Monthly Archives: January 2021

Photo showing installation of a soil nail wall

How Does a Soil Nail Wall Work?

How does a soil nail wall work?
Concept

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.