Naoya Shojo

Wall clay test specimens using different mixing ratios and materials were fabricated using Kumamoto (Yatsushiro, Taragi, Aso, and Amakusa), Saitama, and Kyoto’s new soil and some reused soil. To understand the mechanical properties of the mud wall, a compression test and a double shear test were conducted. Then, multiple regression analysis was performed using this data to calculate the estimation formula of compressive strength (σ_max) and shear strength (τ_max). First, the compression test of a 150×150×60 mm block and the double shear test of a 60×60×180 mm block were conducted, and the following items were confirmed. In the new soil of Yatsushiro,  ･As the compression specimen size decreased in size to a 100×100×60 mm block, the σ_max and its dispersion increased.  ･The σ_max of the second coating soil was higher than that of the first coating soil, but τ_max slightly varied. In addition, the second coating soil displayed brittle fracture behavior, but the first coating soil had a relatively higher degree of toughness.  ･The σ_max of some reused soil was higher than that of new soil, but σ_max and τ_max did not increase even when these soils were mixed.  ･Even when the straw content was increased from 5% to 8%, σ_max and τ_max remained approximately unchanged.  ･In the matured period from 7 to 90 days, σ_max and τ_max increased by 1.2–1.3 times.  ･Even when the quantity of sand in the second coating soil increased, σ_max, Young's modulus, and shear stiffness decreased.  ･When the specimens were classified into coarse-grained soil and fine grained soil, the σ_max of the coarse-grained soil was higher than that of fine-grained soil. However, the τ_max/σ_max of the coarse-grained soil and the reused soil were 0.6–0.7, that of the fine-grained soil was 0.8–1.0, and their strength ratios were classified into two groups.  ･The influence of soil appeared to be significant in the new Kyoto soil.  The following results were obtained using multiple regression analysis:  ･The σ_max of the first and second coating soil had the highest correlation with the clay mass percentage and the water content of the mud plaster (correlation coefficient R = −0.9). τ_max had the highest correlation with the plasticity index and the air-dried water content of the wall clay (R = −0.85).  ･It was possible to estimate σ_max using the explanatory variables of (1) and (2), where (1) is the fine sand mass percentage and density and water content of mud plaster of first and second coating soil, and (2) is the coarse fraction mass percentage, plastic limit, and air-dried water content of first coating soil.  ･τ_max could be estimated by explanatory variables of mud plaster density, fine sand mass percentage, plasticity index, and air-dried water content of first coating soil, but it could be also estimated by the average of σ_max, which is classified as coarse-grained or fine-grained soil.  However, these multiple regression equations have a relatively low degree of general versatility because the specimen shape, the type of material, and mixing ratio are limited. In the future, we will further increase the soil type and derive a multiple regression equation of high general versatility.