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Micropile Underpinning

Micropile underpinning strengthens or replaces an existing foundation by transferring load to deeper bearing strata through small-diameter drilled-and-grouted piles, allowing settled or undersized foundations to be repaired without demolishing the structure above.

500

Kip Capacity

6 ft

Min Headroom

Low

Vibration

Immediate

Load Transfer

Overview

Understanding Micropile Underpinning

Micropile underpinning is the engineering practice of strengthening, replacing, or augmenting an existing foundation by transferring its load to a deeper bearing stratum through small-diameter drilled-and-grouted micropiles. The technique allows a settled, undersized, or load-deficient foundation to be brought back into service without demolition, while the structure remains occupied. Drilled installation in clearances as low as six feet is what differentiates micropile underpinning from driven-pile alternatives and lets crews work inside basements, under bridges, and around historic structures where vibration and access constraints would otherwise rule out foundation repair. The system is governed in US practice by FHWA Geotechnical Engineering Circular No. 7 and the deep-foundation provisions of the International Building Code.

What Is Micropile Underpinning?

Foundation underpinning is the engineering practice of strengthening or extending an existing foundation by transferring some or all of its load to a deeper, more competent bearing stratum. It is used to repair settled or distressed foundations, to add capacity for vertical building expansion, to retrofit pre-code structures for seismic loads, and to stabilize buildings threatened by adjacent excavation or slope movement. Modern underpinning relies primarily on drilled deep-foundation systems that can be installed in restricted access without disturbing the structure above.

Within the FHWA classification, micropile underpinning is a Case 1 application: a drilled-and-grouted pile directly supports an existing structure rather than reinforcing the surrounding soil mass (Case 2). The technique was developed in postwar Italy by Fernando Lizzi as the original pali radice, or root piles, for stabilizing settled masonry buildings, and entered US infrastructure practice through FHWA's seismic retrofit program and the subsequent codification in NHI-05-039. An underpinning pile is not a generic deep foundation. It is a small-diameter (typically 4 to 12 inches) high-strength element specifically engineered to transfer load through a structural pickup connection, the pile cap, needle beam, bracket, or grade beam that physically ties the new pile into the existing footing. Capacity per pile typically ranges from 50 to 500 kips, depending on bar size, grout strength, bond length in bearing material, and the FHWA grouting type (A through D, classified by the pressure and frequency of grout placement).

Key Benefits

Minimal vibration safe for sensitive structures
Works in restricted access and low headroom
Immediate load capacity after grouting
Can arrest and reverse ongoing settlement
Suitable for any soil or rock condition

Used In Our Services

Drilling

Grouting

The Engineering

How Micropile Underpinning Works

How the system carries load in service, and how we build it on site.

Construction sequencing differs from new-foundation pile work because the existing structure must remain operational and the geometry is fixed by what is already there. After subsurface investigation establishes depth to competent bearing material and the existing foundation is field-verified, the underpinning layout is selected from one of three configurations: through-footing, side-pickup with a needle beam or bracket, or grade-beam pickup (covered in System Variants below). Low-headroom rotary or rotary-percussive rigs operating in clearances as low as six feet then drill the boreholes from inside the basement or from a perimeter excavation, with casing advanced through caving overburden as needed.

A threaded high-strength steel bar or self-drilling hollow bar is set with centralizers, and neat cement grout is tremie-placed from the toe upward. FHWA Type A grout is gravity-placed; Types B through D apply progressively higher post-grouting pressures to mobilize additional skin friction in the bond zone. Each pile is then proof- or performance-load-tested under static load before being committed to the structure. Once verified, the structural pickup connection is built. Through-footing piles are integrated by a reinforced pile cap, side-pickup piles by a needle beam or steel bracket bolted or epoxy-doweled to the existing wall, and continuous-wall configurations by a reinforced grade beam cast against the underside of the existing footing. In severe settlement cases, hydraulic jacks placed between the foundation and the new pile cap can lift the structure incrementally back toward original grade before the connection is locked off.

