All posts by Jeffrey W. Coonse, PE

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!

Plaxis Model

Considering Movement of Excavation Shoring Systems

Considering Movement of Excavation Shoring Systems

Are you shoring to limit excavation into a tree protection area?  Are you shoring to excavate near an historic masonry church?  Of course, we would all be more concerned about the shoring system chosen at the church, but which system?  The first step is to choose the right system.  Soil nail walls move 0.1% to 0.4% of the height of the wall or ¼” to a 1” for a 20-ft cut.  Anchored soldier piles will move less as anchors are tensioned for the load the wall will encounter by the time you excavate to the bottom of the cut.  For projects where movements are especially critical, shoring walls that eliminate shotcrete lifts or wood lagging such as a secant pile wall may be considered.

But it is still difficult to assess the amount of movement that may occur as construction methods may affect movement more than design.  What is the height of each lift?  Is each lift installed quickly after excavation?  Are the lagging boards placed tight to the soil?  Did a large storm pass halting construction for days? Construction methods have a large effect on movement.

Most shoring systems are designed to a minimum factor of safety.  Permanent retaining wall structures are often designed with a factor of safety of 1.5 and temporary structures with a factor of safety of 1.35.  Higher factors of safety result in a wall system under less stress and therefore less strain or movement.  Sometimes walls are designed to performance requirements.  This requires the designer to model the excavation and shoring system using a finite element analysis software such as Plaxis to estimate movement.

Finite Element Model of Anchored Secant Pile Wall Near a Heavy Building

Whether designed using a factor of safety or performance method, measuring movement as the excavation progresses will alert the contractor to any concerns prior to reaching the bottom of the excavation.  One effective way to measure is using an inclinometer.  Inclinometers track the movement (angle changes) from the bottom to the top of a casing over time.   The casing can be installed in or behind the shoring wall. Contractors can monitor the movement as the excavation and wall installation progress.  Figure 2 shows that the top of the wall leaned out after the first 5-ft cut, then the top was pushed back by the anchor, and as expected the largest movement outward occurred between the anchor and the bottom of the excavation (in this case less than 1/8”).

Wall Movements as Excavation Progresses

Ultimately the successful performance of a retaining wall shoring system depends on selection of the appropriate retaining wall system, competent design, high quality construction techniques,  and monitoring of the progress.


Robert Jones Serves as Field Supervisor for AMPIS Drilling School

We would like to recognize our General Superintendent, Robert Jones, for serving as Field Supervisor for the Anchor and Micropile Installation School (AMPIS).  ADSC annually puts on AMPIS to better the drilling skills of those in our industry.

To learn more about ADSC’s training opportunities, visit ADSC-IAFD

Congratulations to TJ Ju – a leading engineer, businessman and friend on becoming a US Citizen

Congratulations to TJ Ju, PE, on becoming a US Citizen.  TJ is our lead geotechnical design engineer, a partner at Subsurface, a volunteer in his community and our friend.  We sat down with TJ to see what makes him tick.

Briefly describe what you do all day.

I design excavation shoring and deep foundation systems, respond to various field issues for ongoing projects and brainstorm with team members on new proposal ideas.   I stay up to date reading technical papers from geotechnical engineering journals.

What’s the best thing about your job?

Each project is unique and demands the most creative solution for its success. From time to time I encounter a very challenging one, but I work with very talented team members including PMs and superintendents to get it through. This is very enjoyable and rewarding.

Whether I am designing a shoring wall or deep foundation system, I know our work is one of the first activities on site. Success of the whole project always requires a great start. Our design and construction are essential parts for the project’s success.

What’s the best thing about Subsurface?

Subsurface genuinely cares about people, not just our employees, but also our clients and project owners.  Subsurface works together as a team. We have very talented team members, engineers, estimators, PMs and superintendents. We always do our best and learn from each other.

What’s been one of your proudest moments working at Subsurface?

The most memorable moment in my 17 years at Subsurface was the day I was offered partnership. Since starting Subsurface in Spring 2002, I have tried my best to help the company grow. Then I felt that the founders had recognized the work I did for Subsurface. I was very thankful and truly felt Subsurface caring, not only its success, but also each employee’s growth and success.

What do you like to do when you’re not at the office?

I enjoy spending time with my family. We often go biking and swimming all together. NC is the best state for a family and I want to take advantage of it as much as possible. I also like to travel with family and read books.

What inspires you?

God. He always works through people around me to give inspiration.

Why engineering?

I enjoyed math and physics through my K-12 years and wanted to help practically people in need. Also, my father was an engineer (textile engineer, also businessman) and taught me what engineers could possibly do for the world. Truly it is a joy to see my designs being built and later being used by other people.

