estimate from the soil report, useful for ordering shaft and extensions, not a target to stop on. You stop when the torque says the plates are in bearing, then you add extensions and keep going if they are not.

Tension piers carry their own depth rule on top of that. A helical anchor pulling in uplift has to set its plates well below the surface, commonly 5 to 12 helix diameters deep, so there is enough soil over the helices to resist pullout and so seasonal moisture and frost at the surface do not reduce the hold. The engineer sets the minimum depth, the torque confirms the bearing, and both have to be met before the pier is accepted.

When are helical piers used?

Helical piers shine on a handful of jobs where their advantages line up. The first is underpinning a settling foundation, which is the single most common use, because they install through tight basements and crawl spaces and load the repair immediately. The second is new foundations on poor soil, where firm bearing sits below a soft upper layer and a screwed pier reaches it without excavation or spoil. The third is restricted access, including low-headroom interiors and sites a drill rig or a pile hammer cannot reach, because the drive head runs off a mini-excavator or even a handheld machine.

Speed is the fourth. There is no concrete to cure, so the structure can load the pier the same day, which matters on fast-track work and on emergency stabilization. Helical piers handle light to moderate loads comfortably, and they carry heavy loads too when the design calls for larger shafts, more plates, or more piers in a group.

Where they fade is very soft, deep soil with no bearing layer to reach, and very high single-point loads that would need an impractical number of piers. Those are the cases where a drilled shaft or a driven pile may win, and they belong to the engineer's judgment, not a default.

Underpinning a settling foundation

Underpinning is the job helical piers do best. When a footing has settled because the soil under it lost strength, was never compacted, or dried and shrank, helical piers carry the structure down to soil that has not moved. The crew excavates a small pit at each pier location beside the footing, installs the pier to the design torque, then connects it to the footing with a bracket that wraps under the footing and transfers the load to the pier.

Once the piers are in and the brackets are set, the load comes off the failed soil and onto the piers. The contractor can usually hold the structure where it is to stop further settlement, and on many jobs lift it back toward level by jacking against the piers, within what the structure can take without cracking. Lift is a judgment call, not a guarantee. Old brittle finishes and masonry only move so far before they crack again, and the engineer sets how much lift to attempt.

This is repair work, so the diagnosis comes first. The reason the foundation moved governs the fix, and a soils report or a geotechnical engineer's read on the bearing stratum is what tells you the piers will reach ground that will hold. Pier the symptom without finding the cause and the building can keep moving in a new place.

The bracket to the structure

The bracket is the connection that moves load from the structure into the pier, and on a repair it does the work the original footing can no longer do alone. An underpinning bracket is a steel assembly that seats against and under the existing footing and bolts to the top of the pier, so the building's weight runs through the bracket, down the shaft, and into the helices in bearing. On a lift job the bracket is also the point you jack against to raise the structure before locking it off.

New construction uses a cap or bracket too, sized to the column, grade beam, or wall it supports, so the load lands on the shaft concentrically. Eccentric load on a slender shaft invites bending, so the detail keeps the load over the pier.

The bracket is an engineered, manufactured part matched to the shaft, not a shop weldment improvised on site. It is rated for a capacity in the manufacturer's report the same way the shaft is, and the connection has to be at least as strong as the pier it serves. A pier that reaches full torque means nothing if the bracket under the footing is undersized or the bearing surface against the concrete was never cleaned and seated.

Tension anchors, tiebacks, and uplift

Run a helical pier in reverse and it becomes a tension anchor. Instead of bearing down, the plates bear up against the soil above them and resist a pull, which makes the same hardware a guy anchor for towers, a tie-down for solar racking and light structures in wind, and a tieback for retaining walls and shoring. The torque-to-capacity correlation works for tension just as it does for compression, with its own Kt for the load direction.

Depth matters more in tension than in compression. A pier pushing down has the whole soil column below it to lean on, but a pier pulling up only has the soil above the helices, so the plates have to be set deep enough, commonly several helix diameters below grade, to develop the uplift capacity and to stay below the zone where seasonal moisture and frost loosen the ground.