Foundation Evaluation

Assess existing foundation conditions and determine load requirements.

Access Preparation

Create access through floor slabs or from building exterior as needed.

Micropile Installation

Drill and grout micropiles adjacent to or through existing footings.

Load Transfer Connection

Construct pile caps or brackets to transfer loads from foundation to piles.

System Variants

Types of Micropile Underpinning Configurations

Type 01

Through-Footing Underpinning

The pile is cored vertically through the existing footing and integrated structurally via a reinforced concrete pile cap that ties the bar into the footing through development length and dowel bond. This is the most direct load path and the configuration of choice on thicker concrete foundations and on bridge piers, where the existing footing is heavy enough to develop reinforcement bond and the geometry permits a vertical core. Through-footing piles concentrate load directly under the original bearing line, which simplifies stress analysis but requires careful detailing where existing footing reinforcement is encountered.

Type 02

Side-Pickup Underpinning

Piles are drilled adjacent to the existing wall, with load transferred eccentrically through either a transverse steel needle beam (a W-shape passing through a pocket cut in the wall, used for heavier loads, masonry foundations, and historic structures) or a bolted or epoxy-doweled steel bracket attached to the foundation wall (used for lighter residential loads). Side pickup is the configuration of choice when through-coring is precluded by reinforcement, finishes, or wall thickness, and is the dominant approach on residential foundation repair and on listed historic buildings where the original footing must remain intact.

Type 03

Grade-Beam Underpinning

A continuous reinforced concrete grade beam is cast against the underside of the existing footing, spanning between micropile pairs spaced along the wall. The grade beam distributes load uniformly along the foundation and tolerates variable pile spacing where existing utilities, column footings, or geometric constraints prevent regular pile placement. This configuration is specified when continuous load transfer along a long wall is required, particularly under industrial slabs, continuous bridge abutments, and load-bearing masonry walls where point pickup at discrete piles would induce unwanted bending stresses in the original foundation.

Side By Side

Foundation Underpinning System Comparisons

Micropile Underpinning vs Helical Piers

Both are deep-foundation underpinning systems, but they differ in how they develop capacity. Helical piers are torque-installed steel shafts with bearing plates that gain capacity from the helix bearing in soil, with typical capacities up to 50 kips per pile and a low spoils footprint. Micropiles are drilled and grouted, developing capacity from skin friction along the bond zone in dense soil or rock, with capacities of 50 to 500 kips per pile. Helical piers are well suited to lighter residential foundations on uniform soft or medium-stiff soils within reach of the helix bearing depth. Micropiles are the right tool when bearing material is at depth, when high capacities are required, when rock or cobble layers preclude torque installation, or when load-test verification of every production pile is required by specification.

Micropile Underpinning vs Push Piers (Resistance Piers)

Push piers, also called resistance piers, are steel pipe segments driven hydraulically by jacks that push against the underside of the existing footing as the reaction force. They install fast and inexpensively but capacity is limited to what the existing foundation can react against, typically 30 to 60 kips per pier, and the system depends on ground that allows penetration to refusal at depth. Micropile underpinning develops full design capacity in any ground condition and does not depend on the existing foundation for installation reaction, which is why micropiles are the default selection on commercial buildings, bridge structures, and any project where the existing footing is too light, too distressed, or too heavily loaded to safely react significant pile load.

Micropile Underpinning vs Replacement Foundation

In severely deteriorated structures the engineer's choice is between underpinning the existing foundation and demolishing and replacing it. Replacement gives the most predictable end result but typically requires temporary soldier pile shoring, substantial demolition, and weeks to months of structure displacement. Micropile underpinning preserves the existing structure in place, with work proceeding from inside or perimeter access while the building remains occupied. The cost-benefit calculation favors underpinning whenever historic value, occupancy continuity, or demolition complexity are factors, and is often required outright on listed historic structures and on buildings where business interruption costs would dominate the project budget.

Not sure which system fits? We'll walk through the tradeoffs for your site conditions.

