Typical Vibro Pier construction begins with pre-drilling a full-depth hole with a diameter that is equal to the final pier design diameter. In soft soils, a slightly smaller diameter may be used due to pier enlargement during compaction. Aggregate is then introduced to the hole and compacted in lifts by repetitive ramming with a powerful, specially-designed vibrator. For soils in which the pre-drilled hole will not stay open, the bottom feed process can be used to avoid the need for casing. In the bottom feed process, aggregate is fed through a tremie pipe attached to the vibrator.

The technique will yield reinforced ground conditions to increase bearing capacity and shear resistance, and reduce settlement from new loads. Anchor bars are incorporated during pier construction when tension resistance is required.

For seismic applications, Vibro Piers can be very effective in reducing dynamic settlement. If loose granular layers are present, the process is a very effective densification technique, reducing the liquefaction potential.

Typical Arrangement of Vibro Piers

Schematic of Subsurface Stress Distribution

Vibro Pier Frequently Asked Questions

What is a Vibro Pier^TM? 

A Vibro Pier is a dense aggregate pier constructed to provide subsurface reinforcement for light to heavily loaded shallow foundations. Vibro Piers increase bearing capacity, reduce structural settlement, and provide liquefaction mitigation.    

How are Vibro Piers installed?

An auger is typically used to pre-drill a designed diameter hole in the soil in which the Vibro Pier is constructed. Aggregate is then placed in the hole in lifts and a down hole vibrator is repeatedly rammed into each lift of stone. This displaces the aggregate both downward and outward, densifying both the aggregate and the surrounding soil. Additional aggregate is added in incremental lifts and the vibrator is repeatedly raised and rammed to construct a dense aggregate column.  

Can you install uplift elements in Vibro Piers?

Yes.  There are several ways of providing uplift resistance.  Anchor systems have been developed to provide uplift capacity from Vibro Piers.  Vertical high strength bars fastened to a bottom plate are placed in the densified aggregate during construction.  The bars are then connected structurally to the foundation.  Alternately, high capacity soil or rock anchors can be installed between the piers to provide even higher uplift resistance.

Can you build Vibro Piers under the ground water?      

Yes.  When the pre-drilled hole will not stay open, Vibro Piers can be constructed utilizing a “bottom-feed” system. This system does not require special tools or casing, which other aggregate pier systems may charge extra for (as a change order) after the initial design and award.

How deep can Vibro Piers treat?      

Vibro Piers are considered intermediate foundation systems, so by definition they typically extend no deeper than 20 to 30 feet.  However, vibro-installed aggregate piers can be installed to depths greater than 100 feet.  This allows owners and engineers to provide improvement for static loads or seismic mitigation for sites with liquefiable soils located at greater depths.

Can Vibro Piers control liquefaction of sandy soil?      

Yes. This is the only aggregate pier technology approved by the State of California for treating liquefaction on hospital and school sites.  This technology is the most respected, proven liquefaction mitigation method used in North America. The vibro-installation of Vibro Piers imparts sufficient energy to the surrounding soils to densify cohensionless materials. This mitigates potential liquefaction of the surrounding soils. In less densifiable soils, such as silts and clays, the liquefaction induced settlement is reduced due to the reinforcement of the soils by the dense aggregate piers.

How do I know if my site has been successfully treated for liquefaction mitigation?

Properly designed and executed post-construction quality control and quality assurance (QA/QC) testing is the only way to ensure successful liquefaction mitigation.  Owners and Owner Representative Engineers (consulting engineers) must insist on proper post construction testing.  Industry standards such as  California’s SP-117, “Guidelines for Evaluating and Mitigating Seismic Hazards in California”  and the National Center for Earthquake Engineering’s guidelines (NCEER, 1997) provide guidance regarding the required Standard Penetration Test (SPT), Cone Penetration Test (CPT) or shear wave velocity results that must be observed after installation.   A responsible engineer will ensure adequate post-construction verification for the owner using these industry standards.

How do QC methods vary between a Vibro Pier and other aggregate pier systems for Liquefaction Mitigation?

