A micropile is a small diameter drilled and grouted pile, typically 100 to 300 mm across, in which the structural load is carried by steel reinforcement and transferred to the ground almost entirely through the bond between grout and soil along the shaft. The name suggests a scaled-down bored pile. The load transfer mechanism is not the same.
In a bored pile of conventional diameter, a substantial share of the capacity comes from end bearing, and the concrete section carries the structural load. In a micropile the base area is too small to contribute meaningfully, and the grout is there to transfer load to the ground rather than to resist it. The steel does the structural work.
That difference decides where micropiles are the right answer, how their capacity is calculated, and which checks govern.
A micropile is not a small pile. It is a bond length with a steel core.
What Is a Micropile?
A drilled hole, a steel element, and grout.
The hole is advanced by rotary or rotary-percussive drilling, usually with temporary or permanent casing through the upper soils. Reinforcement is installed as a single high-strength threaded bar, a group of bars, a steel casing left in place, or a combination of casing over the upper length and bar over the lower. Grout is then placed, and in some systems injected under pressure.
Diameters are small enough that a rig can work in a basement with limited headroom, on a slope reachable only by a small track machine, or within a metre of an existing wall. That access is the reason the technique exists.
Why Is a Micropile Not Just a Small Pile?
Because the two components of pile capacity scale differently with diameter.
Shaft resistance is proportional to the perimeter, so it falls linearly as the diameter reduces. Base resistance is proportional to the area, so it falls with the square. Halving the diameter halves the shaft contribution and quarters the base contribution. At micropile diameters the base term becomes small enough that design practice neglects it entirely and takes the whole geotechnical capacity from the bond along the shaft.
The structural side changes as well. A 200 mm micropile with a heavy threaded bar and a permanent casing has a steel ratio no bored pile would have. The section is designed as a composite steel element with grout confinement rather than as a reinforced concrete section, and the governing structural checks are those of a steel member.
How Is Geotechnical Capacity Determined?
From the grout-to-ground bond over the bond length.
Pg = π · D · Lb · αbond
where Pg is the nominal grout-to-ground bond capacity, D the diameter of the drilled hole in the bond zone, Lb the bond length and αbond the grout-to-ground ultimate bond strength for the soil or rock type and the grouting method.
Three things about this expression matter more than the arithmetic. The diameter is the drilled diameter in the bond zone, which in pressure-grouted systems exceeds the nominal hole size because grout permeates the surrounding ground. The bond length is measured below the cased length, since a cased section transfers no load to the soil. And αbond is not a calculated quantity: it comes from published tables by soil type and grouting method, and it carries a wide range within each category.
That last point sets the character of micropile design. The capacity depends on a tabulated bond value with substantial scatter, which is why load testing is not optional on projects of any size.
What Governs the Structural Capacity?
The steel, checked for two things.
Axial capacity comes from the combined section of casing and bar, with the grout treated as confinement rather than as a load-carrying element. In compression the section is checked against yield; in tension the bar governs, and micropiles carry tension well, which is one of the reasons they appear in uplift and tie-down applications.
Buckling is the check that distinguishes micropile design from other pile design. A slender steel element in soil is restrained laterally by the ground, and in competent soil that restraint is more than sufficient. In very soft clay, in peat, or where a void or liquefiable layer exists, the lateral restraint may be too low to prevent buckling of the unsupported length. A micropile passing through a soft stratum needs that check explicitly rather than by assumption.
Does the Grouting Method Change the Capacity?
Substantially, and it is specified rather than assumed.
Gravity-placed grout fills the hole under its own head and produces the lowest bond values. Grout placed under pressure through the casing as it is withdrawn increases the effective diameter and the bond. Post-grouting through a separate tube after the primary grout has set allows further pressurisation of the bond zone and gives the highest values.
Each step up increases capacity and cost. Because the published bond values are tabulated by method as well as by soil type, the method is part of the design rather than a contractor’s choice, and a change in method after award is a design change.
Where Do Micropiles Beat Bored Piles?
| Situation | Why the micropile wins |
|---|---|
| Low headroom, basements, inside buildings | Small rigs work where a bored piling rig cannot stand |
| Underpinning an existing foundation | Installed close to and through existing structures with low vibration |
| Restricted access or steep terrain | Compact equipment, small working platform |
| Obstructions, fill with boulders, old foundations | Rotary-percussive drilling penetrates what augers cannot |
| High tension or alternating loads | Steel section carries tension efficiently |
| Seismic retrofit of existing foundations | Capacity added without removing the structure above |
The first two rows carry most of the real applications. Where a conventional rig can stand and work, a bored pile is usually cheaper per kilonewton of capacity, and the comparison is not close.
How Are Micropiles Tested?
By loading them, because the design rests on a tabulated bond value.
Verification tests are carried out on sacrificial piles before production to confirm the bond assumption, loaded well beyond the design load. Proof tests on a proportion of the production piles confirm that the installation is reproducing the tested behaviour. Where tension governs, testing in tension is what matters, since a compression test does not verify the same mechanism.