Talk Through Your Options

Where It Fits

Common Applications and Project Types

Foundation settlement repair on commercial and residential buildings is the highest-volume application category, particularly in regions with collapsible loess, expansive clay, or organic soils where original shallow foundations have failed. Bridge pier and abutment rehabilitation under DOT contracts is a major segment, with FHWA NHI-05-039 the governing document on transportation projects. Historic structure preservation depends on micropile underpinning because vibration-free drilled installation is compatible with masonry and unreinforced foundations that cannot tolerate driven-pile work, and preservation projects typically require coordination with state historic preservation offices and federal Section 106 review. Capacity upgrades for vertical building expansions, heavy industrial equipment foundations, and seismic retrofit of pre-code buildings round out the project mix. Owners selecting an underpinning contractor for a foundation repair project should confirm low-headroom drilling capability, in-house static load-test capacity, and structural engineering depth on the foundation pickup connection.

Foundations showing settlement or cracking

Bridge abutment rehabilitation

Slope movement affecting buildings

Increasing capacity for renovations

Seismic upgrades for critical infrastructure

Historic building stabilization

Benefits

Key Advantages

Low-Headroom Capability

Specialized equipment operates in spaces as low as 6 feet, allowing work inside basements and under existing structures.

Minimal Vibration

Drilled installation produces no damaging vibration, safe for historic buildings and adjacent structures.

High Capacity

Micropiles develop capacities up to 500 kips, supporting heavy loads from small-diameter elements.

Settlement Arrest

Load transfer to deep bearing strata stops ongoing settlement and can lift structures back toward original grade.

Universal Application

Works in any soil or rock condition from soft clay to hard granite, adapting to variable subsurface conditions.

Engineering

Technical Considerations

Soil/Rock Conditions

Piles extend to competent bearing strata. Bond zone in rock or dense soil develops design capacity.

Groundwater

Grouting methods adapted for wet conditions. Hollow-bar systems allow drilling and grouting in difficult ground.

Load Capacity

Capacity depends on pile diameter, reinforcement, grout strength, and bond length. All piles load-tested to verify performance.

Spacing

Pile configuration determined by existing foundation geometry and required load distribution.

Installation Method

Piles drilled adjacent to or through existing footings. Structural connection via pile caps, brackets, or grade beams.

Equipment Used

Low-headroom drill rigs
Grout mixing and pumping equipment
High-strength reinforcing steel
Pile cap formwork and reinforcement
Load testing equipment

Limitations

Requires access for drilling equipment
Structural connection design critical
Competent bearing material required at depth
Cost higher than shallow foundation repair

Technical Specifications

Pile Diameter

4-12 inches

Capacity

50-500 kips

Installation

Low headroom capable

Grout Type

Neat cement / Sand-cement

Codes And References

Engineering Standards and References

FHWA

NHI-05-039

Micropile Design and Construction Reference Manual

Geotechnical Engineering Circular No. 7. The canonical practitioner document for micropile design and construction in US practice. Defines the Case 1 / Case 2 application classification and Type A through D grouting methodology cited by virtually every state DOT specification.

AASHTO

LRFD §10.9

Bridge Design Specifications, Drilled Shafts and Micropiles

Provides load and resistance factors, design life framework, and load-testing requirements for micropile foundation support on bridge and transportation structures.

ICC

IBC §1810

International Building Code, Deep Foundations

Sets the building-side requirements for deep foundation capacity, installation, testing, and structural connection design on micropile underpinning of buildings.

Authoritative Sources

FHWA Office of Bridges and Structures AASHTO Publications

Combinations

Integration With Other Systems

Pressure Grouting

Grouting can fill voids and improve soil around foundations in conjunction with underpinning.

Learn More

Horizontal Drains

Drainage addresses groundwater contributing to settlement or slope movement affecting foundations.

Learn More

Soil Nailing

Slope stabilization protects underpinned structures from ongoing earth movement.

Learn More

Structural Repair

Foundation crack repair and waterproofing complement underpinning for complete restoration.

