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 paper aims at investigating the content of particles and minerals of soft clay soil in the Mekong Delta coastal provinces, southern Vietnam, as well as improving soil stability using inorganic adhesives, i.e.lime and cement-based stabilization. To study the composition of soft clay soil, a series of different laboratory methods were carried out and revealed various characteristics related to mineral composition, pH value, soluble salt content, and grain particle composition of soft clay soils. The results demonstrated five soil subtypes, namely high – saline soil (S2), low-saline soil (S1), acid sulfate soil (A), acid sulfate - saline soil (S-A), soil without salt andacid sulfate content (S0- A0). The soft clay soil (C) included 5 subtypes, which were C-S2, C-S1, C-A, C-AS, and C-S0-A0, whereas the soft sandy clay soil (SC) included three subtypes, namely SC-S2, SC- S1, and SC-S0-A0. Analysis of the above results showed that the high - saline soil, acid sulfate soil, and acid sulfate - saline soil are not suitable for lime and cement stabilization. This observation was illustrated by the initial experiment of cement – soil mixture properties.

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.

Ground granulated blast furnace slag (GGBFS) is the by-products from pig iron plants which can cause a serious problem for land, soil, and water. Thus, the study for reusing GGBFS is very nessesary. This paper presents the utilisation of GGBFS for soft soil improvement by cement deep mixing method. Portland cement was replaced by GGBFS from 0 to 100% and a total of 33 specimens were used to determine the unconfined compressive strength and deformation modulus of treated soil. The experimental results showed that replacement of GGBFS from 0 to 60% cement could increase the unconfined compressive strength and deformation modulus of treated soil. The optimum GGBFS content was found to be 30% of cement content. In general, the utilisation of GGBFS for soil improvement could increase the properies of soft soil and soil treated with cement. The result of this study will be basic for utilisation of GGBFS in ground improvement by cement deep mixing method.

The horizontal coefficient of consolidation is the most important parameter for designing the improvement of soil soft by prefabricated vertical drains (PVDs) combined with surcharge and vacuum preloading. This paper presents the experimental study on the horizontal coefficient of consolidation (ch) of some soft soils distributed in Vietnam. The ch value was determined by the laboratory test and CPTu dissipation test. The laboratory tests included the Rowe consolidation cell test and constant rate of strain consolidation with radial drainage test. Two types of consolidation laboratory tests were performed. The experimental results indicated that the ch value is always larger than the vertical coefficient of consolidation of soil (cv). The ratio of ch/cv depends on the consolidated pressure, type of soil, and the anisotropy of soil. The ratio of ch/cv is different in different types of soft soil in Vietnam. In the normally consolidated state, the ch/cv ratio ranges from 1.35 to 10.59. It was necessary to choose the ch value at the consolidated stress level for calculating the PVD spacing.