All posts by Jeffrey W. Coonse, PE

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.


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 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


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.

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.


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!


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.

Photo showing installation of a soil nail wall

How Does a Soil Nail Wall Work?

How does a soil nail wall work?

Soil nail walls are typically designed using a limit equilibrium analysis.  Think of the sliding block experiment in high school physics lab where you may have calculated what force is required to overcome friction and move a sandpaper block along an incline.  In a soil nail wall, as a wedge of soil (see the curved plane in the diagram below) tries to slide into the excavation, the wedge of soil is retained by the soil nails and the friction of the ground moving against itself.  The soil nails get their capacity from the friction between the grouted nail hole and the ground.


Diagram showing a soil nail wall and the slip surface of the soil.

Soil Nail Wall Diagram

Soil nails are loaded as the excavation progresses

As the excavation proceeds deeper from lift 1 to lift 2 to the final lift, the slip surface moves further back from the face of the wall.  The wall moves outward and the soil nails go into tension and begin resisting the movement.  The portion of the nails beyond the slip surface resists the movement by the friction between the drilled nail and the ground surface.  Note that we typically make the top rows longer than the lower rows (unlike Figure 5.1) to reduce the lateral movement of the wall.  Soil nail walls typically move 0.1% to 0.4% of the wall height or 0.25″ to 1″ for a 20′ tall wall.

Diagram showing how the soil nails are tensioned as the excavation progresses.

Figure 5.1 from FHWA Soil Nail Manual

Why are soil nail wall faces so thin?

Interestingly, while the friction between the soil nail wall and the ground provides the resisting forces to hold up the wall, the friction also works in reverse and reduces the nail tension at the face of the wall.  The soil that moves in front of the slip surface drags along the soil nail wall.  As you can see in Figure 5.4, nail tension is at a maximum about 35% of the height behind the wall and is reduced as you move along the length of the soil nail toward the face.  This reduction in tension force in the soil nail reduces the punching shear force of the soil nail at the face of the wall.

Diagram showing that the maximum tension in the soil nail wall is at the slip surface.

Figure 5.4 from FHWA Soil Nail Manual

Soil nail wall design

There are several commercial software products used to design soil nail walls.  The majority of the soil nails are designed using a limit equilibrium analysis.  For example, the design insures that you have 35% more resisting force than driving force for a factor of safety of 1.35.  Occasionally, soil nail walls are designed based on performance to limit movement.  In this case the wall would require a design approach that accounts for the stiffness (modulus) of the soil and of the soil nail wall elements.  For this type of design a numerical modeling software, such as Plaxis would be used.

SnailWin Soil Nail Software Output

SnailWin Soil Nail Software Output

Learn more about soil nail walls.

Photo of Alex Smith

Alex Smith is the New Managing Member

Congratulations to Alex Smith on becoming the new Managing Member at Subsurface Construction.  Alex is a professional engineer, a partner at Subsurface, a volunteer in his community and our friend.  We sat down with Alex to see what makes him tick.

Briefly describe what you do all day.

For the first 12 years at Subsurface, I was a project manager and estimator who had to learn and gain experience by relying on mentors and jumping in and figuring things out for myself. I learned quickly by making some mistakes and asking a lot of questions. I was fortunate to learn from some of the best leaders in the industry, right here at Subsurface. In the past year or so, I have been involved in implementing a new operating system for the business and learning about other facets of the business that I had no experience with. This has opened my eyes to how many different people and processes it takes to make a business operate effectively and safely.

What is the best thing about your job?

Hands down it is the people. I really enjoy meeting new people and helping people achieve their goals and aspirations. I really enjoy getting feedback from others that we have made a positive impact. I want everyone who comes in contact with Subsurface, whether it is a client, vendor, or team member, to have an awesome experience and believe that we as a team have added value to the projects and more importantly to their lives

What is the best thing about Subsurface?

Once again that is the people who I get to know and work with daily. The relationships that have been made over the past 14 years are what I cherish the most. Being able to contribute to a team to build great projects, learn from one another, help people achieve their goals, and develop friendships has been fulfilling. Having fun in what I do is awesome as well.

What has been one of your proudest moments working at Subsurface?