Expertise

Why Choose Rock Supremacy for Micropile Underpinning

Low-Headroom Expertise

Specialized equipment and experienced crews work inside basements and confined spaces.

Structural Integration

We design and construct load transfer connections that integrate with existing foundations.

Complete Testing

All piles load-tested to verify capacity with documentation for engineer and owner records.

Minimal Disruption

Low-vibration installation and clean work practices minimize impact on building occupants.

Integrated Solutions

Underpinning combined with drainage, slope stabilization, and structural repair for complete foundation restoration.

Case Studies

Our Work

See how we've applied this technique and others to solve real-world geotechnical challenges.

Transportation

Glenwood Canyon, CO|2023

I-70 Corridor Stabilization

Installed 50,000 sq ft of high-tensile mesh and performed extensive scaling after a major weather event.

View Case Study

Civil

Lava Hot Springs, ID|2024

Lava Hot Springs Slope Stabilization

Multi-phase slope stabilization project protecting the historic hot springs resort and Highway 30 from an active landslide.

View Case Study

Rail

Winter Park, CO|2023

Moffat Subdivision Railroad Stabilization

Large-scale railroad stabilization project with over 10,000 linear feet of drilling to secure train tracks entering the historic Moffat Tunnel.

View Case Study

View All Work

Questions

Micropile Underpinning FAQ

What is foundation underpinning?

Foundation underpinning is the engineering practice of strengthening or extending an existing foundation by transferring some or all of its load to a deeper, more competent bearing stratum. It is used to repair foundations that are settling, to add capacity for vertical building expansion, to retrofit pre-code structures for seismic loads, and to stabilize structures threatened by adjacent excavation or slope movement. Modern underpinning relies primarily on drilled deep-foundation systems, micropiles, helical piers, or push piers, that can be installed in restricted access without disturbing the structure above.

How do I know if my foundation needs underpinning?

Visible warning signs include diagonal cracks in basement walls or above doors and windows, doors and windows that stick or no longer latch, sloping or uneven floors detectable with a level, gaps between walls and ceilings, and bowing or stepped cracking in masonry foundation walls. Settlement is rarely uniform, so the pattern of distress matters more than any single crack. A licensed structural engineer should diagnose the cause with subsurface investigation before repair scope is set, since foundation movement from soil moisture cycling, slope creep, or sinkhole development each requires a different remediation path.

How does micropile underpinning work?

Small-diameter steel piles are drilled through the soil overburden into a competent bearing stratum, typically dense soil or rock, and grouted in place to develop bond capacity. Each pile is then load-tested before being structurally tied to the existing foundation through a pile cap, needle beam, bracket, or grade beam. Load that previously bore on the original shallow foundation now transfers down the pile to deep bearing material. Because installation uses drilled rigs rather than driven hammers, the work can proceed inside basements with as little as six feet of headroom and without vibration that could damage adjacent structures.

What is the difference between micropile underpinning and helical piers?

Both are deep-foundation underpinning systems, but they differ in how they develop capacity. Helical piers are torque-installed steel shafts with bearing plates that gain capacity from the helix bearing in soil, with typical capacities up to 50 kips per pile. Micropiles are drilled and grouted, developing capacity from skin friction along the bond zone in dense soil or rock, with capacities of 50 to 500 kips per pile. Helical piers are well suited to lighter residential foundations on uniform soft soils. Micropiles are specified when bearing material is at depth, when high capacities are required, or when ground conditions preclude torque installation.

What is the difference between micropile underpinning and push piers?

Push piers, also called resistance piers, are steel pipe segments driven hydraulically by jacks that push against the underside of the existing footing as the reaction force. They install fast and inexpensively but capacity is limited to what the existing footing can react against, typically 30 to 60 kips per pier, and the system depends on ground that allows penetration to refusal. Micropile underpinning develops full design capacity in any ground condition and does not depend on the existing foundation for installation reaction, which is why micropiles are the default selection on commercial buildings, bridge structures, and any project where significant capacity is required.