When Vibro methods are used to mitigate liquefaction, a rigorous QC program is undertaken to assess the condition of the soil.  Once the Vibro Piers are installed in granular soils, Standard Penetration Tests (SPT) or Cone Penetrometer Tests (CPT) are conducted throughout the site to measure post treatment densification. Hayward Baker conforms to SP-117, “Guidelines for Evaluating and Mitigating Seismic Hazards in California ” and NCEER, 1997.

How are Vibro Piers different than other aggregate pier systems?

When properly executed, each aggregate pier system constructs a dense aggregate pier in the soil and engages the aggregate and soil interface.  Vibro Piers use a high power, down hole vibrator that is capable of developing very high vertical and lateral compactive effort.  The applied loading is typically 2 to 10 times the energy of vertical tamping methods, which results in a very dense aggregate column and stiffer soil/aggregate boundary. 

Does the vibro process remold clays or disturb the soil around the column?

In clays, the Vibro Pier installation process typically uses a pre-drilled hole into which the aggregate is placed and densified using the vibro probe to avoid disturbance or remolding of the soil. 

How do Vibro Piers perform in Earthquakes?

Hundreds of projects have been constructed on the west coast of the U.S. in the past 30 years on vibro-constructed aggregate piers.  Many of these structures were tested during the Loma Prieta, Northridge and Nisqually Earthquakes.  Every structure built on our vibro-constructed aggregate piers performed very well with no damage due to foundation performance issues.

How does the stiffness of a Vibro Pier compare to that of other aggregate pier systems?

Vibro Piers have been proven to have a high modulus equal to those of other aggregate pier systems.  Vibro Piers don’t rely on a heavy crowd pressure to densify the aggregate; rather, they obtain their stiffness from the vertical and horizontal loading imposed by the repeated ramming with the down hole vibrator.  Structures have been built on Vibro Piers worldwide, meeting the exact same high modulus and settlement specifications.  Hayward Baker can meet or exceed any design specification that a responsible engineer requires.  Owner representatives can save their clients money and time by avoiding ‘sole-source’ specifications of densified aggregate piers.  Legally, publicly financed or publicly owned structures cannot specify ‘sole-source’ aggregate piers, as Vibro Piers are an equivalent product that is often more economical.

I recently received a quote on a Vibro Pier foundation and another aggregate pier system, and I noticed that the Vibro Piers were deeper.  Why were the Vibro Piers deeper?   

Since the stiffness of both systems is identical, the compression of the treatment zone will be equal with either system.  However, a responsible design-build contractor is required to address (and design for) settlement below the treatment zone in addition to the treated zone.  Hayward Baker uses classical soil mechanics principles to determine the increase in stress below the treatment zone.  Owner Representative Engineers must ensure that they agree with the principles and mechanics employed to determine the compression of soils below the treatment zone.  Rather than design short columns and attribute excessive settlement to the soils below, we ensure that we design our treatment to limit total settlement.  This provides a true turnkey solution rather than shifting design liability to the design engineer.  

I understand that some aggregate pier systems can be installed in peat, clay, silt and sand.  Do you install Vibro Piers in all these soil profiles?

Vibro Piers can and have been constructed in these soil types.  However, there are different ground modification methods that are also appropriate for various ground conditions.  Since Hayward Baker is a full-service geotechnical contractor, we work with owners and consulting engineers to select the tool that provides the most value for the client. Others that only install aggregate piers will promote them when other techniques may be more technically appropriate and provide better value to the customer.

Why should I choose Hayward Baker?

Hayward Baker’s network of regional offices and full-service equipment yards means fast mobilization and reduced startup costs. From the job start-up to installation of the last Vibro Pier, attention to quality control ensures that project specifications are achieved. Using Vibro Piers as part of your foundation system significantly reduces construction schedules and project costs by permitting the use of shallow spread footings rather than a traditional deep foundation system.

Hayward Baker, North America ’s leader in specialty geotechnical construction, is committed to providing the most economical solution that satisfies the technical requirements of each project. Whether a common situation, or one that requires unparalleled experience and creativity, Hayward Baker assists engineers, contractors and owners with identifying and implementing the right solution for their project.