Masamitsu Fujimoto

SEEPAGE FLOW-STABILITY ANALYSIS OF THE RIVERBANK OF SAIGON RIVER DUE TO RIVER WATER LEVEL FLUCTUATION

The Saigon River, which flows through the center of Ho Chi Minh City, is of critical importance for the development of the city as forms as the main water supply and drainage channel for the city. In recent years, riverbank erosion and failures have become more frequent along the Saigon River, causing flooding and damage to infrastructures near the river. A field investigation and numerical study has been undertaken by our research group to identify factors affecting the riverbank failure. In this paper, field investigation results obtained from multiple investigation points on the Saigon River are presented, followed by a comprehensive coupled finite element analysis of riverbank stability when subjected to river water level fluctuations. The river water level fluctuation has been identified as one of the main factors affecting the riverbank failure, i.e. removal of the balancing hydraulic forces acting on the riverbank during water drawdown.

A METHOD TO JUDGE SLOPE FAILURES USING SOIL MOISTURE CHARACTERISTICS

Slope failures often occur in Japan, and they frequently result from an increase in the moisture content of the soil. Thus, it is important to consider soil water content. In this study, we carried out a series of laboratory experiments to evaluate changes in soil moisture during rainfall. In the experiments, we constructed a model slope in which we installed 10 soil moisture sensors. The results indicated that the volumetric water content in the slope increased with rainfall, and the increments in volumetric water content were affected by the intensity of the rainfall. Then, using the experimental results, we developed a technique for judging slope failure risk based on absolute values of volumetric water content and rainfall characteristics. Our proposed method will be useful in judging the risk of slope failure because the criteria for the method are based not only on the precipitation but also on the soil moisture in-situ.

Effect of rice straw length on mechanical properties of quick lime and rice husk ash stabilized soil

Riverbank collapse often occurs along the Saigon River which flows through Ho Chi Minh City (HCMC). One of the influencing factors is the soft ground which is the riverbank composed of. Our research about the ground improvement method using quicklime, rice husks ash so far. Rice straw is newly added to the improved soil in this paper. The Rice straw is expected to improve the failure strain property that is to restrain brittle failure. These mixing materials can be obtained easily and cheaply in Vietnam. A series of unconfined compression tests are carried out to improve the mechanical properties of improved soil. Moreover pH tests are conducted to validate the chemical action of the improved soil and evaluate the effect of improved soil on surrounding environment.

Improved measurement of soil moisture using an ultrasonic waveguide to predict rainfall-induced slope failure

Field monitoring of soil moisture and groundwater level is important to predict shallow slope failure stemming from heavy rainfall. We previously proposed a monitoring technique using ultrasonic waveguides in which soil moisture and groundwater levels are monitored on the basis of the intensity and propagation time of ultrasonic waves reflected from the underground soil surface, respectively. In field monitoring, the reflective intensity depends on the depth of monitoring points and the inhomogeneity of grains, so it has been difficult to examine the reflective intensity change quantitatively. In the present work, we propose improvements that will allow us to measure the intensity change quantitatively. Specifically, we utilize an ultrasonic waveguide of a constant length by using casing pipes with different lengths depending on the monitoring depth and cover the surface of coarse-grained soil with a homogeneous fine soil to obtain repeatable data for wetting and drying cycles. The positive effects of these improvements were confirmed by a field monitoring test.