Can underpinning lift a settled foundation back to level?

In many cases, yes. After the underpinning piles are installed and load-tested, hydraulic jacks placed between the existing foundation and the new pile cap can be used to lift the structure incrementally back toward original grade before the connection is locked off. Whether full re-leveling is feasible depends on the rigidity of the existing foundation, the magnitude of differential settlement, and the condition of finishes that have already cracked or distorted. A structural engineer should set acceptable lift limits, since aggressive jacking on a brittle masonry foundation can cause more damage than it corrects.

How long does foundation underpinning take?

Schedule depends on access, pile count, drilling depth, and the structural pickup detail. A typical residential basement underpinning project with eight to fifteen piles runs one to three weeks from mobilization to demobilization, including drilling, grouting, load testing, and the structural connection. Commercial buildings, bridge piers, and historic structures vary widely, with larger projects extending over several months when work must be staged around occupied areas or operating traffic. Grout reaches design strength within seven to fourteen days of placement, so structural pickup typically follows soon after the last pile is installed.

Can underpinning be done while the building is occupied?

Most micropile underpinning projects are completed with the building in service. Low-headroom drill rigs operate in basements with as little as six feet of clearance, and drilled installation produces no damaging vibration, so adjacent rooms and historic finishes are unaffected. Work areas are isolated, dust-controlled, and cleaned at the end of each shift. Owners typically need to provide access to the work area, water, and electrical power for the drill rig and grout plant. Occupants in unaffected areas of the building generally continue normal use throughout the project.

What standards govern foundation underpinning design?

In US practice, the canonical document is FHWA Geotechnical Engineering Circular No. 7 (NHI-05-039), the Micropile Design and Construction Reference Manual, which defines the Case 1 (direct structural support) versus Case 2 (in-situ reinforcement) classification and the Type A through D grouting methodology. AASHTO LRFD §10.9 governs deep-foundation design on bridge and transportation projects. International Building Code §1810 covers deep foundations on buildings, including capacity, installation, testing, and connection requirements. ACI 318 governs the reinforced concrete pile cap and grade beam design used for the structural pickup connection.

How long does micropile underpinning last?

Micropile underpinning is engineered as a permanent foundation system. Service life is set by corrosion protection of the steel reinforcement, the durability of the cement grout, and the integrity of the structural pickup connection, all of which are designed under FHWA NHI-05-039 and AASHTO LRFD provisions for the same design life as the supported structure. Permanent piles use double corrosion protection, encapsulation in grout plus epoxy or galvanized coating on the bar, where corrosive groundwater is identified. Periodic inspection of the foundation pickup connection is the only routine maintenance most projects require.

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"Definitely a five star rating for this group. Even though my residential project was smaller scale, they approached the task with all of the consideration and professionalism I could hope for."

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Methods

Related Techniques

Explore other engineering methods we use to deliver comprehensive geotechnical solutions.

Foundation Underpinning

Load-transfer technique that carries existing foundation loads past failing soil into deeper bearing strata or anchored ground. Includes micropile underpinning, battered soil anchors, and foundation tie-backs.

Learn More

Horizontal Drains

Horizontal drains are slightly upward-inclined drilled drains (50 to 500 plus ft, 2 to 4 inch slotted PVC or HDPE) installed into a slope or embankment to lower the groundwater table and relieve the pore pressure that drives landslide movement. Standard slope-drainage tool per TRB Special Report 247 and FHWA-NHI-08-097.

Learn More

Micropiles

Micropiles are small-diameter, high-strength deep foundation elements used to transfer loads into competent ground. Their versatility makes them ideal for underpinning, slope stabilization, and situations with limited access or difficult geology.

Learn More

Pressure Grouting

Pressure grouting injects fluid grout into soil pores, rock fractures, or open voids to densify loose ground, seal water pathways, fill cavities, and lift settled foundations. Compaction, permeation, fracture, and chemical grouting variants address different host-ground conditions.

Learn More

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