Archives

Biogen Soil Nail Walls

Soil Nail Walls

What is a soil nail wall?

A soil nail wall is a a “top-down” retaining wall constructed in 5′ lifts by drilling rows of nails and applying a shotcrete face.  The nails start at the existing ground surface and are installed as the ground in front of the wall is excavated in lifts.  The purpose is to construct a retaining wall (temporary or permanent) where sloping per OSHA is not possible due to existing structures, utilities, or property lines.

The first soil nail wall was constructed in 1972 near Versailles, France to stabilize a 60′ deep excavation for a railroad widening project.  Prior to soil nails, most excavations were stabilized with driven sheet piles or H-piles with wood lagging that were either anchored or internally braced.

Temporary or Permanent Wall

Advantages
    • Soil nail walls are installed more quickly and cost effectively than other shoring systems.
    • Soil nails are an excellent alternative when soil overlies hard material or rock.
    • A wide variety of facings can be used..  Permanent walls can have a finished shotcrete face or any veneer such as brick, stone, or modular blocks.
    • The nails can be drilled and grouted or driven members.
    • Soil nails work well in the piedmont residual soils found in our region of Virginia, North Carolina, South Carolina, Tennessee and Georgia.

  • Read more about the advantages of soil nails

Limitations
    • For construction, the soil must stand in 5′ lifts during construction.  Therefore, soil nails walls are not typically used in clean sands such as might be found in coastal plain geology.
    • Since the nails are installed on a typical 5′ x 5′ pattern, it is not a good solution where multiple utility conflicts may be encountered in the zone of nails.

Construction

Contractors drill nails on a close pattern and applying the shotcrete facing in lifts.   A 5′ lift is excavated.  A row of nails are then installed on that lift.  Steel reinforcement such as WWF and rebar are then placed at and shotcrete is applied.  That lift is now complete and the site is excavated an additional 5′ and the process is repeated.

Nails are typically drilled on a 5′ x 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. The nails are installed by drilling 4″ to 6″ diameter holes into which a threaded rod is inserted.  The hole is then grouted to attach the threaded rod to the ground.

    • Excavate 5′
    • Drill 4″ to 6″ diameter holes and grout in place an all thread bar
    • Place reinforcement and shotcrete
    • Proof test 5% of the nails
    • Repeat process to the bottom of the excavation

Temporary Drilled Soil Nail Wall

 

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 grouted nail hole and the ground.  The walls are designed to have a factor of safety of 1.35 to 1.5, meaning 35% to 50% 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.

Learn more about soil nail wall design

Learn simple QC measures for a successful wall

Other Resources

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


Photos

When we need a soil nail wall sub again, I highly recommend Subsurface.  They worked with our ever changing schedule without complaining.

Eric Perkinson, Brasfield & Gorrie

Photo of an Anchored Soldier Pile Wall

Soldier Pile Walls

Anchored soldier pile walls are offered as a solution when the site requires a stiff excavation shoring system, especially with multiple utility conflicts.

What is a soldier pile wall?

A soldier pile wall is a a “top-down” retaining wall consisting of vertical steel piles, wood lagging, and anchors as required.  The purpose is to construct a retaining wall (temporary or permanent) where sloping per OSHA is not possible due to existing structures, utilities, or property lines.  Soldier piles are the most common shoring system used for excavations in dense downtown areas.

Soldier piles were the original excavation shoring system.  In the past, this system was commonly referred to as a Berlin Wall owing to its use in Europe more than a century ago.   These walls were braced internally to retain the earth.  Anchors were developed for a dam in Africa in the 1930’s.  Anchors were later used with soldier pile walls to eliminate internal bracing.

