Monthly Archives: February 2021

photos of micropiles for supporting a tower crane

Micropile Design

Micropile Design
Structural Design

Design Criteria
Micropiles are designed by two standards, the Federal Highway Administration (FHWA) and the 2015 International Building Code (IBC).  Of course, the FHWA standard applies to highway projects and the IBC standard applies to building construction.  The main difference in the two standards are the allowable stresses.  FHWA allows for slightly higher stresses to be imposed on the steel and grout.  The following equations shown the difference.

FHWA NHI-05-039 December 2005
0.47*Asteel*Fy-steel + 0.40*Agrout*fc-grout = Compression Capacity

IBC 2015
0.40*Asteel*Fy-steel + 0.30*Agrout*fc-grout = Compression Capacity

Note that Fy-steel is limited to a maximum of 87 ksi.  This is to limit the strain to that which can be tolerated by the grout.

Cased Zone
Within the cased micropile zone, both the steel casing and the inner reinforcing steel are combined for the total area of steel. Be mindful that the steel casing and the reinforcing steel may have different yield strengths.  The lower strength value must be used to determine the steel contribution.  Again, note that the maximum allowable steel strength that can be used is 87 ksi.

Asteel = Acasing + Areinforcing

Uncased Zone
Within the uncased micropile zone, only the steel reinforcing and the grout are available to carry the load.

Asteel = Areinforcing

Geotechnical Design

The geotechnical capacity of the micropile is typically determined by the bond to rock within the rock socket.  While end bearing can be significant in some cases, micropiles are typically designed using only the bond to rock given the small diameter of the piles.

While the best estimate of the bond capacity in rock is based on geotechnical data and local experience, allowable bond stresses within the rock socket vary from 3 ksf (20 psi) for fine grained partially weathered rock such as a silt stone to 20 ksf (138 psi) or higher for granite.  Estimation of the bond value can be made by looking at the % sample recovery and rock quality designation (RQD). With that said, nothing trumps past local test data.

Micropiles can also be supported along the continuous length of the pile.  The most common approach for this method is a hollow bar micropile.  Hollow bar micropiles are installed by advancing a hollow threaded rod with a sacrificial bit into the ground.  The pile is grouted as it is drilled into the ground.  Once the hollow bar has reached its design depth, the pile installation is complete.  While the material cost is higher than some micropile systems, the hollow bar avoids the need for casing and is therefore especially helpful in coastal soil profile where rock may be incredibly deep.

Resources

FHWA Micropile Manual
IBC 2015 Chapter 18 Deep Foundations
Guide to Drafting a Specification for Micropiles
Learn more about micropiles.

photo of a small drill rig installing micropoiles

Micropile Advantages

What are the advantages of micropiles?

 

Overcoming Difficult Ground Conditions

Micropiles can overcome ground conditions such as debris fill, natural boulders, and karst. Other foundations systems such as auger cast piles and driven piles are not suitable in these soil conditions.

Debris Fill
Subsurface installed micropiles to support a hotel developed on an old debris fill.  The fill consisted of soil, boulders, and concrete debris.  Micropiles can be installed through such difficult fill conditions using overburden drilling systems.  These drilling technologies undercut the casing to often allow for pile installation through the obstructions.

Karst
Karst is water soluble rock.  In the work area of Subsurface Construction, karst is commonly encountered in the mountains of Virginia and eastern Tennessee.  Due to voids, softer layers, and rock pinnacles, deep foundations such as auger cast piles or driven piles are often not feasible in karst geology.  Because micropiles are installed by advancing casing and by using overburden drilling systems that can penetrate rock layers, micropiles can be advanced through the voids and soft layers and into competent rock.

image of karst geology showing layers, voids and sink holes

Karst Geology – Figure 1 in Taylor, Charles J., and Earl A. Greene. “Hydrogeologic characterization and methods used in the investigation of karst hydrology.” US Geological Survey (2008). Chapter 3 of Field Techniques for Estimating Water Fluxes Between Surface Water and Ground Water, Edited by Donald O. Rosenberry and James W. LaBaugh, Techniques and Methods 4–D

Underpinning

Micropiles can be used for underpinning failing foundations, seismic retrofits, and to support new columns for vertical expansions.  Micropiles can provide 400 kip vertical loads in as little as 9 feet of head room using 10″ diameter micropiles.

Underpinning
Micropiles can be used to underpin existing foundations by coring through the existing foundations and installing micropiles.  The piles would then be bonded to the existing foundations to connect the piles to the existing building.

