Category Archives: 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.

Read the full article >