Vol.9 No.2 2016

Research paper : Three-dimensional urban geological map (T. NAKAZAWA et al.)−82−Synthesiology - English edition Vol.9 No.2 (2016) Fig. 9 3D geological model of the subsurface valley fill deposits around Kashiwa, Chiba Prefecturewell known to exist beneath uplands, so this may be a blind spot of the urban ground. It is known that such valley filling sediments are distributed from east to west (from Kashiwa to Narita) in northern Chiba (Fig. 9), and it will be interesting to learn how the facies change, how the facies changes are reflected in the physical properties, and how this is expressed as differences in seismic motion.The landfill layer is widely distributed along the coastline of the Tokyo Bay area of northern Chiba. Underneath the landfill layer, Chuseki-so exists in several strands of incised valleys. The landfill layer of this region caused serious liquefaction in the 2011 Off the Pacific Coast of Tohoku Earthquake, and in some areas, the urban infrastructure was paralyzed. In general, the stratum that is subject to liquefaction is the loosely packed sand layer saturated with underground water, and such a sand layer is commonly seen in the landfill layers of the coastal and riverside areas. The landfills in the Tokyo Bay area were made of sand and gravel dredged from offshore by a sand pump method.[26] They represent alternating layers of sand and mud, and may resemble a natural stratum at a glance, but it can be identified by detailed observation of the sedimentary structure and fossil occurrence in the core sample.[27] Also, in the case of the sand pump method, there is unevenness of the distribution of sand and mud in the landfill layers depending on the position of the pump outlet, and it is said that liquefaction is likely to occur in areas that are predominantly sand.[28] Through the series of drilling surveys that we conducted, it is possible to establish identification standards for the landfill layer by the sand pump method, and the pattern of unevenness of sand and mud in the landfill can be clarified through the analysis of borehole data and past aerial photographs. Moreover, soft muddy sediments (Chuseki-so) exist irregularly beneath the landfill layer, and the change in the thickness of the muddy sediments may be of effect on the liquefaction of the landfill layer on top.5.2 Cooperation with the local governmentsCurrently, we are working on the management of borehole data and their analysis with the cooperation from the Chiba Prefectural Environmental Research Center. This Center has worked on research for geopollution, liquefaction, etc. It is also a pioneer in establishing a borehole database because it has gathered borehole data of public works in Chiba Prefecture and has disclosed them on the web since 1991. The “Chiba Information Bank for Geological Environments” managed by the Center provides borehole data for about 20,000 sites in Chiba. Since the data stored in this database is in an original format, we are attempting to convert them into the JACIC spec XML that is the current standard format (Fig. 7). We are working along with the researchers of the Center to convert to a database that stores in the JACIC spec XML format in the future. The specification of the aforementioned borehole data management system was decided while discussing with the researchers of the Center. Initially, the system that uses the web was considered, but as a result of meetings with the person in charge, we shifted to the specification that runs on stand-alone computers due to security concerns. Such demands became apparent after exchanging opinions with the local government, and it was a good opportunity to learn how extremely important it is to understand the situation of the local government. The Center has abundant experience in geological surveys, and has strong views that the organization of standard borehole data based on investigation of actual sediment core samples, as well as the collection of existing borehole data of public works, are extremely important for geological environment and disaster measures such as dispersal of soil contamination and liquefaction prediction. We are engaging in the topic together with such shared awareness.5.3 Prototype of the 3D geological mapFor the establishment of the algorithm for the 3D geological model creation of northern Chiba, we conducted practical research in the Kisarazu district of Chiba Prefecture that is a stratotype of the Pleistocene strata that comprises the Kanto Plain. First, we created the algorithm for constructing a 3D geological model based on the stratigraphy in the Kisarazu district described in the published 1:50,000 geological map, and a 3D geological model was constructed using the outcrop data same as the ones used to create the geological map. The created 3D geological model was compared digitally and visually with the published, hand-drawn geological map. As a result, this 3D geological model created by a logical method was found to have no problem stratigraphically, and have no human error compared to the conventional manual-work drawing. The geological boundary can be drawn precisely and faithfully to the data, and its practicality was demonstrated.[29] The algorithm constructed for the Kisarazu district that is a stratotype of the Pleistocene strata in the Kanto Plain is thought to be widely applicable to the entire Kanto Plain. ★ Standard borehole log★★★★Grey: Lower part of the Kioroshi FormationSoft mud layer of the valley fillKashiwa StationN500 m


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