Overview
What Are Hollow Bar Micropiles?
Hollow bar micropiles use a continuous threaded hollow steel bar as both the drill string and the permanent structural reinforcement. Grout is pumped through the center of the bar and out through a sacrificial drill bit during drilling — simultaneously advancing the hole and grouting in a single pass. The bar stays in the ground as the finished pile.
This drill-and-grout-simultaneously method is the key advantage of hollow bar systems. Because grout stabilizes the hole as the bar advances, hollow bar micropiles can be installed in caving soils, loose fill, high water table conditions, and other ground where open-hole drilling would collapse before casing or reinforcement could be placed.
Rocky Mountain Micropiles installs hollow bar micropiles in diameters from 30mm to 76mm (approximately 1.2″ to 3″ bar OD), with finished pile diameters of 5″ to 12″+ depending on the drill bit and ground conditions. The pressurized grout creates an enlarged grout body around the bar that significantly increases bond capacity beyond what the drill hole diameter alone would provide.
Technical Data
Hollow Bar Specifications
Key parameters for our hollow bar micropile systems. Actual design values are project-specific based on geotechnical conditions and structural requirements.
Continuous threaded hollow steel bars from 30mm (~1.2″) to 76mm (~3″) OD. Bar selection is based on required structural capacity, pile length, and grout body diameter needed.
Sacrificial drill bit diameter and pressurized grout create a finished pile body significantly larger than the bar itself — increasing bond surface area and load capacity.
Individual pile capacities routinely exceed 300 kips in compression depending on bar size, grout body diameter, bond zone length, and ground conditions in the bearing stratum.
Grout is pumped through the hollow bar during drilling, stabilizing the hole as it advances. No open hole, no casing, no separate grouting step — one continuous operation.
Every production pile is proof-tested to 200% of design load per IBC and project specifications. Verification tests available on designated test piles for critical projects.
Loose fill, collapsible soils, running sands, high water table, cobble/boulder zones — anywhere the hole won’t stay open long enough for conventional reinforcement placement.
Applications
When to Use Hollow Bar Micropiles
Hollow bar is the preferred micropile system when ground conditions make open-hole drilling impractical — or when speed of installation is a priority. The simultaneous drill-and-grout method eliminates the risk of hole collapse and reduces installation time per pile.
Discuss Your ProjectCaving & Collapsible Soils
Loose fill, Lake Bonneville silts, collapsible loess, and other soils that won’t hold an open hole. The grout stabilizes the borehole in real time as drilling advances.
High Water Table
Groundwater makes open-hole drilling difficult and degrades grout quality in conventional piles. Hollow bar’s pressurized grouting displaces water and ensures continuous grout contact with the bar and surrounding soil.
Fast-Track Schedules
One-pass installation is significantly faster than two-step solid bar methods. When the project schedule is compressed, hollow bar can reduce the number of days on site per pile.
Cobble & Boulder Zones
Alluvial deposits with cobbles and boulders that deflect conventional drill bits. Hollow bar bits can penetrate and displace obstructions that would stop open-hole drilling.
New Construction Foundations
Commercial and industrial structures requiring deep foundation support — including compression, tension, and lateral loading. Ideal for silo foundations, equipment pads, and structures with combined vertical and wind loads.
Underpinning & Retrofit
Stabilizing existing foundations experiencing settlement or adding capacity for new loading. Hollow bar can be installed in low-headroom conditions using segmental couplers to extend bar lengths.
Installation
Hollow Bar Installation Process
The single-pass drill-and-grout method eliminates multiple steps — no casing, no separate grouting, no open-hole risk.
Set Up & Locate
Position the drill rig at the pile location. Verify coordinates, elevation, and alignment per the foundation plan. Attach the sacrificial drill bit to the first section of hollow bar.
Drill & Grout
Advance the bar while pumping grout through its hollow center and out through the drill bit. The grout stabilizes the borehole and encapsulates the bar simultaneously. Add bar sections with threaded couplers as depth increases.
Flush & Top Off
Once the bar reaches design depth, continue pumping grout until clean, consistent grout returns appear at the surface. This confirms the entire annulus is filled with no voids or water inclusions.
Cure & Prep
Allow grout to reach required compressive strength. Cut bar to specified elevation and install bearing plate, nuts, and any required connection hardware for the pile cap or foundation interface.
Proof Test
Load-test the pile to 200% of design load per IBC and project specifications. Record load vs. displacement at each increment. Submit test data to the engineer of record for acceptance.
Comparison
Hollow Bar vs. Solid Bar Micropiles
Both are high-capacity micropile systems. The right choice depends on ground conditions, required capacity, and project constraints. We install both and recommend based on what your site actually needs.
Choose Hollow Bar When:
Ground is caving or collapsible and won’t hold an open hole. Water table is at or above the drilling elevation. Schedule requires maximum installation speed. Cobbles or boulders are present in the soil profile. Moderate capacities (up to ~400 kips) are sufficient.
Choose Solid Bar When:
Very high capacities are required (500+ kips). The hole can be drilled and held open with casing or in stable rock. Larger bar cross-sections are needed for structural steel capacity. Corrosion protection requirements demand specific bar coatings or encapsulation.
Hollow Bar Advantages
Faster installation (single pass). No open hole risk. Works in conditions where solid bar can’t. Pressurized grout creates enlarged grout body for increased bond. Segmental couplers allow low-headroom installation.
Solid Bar Advantages
Higher ultimate steel capacity with larger bar diameters. Better corrosion protection options. Post-grouting available for additional bond enhancement. Preferred for the highest-capacity applications (1,000+ kips).
Featured Project
Industrial Silo Foundation, Wasatch Front
Rocky Mountain Micropiles installed an array of 7″ OD hollow bar micropiles to support new industrial silos at an existing facility. Each pile was designed to carry nearly 300 kips in compression, with additional lateral and shear capacity to resist wind-induced overturning forces — the silos extend well above the adjacent building roofline, creating significant wind loading on the foundation system. The tight working corridor between existing buildings required precise pile placement and compact equipment.