Micropile underpinning needs an effective load path through the existing structure, pile head, steel and grout, bond zone and into the supporting ground.
Some common engineering challenges are:
Structural-support micropile: small-diameter drilled and grouted pile with high-capacity steel reinforcement installed that transfers load through both the reinforced pile section and along the grout-to-ground bond length. The typical underpinning load path follows as:
Existing Structure → footing or transfer beam → micropile head → reinforced pile section → grout-to-ground bond zone → competent bearing stratum
Possible components of a full micropile reinforcement system can include:
When specifying a threaded bar or casing tube as a micropile, make sure that you are referring to the entire installed system.
Steel reinforcement and connection components supplied by Wanfeng can be utilized as part of an engineered micropile system designed by a professional engineer.
Steel reinforcement (central bar or casing) can be designed to resist a portion or all of the structural load. The approved micropile design will specify details. Grout surrounds the reinforcement to protect steel if required and transfers load to the ground across the micropile bond zone. Pile-head plates, brackets caps or beams are used to connect the micropile to the structure being supported.
Intended System Functions
Micropiles can be part of an engineered solution to:
Engineer’s Disclaimer
Installed micropile diameter, length, reinforcement details (size, quantity), casing size, slope, bond length, grouting method, pile head connection details, spacing, load-transfer sequencing and testing programs should be designed or reviewed by a registered geotechnical engineer and/or structural engineer.
Geotechnical and structural engineering design should consider:
While Wanfeng will provide components to confirmed specifications we do not determine installed pile capacity, bond strength values or safe underpinning/load sequencing.
These systems can use similar steel elements but perform different engineering functions.
| Characteristic | Micropile | Ground anchor | Soil nail |
| Primary function | Deep foundation or in-situ ground reinforcement | Prestressed tensile restraint | Passive reinforcement of soil mass |
| Typical loading | Compression, tension or combined loading | Primarily tension | Primarily tension developed through ground movement |
| Load connection | Pile cap, footing, bracket or structural connection | Anchor head and waler or structure | Nail head and facing |
| Grouting | Grouted pile with structural reinforcement | Defined bond and free lengths | Commonly full-length grout |
| Prestressing | May be preloaded during underpinning but is not defined solely as a prestressed anchor | Normally tested, stressed and locked off | Generally not prestressed |
| Typical application | Foundations and underpinning | Retaining walls and uplift restraint | Slopes and excavations |
| Field testing | Compression, tension or lateral load testing as specified | Verification, performance, proof and creep testing | Verification and proof testing |
1. Solid Threaded Micropile Bars
Solid steel bars provided as central reinforcement within a drilled and grouted micropile.
Recommended Spec Fields
Additional Fields
2. Coupled Micropile Bars
Threaded bars of specific sectional length coupled together with approved couplers to create restricted-headroom or long-pile configurations.
Recommended Spec Fields
Additional Fields
Note to Specifier
Coupled assembly must meet specified axial strength and fit within desired borehole, casing and grout- cover geometry.
3. Hollow-Bar Micropiles
Threaded hollow bars that may serve as drilling member, grout delivery tube and permanent reinforcement in certain self-drilling micropile applications.
Recommended Spec Fields
Additional Fields
Availability note
Do not list hollow-bar micropiles until Wanfeng’s bar, coupler, drill-bit and system compatibility has been confirmed. A hollow bar should never be sold as a self-drilling micropile without confirmed compatible installation components and technical information.
4. Micropile Steel Casing
Steel casing provided as permanent structural reinforcement, temporary borehole support, or both.
Recommended Spec Fields
Additional Fields
Application note
Only list casing products that fall within Wanfeng’s confirmed manufacturing capabilities. Connection strength and installation method should be detailed in the project specifications.
5. Casing Couplers and Threaded Connections
Specialized connections used to join sections of casing or reinforcement.
Recommended Spec Fields
Additional Fields
6. Centralizers and Spacers
Used to keep reinforcement centered and within required grout cover inside the micropile borehole or casing.
Recommended Spec Fields
Additional Fields
7. Micropile Head Plates
Steel plates used in connection between micropile reinforcement and footing, pile cap, underpinning bracket or transfer beam, as designed by the project team.
