Monthly Archives: March 2021

photo of an anchored soldier pile wall

Soldier Pile Wall Advantages

What are the advantages of a soldier pile wall?
Flexibility

Soldier Pile Walls are the go-to excavation shoring solution in densely populated urban areas such as city centers, hospital campuses, and universities.  Soldier piles allow for flexibility when working around existing buildings and existing utilities.  For taller walls, anchors can be installed 10′ below existing grade and can be sloped up to 45 degrees to get below existing utilities.

In the sketch below, the top row of anchors (red) were installed around existing utilities.  The 2nd row of anchors (yellow) were angled more steeply and were installed below the existing utilities.

3-D sketch showing anchored soldier piles with underground utilities

A 3-D model of soldier piles and anchors installed around existing utilities.

Stiffness

Solder pile walls are more stiff than soil nail walls.  The anchors for the soldier piles are post tensioned for the load of the full wall height.  By post tensioning the anchors as the excavation progresses, wall deflections are limited.  For this reason, soldier pile walls may used when shoring near existing buildings and sensitive utilities.

While limit equilibrium analyses are most often used to design soldier pile walls, performance designs can also be done using numerical modeling software such as Plaxis to estimate the movement of the wall.

Output of numerical modeling of an anchored pile wall

Performance design of an anchored wall.

Wide Variety of Face Options

A wide variety of facings can be used on permanent soldier pile 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 soldier pile wall and “bottom-up” retaining walls such as segmental block walls or cast concrete retaining walls.  Since multiple veneers are possible, the all the site walls can match.  We really like architectural shotcrete finishes on permanent soil nail walls such as those constructed by BoulderScape.

Retaining Wall with architectural shotcrete finish.

Ideal Soil Conditions in our Region

Soldier pile walls work well in the piedmont residual soils found in our region of Virginia, North Carolina, South Carolina, Tennessee and Georgia.  Because they are constructed in 5′ lifts, the soil must have sufficient apparent cohesion to stand for 24 to 36 hours while the wood lagging is installed.  With good geotechnical data and thoughtful design, unsaturated soil mechanics can be used to optimize the design.

Micropile Drilling

Quality Control for Micropiles

Quality Control for Soil Nail Walls

Using some basic quality control measures will help your micropile project move forward with fewer mistakes and will avoid dreaded rework.  With that in mind, here are some basic QC steps for micropiles

Receipt of Materials

Ensure that the threaded steel bars and casing pipes are the correct size and grade to meet the design drawings.  Check the mill certifications for the threaded rods against the project specifications.  Provide coupon testing as required by the specifications for the secondary mill pipe used as casing. Be mindful of Buy America requirements and prime steel requirements, especially with regard to the micropile casing.  Buy America and prime pipe are most commonly see for DOT projects.

Critically – be sure that any casing that must be bonded to the grout is free of coatings.  Sometimes secondary mill casing has a black coating meant to prevent rust.  Grout will NOT stick to this coating.  If casing pipe show up without significant rust (brown color), BEWARE!  If pipes are coated and the grout must bond to them, then the pipes must be returned so they can be sand blasted clean of any coatings.

Storage

Be cautious in storing steel bar, particularly corrosion protected bar such as epoxy coating or encapsulated bars.  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.  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

Check the drill holes and casing diameters to verify that the diameters match the design drawings.  Check the length of the reinforcing bars to be inserted into the micropiles. To be sure that the rock socket has not collapsed, observe that the reinforcing bar goes easily into the drilled holes.  Mark the grout tube with paint to know that the tube is inserted to the tip of the micropile 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.  Having the mud balance data is critical to back up the grout cube compression testing as the compression tests occasionally have an unexplained poor break.

Micropile Testing

Verification Testing – Verification tests are done on sacrificial micropiles prior to the start of production.  There are two testing options – full compression tests (ASTM D-1143) that typically require 4 tie-downs and tension tests (ASTM D-3689).  The compression test has the advantage of providing the performance of the piles under design compression load, but the disadvantage of significant cost.  Tension verification testing can be used to verify the bond to rock and since only one pile is drilled, a tension test is significantly less expensive.

Proof Testing – Proof tests are typically done by loading production piles to 133% of design load.  Piles can be proof tested in tension or compression.  Typically, 5% of production piles are proof tested or a minimum of 1 pile in each area of piles (e.g. proof testing one micropile in each bridge end bent).

Field Quality Control of Materials Checklist
  • For steel components, obtain coupon samples for testing (when specified) and check all Mill Test Certifications for compliance with the specifications.
  • Visually check all reinforcing steel for damage and defects upon delivery and prior to use.
  • Be sure that any steel pipe used is coating free so that the grout will stick.
  • Mud balance grout batches.
  • When specified, take grout (cubes)  for testing.
  • Verify adequacy of field storage of construction materials to prevent damage or degradation.


Learn more about micropiles.

Other Resources

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