Category Archives: Micropiles

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.

Micropiles

Micropile Foundations – Are they a good fit for this project?

Micropile Foundations

Is this project a good fit for micropiles? This is a question that I’m asked quite often.  Micropiles are an excellent deep foundation option to solve two common problems – difficult ground conditions or limited access.

What are Micropiles?

Let’s start with a brief description.  A micropile is a drilled and grouted replacement pile less than 12 inches in diameter.  Capacities typically range from 50 kips to 500 kips with higher loads sometimes achieved with larger micropiles (>12”).  To install a pile, contractors typically advance casing through the overburden material to the top of rock then drill a rock socket. Steel reinforcing is lowered into the casing and rock socket then grout is tremie pumped from the pile tip to the top of the pile displacing any ground water. The grout bonds the reinforcing steel to the rock socket. The load is predominantly carried by steel reinforcing – a pipe, threaded rod(s), or both.  The grout also contributes.

Micropiles for Difficult Ground Conditions

When deep foundations are required, and other pile solutions are not feasible due to difficult ground conditions, then micropiles are a great option.  Micropiles are commonly installed in situations where driving piles or installing auger cast piles are prohibitive due to obstructions such as construction debris or boulders.  Advanced overburden drilling technologies developed by the mining industry allow for installation of casing through concrete or boulders. A recent micropile project we completed required installation of micropiles by drilling through 40’ to 50’ of debris laden fill to reach original ground.

Another common use of micropiles due to difficult ground conditions is limestone karst.  Karst is water soluble rock and is encountered in our region in the mountains of Virginia, West Virginia and eastern Tennessee. In geologic time, ground water has dissolved rock leaving behind pockets of soft soil and voids. The ground layers are often tilted or folded creating a situation where a pile will go through soil, then rock, then a void, and then rock again. Due to the overburden drilling systems, micropiles can be advanced through these varied conditions to competent rock. Often in karst, depth to rock varies wildly. Since micropiles are installed in sections, the pile length can be increased as required.

Micropiles for Limited Access

Micropiles are also a great solution when deep foundations are required but access is limited. When a vertical expansion is needed for an existing structure, micropiles can be installed in low headroom conditions (typically 8’ to 9’) to reinforce existing footings or to add new columns for the vertical expansion. The small rigs operate with umbilical cords so that the diesel fumes are on the outside of the building. Other limited access conditions may include drilling below bridges, between structures or in courtyards. We have installed micropiles for a hospital vertical expansion while patients were treated on the other side of the wall.

See Our Micropiles Service

Other Micropile Resources

DFI Micropile Committee

ADSC Micropile Resources

Chapter 18 IBC (now includes micropiles)

FHWA Micropile Design Manual 2005 FHWA-NHI-05-39