For solar and other wind-driven structures the controlling case is often uplift and overturning, not gravity, so the anchor is sized for the pull. The same engineering applies: the configuration and depth come from the design, the torque confirms it during install, and a tension load test verifies it where the project or the AHJ requires.

Helical piers in new construction

Helical piers are not just a repair tool. On new work they carry decks, room additions, light commercial buildings, modular structures, boardwalks over wetlands, and solar fields, anywhere the upper soil is poor or the schedule will not wait on concrete. Because the install makes no spoil and almost no vibration, they suit sites where you cannot truck out drill cuttings or disturb what is next door.

The speed is the draw. A deck or an addition can sit on piers and start framing the same day, with no footing excavation, no forms, and no cure time before the load goes on. On a wetland boardwalk or a remote site, hand-portable and small-machine installs put a foundation where a concrete truck cannot go.

The trade-off is that new helical foundations still need the same design discipline as a repair. The engineer sizes the piers to the column and wall loads off the soil report, the crew installs to torque and records it, and the AHJ inspects the deep foundation like any other. The fact that a deck pier goes in fast does not make it an unengineered post in a hole.

Helical piers vs driven and drilled piles

Against driven piles and drilled shafts, helical piers win on disruption and on knowing the capacity as you go, and lose on raw capacity per pier. Driven piles are hammered or vibrated in, which is fast over a large area and densifies the soil, but the noise and ground vibration can damage neighbors and rule them out in tight urban sites. Drilled shafts, cast by drilling a hole and filling it with rebar and concrete, reach great depth and large diameter, but they generate spoil to haul off and need cure time before they carry load.

Helical piers make no spoil, run at low vibration and noise, and load immediately because there is nothing to cure. The torque gives a real-time capacity read that neither driven nor drilled piles offer during install. For light to moderate loads, tight access, and fast turnaround, that package is hard to beat.

Where the others win is high load and deep soft soil. A single large drilled shaft can replace a cluster of piers and simplify the cap, and driven piles cover big footprints quickly. The choice is the engineer's, made on the soil profile, the loads, the site constraints, and cost, not on which system a contractor happens to own.

The soil and the geotech control feasibility

The soil decides whether helical piers work at all, so the geotechnical picture comes before the pier design. What matters is the soil profile with depth, where the firm bearing stratum sits and how deep you have to go to reach it, and whether anything in between stops the pier. A subsurface investigation or a geotechnical engineer's report is what answers that, and on anything past a small residential repair you want it in hand.

Two soil facts kill or complicate a job. The first is no reachable bearing layer: very soft or loose soil all the way down gives the plates nothing to bear on and the shaft to buckle in, so the site may need a different foundation. The second is early refusal, where shallow rock, cobbles, or fill stop the pier before it reaches design depth, which can be fine if the torque is there and a problem if a single boulder blocks one pier in a row.

Soil corrosivity belongs to the geotech read too, because it sets the design life of the steel in the ground. The soil decides feasibility, the depth, and how long the pier lasts, and none of that is a field judgment made off the back of the truck.

The engineer designs the pier

A structural or geotechnical engineer designs the helical pier system, and that is not optional on real work. The engineer sets the shaft size, the helix configuration, the design capacity, the minimum installation torque, and the minimum depth, working from the soil report and the structure's loads. The crew's job is to install to that design and prove it with torque, not to invent the capacity in the field.

The manufacturer's ICC-ES evaluation report is the other half of the design basis. It lists the allowable shaft and bracket capacities, the Kt torque factors, the helix areas, and the limits the product was tested to, and the engineer designs within it. A pier installed without an engineer's design and a matching product report is a guess wearing steel.

This is where the call gets hedged, on purpose. The capacity, the Kt, the depth, and the torque target are the engineer's and the manufacturer's territory, confirmed by the AHJ through permit and inspection. A contractor who reads a torque off the gauge and declares a capacity without the engineer's design behind it has skipped the step that makes the number mean anything.

