Abstract: This paper presents the geotechnical engineering investigation and evaluation results of a collapsible loess project site in the east Ningxia before and after the soil improvement by the dynamic compaction. The investigation methods included laboratory soil tests, standard penetration tests, Becker penetration tests as well as insitu plate loading tests before and after the soil improvement. The settlement of the site was also evaluated. The following results can be obtained: 1)the unit weight of the loess soils was significantly increased. The compressibility of the loess soil was low and the collapsible potential of the surficial loess soils were reasonably eliminated. The bearing capacity of the improved loess soils reached the design requirements. The dynamic compaction was suitable for improving the east Ningxia loess soils. 2)for sites that were backfilled with collapsible loess soils, a layer of cushion material is necessary at the bottom of the foundation so that the minor settlement induced by the surface water infiltration could be avoided. Gravel is the desired cushion material. 3)The consolidation induced by the backfill material and soaking of the collapsible loess filling material could also induce settlement in loess soils.

Key words: Loess Soils, Dynamic Compaction, Plate Loading Tests, Standard Penetration Tests, Becker Penetration Tests, Settlement Analysis