Model tests for observing the deformation characteristics of micropile boreholes during drilling in a soil-limestone mixture

Little attention has been paid to investigating borehole deformation characteristics during drilling in soil and rock mixtures commonly encountered in the construction of ground anchors, micropiles, etc., raising questions as to the pile quality. This unique study investigates the deformations of micropile boreholes during rotary drilling in a compacted soil-limestone mixture (S-LM) by developing model tests. Three sets of experiments were carried out, in which three groundwater conditions (no water flow, 40 cm water level, and 80 cm water level) were applied at each of the three boreholes, which had diameters of 32, 44, and 56 mm. The variations in vertical displacement, deep horizontal displacement, pore water pressure, and soil pressure were recorded during the drilling stage. The test results showed that, in general, the vertical displacement at the ground surface, radial horizontal movement, and soil pressure induced by rotary drilling were strongly correlated with the borehole diameter. It was observed that the larger the borehole diameter was, the higher the values of these parameters. Furthermore, the 80-cm water level resulted in borehole collapse during the drilling stop periods since a negative displacement area and a soil pressure varying from negative to zero were observed. This research broadens the limited study of borehole deformation conducted on soil and rock mixtures via model tests. Additionally, it can provide a reference for micropile designers and contractors planning to drill in similar soil-rock mixtures.