Soldier Pile Wall Advantages
    • For modest heights, 14′ or less, soldier piles can be cantilevered to eliminate the need for temporary easements from adjacent property owners.
    • The prestressed anchors limit movement as compared to soil nails that are not prestressed.
    • Soldier pile walls are helpful in avoiding existing utilities.  The top row of anchors can be started 8′ to 10′ below the top of wall and angled as steep as 45 degrees to get under existing utilities.
    • A wide variety of facings can be used.  Permanent walls can have a finished shotcrete face or any veneer such as brick, stone, or modular blocks.
    • Soldier pile walls work well in the piedmont residual soils found in our region of Virginia, North Carolina, South Carolina, Tennessee and Georgia.
    • More advantages
  • Limitations

    • For construction, the soil must stand between the piles in 5′ lifts during construction.  Therefore, soldier pile walls are not typically used in clean sands such as might be found in coastal plain geology.
    • The cost is higher than soil nail walls and can increase significantly when rock is encountered.

 

Construction

Contractors drive or drill and set piles depending on ground conditions, vibration restrictions, and noise limits. Piles are typically installed on an 6′ to 10′ horizontal spacing.  Wood lagging is installed in two 5′ lifts to a depth of 10′.  At this point, anchors would be drilled and prestressed for the load required for the full height of the excavation.  Wood lagging would then continue in 5′ lifts to the bottom of the excavation.

    • Drive or drill and set vertical steel piles on an 6′ to 10′ horizontal spacing.
    • Install wood lagging in 5′ lifts
    • Drill anchors starting approximately 10′ below the top of the wall.
    • Anchors are installed on the same horizontal spacing as the piles and additional rows are used every 10′ or more vertically.
    • Continue installing wood lagging in 5′ lifts to the bottom of shoring elevation.

More Construction Details

 

Sketch showing cross section of a soldier pile wall

Safe Installation of Walls

Design

Contractors typically design soldier pile walls using a limit equilibrium analysis.  Think of the sliding block experiment in high school physics lab where you  calculated the force required to overcome friction and move a sandpaper block along an incline.  As a wedge of soil tries to slide into the excavation, the wedge of soil is retained by the steel piles, anchor force, and the friction of the ground moving against itself.  The piles provide resistance by pushing against the soil in front of the wall below the excavation.  The anchors get their capacity from the friction between the grouted anchor and the ground.  The walls are designed to have a factor of safety of 1.35 to 1.5, meaning 35% to 50% more resisting force than is required to stabilize the wall.  Since the anchors are prestressed, wall movement is limited.

Other Resources

FHWA Ground Anchors and Anchored Systems
DFI Anchored Earth Retention Committee


Photos
What is a soldier pile wall?

A soldier pile wall is a a “top-down” retaining wall consisting of vertical steel piles, wood lagging, and anchors as required.  The purpose is to construct a retaining wall (temporary or permanent) where sloping per OSHA is not possible due to existing structures, utilities, or property lines.  Soldier piles are the most common shoring system used for excavations in dense downtown areas.

Soldier piles were the original excavation shoring system.  In the past, this system was commonly referred to as a Berlin Wall owing to its use in Europe more than a century ago.   These walls were braced internally to retain the earth.  Anchors were developed for a dam in Africa in the 1930’s.  Anchors were later used with soldier pile walls to eliminate internal bracing.

Soldier Pile Wall Advantages
    • For modest heights, 14′ or less, soldier piles can be cantilevered to eliminate the need for temporary easements from adjacent property owners.
    • The prestressed anchors limit movement as compared to soil nails that are not prestressed.
    • Soldier pile walls are helpful in avoiding existing utilities.  The top row of anchors can be started 8′ to 10′ below the top of wall and angled as steep as 45 degrees to get under existing utilities.
    • A wide variety of facings can be used.  Permanent walls can have a finished shotcrete face or any veneer such as brick, stone, or modular blocks.
    • Soldier pile walls work well in the piedmont residual soils found in our region of Virginia, North Carolina, South Carolina, Tennessee and Georgia.

 

  • Limitations

    • For construction, the soil must stand between the piles in 5′ lifts during construction.  Therefore, soldier pile walls are not typically used in clean sands such as might be found in coastal plain geology.
    • The cost is higher than soil nail walls and can increase significantly when rock is encountered.

