Driven Soil Nail Walls

photo of a driven soil nail wall
What is a driven soil nail wall?

Driven soil nail walls consist of driven steel bars with a composite geotextile fabric and wire mesh face, improving installation time by 50% over conventional shoring methods.

photo of a driven soil nail wall

Driven Soil Nail Wall

Just like other shoring systems, a driven soil nail wall is a a “top-down” retaining wall constructed in 5′ lifts by driving rather than drilling rows of steel bars.  The nails start at the existing ground surface and are installed as the ground in front of the wall is excavated in lifts.  Rather than shotcrete, driven soil nail walls use a composite geotextile fabric and wire mesh face.  This saves prep time and avoids waiting on shotcrete delivery and cure time.

Unlike other soil nail walls, driven nail walls can be excavated immediately after installation, saving time and money.

Why You Should Use Driven Soil Nail Walls (video)

Advantages
    • Driven soil nail walls are installed more quickly and cost effectively than other shoring systems.
    • Driven soil nails avoid waiting for grout and concrete to cure.
    • Grading contractors also save money as the excavation can proceed more quickly.
  • Limitations
    • Driven soil nail walls work best in soil – you can’t drive steel bars in weathered rock.
    • As with other soil nail systems, utility conflicts must be limited so that then nails can be installed.

Construction

After a 5′ lift is excavated, we drive steel bars (soil nails) on a close pattern through a non-woven geotextile fabric and wire mesh face.    That lift is now complete and the site is excavated an additional 5′ and the process is repeated.

Nails are typically driven on a 2.5′ x 2.5′  pattern.  The length of the nails are typically 100% to 125% of the wall height.  The nails may decrease in length near the bottom of the wall.

Due to the close spacing of the driven nails, the composite geotextile and wire mesh face is sufficient rather than shotcrete.

    • Excavate 5′
    • Place geotextile fabric and wire mesh
    • Drive steel bars on a close spacing
    • Proof test 5% of the nails
    • Repeat process to the bottom of the excavation
cross section of a driven soil nail wall

Driven Soil Nail Wall Sketch

Design

Contractors typically design walls 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.  As a wedge of soil (see the curved plane in the diagram above) 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 nails get their capacity from the friction between the driven nail and the ground.  The walls are typically designed to have a factor of safety of 1.35, meaning 35% more resisting force than is required to stabilize the wall.  Since some movement is required for the soil to develop sliding friction against itself and for the nails to engage in friction, soil nail walls typically move 0.1% to 0.4% of the wall height; 1/4″ to 1″ on a 20′ wall.

Other Resources

FHWA Soil Nail Manual
DFI Anchored Earth Retention Committee
Guidance for Design of Temporary Soil Nail Walls, DFI and ADSC


Photos

We wish all subcontractors were as professional and cooperative as your team.

Dwayne Rakestraw, New Atlantic Contracting