Micropiles can be used to support existing foundations to enable shoring near existing buildings.  Underpinning is especially helpful in supporting a soil nail shoring system near a building.

sketch of a footing underpinning with micropiles to aid a shoring sytems

Micropile Underpinning of a Footing with a Shoring System

Micropiles can also be used to underpin failing foundation systems.

photo of underpinning a drilled shaft

Underpinning a Drilled Shaft with 4 Micropiles and Attached with Concrete Beams

Limited Access

Micropiles are often the only possible deep foundation system for areas of limited access.  Micropiles can be installed in as little as 9′ of headroom.  The limited access drilling equipment is run with electricity or exterior hydraulic power packs such that no fumes are introduced inside the building.

low overhead micropile drilling inside a hospital

Low Overhead Micropile Drilling in a Hospital

Learn more about micropiles.

Anchor rig drilling soil nails

Quality Control for Soil Nail Walls

Quality Control for Soil Nail Walls

One of our founders has a saying, “checking is cheap.”  Another one most of us have heard is, “measure twice, cut once.”  Using some basic quality control measures will help your project move forward with fewer mistakes and will avoid dreaded rework.  With that in mind, here are some basic QC steps for soil nail walls.

Receipt of Materials

Ensure that the steel bars are the correct size and grade to meet the design drawings.  Check the mill certifications against the project specifications.  Be mindful of Buy America requirements in the contract documents as some materials are foreign made, particularly hollow bar soil nails.  Make similar inspections for the welded wire fabric and rebar needed for shotcrete.

When receiving shotcrete be sure that the ticket matches the approved mix design for the project and that the concrete is at the right slump and temperature.  Do not hesitate to reject a hot truck on a summer day that has sit in traffic too long.

Inspect the other typical materials such as centralizers (diameter), drain board, and pvc drain outlets to be sure that they match the design drawings.

Storage

Be cautious in storing steel bar, particularly corrosion protected soil nails such as epoxy coating.  Be sure the bars are handled and stored in such a way as to prevent damage to the coatings.  Store materials in an area that will likely not require relocating materials multiple times as the excavation progresses.  Minimizing handling will reduce the likelihood of damage.

Cement must be kept off the ground and wrapped in plastic to ensure that it remains dry.  This will lead to a better grout and will prevent clogs in grout pumps and hoses.  No one wants a 94lb door stop!

Construction

After each excavation, inspect the cut face to see if the soil matches the design soil values assumed.  Be on the lookout for zones of weaker soils, such as a dike or isolated fill area.  Notify the engineer of record immediately if you encounter soils that are obviously weak or appear inconsistent with the surrounding soil.

Check the drill holes to verify that the diameter matches the design drawings.  Check the length of the bars to be inserted in drilled holes. To be sure that the holes have not collapsed, observe that the bars go easily into the drilled holes.  Mark the grout tube with paint to know that the tube is inserted to the end of the drilled hole and observe that the tremie grouting method is used.

Use a mud balance to check that the grout is the right consistency to reach the required design strength.  Assist the owner’s representative/special inspector by providing samples of grout for compression testing.  Havingd the mud balance data is critical to back up the grout cube compression testing as the compression tests occasionally have an unexplained poor break.

Prior to placing shotcrete, ensure the welded wire fabric and rebar are sized and placed per the design drawings.  Check the chair height to be sure the reinforcement will be centered in the shotcrete face.  Assist the owner’s representative/special inspector by providing concrete samples during shotcrete activities.

Nail Testing

Most soil nail walls start with two or more verification nails.  These are sacrificial nails installed prior to the start of production work to verify the design grout to ground bond values.  In soil nail walls, 5% of the soil nails are typically proof tested to verify design grout to ground bond values as the project progresses.

FHWA Field Quality Control of Materials Checklist
  • For steel components, centralizers, and drainage materials, obtain samples for testing (when specified) and check all Mill Test Certifications for compliance with the specifications.
  • Visually check all soil nail tendons and reinforcing steel for damage and defects upon delivery and prior to use.
  • Visually check epoxy coated or encapsulated tendons for compliance with the specifications and for any damage to the corrosion protection.
  • Confirm mix design compliance of soil nail grout and facing shotcrete.
  • When specified, take grout (cubes) and/or shotcrete samples (test panels and cores) for testing.
  • Verify compliance of geocomposite drainage materials with the contract plans/specifications. Verify adequacy of field storage of construction materials to prevent damage or degradation.


Learn more about soil nail walls.