Recommended Spec Fields
Additional Fields
Application note
Just because a steel plate can be used as a standard “bearing plate” doesn’t mean it is suitable to use as a micropile head plate. Check the connection for bearing, bending, punching, shear strength and the method how it will transfer into existing or new concrete.
8. Nuts, Washers and Locking Components
Coupling nuts, threaded washers and other components used in pile-head and project-specific load transfer connections.
Recommended Spec Fields
Additional Fields
9. Transfer Brackets and Connection Components
Fabricated steel components created from shop drawings used to connect micropiles to existing foundations.
Possible Components
Recommended Spec Fields
Availability note
Offer fabricated connection components after drawings have been reviewed and proper welding, inspection and part traceability capabilities are confirmed.
| Typical project condition | Typical requirement | Micropile configuration or related measure that may be relevant |
| Existing footing with inadequate capacity | Transfer load to deeper competent ground | Underpinning micropiles with engineered pile-head connection |
| Restricted headroom | Installation in short sections | Coupled threaded bars or sectional casing |
| Limited site access | Small drilling equipment and compact components | Project-designed micropile system |
| Variable soil with cobbles or boulders | Drilling method suited to obstructions | Cased or specialist drilling system |
| Weak upper ground over competent bearing layer | Transfer loads through weak strata | Reinforced micropile with suitable bond zone |
| Existing foundation settlement | Arrest movement and redistribute load | Micropiles with a controlled structural load-transfer sequence |
| Seismic foundation retrofit | Increased axial and lateral resistance | Micropile group integrated with strengthened pile cap |
| Uplift loading | Verified tension and bond capacity | Tension micropiles with engineered connection |
| Scour-sensitive foundation | Transfer load below potential scour zone | Micropiles extending to suitable deeper ground |
| Very soft soil or voided zone | Buckling resistance and borehole stability | Casing or other structural reinforcement as designed |
| Significant lateral loading | Lateral resistance and pile-cap interaction | Grouped or inclined micropiles where verified |
| Closely spaced pile group | Group interaction and construction effects | Project-specific spacing, sequence and group analysis |
| Aggressive soil or groundwater | Long-term corrosion resistance | Verified protective system for bar, casing and pile head |
| Contaminated ground | Controlled spoil, grout and worker exposure | Specialist environmental and construction procedure |
| Permanent structural support | Long-term durability and traceability | Documented materials, grout, installation and load testing |
Selection Note
This matrix is for general guidance purposes only.
Micropiles can be ideal for limited-access underpinning applications; however, they are not always the best foundation choice for every situation. Factors to consider when selecting include:
Micropiles can be utilized in one of two general ways.
Structural-Support Micropiles
The pile is bonded to a structure which imposes project-defined loads on the pile.
Common applications include:
In-Situ Reinforcement
Clusters/nets of micropiles work to reinforce the soil mass instead of just functioning as individual foundation piles.
Potential applications include:
The two methods involve different analysis, connection techniques, and load-transfer assumptions.
The following is an example of an underpinning sequence that may be required for a specific project:
Underpinning Warning
Cutting, unloading, jacking or shifting loads from existing foundations to micropiles should never be attempted without an engineered sequence approved by a structural engineer. Installing groups of piles too close together at the same time may alter the load path or cause structural instability.
Wanfeng DOES NOT determine what is safe for underpinning sequences or movement of structures.
Micropile capacity often relies upon interaction of steel reinforcement, grout and ground.
Factors that may be important to a project include:
The ultimate grout-to-ground bond resistance depends on site conditions and installation. It will not be achieved based on bar diameter and/or factory strength of steel alone.
