Quality Control for Soil Nail Walls
One of our founders has a saying, “checking is cheap.” Another one most of us have heard is, “measure twice, cut once.” Using some basic 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 soil nail walls.
Receipt of Materials
Ensure that the steel bars are the correct size and grade to meet the design drawings. Check the mill certifications against the project specifications. Be mindful of Buy America requirements in the contract documents as some materials are foreign made, particularly hollow bar soil nails. Make similar inspections for the welded wire fabric and rebar needed for shotcrete.
When receiving shotcrete be sure that the ticket matches the approved mix design for the project and that the concrete is at the right slump and temperature. Do not hesitate to reject a hot truck on a summer day that has sit in traffic too long.
Inspect the other typical materials such as centralizers (diameter), drain board, and pvc drain outlets to be sure that they match the design drawings.
Be cautious in storing steel bar, particularly corrosion protected soil nails such as epoxy coating. 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 as the excavation progresses. 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!
After each excavation, inspect the cut face to see if the soil matches the design soil values assumed. Be on the lookout for zones of weaker soils, such as a dike or isolated fill area. Notify the engineer of record immediately if you encounter soils that are obviously weak or appear inconsistent with the surrounding soil.
Check the drill holes to verify that the diameter matches the design drawings. Check the length of the bars to be inserted in drilled holes. To be sure that the holes have not collapsed, observe that the bars go easily into the drilled holes. Mark the grout tube with paint to know that the tube is inserted to the end of the drilled hole 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. Havingd the mud balance data is critical to back up the grout cube compression testing as the compression tests occasionally have an unexplained poor break.
Prior to placing shotcrete, ensure the welded wire fabric and rebar are sized and placed per the design drawings. Check the chair height to be sure the reinforcement will be centered in the shotcrete face. Assist the owner’s representative/special inspector by providing concrete samples during shotcrete activities.
Most soil nail walls start with two or more verification nails. These are sacrificial nails installed prior to the start of production work to verify the design grout to ground bond values. In soil nail walls, 5% of the soil nails are typically proof tested to verify design grout to ground bond values as the project progresses.
FHWA Field Quality Control of Materials Checklist
- For steel components, centralizers, and drainage materials, obtain samples for testing (when specified) and check all Mill Test Certifications for compliance with the specifications.
- Visually check all soil nail tendons and reinforcing steel for damage and defects upon delivery and prior to use.
- Visually check epoxy coated or encapsulated tendons for compliance with the specifications and for any damage to the corrosion protection.
- Confirm mix design compliance of soil nail grout and facing shotcrete.
- When specified, take grout (cubes) and/or shotcrete samples (test panels and cores) for testing.
- Verify compliance of geocomposite drainage materials with the contract plans/specifications. Verify adequacy of field storage of construction materials to prevent damage or degradation.
Learn more about soil nail walls.