An Updated Effective Stress (β_HBM) Method for Predicting Hollow-Bar Micropiles’ Pullout Capacity in Sandy Soils

The effects of construction parameters on the development of a generalized static analysis method for predicting overall axial pullout capacity of hollow-bar micropiles (HBM) in sandy soils were described in this article. One of the most commonly methods for predicting ultimate shaft resistance of deep foundation in sandy soils is effective stress or “β” method. Beta-coefficient (β), a function of coefficient of lateral earth pressure and the interface friction angle between pile wall and the adjacent soil, is primarily developed from the field test results of driven pile or drilled shaft whose diameter is predetermined. However, the self-drilling HBMs have an additional unknown variable, that is, annulus or diameter, which needs to be evaluated accurately. The annulus of the HBMs varies significantly as the central hollow-bar penetrates soil at different penetration rates and grout pump flow rates. The type of soils and existing density also play important role in forming HBM annulus. The influence of both the construction methods and existing soil conditions was evaluated statistically by multivariate regression analysis, and optimized models were developed to predict the unknown diameter of HBMs and the beta-coefficient for the effective stress method. Eight HBMs, four instrumented and four non-instrumented, were constructed at a site in the Outer Banks of North Carolina in two phases. The field-testing program included installation of HBMs to a depth of 25 ft (7.62 m) while varying the drilling or drill-bit insertion rate (I_R) and the grout flow rate (Q_R). In addition, the installation methods included HBMs that were continuously drilled and grouted with neat grout water-cement ratio (w/c) of 0.4, and others that were first drilled and grouted continuously with thinner grout (w/c-0.7) and then flushed from bottom to top with thicker grout (w/c-0.4). Eight HBMs, designated as Fast/Fast (w/c-0.4), Fast/Slow (w/c-0.4), Slow/Fast (w/c-0.4), Slow/Slow (w/c-0.4), Fast/Fast (w/c-0.7/0.4), Fast/Slow (w/c-0.7/0.4), Slow/Fast (w/c-0.7/0.4), and Slow/Slow (w/c-0.7/0.4), were tested for ultimate pullout capacity. Four of those instrumented HBMs were retrieved from the field after the load test to measure the diameter of the HBM at every foot. The back-calculated diameter and pull-out test results were regressed by multivariate regression analysis to develop modified effective stress (β_HBM) method for predicting uplift pullout capacity of those non-instrumented HBMs. The newly pullout capacity prediction models predicted pullout capacity within a range of ±30% of the measured ultimate pullout capacity.