2510109532

THE STATE OF GEOTECHNICAL

ENGINEERING IN JAPAN

Prof. Masami Fukuoka President,

Japanese Society of Soil Mechanics and Foundation Engineering

Distinguished Guests, Ladies and Gentlemen.

On this occasion of the Ninth International Conference on Soil Mechanics and Foundation Engineering in Tokyo, it is my honor, as the President of the Japanese Society of Soil Mechanics and Foundation Engineering, to address such an eminent audience on the theme of the State of geotechnical engineering in Japan.

Our country, Japan, is located on the eastern end of the Asian Continent facing the Pacific Ocean. It consists of four major islands and numerous smali islands, stretching north to south some 2,500 kilometers, and east to west about 1,500 kilometers. The total area of this country is about 370,000 square kilometers with the population of approximately 110 million.

The climate is quite varied. In the south, it is subtropical, whereas in the north, it is sub-frigid. The climate in the central part of Japan is generally moderate. Because of the presence of the backbone mountain rangę running through the main island, however, we have much snowfall on the side of the Sea of Japan in winter.

The geology is extremely complex, with prac-tically every type of rock and soil ranging from the Palaeozoic to the Quaternary.

There are many volcanoes, erupting and spreading volcanic ashes which distribute throughout Japan. Some of the rocks that constitute mountains have such Iow strengths that there are many faults and zonesof shat-tered rocks. Throughout the country, there-fore, there are several tens of thousands of landslides.

Japan is situated in the circum-Pacific earthquake belt. We have had freąuent earth-quakes including the great Kanto Earthquake in 1923 and the Niigata Earthquake in 1964.

In the summer season, typhoons generated in the tropical region move to the north, bringing strong rainfalls ranging from 500 to 1,000 millimeters per day, which cause floods and landslides as well as erosions along the coasts.

Because of the excessive pumping of the groundwater, subsidence occurs in many allu-vial plains in this country. In some of the areas in Tokyo, the subsidence up to 4.5 meters has been recorded for the past 50 years. Along with the industrial develop-ment, we have problems of polluted air and water, and noises. In estuary deltas, there are loose sand strata, which liąuefy during earthquakes. In lakes, Coastal regions or valleys, there are peat strata which cause embankments to settle excessively and even to show instability.

In Japan, pile foundations were used for the first time around 1870, in connection with construction of the pierś for a railroad bridge, using cypress wood. Thereafter pine trees were also used quite extensively. At present, pre-cast concrete piles and Steel pipę piles are being used at an annual ratę of about six million tons each. Also cast-in-place concrete piles are employed exten-sively.

Since pile driving is not allowed in urban areas, various measures have been devised.

Fig. 1 shows a special pile driving rig, developed by the Japanese Association for Steel Pipę Piles. The hood covers the entire rig to minimize the noise quite successfully.

Cast-in-place concrete piles have an advantage that they do not produce noise, but the dis-advantage is that slime tends to accumulate at the bottom of the borehole to such an extent that it reduces the point bearing capacity. To cope with this problem, a method has been developed that first solidifies the bearing stratum by grouting, and then installs cast-in-place concrete piles, thus the point bearing capacity of which is improved remark-ably.

There is also a method to reduce the driving resistance of pre-cast concrete piles; water jet with pressures as high as several hundred atmospheres are ejected from the tip of the pile while a static load is applied to the head of the pile. When the tip of the rig reaches the desired depth, high pressure jet is stopped and the pile is jacked down against the weight of the driving rig, Fig. 2.

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