Monitoring and recording installation torque

Monitor and record the installation torque on every pier, not a sample. The torque is the proof of capacity, so a pier with no torque record is a pier with no proven capacity, full stop. The crew reads torque continuously during the drive and logs the final torque over the last increment of penetration, because the value at bearing is what the Kt factor multiplies.

How you read it matters. A calibrated in-line torque transducer between the drive head and the shaft gives the most reliable number and keeps the correlation tight. A differential hydraulic pressure gauge on the drive motor is common and convenient but less accurate, so where the design leans on the torque, the more accurate instrument is worth it. Either way the instrument has to be calibrated, and the calibration is part of the record.

Capture each pier's final torque against its location, the depth reached, the shaft and helix configuration, and the calculated capacity, while the crew is still on it. A field platform like FieldOS keeps that log per pier with photos and a timestamp, so the engineer's sign-off and the inspector's review run off one record instead of a clipboard that may not survive the drive home. The pier you cannot show the torque for is the pier you will be back to load test.

The load test

A load test is the physical proof that a pier holds its design load, and it is where the torque correlation gets verified against the actual site instead of the manufacturer's tables. The test jacks the pier against a reaction frame, applies load in steps to a multiple of the design load, and measures deflection at each step. Acceptance is set by the design and the criteria the project follows; ICC-ES AC358, for example, defines ultimate at a net deflection of 10 percent of the average helix diameter.

Not every pier gets tested. On a small repair the torque record may be the whole proof. On a larger project, or where the soil data is thin, the engineer or the AHJ commonly calls for a load test on one or more sacrificial or production piers to confirm the torque-to-capacity relationship before the rest are accepted on torque alone. If a pier hits an obstruction or behaves oddly, load testing that pier is the way to show its capacity was not compromised.

The test answers a question the torque only estimates: does this pier, in this soil, hold this load at acceptable movement. Where the project requires it, the test result, not the gauge reading, is the acceptance.

The Kt factor in more detail

The Kt factor, the torque correlation factor, is the multiplier that turns final installation torque into ultimate capacity, and its value is the most misused number in the trade. It is empirical, derived from load tests, and it depends on the shaft. ICC-ES AC358 publishes default values that fall as the shaft grows: roughly 10 per foot for a 1.5 to 1.75 in square shaft, about 9 for a 2.875 in round shaft, 8 for a 3.0 in, and 7 for a 3.5 in. A manufacturer may publish a higher tested value for its specific product, and that value, from its evaluation report, is the one to use.

The reason to hedge it hard is that the correlation is approximate by nature. Research has shown the published Kt can underestimate capacity at low torque and overestimate it at high torque, and the real factor varies with helix configuration and load direction as well as shaft size. Treating a single Kt as exact, then back-calculating a capacity to two significant figures, reads precise but is not.

Use the engineer's and manufacturer's Kt for the exact pier, apply it to a reliably measured torque, and verify with a load test where the stakes or the AHJ demand it. The factor is a tool for the engineer, not a shortcut around one.

Corrosion, galvanizing, and design life

Steel in the ground corrodes, so the design life of a helical pier depends on the soil it sits in. In ordinary soils a galvanized pier has a long service life, commonly cited at 75 to 100 years, but that figure assumes the soil is not aggressive. Corrosivity is a soil property the geotech evaluates, and ICC-ES AC358 flags soils as corrosive when resistivity is below 1,000 ohm-cm, pH is below 5.5, the soil is high in organics, sulfate exceeds 1,000 ppm, or the site is fill, landfill, or mine waste.

The common protection is hot-dip galvanizing to ASTM A123 for the shaft and brackets and A153 for the hardware, which can roughly double the life over bare steel by giving the zinc up before the steel. In aggressive ground the design can also add sacrificial steel thickness for the anticipated loss over the design life, or use sacrificial anodes for cathodic protection.