 

Construction

Contractors drive or drill and set piles depending on ground conditions, vibration restrictions, and noise limits. Piles are typically installed on an 6′ to 10′ horizontal spacing.  Wood lagging is installed in two 5′ lifts to a depth of 10′.  At this point, anchors would be drilled and prestressed for the load required for the full height of the excavation.  Wood lagging would then continue in 5′ lifts to the bottom of the excavation.

    • Drive or drill and set vertical steel piles on an 6′ to 10′ horizontal spacing.
    • Install wood lagging in 5′ lifts
    • Drill anchors starting approximately 10′ below the top of the wall.
    • Anchors are installed on the same horizontal spacing as the piles and additional rows are used every 10′ or more vertically.
    • Continue installing wood lagging in 5′ lifts to the bottom of shoring elevation.

 

Sketch showing cross section of a soldier pile wall

 

Design

Contractors typically design soldier pile walls using a limit equilibrium analysis.  Think of the sliding block experiment in high school physics lab where you  calculated the force required to overcome friction and move a sandpaper block along an incline.  As a wedge of soil tries to slide into the excavation, the wedge of soil is retained by the steel piles, anchor force, and the friction of the ground moving against itself.  The piles provide resistance by pushing against the soil in front of the wall below the excavation.  The anchors get their capacity from the friction between the grouted anchor and the ground.  The walls are designed to have a factor of safety of 1.35 to 1.5, meaning 35% to 50% more resisting force than is required to stabilize the wall.  Since the anchors are prestressed, wall movement is limited.

Other Resources

FHWA Ground Anchors and Anchored Systems
DFI Anchored Earth Retention Committee


Photos

The Subsurface Construction team exceeded our expectations in every phase of the project. When challenges presented themselves, the field and office teams were proactive in overcoming them and stayed on schedule. From the bottom to the top, they are driven...

Andrew Hagood, Balfour Beatty

Micropiles

Micropiles

What is a micropile?

A micropile is a high capacity small diameter pile that can be installed through difficult soil conditions and in limited access.  Micropiles are typically installed by advancing casing pipe to rock, drilling a rock socket, then grouting reinforcing steel in the rock socket.

In spite of their small diameter, less than 12″, micropiles capacities typically range from 100 kips to 400 kips.

sketch of a typical micropile

Typical Micropile

Advantages
    • Micropiles can be installed through boulders and debris.  Unlike other deep foundations, contractors can use overburden drilling systems that cut ahead of the casing to get through very hard boulders and concrete debris.
    • Micropiles are the best deep foundation system in challenging karst geology.  Karst is water soluble rock.  In Subsurface Construction’s service area, karst is found in the mountains of Virginia and eastern Tennessee.  Because karst contains layers of hard rock and soft soil or voids, micropiles can be cased through the soft zones to competent rock.
    • Micropiles are ideal in limited access areas such as basements.  The piles are often used to underpin failing foundations or to support new columns for vertical expansions.  400 kip piles can be installed in 9-feet of headroom.  Contractors can use equipment that is powered by electricity or hydraulic power packs stored outside the building to avoid introducing fumes inside the existing building.


Is my project a good fit for micropiles?

Advantages of Micropiles

Limitations
    • Due to their small diameter, typically 6″ to 12″, vertical micropiles have limits in resisting lateral loads.  One solution is the embed the piles 2 diameters into the pile cap to create a fixed-head condition, cutting the lateral deflection in half in comparison to a free-head condition.  Another option is to batter some of the micropiles.  1/4(H):1(V) or 15 degree batters are typical.  The battered piles are then able to use the resultant horizontal force to resist lateral loads.
    • Micropiles are the most economic and sometimes only solution in difficult ground conditions and limited access.  For sites that have more homogenous soil profiles, other foundations such as auger cast piles or driven piles are preferred.