I would say that even though I have had some very difficult times over the years, whether it be project related or personal, that I have never given up and never had to compromise mine nor the company’s values. I am proud to be associated with a company in which the leaders encourage others to always do what is right and to uphold our core values of Honesty, Creativity, Respect & Dignity, and Self-Motivation. I am proud to be associated with some of the best people I have ever met and that we are a company where individuals bring their own talents and gifts together to accomplish great work, but more importantly help one another grow as individuals.

What do you like to do when you are not at the office?

I enjoy spending time with my beautiful wife Kelly, my son Landon (7 years old), and Hallie (3 years old). I enjoy duck hunting and spending time outdoors.

What inspires you?

Life. I am thankful that God has given me so many opportunities and that I can live everyday knowing that He has a purpose for me and that in some way I can help others achieve or find their purpose as well. I am inspired by seeing how others have special gifts and how they use those gifts to benefit the team. There is no one person who can do all things and seeing others become successful and add value through their gifts inspires me.

Why engineering?

I think it was what I was called to do. I was never overly excited about engineering, but it was the path that was laid out before me. The engineering path has prepared me to solve problems but has also opened many doors for helping people. I am glad I went this route.

What is the most important attribute of a good leader?

I would say always do what is right. We all make mistakes, but if you make decisions on what you believe is right and truthful, usually things will work out for the best.

Also, listening to and caring for people. There are a lot of things we cannot change about this world but having a positive impact on someone’s life is a good start. We are all leaders in some way and have influence on others. The way we treat and serve people will have an impact on future generations. One of our best opportunities to make a positive impact is with the people we work with.

Permanent soil nail wall during shoring construction project

Soil Nail Wall Advantages

What are the advantages of a soil nail wall?
Speed and Cost

Soil nail walls are installed more quickly and cost effectively than other shoring systems.  Soldier pile walls or sheet pile walls require large equipment to drill in place or drive the piles or sheets.   Soil nails avoid the time and cost of installing these vertical elements.  The material cost of soil nail walls is less than that of anchored soldier piles with wood lagging and substantially less than that of sheet pile walls.  Driven soil nail walls are even faster, with installation times reduced by 50%.

Anchor rig drilling soil nails

Anchor rig drilling soil nails at high rate of productivity

Encountering Rock

Soil nail walls are an excellent alternative when soil overlies hard material or rock.  Often, below grade excavations for basements or underground parking extend to or into rock.  Rock at the bottom of shoring elevation precludes the use of sheet piles as the sheets cannot be driven into the rock.  Soldier pile walls will work in in this situation, but the piles must be drilled below the bottom of shoring and toed into the rock.  Soil nail walls can be installed in the soil and into the rock should the excavation extend into rock.  The small diameter drilling required for soil nails is more easily done than the large diameter drilling required for soldier piles.

Waltonwood Apartment Complex Drilling

Permanent soil nail wall with 4th lift in weathered rock

Wide Variety of Face Options

A wide variety of facings can be used on soil nail walls.  Permanent walls can have a finished shotcrete face or any veneer such as brick, stone, or modular blocks. Many sites required a mix of “top-down” retaining walls such as a soil nail wall and “bottom-up” retaining walls such as segmental block walls or cast concrete retaining walls.  Since multiple veneers are possible on soil nail walls, the soil nail wall can match the other site walls. In particular, the finished shotcrete face can act as the permanent face of the soil nail wall, saving in the cost of shotcrete.  We really like architectural shotcrete finishes on permanent soil nail walls such as those constructed by BoulderScape.

Soil nail wall with architectural finish

Ideal Soil Conditions in our Region

Soil nails work well in the piedmont residual soils found in our region of Virginia, North Carolina, South Carolina, Tennessee and Georgia.  Because soil nail walls are constructed in 5′ lifts, the soil must have sufficient apparent cohesion to stand for 24 to 36 hours while the nails are drilled and the shotcrete is applied.  With good geotechnical data and thoughtful design, unsaturated soil mechanics can be used to optimize the soil nail design.

Learn more about soil nail walls.

photo of Greg Sullivan

Congratulations to Greg Sullivan

With admiration, gratitude, and sadness we announce that Greg Sullivan will retire at the end of 2020.

Greg has led Subsurface Construction as the Founder and Managing Member for 25 years.  He has inspired us with care for people, servant leadership, engineering expertise and wisdom.  Greg plans to spend time with family and friends in Morehead City and Raleigh upon his retirement.  He and Terry also look forward to traveling once it is safe to do so.  Please join us in congratulating Greg on his retirement.  Thank you, Greg, for inspiring us to be better individuals, teammates, and family members.  Well done!!