This is an engineering decision tied to the soil test and the required service life, not a coating you pick by habit. Where the soil is corrosive and the structure is permanent, the corrosion design is as much a part of the pier as the capacity, and it belongs to the engineer with the soil data in hand.

Refusal, obstructions, and shaft buckling

Two install problems sit at opposite ends of the soil. The first is refusal, where the pier stops advancing before it reaches design depth because it hit rock, cobbles, a boulder, or old fill and the torque spikes. Sometimes that early refusal is fine, if the torque proves the capacity and the depth meets the minimum. Sometimes it is an obstruction blocking one pier short of where it needs to be, and the options are to relocate the pier, advance past the obstruction, or load test the affected pier to show its capacity held.

The opposite problem is soft soil. A shaft turning in weak ground without reaching firm bearing can be pushed past the point where its slender section is stable, and the structural capacity of a helical pier in soft soil is governed by the buckling strength of the shaft and its couplings. A pier that will not build torque is not a pier to keep forcing deeper. It is a sign the bearing layer is not there or the wrong shaft was chosen.

Both cases are the engineer's call, not the operator's. Refusal short of design, torque that will not build, or a shaft that buckled all get reported and resolved against the design, not torqued through and hoped over.

How they get installed

A helical pier goes in with a hydraulic drive head, a torque motor that clamps the shaft and turns it while pushing down with light crowd pressure. The drive head mounts on a carrier sized to the job, a mini-excavator or skid steer for typical work, a handheld or cart-mounted machine for low-headroom interiors and tight repairs. The carrier supplies the down pressure and the operator keeps the pier turning at a steady rate.

Alignment is set at the start and held throughout. The lead section enters at the design batter, vertical for most foundations or raked for a tieback, and the operator keeps the shaft on line as it advances, because a pier that wanders off plumb loads its bracket eccentrically. Extensions are coupled on as the lead goes down, each one made up at its coupling, until the helices reach bearing and the final torque is read.

Rate matters for the correlation. The pier should advance close to one helix pitch per revolution so the plates bear on undisturbed soil instead of augering an oversized hole that lowers the torque and the capacity. An operator who spins the shaft faster than it advances is grinding away the very soil the pier needs.

What the inspector checks

Helical piers are a deep foundation, so the AHJ inspects them, and on engineered work a special inspector commonly observes the installation. The inspector's first interest is the torque, because that is the proof of capacity, so they check that the torque was measured with a calibrated instrument and that each pier met the minimum installation torque the engineer specified. They confirm the depth reached, that the pier is on location and within plumb or batter tolerance, and that the configuration matches the design.

Above ground the inspection moves to the bracket: the right bracket for the shaft, seated and bolted to a clean footing or cap, and the connection made up per the manufacturer's instructions. Where the design requires a load test, the inspector witnesses it and the result against the acceptance criteria.

The record is half the inspection. A special inspection report ties each pier's torque, depth, and capacity to its location and to the engineer's sign-off, and the AHJ relies on it for the deep-foundation approval. Confirm the inspection scope and the adopted code edition with the AHJ before the install, because what gets witnessed and recorded varies by jurisdiction and by how the engineer wrote the requirements.

Where helical piers run out of room

Helical piers are not the answer to every foundation. Very soft, deep soil with no firm stratum within reach gives the plates nothing to bear on and the shaft a length it can buckle in, and that site may need a different deep foundation or ground improvement first. Very high single-point loads can drive the pier count or the shaft size past the point where the system is practical or economical against a drilled shaft.

Lift on an underpinning job has limits too. You can stop settlement reliably, but how far you can raise a settled structure back toward level is bounded by what its finishes and framing can take without new cracking, and on old masonry that ceiling is low. Sites with shallow obstructions, dense cobbles, or buried debris can defeat the install one pier at a time.

The engineer judges where the limit is, weighing the soil profile, the loads, and the alternatives. A good contractor knows the system's edges and says so, because pushing helical piers onto a site that needs something else is how a repair becomes a callback.

