Compaction by deep vertical vibration increases not only the soil stiffness but also the horizontal effective stress. Based on the first practice of deep vibratory compaction in the treatment of collapsible loess foundation in northwest China

this paper discusses the influence of deep vibration compaction on the change of horizontal stress of collapsible loess and reveals the mechanism of compaction of collapsible loess by vibration. The results obtained from the field test showed that the cone stress and the sleeve resistance of the soil increased by more than 2 times within the treatment depth (–8 m)

and the friction ratio increased by 20%–50%. The value of the sleeve resistance ratio was relatively large above –3 m depth

and corresponding to a higher horizontal stress change ( )

indicating an increase of horizontal stress after compaction. At the depth from –3 m to –7 m

the sleeve resistance ratio was maintained at about 4

and the corresponding was between 3 and 4. The value of the soil below –7 m gradually decreased and became close to 1. The horizontal stress change was greatly affected by the sleeve resistance ratio but was less affected by the effective friction angle. The increase of horizontal stress caused the “pre-consolidation” effect of the soil

which was manifested in the microscopic view that loess changed from a granular structure to a mosaic structure. The large pores dominated by trellis pores disappeared or evolved into small pores such as mosaic pores

and the soil layer gradually became dense.