An Approach to Predict Unknown Diameter of Hollow-Bar Micropiles (HBMs) in Sandy Soils Considering Installation Parameters

摘要

A common method for predicting ultimate shaft resistance of pile foundation in sandy soils is based on the use of effective stress and utilizing a beta-coefficient (β) that incorporates the soil-pile interface effects. The β is a function of coefficient of lateral earth pressure (defined as the ratio of the effective radial stress acting on the pile wall to the effective overburden stress, σ'v) and the interface friction angle between pile wall and the soil. In the case of the self-drilling HBMs, the diameter of the formed pile is not known. The diameter of the HBMs varies significantly as the central hollow-bar penetrates soil at various rates and grout pump flow rates. The soil types and existing density also play an important role in forming the diameter. TITAN Injection Pile or HBM manufacturer company "Ischebeck" introduced a new term describing a widening value "a". This value quantifies the difference between diameters of the formed HBM and the used drill-bit. The "a" value ranges from 0.8-inch (20 mm) minimum for any type of soil to 2.0 inches (50 mm) for sandy soils and 3 inches (75 mm) for gravelly soils. However, the effect of installation parameters on the formation of diameter of HBM is not addressed in previous studies. Therefore, utilizing field-measured data, work presented herein examined the effect of both the installation parameters and soil conditions on the formed HBM diameter. Multivariate regression analysis was used to develop models to predict the formed diameter of HBMs used to assess the axial pullout capacity of HBMs. Eight HBMs, four instrumented and four non-instrumented, were used in the study. The field testing program included installation of micropiles to a depth of 25 ft. (7.62 m) while varying the 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). The eight HBMs were tested for ultimate pullout capacity. Four of those instrumented HBMs were retrieved after the load testing to measure the formed diameter per foot of installation. The back-calculated diameter and pull-out test data were used multivariate regression analysis to predict the diameter of those non-instrumented and non-retrieved HBMs.