The problems of embankment construction over peat ground with high water content mostly relates to stability, and long-term settlement. Vacuum consolidation method is commonly used to assist the embankment construction over the soft ground for the purpose of accelerating a consolidation process as well as increasing an overall stability of soft ground for a rapidly embankment construction. Theoretically, the vacuum pressure is considered to be a surcharge load applying to the ground. This load will be removed immediately after the termination of vacuum operation. Basically, the settlement after construction can be minimized by waiting the primary consolidation process to complete at the higher degree prior the removal of surcharge resulting in the increase of over consolidation ratio. Nevertheless, the effect of vacuum consolidation as a surcharge load related to a long-term settlement characteristic has not been clearly investigated. This paper presented the measurement of the long-term settlement after the embankment constructions over peat ground assisted by vacuum consolidation. The effect of vacuum operation period was observed based on the field monitoring data. Moreover, the effect of surcharge by embankment and vacuum consolidation method was also studied and compared. It was found that the increasing vacuum pumping period and increasing the amount of surcharge by vacuum consolidation gave a very good performance to reduce the long-term settlement after the construction.

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Vacuum consolidation is a soft ground improvement method. The vacuum consolidation of soil above the groundwater level (unsaturated soil) has not been sufficiently examined in earlier studies. Thus, in this study, laterally confined vacuum consolidation tests were conducted for unsaturated clay slurry with Sr ≈ 80%. During the tests, the air bubbles on the sides of the specimens were recorded, and the water content and diameter were measured at the end of the tests. Surcharge consolidation and consolidation tests of saturated clay slurry were also conducted. Main conclusions are as follows: Surcharge consolidation of unsaturated clay slurry is one-dimensional. Although the settlement curves and water content distribution curves of unsaturated clay slurry under vacuum consolidation and surcharge consolidation are almost identical, lateral shrinkage is observed and the deformation is three-dimensional.

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Electrode material has significant influence on the behavior of soil during electro-osmotic consolidation. Four electro-osmotic consolidation experiments on a sodium bentonite were conducted with copper, iron, graphite and stainless steel electrodes. The result indicated that the copper electrodes achieved the best drainage performance. The change in drainage rate, current and voltage loss near the anode was similar for the four electrodes, with a rapid change stage in the first 2 hours followed by a slow change stage in the next 6~8 hours and finally a stable stage. The rapid voltage loss in stage 1 was mainly caused by the formation of vertical and horizontal cracks, while in stage 2, the relative less voltage loss was due to the corrosion of electrode and the development of cracks. The electro-osmosis conductivity decreased along with the drainage of water and varied in the range of 1×10^-4 to 1×10^-5 cm²/ (V∙s). The average electro-osmosis conductivity and the average energy consumption per milliliter water discharge were independent of electrode material.

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Ground improvement techniques are widely adopted in geotechnical engineering practices for improving the strength, density, and/or reducing drainage characteristics of the soil. Among the various options available for improving the soil, dynamic compaction (also referred to as impact densification, heavy tamping, and dynamic consolidation) has evolved as a widely accepted method of soil improvement in the past decade for treating poor soils in situ. This method is often an economically attractive alternative for utilizing shallow foundations and preparing subgrade for construction, as compared to other conventional expensive solutions like pile foundations, excavation and replacement, densification etc. Moreover, dynamic compaction has some unique applications, including treatment of reclaimed land, liquefaction mitigation and heterogeneous fill materials, and displacing unsuitable materials such as peat, and collapsing sinkholes. In general, the ultimate goals of dynamic compaction are to increase the bearing capacity of the soil, and decrease the total and differential settlements within a specified depth of improvement. Till date, the effective depth of improvement achieved through this technique has been restricted to about 5m of the soil. To increase the effectiveness of dynamic compaction, the soil condition and the energy configuration (which is decided by the surface area and the shape of the tamper used) has to be taken into account. In the present paper, an attempt has been made to investigate the impact of tamper base area in improving the influence depth during dynamic compaction on sandy soil. For this purpose, an innovative dynamic compaction set-up was developed in the laboratory for carrying out small-scale physical model tests on low energy dynamic compaction using circular steel tampers of three diameters (50 mm, 75 mm and 100 mm). This paper describes details of the dynamic compaction set-up developed, and its advantages over other compaction set-ups developed till date with respect to evaluation of dynamic compaction technique in the laboratory. In general, it was observed that, the width of area influenced by dynamic compaction is proportional to almost 2.5 times the tamper diameter. However, the tamper base area was found to exhibit marginal influence on the depth of improvement, provided the impact energy intensity was kept constant in all the three tests.

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Dynamic compaction has been carried out on reclaimed soft ground in Chiba prefecture, Japan to reduce liquefaction potential and ground settlement due to consolidation. The ground was reclaimed with up to 12m of inhomogeneous fine-grained soil 30 years ago. The dynamic compaction process, carried out prior to construction of a factory building, entailed lifting a drop weight of 250kN with a crane and dropping it repeatedly on the ground from a height of 25m. Impact points were spaced at intervals of 5m. Geotechnical site investigations were performed before and after compaction to confirm its efficiency. These investigations reveal that (1) an increase in SPT N-value is observed in sandy soil, (2) increased consolidation yield stress is observed in clayey soil, and (3) increased Nsw values by the Swedish Weight Sounding test are observed in both sandy and clayey soils. These results demonstrate that dynamic compaction is an effective method of improving both sandy soils and clayey soils.

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