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ABSTRACT: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.

Field monitoring of soil moisture and groundwater levels is one of the countermeasures against slope failure caused by heavy rainfall. We developed a new monitoring method for soil moisture and groundwater level using ultrasonic waves. An ultrasonic detector in this measuring method is easily installed underground and at low cost, and therefore, this method is useful for multipoint monitoring in a wide field area. However, since the measurement data are greatly influenced by temperature, this temperature-dependent effect must be corrected. Previously, the dummy pair system using two detectors had solved this problem. In this paper, we propose another improved temperature correction method using one detector and evaluate the usefulness of both methods in detail.

Multistep excavations were implemented at the toe of a large-scale slope model, and the surface displacements in the slope were measured to examine the validity of the relationship between the velocity and acceleration proposed by Fukuzono for excavated slopes. The surface displacement increased both during and after slope excavation, among which the latter was due to creep deformation under a constant stress. The rate of increase in the surface displacement was initially high and then decreased to zero during creep deformation after the excavation without slope failure. However, the surface displacement exhibited an accelerated increase during creep deformation after the final excavation prior to slope failure; the surface displacement increased with small fluctuations even before slope failure occurred. The surface displacement velocity and acceleration also fluctuated notably due to variations in the surface displacement. The trendlines for the derived relationships between the velocity and acceleration were in good general agreement with the measured data at certain locations in the model slope. These relationships were unique at different locations on the slope, while the inclination of the relationship trendline suddenly decreased just prior to slope failure. The steeper trendlines predicted an earlier failure time if the displacement was large and close to the failure condition, whereas they resulted in worse predictions if the displacement was small and far from causing slope failure according to the prediction method proposed by Fukuzono.