Micropile Foundation Quality Control
Using some basic micropile foundation quality control measures will help your 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.
Other Resources
FHWA Micropile Manual
IBC 2015 Chapter 18 Deep Foundations
Guide to Drafting a Specification for Micropiles