Construction

Micropiles are constructed by advancing casing through the overburden soil to the top of rock.  The casing is threaded together in sections as the piles are advanced.  Once the top or rock is reached, a hole is drilled in rock with a bit that fits inside the casing.  The rock socket is an open hole in rock that is typically 5″ to 8″ diameter depending on the casing size and pile capacity needed.  Rock sockets are 5′ to 20′ or more in length depending on the quality of the rock and the pile capacity.  Once the casing has been advanced to rock, the inner drill rod, down hole hammer, and bit are extracted leaving the cased hole and open rock socket.  Steel reinforcement is then lowered through the casing and into the rock socket.  The hole is then tremie grouted displacing any ground water.  The grout bonds the reinforcing steel to the rock socket, provides corrosion protection, and assists the steel in carrying compression load.

    • Duplex drill casing through the overburden soil to the top of rock.
    • Continue drilling without the casing to make the rock socket.
    • Extract the drill steel, down hole hammer, and bit.
    • Lower reinforcing steel (typically a threaded rod) into the casing and rock socket.
    • Tremie grout the full length of the pile.

Learn more about QC of Micropiles

photo of a small drill rig installing micropoiles

Limited Access Micropile Drilling

Design

The majority of the compression load is carried by steel with a contribution from the grout.  Micropile design is governed by two main bodies, the FHWA and International Building codes.  The structural capacity  is determined by combining the allowable contribution of the steel and grout.

FHWA NHI-05-039 December 2005
0.47*Asteel*Fy-steel + 0.40*Agrout*fc-grout = Compression Capacity

IBC 2015
0.40*Asteel*Fy-steel + 0.30*Agrout*fc-grout = Compression Capacity

Note that Fy-steel is limited to a maximum of 87 ksi.

Other Resources

FHWA Micropile Manual
IBC 2015 Chapter 18 Deep Foundations
Guide to Drafting a Specification for Micropiles

Photos

Due to the location of the project, North Carolina State University, I was very impressed at the crew’s manners and respect for their surroundings. They acted very professionally.

Charlie McIntosh, Environment Quality Resources

photo of a driven soil nail wall

Driven Soil Nail Walls

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

micropile underpinning and soil nail shoring

Underpinning

Underpinning with Excavation Shoring

All excavation shoring systems move……a little, not a lot, but they do move.  Soil nails move about 0.1% to 0.4% of the wall height or a 0.25″ to 1″ for a 20′ tall wall.  When shoring near an existing building, underpinning the existing building limits the risk of settlement associated with movement of the shoring system.

We often underpin lighter buildings, say 2 or 3 stories, with Atlas Resistance Piers or Helical Piers.  Both pier types are advanced to a suitable bearing stratum below the proposed excavation then are locked off against the weight of the building.

To underpin large buildings prior to excavation and shoring, we will underpin using micropiles.  To underpin with micropiles, we core the existing footings then install the micropiles through the cored holes.  Once the micropiles are installed, we epoxy the micropiles to the existing footing.

Cross section showing micropiles and soil nail shoring

Micropile Piles and Soil Nail Shoring


Photo of micropiles and soil nails

Photo of the Micropile Underpinning and Shoring

Conventional Underpinning

Conventional staged concrete underpinning is a good option to lower the bearing elevation of an existing wall footing.  This is especially helpful to construct zero lot line shoring against an existing building.  This underpinning method requires undermining wall footings in pits and backfilling with concrete in stages.

photo of staged underpinning

Beginning Staged Underpinning

photo showing later stages of conventional underpinningLater Stages of Conventional Underpinning

Underpinning to Stop Settlement

When buildings settle due to poor design or settlement of poorly compacted fill, underpinning with Atlas Piers, Helical Piers, or Micropiles are a good option.

As the Piers are loaded against the dead load of the structure, they transfer existing building load from the footings to the Piers.  Since micropiles are grouted directly to the footings, any future settlement of the building will be limited to the elastic shortening of the piles (the micropile steel compressing under the load).

Subsurface did a great job upfront making sure that we were installing the most effective solutions for our project. And despite a challenging schedule and unexpected underground conditions, the job was still completed ahead of schedule.

Greg Shenkler, Skanska USA Building