The result that matters is not only the maximum load reached but the load-displacement shape. A pile that reaches its load with continuing creep is behaving differently from one that stabilises, and the difference points to the bond rather than to the steel.
Do Micropiles Work in Groups the Same Way?
Mostly, with two differences worth noting.
Group efficiency applies as it does for other piles, with the block failure and individual failure mechanisms both checked. Because micropiles are often installed at close spacing to make up capacity with numbers, the group check is more likely to govern than it is for large-diameter piles.
The second difference is negative skin friction. A micropile relying entirely on shaft bond is fully exposed to downdrag where a compressible layer settles around it, since there is no base resistance to fall back on. The neutral plane position and the resulting dragload belong in the design where fill, recent alluvium or a dewatered layer is present.
When Is a Micropile the Wrong Choice?
When the site has room and the loads are vertical and large.
Cost per unit of capacity is higher than for bored piles, sometimes considerably. A scheme that replaces four bored piles with twenty micropiles because the design was done that way rather than because access demanded it is paying for a capability it does not use.
Lateral capacity is the other limit. A slender element resists lateral load poorly compared with a large-diameter pile, and a micropile group taking significant horizontal load usually needs inclined piles arranged to carry it axially rather than in bending.
Common Errors in Micropile Design
| Error | Consequence |
|---|---|
| Including base resistance in the capacity | Capacity overstated by a term the diameter cannot deliver |
| Measuring bond length over the cased section | Load assumed to transfer where the casing prevents it |
| Taking a mid-range bond value without verification testing | Design rests on a tabulated figure with wide scatter |
| Omitting the buckling check through soft strata | Slender steel element unrestrained over an unsupported length |
| Changing the grouting method after design | Bond values in the design no longer apply |
| Ignoring downdrag on a shaft-only element | No base resistance available to absorb the dragload |
| Specifying micropiles where a bored piling rig could stand | Higher cost per kilonewton with no access benefit |
Where SETAF2018 Fits
SETAF2018 treats micropiles as one of the deep foundation element types available within the same model as bored piles, deep mixing and jet grout columns, rather than as a separate module with its own ground data.
Micropiles are placed beneath rectangular, circular or polygonal foundations, positioned individually by coordinate or generated as a group at a chosen spacing, with layouts adjustable afterwards. Axial capacity is calculated by the shaft and base methods used for piles, with the α, λ and β approaches in cohesive and granular soils and group efficiency by Converse–Labarre or the Terzaghi block mechanism. Where a compressible layer is present, negative skin friction and the neutral plane are evaluated for the element as designed.
The ground data is the shared layered profile, so the SPT records with their corrections, the strength parameters and the groundwater conditions used for the micropile check are the ones used for the rest of the project. Settlement of the supported foundation is calculated through the same profile, so a micropiled foundation is compared against a shallow or raft alternative without rebuilding the model, within the wider foundation analysis workflow.
Calculation reports print the equations, the layer-by-layer resistances and the intermediate values, which is what allows a bond assumption to be checked against a load test result rather than accepted. Reinforcement minimums follow TS 500, and layouts and quantities are produced from the same model.
Frequently Asked Questions
What is a micropile?
It is a small diameter drilled and grouted pile, typically 100 to 300 mm across, reinforced with a high-strength bar, a permanent steel casing or both. The structural load is carried by the steel and transferred to the ground through the bond between grout and soil along the shaft.
How is micropile capacity calculated?
From the grout-to-ground bond over the bond length: the bond strength for the soil type and grouting method multiplied by the perimeter and the bond length. Base resistance is neglected, because at these diameters the base area is too small to contribute meaningfully.
Why is base resistance ignored in micropile design?
Because shaft resistance scales with the perimeter and base resistance with the area. Reducing the diameter reduces the base contribution with the square, so at micropile diameters it becomes a small fraction of the total and standard practice omits it.
What is the difference between a micropile and a bored pile?
Diameter, load transfer and structural section. A bored pile draws significant capacity from end bearing and carries load through a reinforced concrete section. A micropile carries load through shaft bond alone, with the structural load taken by a steel bar or casing acting as a composite section with the grout.
Does the grouting method affect micropile capacity?
Yes, considerably. Gravity-placed grout gives the lowest bond values, pressure grouting during casing withdrawal increases the effective diameter and the bond, and post-grouting through a separate tube gives the highest. Published bond values are tabulated by method, so it is a design decision rather than a site choice.
Do micropiles need load testing?
Yes on any project of size, because the design rests on a tabulated bond value with wide scatter within each soil category. Verification tests on sacrificial piles confirm the assumption and proof tests on production piles confirm the installation reproduces it.
When does buckling govern micropile design?
Where the element passes through very soft clay, peat, a void or a liquefiable layer, so the lateral restraint from the ground over that length may be insufficient for a slender steel section. In competent soil the restraint is more than adequate and buckling does not govern.
Are micropiles more expensive than bored piles?
Per unit of capacity, usually yes. Their advantage is access: low headroom, work inside existing buildings, restricted sites, steep terrain and drilling through obstructions. Where a conventional piling rig can stand and work, a bored pile is normally the cheaper solution.
