Seismic analysis of the 25 Stone Buddhas Group at Hakone, Japan

On the slopes of an ancient volcano near the town of Hakone, Japan, is an outcropping of andesite. Approximately 700 years ago, 25 images of Buddha were carved on the sides of the rock outcrop. The carvings are collectively referred to as the 25 Stone Buddhas Group. Over the centuries, joints in the rock outcrop have widened. Some of the joints exceed 10 centimeters in width and are filled with a soft volcanic soil. It is the concern of the town of Hakone that the outcrop is becoming increasingly vulnerable to earthquake damage. This paper discusses the steps taken to build a 3DEC (Itasca 1994) model of the outcrop, perform a seismic stability analysis, and model remedial countermeasures. Results of the seismic analysis indicate that the presence of the soil in the joints puts the carvings at risk of severe damage in the event of a magnitude 7.5 earthquake. The study also concludes that countermeasures involving replacing the soil with a high strength polymer grout are sufficient to prevent damage to the carvings. The use of rock bolts without the removal of the soil was found to be unacceptable.