The pier log and what to document

A helical pier job lives or dies on its records, because the capacity is invisible once the pier is in the ground. The pier log is the proof, and it has to tie each pier to a torque, a depth, a configuration, and a location, so the engineer can sign off and the next person can find out what was actually installed.

Capture per pier: the location or pier number against the plan, the shaft size and helix configuration, the lead and extensions used and the total length, the depth reached, the final installation torque and the instrument that read it, the calculated capacity from the engineer's Kt, the bracket type, any load test result, and the date with the installer. A field platform like FieldOS holds that per-pier record with photos and a timestamp and rolls it into the special inspection report, so the torque proof is one dataset instead of a torn page from a notebook in a truck.

Common mistakes

• Installing without an engineer's design and guessing the capacity off the gauge.
• Not recording the installation torque on every pier, so the capacity can never be proven.
• Screwing to a fixed depth off the drawing instead of to bearing and the minimum torque.
• Ignoring soil corrosivity and design life, so the steel outlives its coating in aggressive ground.
• Forcing past refusal on cobbles or a boulder instead of relocating or load testing the pier.
• Undersizing the pier count, the plate area, or the bracket for the load.
• Using a generic Kt instead of the manufacturer's value for the exact shaft and load direction.
• Spinning the shaft faster than it advances and augering away the soil the plates need to bear on.

Field checklist

Standards and references

The framework lives in the building code and the manufacturer's evaluation report. The International Building Code covers soils and foundations in Chapter 18, with deep foundations, helical piles among them, in the Section 1810 provisions, which require the pier to be designed and built by accepted engineering practice to resist installation and service loads. The adopted code edition and local amendments control, so confirm them with the AHJ before you cite a section on a submittal.

The product side is the ICC-ES evaluation report, written against Acceptance Criteria 358 (AC358), which sets how helical piles are tested and evaluated, defines the default Kt torque factors by shaft, and fixes the ultimate-capacity deflection limit at 10 percent of the average helix diameter. Corrosion protection references ASTM A123 for hot-dip galvanizing of the steel members and A153 for the hardware. The torque-to-capacity correlation and the load test practice that confirms it are detailed in the deep-foundations literature and the manufacturer's design manual.

Three things stay with the people who own them. The capacity, the Kt, the depth, and the torque target come from the structural or geotechnical engineer and the manufacturer's report, not a field rule. You screw to bearing and record the torque on every pier, because the torque is the proof. And the soil profile and its corrosivity govern feasibility and design life, which is why the geotech and the AHJ are in the loop from the start.

Units and terms

Helical pier work mixes a few unit systems and a vocabulary that shifts between the soil report, the manufacturer's tables, and the drawings, so the same idea can read differently across a set.

Installation torque is in foot-pounds (ft-lb) or newton-meters; capacity is in pounds, kips (1 kip is 1,000 lb), or kilonewtons. The Kt torque factor carries units of per foot in US practice. Soil resistivity for corrosion is in ohm-centimeters. Helix diameter and shaft size are in inches, while a metric soil report may give them in millimeters.

Helical pier / screw pile

A steel shaft with one or more helical plates, screwed into soil until the plates bear in firm strata, carrying load in compression or tension.

Helical plate (helix)

The steel plate welded to the shaft at a true pitch that bears on the soil; its diameter and number set the bearing area and the capacity.

Torque-to-capacity / Kt

The empirical correlation Qult equals Kt times final installation torque; Kt is the manufacturer's and engineer's factor for the specific shaft and load direction.

Bearing stratum

The firm soil layer the helices must reach to develop the design capacity; the depth to it is set by the soil, not the drawing.

Underpinning

Carrying an existing settled foundation down to firm soil on new piers, connected by brackets under the footing, to stop or reverse settlement.

Tension anchor / tieback

A helical pier loaded in uplift, where the plates bear on the soil above them, used for guy anchors, solar tie-downs, and retaining-wall tiebacks.

Refusal

When a pier stops advancing before design depth against rock, cobbles, or fill; resolved by relocating, advancing past, or load testing, not forcing.

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