Recently in Japan, accompanying the development of such major traffic networks as super-express railways, express highways, and trans-strait bridges, many long span bridges have been constructed on weak foundation rock. It has thus become necessary to carry out surveys and tests on the strength and deformation characteristics of weak rock. In Japan a standard survey and test method for weak rock as bridge foundation ground has not yet been established. The public corporation concerned has carried out the required surveys and tests on the foundation ground, and the design and construction of foundations has been carried out accordingly.

In order to assess the present situation regarding surveying for, and the design and construction of, long-span bridges built on weak foundation rock in Japan, this Working Group investigated the case histories of bridges built on weak foundation rock with a span length of 50 meters or more that have either been completed in the last ten years, or are presently under construction. Based on the results, the situation in Japan with regard to survey and test methods relating to bridge scale and geological conditions, and the prediction of mechanical parameters for engineering purposes has been briefly summarized here, both of which are required for foundation design, and the selection of foundation types and construction methods in accordance with site conditions. Case studies were made on five bridges built on weak rock such as Tertiary sedimentary rock, weathered Granite and pyroclastic rock. As a special case, the elevated bridge of Shin-Kobe station which has been built on an active fault, is included.

Geological conditions, and locations of long-span bridges

Recently, long-span bridges constructed in Japan have been for roads, but in some cases they are combined bridges for road and railway.

The 76 bridges surveyed throughout the nation are shown in Figure 2.1, showing their locations, the geographical situation and geological conditions.

(Figure in full paper)

The figure shows that the foundation ground of the majority of the long-span bridges is soft, weak, sedimentary rock, followed by weathered or altered rock. With regard to the geographical situation, mountainous locations are concentrated in the North and Central regions, and large scale trans-strait bridges are mainly to be found in the island areas of West Japan.

As the above indicates, the use of weak rock as a foundation ground for long-span bridges is common in Japan.

Geological survey methods and rock classification
Present geological survey method

The case history data from the 76 bridges reveals a wide variety of types and ranges of geological survey relating to long-span bridge foundations. In summary, drilling surveys for individual bridges consisted of anything from 1 to 73 drilling sites, the total drilling depth varied from 20 to 3,700 meters, and various other methods were also employed. Table 2.1 shows the details according to the geographical situation and bridge scale.

The foundation ground survey method and purpose in the planning and design stage generally included rock classification

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