Effect of Aggregate Uniformity on Pullout Resistance of Steel Strip Reinforcement

Results are presented of large-scale laboratory pullout tests on ribbed steel strip reinforcement embedded in six types of aggregates with uniformity coefficients (ratio of the particle size at 60% finer to that at 10% finer) ranging from 1.4 to 14. These aggregates satisfied the AASHTO standard gradation requirements for select backfill. Each backfill material was placed in two lifts and compacted until the required degree of compaction was achieved. The normal stress was applied with a pressurized air bag placed on top of the compacted backfill. A double-acting hydraulic jack was used to apply the pullout load, which was measured with a load cell. Displacement transducers were installed to measure the rear and front strip displacements. The test results demonstrated that the uniform aggregates generally behaved the same way as did the well-graded aggregates in terms of pullout resistance. The effect of uniformity was more obvious in the tests under a lower normal stress than those under a higher normal stress. Moreover, the pullout resistance factors, F*, from this study lie to the right of the default F*-values for ribbed strip reinforcement suggested by AASHTO. The comparison also shows that the pullout resistance factor for ribbed steel strips decreased with depth in the same way as that suggested by AASHTO. However, the F*-values recommended by AASHTO are conservative as compared with the test results when gravel backfill is used. The formula suggested by AASHTO can be used to conservatively estimate the F*-value with the uniformity coefficient ranging from 1.4 to 14.