Tag Archives: design

photos of micropiles for supporting a tower crane

Micropile Design

Micropile Design

Micropile design includes structural and geotechnical design. Let’s discuss the importance of these two types of design for your next commercial construction project.

Structural Design

Design Criteria
Micropiles are designed by two standards, the Federal Highway Administration (FHWA) and the 2015 International Building Code (IBC).  Of course, the FHWA standard applies to highway projects and the IBC standard applies to building construction.  The main difference in the two standards are the allowable stresses.  FHWA allows for slightly higher stresses to be imposed on the steel and grout.  The following equations show the difference.

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.  This is to limit the strain to that which can be tolerated by the grout.

Cased Zone
Within the cased micropile zone, both the steel casing and the inner reinforcing steel are combined for the total area of steel. Be mindful that the steel casing and the reinforcing steel may have different yield strengths.  The lower strength value must be used to determine the steel contribution.  Again, note that the maximum allowable steel strength that can be used is 87 ksi.

Asteel = Acasing + Areinforcing

Uncased Zone
Within the uncased micropile zone, only the steel reinforcing and the grout are available to carry the load.

Asteel = Areinforcing

Geotechnical Design

The geotechnical capacity of the micropile is typically determined by the bond to rock within the rock socket.  While end bearing can be significant in some cases, micropiles are typically designed using only the bond to rock given the small diameter of the piles.

While the best estimate of the bond capacity in rock is based on geotechnical data and local experience, allowable bond stresses within the rock socket vary from 3 ksf (20 psi) for fine grained partially weathered rock such as a silt stone to 20 ksf (138 psi) or higher for granite.  Estimation of the bond value can be made by looking at the % sample recovery and rock quality designation (RQD). With that said, nothing trumps past local test data.

Micropiles can also be supported along the continuous length of the pile.  The most common approach for this method is a hollow bar micropile.  Hollow bar micropiles are installed by advancing a hollow threaded rod with a sacrificial bit into the ground.  The pile is grouted as it is drilled into the ground.  Once the hollow bar has reached its design depth, the pile installation is complete.  While the material cost is higher than some micropile systems, the hollow bar avoids the need for casing and is therefore especially helpful in coastal soil profile where rock may be incredibly deep.

Resources

FHWA Micropile Manual
IBC 2015 Chapter 18 Deep Foundations
Guide to Drafting a Specification for Micropiles
Learn more about micropiles.

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

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.