Taro Uchimura

Effects of wetting and drying on the unsaturated shear strength of a silty sand under low suction

Shear strength properties of unsaturated soils are important in evaluating the stability of geotechnical structures such as natural slopes, embankments, retaining walls, excavations, and footings. This importance is more in countries, where intense rainfalls cause the instability of natural and man-made slopes. An unsaturated soil behaves differently at wetting and drying and this difference in behavior is referred to as hysteresis. Hysteresis is often exhibited in soil-water characteristic curves of unsaturated soils. The hysteresis in the shear strength behavior of unsaturated soil under wetting and drying has not been fully investigated. Shear strength tests on unsaturated soils have usually been conducted following the drying process. For example, in a multi-stage shear test, the soil is subjected to increasing matric suctions at subsequent stages. Very few studies have been conducted on the unsaturated shear strength parameters on wetting process by which slopes become more unstable. Re-compacted soil specimens of silt-sand were tested in a modified triaxial apparatus to examine the effects of wetting and drying on shear strength. Identical soil specimens were tested at wetting and drying under controlled air and water pressures. The study was conducted for low suction range, (i.e. 0–50 kPa). The soil at the drying had lower shear strength as compared to the soil at the wetting under the same matric suction. Furthermore, the study was extended to investigate the effects of suction and the hysteresis of SWCC on shear strength parameters such as effective friction angle and the apparent cohesion, c. Results exhibited no significant effects of suction and the hysteresis of SWCC on the effective friction angle. It was observed that the apparent cohesion at the wetting was higher than that was obtained at the drying under the same suction. The c increases as the suction increase with the decreasing rate at wetting.

Direct Shear Testing on Unsaturated Silty Soils to Investigate the Effects of Drying and Wetting on Shear Strength Parameters at Low Suction

AbstractA modified conventional direct shear device was used to measure unsaturated shear strength of two silty soils at low suction values (∼0–50  kPa) that were achieved by following drying and wetting paths of soil-water characteristic curves (SWCCs). The results revealed that the internal friction angle of the soils was not significantly affected by either the suction or the drying-wetting SWCCs. The apparent cohesion of soil increased with a decreasing rate as suction increased. Shear stress–shear displacement curves obtained from soil specimens subjected to the same net normal stress and different suction values showed a higher initial stiffness and a greater peak stress as suction increased. A soil in wetting exhibited slightly higher peak shear stress and more contractive volume change behavior than that of soil in drying at the same net normal stress and suction.

Residual Earth Pressure on a Retaining Wall with Sand Backfill Subjected to Forced Cyclic Lateral Displacements

A pair of about 11 m-high soil retaining walls of an U-shaped underground reinforced concrete (RC) structure in Tokyo exhibited a large residual inward (i.e., toward the active side) displacement with potential structural damage, which became 18 cm between the tops of the two walls about three years after its completion. Noticeable settlements of the backfill were observed behind the walls. A series of small-scale model tests was performed in the laboratory to understand this field behaviour. The results from in-situ investigation and model tests showed that this wall behaviour can be attributed to a gradual increase in the residual lateral earth pressure, resulting from cyclic lateral displacements of the walls caused by a small number of relatively large seasonal thermal cyclic displacement of the RC wall facing and bottom slab of the structure, not by a great number of relatively small daily displacement. Three factors for the mechanism of this wall behaviour (i.e., ratcheting, cyclic hardening and cyclic loading-induced residual deformation of the backfill) were identified and analyzed based the model test results. The settlement in the backfill observed in the model tests is consistent with the field behaviour.

Development and Performance Evaluation of Disk-Type Piezoelectric Transducer for Measurement of Shear and Compression Wave Velocities in Soil

A piezoelectric transducer capable of measuring both shear and compression wave velocities in soil simultaneously in triaxial testing conditions is presented. Performance evaluation of disk transducer system showed that the use of low-noise coaxial cables, proper grounding, and high resolution wave recorder can significantly enhance signal quality and eliminates crosstalk deterioration. Distortions due to near-field effects were found to diminish by increasing input frequency and by using sinusoidal input waveform, compared to square input. Disk-type piezoelectric transducers show significant future potential for laboratory determination of shear, and compression modulus of soil because of their robustness and noninvasive nature.

Detection of water infiltration and deformation of unsaturated soils by elastic wave velocity

Intense rainfall is the most important landslide trigger. In many mountainous environments of the world, heavy rainfall has caused many landslides and slope failures in a matter of seconds without warning. Therefore, an early warning system can be an effective measure to reduce the damage caused by landslides and slope failures by facilitating the timely evacuation of people from landslide-prone areas. In this study, we propose an idea to correlate soil moisture changes and deformations in slope surface by means of elastic wave propagation in soil. Constant shear stress drained triaxial tests where water was infiltrated from the bottom of specimen until failure, and slope model tests under artificial rainfall were performed to investigate the response of elastic wave velocities during pre-failure phases of rainwater infiltration and deformation. Analysis of the results has established that the elastic wave velocity continuously decreases in response of moisture content and deformation, and there was a distinct surge in the decrease rate of wave velocity when failure was initiated. Possible mechanisms were interpreted based on the test results. It is proposed that a warning be issued at switch of wave velocity decrease rate. This approach can thus serve as the basis of an early warning system for landslides and slope failure considering both moisture content and deformation.

