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Soft soil is widely distributed in Vietnam's coast area, which normally contains salt, organic matters, and its properties can be affected by the saltwater intrusion. This article presents experimental laboratories to investigate the salt, alum contents in some soft soils distributed in the coastal areas of Vietnam, and their effects on the properties of soft soil. To investigate the effect of salt content and pH values on soil properties, the soil was submerged into different solutions of salt content and pH value for 10 days before testing. The experimental results indicated that the average salt content in natural soils in the studied area changedfrom 0.31% to 1.58% and had a different level of salt contamination. In some places, the soil had a low pH value (pH<4.5) and high sulfate content. The research also showed that the increase in salt and alum contents caused a decrease in undrained shear strength and an increase in liquid limit and compression index.

The North Central Coast of Vietnam has a wide distribution of loose sand which is often exposed on the surface. The thickness changes from a few meters to over ten meters. This sand with the loose state can be sensitive to the dynamic loads, such as earthquakes, traffic load, or machine foundations. It can be liquefied under these loadings, which might destroy the ground and buildings. The Standard Penetration Test (SPT) is widely used in engineering practice and its values can be useful for the assessment of soil liquefaction potential. Thus, this article presents some ground profiles in some sites in the North Central Coast of Vietnam and determines the liquefaction potential of sand based on SPT and using three parameters, including the Factor of Safety against Liquefaction (FSLIQ), Liquefaction Potential Index (LPI), and Liquefaction Severity Number (LSN). The research results show that the FSLIQ, LPI, and LSN values depend on the depth of sand samples and the SPT values. In this study, the sand distributed from 2.0 to 18.0m with (N1)60cs value of less than 20 has high liquefaction potential with FSLIQ<1, LPI is often higher than 0.73, and LSN is often higher than 10. The results also show that many soil profiles have high liquefaction potential. These results should be considered for construction activities in this area.