Circumstances contributing to the deterioration of old cob structures in Japan

Abstract

The benefits of using earthen material such as cob in architecture are increasingly acknowledged, yet remaining concerns about safety prevent its broader use. This is particularly true in areas that experience earthquakes and heavy rain. To better understand the structural characteristics of cob in such harsh environmental conditions, this study examines cob structures found in rural areas of Japan, which are subject to those conditions. On average, we found they have been standing for 80 years. First, through interviews and surveys, this study verifies that amateurs built these structures; that is, they were built under worst-case scenarios for quality control. Second and mainly, field work measurements provide data to show deterioration patterns over decades, and sometimes centuries. Major findings were as follows. As expected, the bottom of the walls is most susceptible to deterioration. Unexpectedly, the data appears to show that deterioration is minimized when foundations are short, using small stones, 80 mm to 140 mm in height. The average deterioration at the bottom for walls with tall foundations (150 mm in height and over) is 64 mm, and for walls with a short foundation is 25 mm.

Keywords

• earthen construction
• cob
• deterioration
• japanese cob structure