Category Archives: Soil Nail Walls

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.

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.

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.

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.

Shotcrete
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.

Appearance
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.

 

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.

 

Anchor rig drilling soil nails

The things we (and you) should ask when Preparing and Evaluating a Shoring Bid

Common questions we ask ourselves when preparing a shoring bid:

1.Have we clearly communicated the scope of work included in our shoring bid proposal?

2.Will the ground water table cause trouble?

3.How deep is rock?

4.Open hole drilling or casing?

5.Are there reasons to expect drilling obstructions?

6.Can sloping be used to avoid utilities and/or reduce shoring quantity?

7.What utilities will need to be located, exposed?

8.Shotcrete quantity?  Is our assumption reasonable?  Add and deduct.

9.Will rock be encountered at the face of excavation?  Line drilling, blasting, trimming in PWR/rock.

10.Aesthetics of permanent shotcrete?

11.One side forming?  Foundation wall overpour.

12.Will the owner have easement(s)/permission to drill under adjacent property/ROW?

13.Access, ramps, phasing of ramps?

14.Are there environmental concerns that could impact safety and/or scope of work, e.g. Brownfield sites?

15.LF of micropiles in base bid. What triggers add/deduct?

16.What is our client’s bid situation?

17.Are our assumptions reasonable?

18.What information do we need to get started?

Subsurface South Carolina Construction News

Subsurface is featured in SC Construction News

Engineers build successful Carolinas shoring construction business with participation of employee owners

South Carolina Construction News special feature

Formed in 1995 by three engineers, two structural and one geotechnical, Subsurface Construction Company LLC has grown over time to two regional offices, a staff of 35 and the capability of handling multiple, complex shoring and deep foundation projects at once.

The three founders started in the business as consulting engineers in Raleigh, NC, designing shoring systems for installation by contractors. Greg Sullivan, P.E., Subsurface’s managing member, says the group recognized that there were very few shoring contractors in the area, saw the market need, and launched Subsurface.

“We began in the shoring construction business by installing our driven soil nail walls, a unique shoring system that was developed by our company,” he said.

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