Influence of soft layer thickness on the aggregate displacement in the backfill material of dynamic replacement columns – results of laboratory model tests

Abstract

The dynamic replacement columns are formed by driving a coarse-grained material into a soft soil by means of repeatable drops of a pounder. The final shapes of the columns are non-cylindrical and depend on the subsoil conditions. This paper presents results of the laboratory study on influence of the thickness of the soft soil on the displacements of the backfill aggregate during the driving process.A test box with one acrylic-glass wall was prepared, in which, over a load-bearing sand layer, a soft soil of various thicknesses (𝐻 𝑠 = 0.3, 0.4 or 0.5 m) was modelled using a semi-transparent acrylic polymer. The displacements of the backfill gravel particles were tracked by means of a high-speed camera. The material was driven by dropping a 0.2 m high (𝐻 𝑝 ) pounder. The results revealed that the distance between the bottom of the first crater and the top of the sand layer played an important role in directing the particles. At 𝐻 𝑠 /𝐻 𝑝 = 2.5 pear-shaped floating columns were formed as the grains in the side zones were less affected by the pounder drops and their paths deviated from the vertical axis by not more than 50 • . In case of 𝐻 𝑠 /𝐻 𝑝 = 2.0 and 1.5, the column bases reached the bearing layer and the impact energy caused much larger vertical and horizontal displacements of the backfill material in the side zones -the observed largest angles were equal to 64 • and even 90 • , respectively. Eventually, the final column shapes resembled a non-symmetrical barrel and a truncated cone.