The northern part of the Kanto Plain is the area where the most severe land subsidence have been taking a place in recent years. This paper presents the relation between the change of circumstances of groundwater recharge which is caused by the change of the land use and the mechanism of the land subsidence. The land subsidence in this area is classified into two types by the depth of the compacted beds. The one is the compaction of alluvium strata that is about 30 meters in thickness and developed only the lowland. The rest is the compaction in the Pleistocene beds. This compaction is mainly occurring more than 200 meters beneath and spreading widely over the whole area. The groundwater head become lower according to the depth of the aquifers. In the aquifer from 200 to 300 meters under the surface, groundwater head is lowest and it ranks about 25 meters below sea level. Because of the wide lowering of the groundwater head of the deep aquifers, the groundwater is recharged vertically from the adjacent aquifers by leakage. The original source of the groundwater is rainfall and irrigation water. Especially, the recharge from paddy fields takes an important role for raising groundwater table of the unconfined aquifer. For the effective prevention of the land subsidence, it is necessary not only to reduce the amount of the groundwater use from the deep aquifers with low groundwater head, but also to manage the proper land use that is maintain the function of the groundwater recharge.
In order to utilize the land on natural soft clay deposits or artificial reclamation fills as urban development purposes, prediction of long-term subsidence is important. Purpose of this study is to find out quantitative consolidation behavior by the analysis based upon natural analogue point of view. Taking inner and central parts of Tokyo Bay area on Alluvial soft muddy formation as the sample sites, the authors made thorough review and compilation of the historical results of borehole measurements and laboratory tests. At four representative model sites, the consolidation behavior was examined in detail by plotting the relationship between void ratio and effective overburden pressure, normalized by constant liquid limit or clay content. The results showed us that time-dependent change of field scale compression index values could be obtained from the existing data.
Recently, constructions of highway in a city have been built in semi-groundstyle as one of environmental methods against noise pollution. However, in some cases, these constructions can influence harmfully on the groundwater environmental protection, because they intercept the groundwater flow system. Semi-underground highway of Touhoku Line in Nerima section intercepted the aquifer, which was expected that the maximum water table rose 3m at up-stream and went down 2m at down-stream. Therefore, groundwater recovering countermeasure was necessary. Based on the results of field surveying observation and numerical simulation, this paper explains the groundwater recovering countermeasure and its effects which can respond well to the underground structure, hydrogeological conditions and the variations of water table. As results, we took following steps by considering both situations in nature and in heavy rain. (1) Walls used for S. M. W (Soil Mixing Wall) in up-stream and down-stream were cut 3m and 3.5 m depth from the ground surface, respectively; (2) Underdrain was constructed; (3) The connected conducts with 200mm in diameter were installed in every 50m; and (4) Discharges in downstream was conducted through infiltration boxes or recharging wells set every 50m. During the construction, maximum water table rose in 1.5m at up-stream and went down in 1m at downstream, respectively. After this countermeasure was built, the water table is stabilized at the same depth as before. Therefore, the effect of engineerings for recovering groundwater system is obviously satisfactory.
Geological conditions should be considered when a storage cavern is constructed in rock ground for compressed air energy storage gas turbine generator which is hopeful as electric power storage in near future, and muddy slurry excavation and steel pipe shaft method is proposed in soft rocks. The water replacement type and dry type are typical in the method and their basic characteristics are studied in this paper. Most important things in realizing this method are both feasibility of excavating a cavern and its economic advantage, especially present evaluation of cavern stability in muddy slurry excavation and economically optimum depth in positioning a cavern are studied in this paper.