This paper discusses the use of vacuum consolidation to improve soft ground and increase its undrained shear strength (Su). Since soft soils, particularly peat and organic clay soils, have very low Su values, the construction of embankments and structures on these soils often cause ground failure. For the trial embankment conducted in this research project, soft ground composed of peat, organic clay and clay was improved by using vacuum consolidation, resulting in a significant increase in Su values, and an embankment 10.7 m in height could be constructed on the improved ground without causing ground failure. The initial Su values of the peat and organic clay were lower than the strength of the clay, namely, 48% (peat) and 59% (organic clay) of that of clay. However, after vacuum consolidation of these soils, their final Su values greatly increased to 199% (peat) and 154% (organic clay) of the final strength of clay. It was demonstrated that the increased Su was the result of synergy of the relatively high normalized undrained shear strength (Su/σ′v, where σ′v is the effective overburden stress) of peat and organic clay, and the unique loading mechanism of vacuum consolidation. It was also demonstrated that in case of a relatively narrow embankment (e.g., road embankment and river levee), the effect of isotropic consolidation also contributes to increase the stability of the vacuum consolidated peaty ground. It was also found that if the (Su/σ′v)NC of the original ground and the over-consolidation ratio (OCR) are known, the (Su/σ′v)OC of the given ground over-consolidated by vacuum consolidation can be predicted.

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In this paper, we investigate the relationship between S wave velocity distribution and dry density immediately after completion of two test embankments constructed by changing degree of compaction. In addition, we compare the change of S-wave velocity distribution before and after the rainfall, and examined the applicability of surface wave exploration to the change of the properties inside the embankment. As a result, the distribution of the dry density and the S wave velocity distribution obtained from the surface wave survey were well matched. In low embankment, the saturation increased due to rainwater infiltration, and the S-wave velocity remarkably decreased after rainfall. From these results, it is possible to estimate the property change in the embankment by the S-wave velocity distribution obtained from the surface wave survey, and it was confirmed that the surface wave exploration can be applied as the primary screen for the property evaluation of the embankment.

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This paper is to study about the way to minimize the total cost, including opportunity cost, of public works. The author compares the costs between of short-time high-construction-cost case and of long-time and low-construction-cost case, in an existed road construction project. The author also compares the costs between of daytime maintenance and of nighttime maintenance, in existed road maintenance.From these comparisons, the author finds out that in order to select minimum cost case it is necessary to 1) estimate the benefit from the project, 2) calculate the direct cost of the project and 3) forecast the duration of the project. In-house engineers have to select the best way with considering cost minimization including opportunity cost.