Permanent soil nail wall during shoring construction project

Permanent or Temporary Soil Nail Wall?

Do you need a permanent or temporary soil nail wall?  Here are the similarities and differences.

Both Remain in Place
Both permanent and temporary soil nail walls remain in place.  Temporary soil nails are backfilled against and abandoned in place.  Permanent soil nail walls remain in place to serve as site retaining walls or to keep earth pressures off basements.

Design Differences
Permanent soil nail walls are designed with higher factors of safety and often with more conservative soil properties than temporary soil nail walls.  Permanent walls are designed using little or no cohesion as future water content changes in the soil behind the wall would affect the apparent cohesion of the soil.

Corrosion Protection 
Permanent soil nail walls require corrosion protection of the soil nails.  Three common options are epoxy coated soil nails, galvanized soil nails (especially with hollow bar soil nails) and multiple corrosion protection.  Williams Form offers some nice details on corrosion protection options for soil nails and ground anchors at this link, corrosion protection.

The shotcrete face of a permanent soil nail wall is designed per ACI to ensure that the rebar and WWF have sufficient concrete coverage to limit corrosion.  ACI restrictions on placing shotcrete must be strictly followed for permanent work.  Permanent soil nail walls should receive curing compound.  With regard to construction joints, joints are not used in temporary soil nail walls are are not effective in permanent soil nail walls.  For more on that, see the ADSC position paper on construction joints for soil nail walls.

Temporary soil nail walls are “as-shot” meaning that the surface of the shotcrete will have a very rough texture and will follow the profile of the excavated ground.  The bearing plate connection of the soil nail is most often exposed.  Permanent soil nail walls are often screeded.  For screeded walls, the surface of the shotcrete has a coarse texture and a screed rod is used to cut the shotcrete to alignment wires installed in each lift.  The bearing plate connection of the soil nail is typically not exposed.  Permanent walls are sometimes covered with a veener, such as brick, segmental block, cast concrete, or my favorite, architecturally sculpted shotcrete that closely imitates masonry or native rock.


Empathy is Imperative – Construction Leadership

Expressing genuine empathy is critical to being a successful leader, salesperson and coworker.  Want to lose someone’s confidence?  Show them you’re more focused on short term productivity, closing the deal or getting what you want in spite of their wants and needs – professionally or personally.

Listen to understand , then respond.

When in conversation, how often are we preparing our response well before the other person has finished speaking?  In our fast paced culture we can forget that we must listen first to understand then to respond.  It’s the old joke you heard from your grandmother about God giving you two ears and one mouth.  When we rush ahead we leave the other person feeling undervalued.  Who would want to do that to a direct report, client or coworker?  I have learned the phrase “listen to understand and validate” or LUV.  We can’t help a client or coworker until we understand where they’re coming from and what they need.  We can then top that off by letting them know we have heard and understand their frustration, concern or need, i.e. the validation.

In The 7 Habits of Highly Effective People, Stephen Covey writes, “To relate effectively with a spouse, child, friend or coworker, we must learn to listen.  And this requires emotional strength.  Listening involves patience, openness, and the desire to understand – highly developed qualities of character.”

In the tough and fast paced construction industry, may we all aspire to show one another the respect and kindness of listening to understand.


Read about Level 5 Leaders Here.




OSHA Announces “Targeted Industries in Construction” for silica enforcement.

Silica Enforcement in the Construction Industry

We should consider the OSHA’s new silica law a good thing.  The permissible exposure limit for silica has been cut from 250 ug/m3 to 50 ug/m3 – a five fold decrease.  We depend on our crew members.  The stakes are high.  Silica causes silicosis – lung disease, kidney disease, increased risk for cancer and death.  But there is a way to be safe.  Follow Table A and turn on the vacuum or water.

A special thank you to Rick Marshall of Association of Drilled Shaft Contractors for educating me and sharing the “U.S. Department of Labor Revises National Emphasis Program to Reduce or Eliminate Worker Exposure to Silica” with our members.

Here is a list of the “Targeted industries in Construction.”  And yes, shoring and micropile contractors are on this list.  Be safe and healthy!