Nobutaka Hiraoka

Improved Measurement of Soil Moisture and Groundwater Level Using Ultrasonic Waves

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.

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.

Study of SPH simulation on tunnel face collapse

Maintenance measures for tunnel portal zones have attracted increasing interest owing to the increased collapse risk of slopes by severe torrential rain and inland earthquakes in Japan in recent years. However, these large-scale collapse behaviours cannot be simulated by finite difference method (FDM) analysis. In this study, we applied the smoothed particle hydrodynamics (SPH) method to analyse the collapse behaviour of the tunnel face under construction and the effect of the stabilization method. First, we simulated the collapse behaviour at the tunnel portal zone by two-dimensional SPH simulation and compared it with an aluminium-bar experiment, and we showed that the SPH method is effective to model the overall tendency of the large deformation and the collapse behaviour in the experiments. Second, we simulated the collapse behaviour using the SPH method in the real tunnel portal zone collapse and provided a restraint result for collapse using facebolts. Therefore, we demonstrated that the SPH method is an effective technique to evaluate reinforcement measures.

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.