If you weren’t an engineer, what would you be doing?

Movie director. When I was in college in Korea, I used to work with a group of people making independent movies. I enjoyed it very much. At one point I seriously thought about becoming a movie director but gave it up due to some personal reasons.

What lead you to become a US citizen?

Below is a copy of my short note to a dear friend who congratulated me on becoming a US citizen. I think that this can be my answer to this question.

“I do appreciate your thoughtful and caring words. Now I can tell you this. Truly what led me to become an American today is not that it is the richest country in the world full of great opportunities, but Americans like you, everyone who has supported me, cared for me and treated me as a real friend of theirs for past 20 years. I assure that you are definitely one of them, my dear American friend. Once again thanks a lot for everything you have done for me and my family. May the Lord always be with you.”



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 micorpiles)

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



level 5 leadership pyramid

Are you a Level 5 Leader?

Leading in the Construction Industry


Jim Collins in “Good to Great” defines Level 5 Leaders as those who blend personal humility with intense professional will.  While Collin’s team didn’t set out to focus on leadership, during their research they observed that Level 5 leaders were at the helm of every Good to Great company during their transition to greatness.   The difference between Level 4’s  and Level 5’s is one of direction.  While Level 4’s are effective, they focus inwardly on success.  Level 5’s use their immense energy to forward the larger cause rather than themselves.  Unlike conventional wisdom that may seek a larger than life CEO, successful companies require leaders that are a blend of personal humility and fierce resolve.

Unlike their Level 4 counterparts, Level 5’s see the future of their organizations as being brighter after their departure.  They work hard to set up their successor to be even more successful.  Rather than quick returns and personal accolades, Level 5’s seek to create an enduring organization.

While humble and outwardly looking, Level 5’s have a strong professional will to do what must be done to move the organization forward.  They are fanatically driven to produce results.  With high standards, Level 5’s often set aside tradition and nepotism to recruit the best possible team members.  They will change with the times and rebuild as needed.

Harry S. Truman said, “You can accomplish anything in life, provided that you do not mind who gets the credit.”

Our entire team gets credit for all of our project accomplishments. You can see them here.

Recommended reading: “Built to Last” and “Good to Great” by Jim Collins
anchor rig drilling soil nails

Silica Exposure Control Programs in the Drilling Industry

Silica Exposure Control Programs in the Drilling Industry

We are all gaining familiarity with “Table 1. SPECIFIED EXPOSURE CONTROL METHODS WHEN WORKING WITH MATERIALS CONTAINING CRYSTALLINE SILICA” of OSHA’s Crystalline Silica Rule for Construction –  a title that kind of rolls right off the tongue.  Table 1 was OSHA’s attempt to guide contractors in reducing silica exposure below the new permissible exposure limit (PEL) of 50 ug/m3.  For most of the construction industry, it largely works and boils down to two things – use water or use a vacuum.  Common tasks such as cutting, chipping, drilling and grinding concrete are listed in Table 1 along with the precautions the contractor must do to stay below the PEL.  Do these tasks with these precautions and you’re all set.  But what happens when your task is not listed?  This is the place that the drilling industry finds itself.

The tie-back and micropile industry drills rock on a regular basis.  According to Encyclopedia Britannica, “The mass of Earth’s crust is 59 percent silica, the main constituent of more than 95 percent of the known rocks.”  OSHA requires that exposure to safety or health hazards be eliminated through administrative or engineering controls if possible.  Our industry most commonly drills rock with down the hole hammers.  These hammers are single pistons at the bottom of the drill rods that actuate a bit up and down through the power of high pressure, high volume air compressors.  The primary engineering control is to drill with water.  The volume and pressure of the water must be sufficient to reduce silica exposure below the PEL.  So how do we know that we are protecting our workers during drilling?  We test.

The OSHA rule for silica exposure has two options for assessing employee exposure when not following Table 1 – the Performance Option and the Scheduled Monitoring Option.  The Performance Option requires assessing employee exposure based on any combination of testing and objective (read industry) data.  Unfortunately, we don’t have industry data, but we’re working on it.  More from ADSC on this in the future.  At Subsurface Construction are focusing on the Scheduled Monitoring Option for drilling.  Scheduled Monitoring means testing for respirable silica on individual employees while drilling with engineering controls in place.  Depending on the results, OSHA guides the employer as to what additional testing is required.  Unfortunately, monitoring is not a one-time event as the rock formations and drilling conditions can vary site to site.  To date, we have not been over the PEL when drilling rock with water, but we will continue to wear respirators as we gather more data.

Recommended reading:
Recommended action: Join an upcoming Silica Competent Person class through ABC, AGC or local safety training partners.