Soil comprises soil particles, water, and air. The engineering properties of soil are significantly affected by the ratio of these components. Unsaturated soil mechanics is vital to geotechnical engineering, as it is necessitated for managing problems associated with disaster prevention and the geoenvironment, such as rainfall-induced slope failures and seepage in waste facilities. We aim to investigate the water/heat transfer and deformation of unsaturated soils via experiment and numerical analysis.

The distribution of water content distribution in soil must be predicted because the strength and deformation properties of soil are significantly affected by water content. Soil water retention is associated with the pore structure, and the coefficient of permeability changes by several orders of magnitude with water content under unsaturated conditions. We aim to numerically predict water transfer in unsaturated soil based on hydraulic properties measured from laboratory tests. Near the ground surface, pore water exists in liquid and vapor forms, and the phase change involves heat transfer. Additionally, we aim to model the complex simultaneous transfer of water and heat.

Rain-water shielding systems using geomaterials have been proposed, in addition to those using artificial materials, such as liner sheets. Water diversion at the contact between sand and gravel layers is referred to as a capillary barrier, which has been investigated for application in waste facilities. The capillary barrier has garnered attention as a sustainable water-shielding system that uses natural materials, which complies with the concept of SDGs. We aim to elucidate the essential mechanism of water diversion of the capillary barrier and the barrier design via rainfall model tests and numerical analysis.

Unsaturated soil mechanics has been developed by focusing on soil behavior during the wetting process instead of the drying process. However, understanding soil behavior in both processes is necessary to prepare for future extreme climates, such as heavy rainfall and drought. We aim to investigate desiccation cracking caused by internal stress development when shrinkage is restricted during the drying process. We developed a new laboratory test to measure the internal stress during the cracking process. Furthermore, we aim to create a model for predicting desiccation cracking using measured internal stresses.

The temperature change in underground spaces is less significant than that in the atmosphere because soil is a high-heat-insulation material. However, the ground temperature in shallow zones is affected by atmospheric temperature changes, which can result in severe damage to underground cultural heritage sites. For example, dew condensation caused by temperature changes in masonry burial chambers can damage mural paintings and burial accessories. We aim to numerically predict the hydrothermal environment in underground spaces based on soil thermal properties measured from laboratory tests. We are currently performing temperature monitoring in a burial chamber to validate the numerical model by comparing measured and calculated temperatures.

Two failure modes, i.e., shear and tensile, must be considered near the surface of embankments because unsaturated soil exhibits tensile strength developed from suction. We aim to develop tension tests for unsaturated soil and clarify the seismic behavior of unsaturated embankments via centrifuge model tests and numerical analysis. Earthquake disaster prevention has become an important issue in the preservation of cultural heritage. For example, the seismic resistance of burial mounds with masonry structures in unsaturated mounds is significantly affected by the shear strength between stones. Hence, we aim to develop a nondestructive shear strength estimation method using stone surface impressions.

Suction (i.e., negative porewater pressure) develops in unsaturated soil. Soil particles are bonded by suction, however the interparticle force decreases with moisture increase. This contributes to slope failure during rainfall. We aim to study the deformation mechanism induced by rainfall infiltration and protections by means of laboratory tests to measure deformation properties of unsaturated soil and seepage-deformation analyses. We practically used these methods to restore the Sakafuneishi site in Nara prefecture damaged by rainfall. Our results showed that the soft clayey soil in the site greatly contributes to the slope failure, and thus an effective restoration method was proposed based on the failure mechanism.