Shaking Model Tests on Liquefaction Mitigation of Embedded Lifeline

The gigantic earthquake in 2011 caused significant damage in lifeline in the Tokyo Metropolitan area. In particular, the damage was significant in recent artificial islands where liquefaction affected embedded sewage pipelines profoundly. The encountered problem is that the entire subsoil liquefied in addition to loose backfill soils, and that the damage of branch lines introduced liquefied sand into trunk sewage lines, leading to difficult problem of sand clogging. Moreover, the same problem is expected to occur in other areas where strong earthquakes are expected in near future. The present paper addresses the ongoing model tests by which a variety of mitigation measures for sewage pipelines are examined. In the regions where future earthquake is expected, it is not possible to excavate pipes and reconstruct backfills now because of financial limitations. To cope with this situation, less expensive measures such as mechanical constraint, partial injection of grout, or limited installation of drainage measures are studied.

A new simple method to substantially increase the seismic stability of reinforced soil structures

A preloading and prestressing (PLPS) method has been proposed to substantially decrease the transient and residual vertical compression of geosynthetic-reinforced soil (GRS) structures subjected to long-term traffic load. It is shown that by using a newly developed device (called the ratchet system) in addition to the PLPS procedure, the seismic stability of PLPS GRS structures becomes very high. The ratchet system can not only maintain high prestress when the backfill tends to contract but also prevent the expansion of the backfill, both effectively restraining the shear and bending deformation of the structure subjected to seismic load.

Strength and Deformation Characteristics of Recycled Concrete Aggregate in Triaxial Compression

To investigate the feasibility of the use of recycled concrete aggregate as the backfill material of geotechnical engineering structures, such as embankments, geo-synthetic reinforced soil retaining walls and bridge abutments, a series of drained triaxial compression tests were performed. Both axial and lateral strains were measured locally. It was found that the compressive strength, q max , of recycled concrete aggregate highly compacted at water content in the vicinity of optimum moisture content, w opt , becomes similar to that of well-graded gravel that has been used as the backfill material of highest quality. When compacted at w opt , post-compaction saturation does not have any detrimental effect on q max of recycled concrete aggregate. The viscous aspects of the stress-strain characteristics were evaluated. Upon the restart of MLfollowing each creep stage, t he specimen exhibited very stiff and nearly elastic response for some stress range followed by marked yielding. The changes in the stress ratio upon a stepwise change in the axial strain rate were rather proportional to the instantaneous stress ratio and the logarithm of the ratio of the strain rates after and before a step change. These trends are very similar to those of ordinary type backfill materials.

On Early Detection and Warning against Rainfall-Induced Landslides (M129)

Traditional approaches to prevent rainfall-induced landslides consist of such stabilization of unstable slopes as installation of retaining walls as well as ground anchors. Although having been useful in mitigation of large slope failures, those traditional measures are not very helpful in mitigation of small slope failures which are less significant in scale but numerous in numbers. It is proposed in the present text for people to install slope instability detectors which find precursors of an imminent slope failure and issue warnings so that people may be able to evacuate themselves prior to fatal slope failures. To achieve this goal, model tests as well as laboratory triaxial tests have been conducted in order to understand the behavior of soil prior to failure. Moreover, numerical analyses on ground water percolation and decrease of factor of safety in the course of rainfall were conducted on a sandy slope in order to support findings from model tests. As a whole, a small instrument is proposed for a use of people which can detect minor displacement and change of moisture content prior to failure in a slope and issue warning through internet.

Elastic wave velocity monitoring as an emerging technique for rainfall-induced landslide prediction

Landslides are recurring phenomena causing damages to private property, public facilities, and human lives. The need for an affordable instrumentation that can be used to provide an early warning of slope instability to enable the evacuation of vulnerable people, and timely repair and maintenance of critical infrastructure is self-evident. A new emerging technique that correlates soil moisture changes and deformations in slope surface by means of elastic wave propagation in soil was developed. This approach quantifies elastic wave propagation as wave velocity. To verify its applicability, a series of fixed and varied slope model tests, as well as a large scale model test, were conducted. Analysis of the results has established that the elastic wave velocity continuously decreases in response of moisture content and deformation, and there was a distinct surge in the decrease rate of wave velocity with failure initiation, soil deformation was thus envisaged to have more significant effect on elastic wave velocity than water content. It is proposed that a warning be issued at switch of wave velocity decrease rate. Based on these observations, expected operation of the elastic wave velocity monitoring system for landslide prediction in the field application is presented. Consequently, we conclude that the elastic wave velocity monitoring technique has the potential to contribute to landslide prediction.