Soil Stability Inspection Guidelines

When chemical modification is being applied to dry and improve the workability of a soil as a construction expedient, mix designs and detailed inspection is not typically required. The chemical modifying agent is applied and mixed until the desired drying/workability is obtained.

With soil modification/stabilization, like most paving projects, the primary goal is to produce a strong, durable subgrade or subbase course that is ready to receive the final pavement section.  Inspectors can help achieve this goal by ensuring that the process is in accordance with the contract documents as follows: 

Mix Design: Verify representative samples were obtained throughout the project and that a mix design(s) was performed by a qualified lab resulting in a mixture that meets or exceeds the contract documents.

Surface Preparation: Walk or drive the entire project section to recieve soil modification/stabilization for the day to ensure the area has been properly prepared and that all pre-construction repairs have been performed.

Acceptable Weather: Measure the air temperature and check conditions for anticipated heavy rainfall and freezing weather to determine if construction can start/continue.

Traffic Control: Verify that the proper traffic control is in place to keep the traveling public safe during construction.

Quality Equipment:

• Verify that the on-board modifying/stabilizing agent system of the reclaimer is equipped with a meter capable of recording the rate of flow and total amount of each liquid being added to the stabilized material.
• Verify that nozzles and spray bars of the reclaimer or central plant mixer are working properly and are not plugged.
• Verify that the bulk spreader is properly calibrated and is capable of accurately dispensing the required quantity of dry modifying/stabilizing agent.
• Verify that motor graders used have cross slope indicators and are in accordance with the contract documents.
• Verify that the reclaimer/tiller is properly mixing the material.
• For foamed asphalt, verify that the proper half-life and expansion ratio is being maintained.
• Verify that the compaction equipment is of the required type, number and size and that scrapers and spray bars are properly working.
• Verify that the modifying/stabilizing agent system of the central plant mixer is equipped with a meter capable of recording the rate of flow and total amount of each liquid added to the stabilized base material
• Check the modified/stabilized mixture for uniformity and homogeneity as it is discharged from the central plant mixer
• Verify that the haul trucks are of the required type, size, number and that tarps are working properly

Quality Workmanship:

• Verify that the modifying/stabilizing agent is being properly mixed.
• Monitor the amount of water introduced to maintain the specified range of optimum moisture content.
• Verify that the modified/stabilized mixture is mixed and placed to the proper depth, grade and cross-slope. 
• Check that longitudinal and transverse joints are properly constructed.
• For cementitious stabilizing agents, verify that maximum mixing and compaction time frames are maintained.
• Verify that a control strip was established to determine the compaction procedures.
• Verify that the rolling procedure established in the control strip is followed and that the compacted density is within the specified range. 
• Verify that the completed section is trimmed to the proper grade and cross-slope.

Application Rates: Periodic checks should be made to verify that the amount of modifying/stabilizing agent added to the subgrade is within allowable tolerances.

Curing:

• Verify that the modified/stabilized mixture can support the anticipated loads and will not impair material curing before opening the section to light traffic.
• Verify that the completed modified/stabilized subgrade is properly cured.