Compression
Factors to consider in Compression design may include:
Tension
Factors to consider in Tension design may include:
Strength of steel
Strength of coupler
Pile-head connection
Bond of grout-to-ground
Uplift behavior of cone/group
Cyclic / seismic loading considerations
Lateral Load
Micropiles are capable of providing lateral resistance; however, due to their small diameter,
lateral behavior may govern the design. A few options that should be considered include:
Marketing high lateral capacity micropiles without project specific analysis and test data should be avoided.
| Design consideration | Temporary micropile | Permanent micropile |
| Service period | Defined construction duration | Full structure design life |
| Corrosion protection | Based on actual exposure | Long-term project-specific protection |
| Pile-head connection | Temporary load-transfer detail | Permanent structural connection |
| Grout requirements | Construction-stage performance | Long-term structural and durability requirements |
| Testing | Project-defined temporary capacity | Verified permanent design capacity |
| Documentation | Installation and construction records | Permanent works and traceability records |
| Monitoring | Construction-stage monitoring | Long-term monitoring where specified |
Temporary status should not be used to justify unverified steel, connections, grout or installation quality.
The corrosion protection provided by the system should take into account:
Potential solutions could be:
Note that because of these factors, simply coating one element of the micropile system does not ensure corrosion protection of the entire micropile. Only specify a corrosion-protection class when the entire assembly, as well as documentation, has been reviewed.
1. Receipt of Raw Materials
2. Bar, Casing and Thread Inspection
3. Mechanical and Connection Testing
If required and specified in contract:
4. Dimensional Inspection
5. Control of Surface Treatment
6. Identification and Final Inspection
7. Certification Requirements
If conformance or third-party testing is claimed for your products, please publish:
DO NOT show ASTM, EN, ISO, AS/NZS or other standard marks without listing product specific certifications.
| Test or inspection | Typical location | Purpose |
| Bar tensile test | Factory or laboratory | Verify reinforcement properties |
| Casing mechanical test | Factory or laboratory | Verify casing steel properties |
| Coupler tensile test | Factory or laboratory | Verify connection capacity |
| Nut proof-load test | Factory or laboratory | Verify threaded connection performance |
| Pile-head inspection | Factory | Verify dimensions, material and welding |
| Coating inspection | Factory | Verify specified protection |
| Compression load test | Project site | Verify installed pile response under compression |
| Tension load test | Project site | Verify installed pile response under uplift |
| Lateral load test | Project site | Evaluate lateral behavior where required |
| Grout testing | Project site or laboratory | Verify specified grout properties |
| Installation records | Project site | Document drilling, casing, reinforcement and grout |
| Structural monitoring | Project site | Assess movement during load transfer |
Factory steel strength does not verify installed grout-to-ground bond or foundation movement.
Micropile Installation Records
The project quality plan may require recording:
These site records complement—but do not replace—factory material certificates.
1. What is a micropile?
Micropiles are small diameter drilled and grouted piles with steel reinforcement. Structural loads are transferred through the reinforced pile and into competent ground via the grout-to-ground bond.
2. Why micropiles for foundation underpinning?
Micropiles can be installed through relatively small drilling rigs with sectional reinforcement. Their flexibility allows them to be used where access or headroom is restricted. They can transfer existing foundation loads to deeper competent strata.
3. Can micropiles resist compression and tension loads?
Micropiles can be designed to resist compression, tension or a combination of both. The steel, grout, ground bond, pile head detail and group behavior must all be reviewed for the design load case.
4. Are micropiles appropriate for lateral loading?
Micropiles can resist lateral loads, however their stiffness may be restricted by their small diameter. Consideration should be given to using pile groups, inclining piles, casing or another lateral resisting system.
5. Difference between micropile and ground anchor?
A micropile is designed to be a deep foundation element that may resist both compression and tension loads. A ground anchor is normally designed as a prestressed tensile element with established free length and bond lengths.
6. Can micropile bars be spliced?
Yes, sectional micropile bars can be connected together with compatible couplers. The entire connection must meet specified loads and fit within the casing, borehole and grout cover requirements.
7. How do you confirm micropile capacity?
Compression, tension and/or lateral capacity can be confirmed with load testing at the project specified loads. Load tests verify in-situ performance that cannot be confirmed with factory testing of the pile materials.
8. Can Wanfeng fabricate micropiles from drawings?
Yes. Provided bars, casings, couplers, plates, nuts, centralizers and pile heads are supplied with approved drawings, Wanfeng can review for manufacturing capability. The complete micropile design and foundation design should be reviewed and approved by licensed professionals.
