Tomohito Hori

Centrifuge Model Tests on Seismic Slope Failure

In this study, experimental analyses were carried out to investigate characteristics of slope failure by earthquakes and potential risks of secondary failure. A 1/50 scale model slopes were made of Kanto loam to simulate the shallow unsaturated soil in slopes. Seismic acceleration was applied on a shaking table in centrifuge, so that shear strain in the shallow unsaturated soil increased as the number of seismic cycles. It was found that the slopes failed as a result of accumulation of the plastic shear strain. In addition, changes in height and angle of slopes were measured in the upper remaining section after seismic failures. The potential risks of secondary failure (R v ) were analyzed using Taylor’s stability chart. It was ensured that the value of R v on the upper section decreased as the collapsed soil increased.

Analysis of labour accidents in tunnel construction and introduction of prevention measures.

At present, almost all mountain tunnels in Japan are excavated and constructed utilizing the New Austrian Tunneling Method (NATM), which was advocated by Prof. Rabcewicz of Austria in 1964. In Japan, this method has been applied to tunnel construction since around 1978, after which there has been a subsequent decrease in the number of casualties during tunnel construction. However, there is still a relatively high incidence of labour accidents during tunnel construction when compared to incidence rates in the construction industry in general. During tunnel construction, rock fall events at the cutting face are a particularly characteristic of the type of accident that occurs. In this study, we analysed labour accidents that possess the characteristics of a rock fall event at a work site. We also introduced accident prevention measures against rock fall events.

Section B Fire and Explosion - Observation of Confined Deflagration Phenomena of Flammable Gas Mixtures Under Elevated Gravity

In order to investigate the effects of gravity on ignition and deflagration of flammable mixtures, we conducted flammable gas explosion experiments using constant volume combustion vessels under several gravity levels from normal gravity (1 G) to elevated gravity up to 50 G. The elevated gravity field was formed by a large-size centrifuge system. We examined flame kernel growth, flame spreading, and explosion pressure in hydrogen-air, methane-air, and propane-air mixtures in an elevated gravity environment. The flammability ranges were determined by the records of the explosion pressures, and it was confirmed that the upper and lower explosion limits were approximately the same regardless of the gravity level. However, the shapes of the propagating flames changed dramatically with increase of gravity level due to strong buoyancy effect especially in lean and rich mixture conditions with slow burning velocity. Moreover, it was found that similar flame spreading phenomena were observed in large-size deflagration of rich mixture under normal gravity and small-size deflagration of stoichiometric mixture under elevated gravity.

Analysis of death labour accidents relating with drag-shovels and investigation of the countermeasure for safety

In Japan, we have more than 600,000 drag-shovels and the death labour accidents relating with drag-shovels happened more than 70 per a year until 2006. So that we analysed the accidents relating with drag-shovels in details. As a result, there are 4 types of labour accidents: a drag-shovel falls down due to its unstable condition, a burden hanged hits a worker, slope or trench collapses due to excavation and weight of a drag-shovel, a drag-shovel drives backward or pivots and hits a worker. It is revealed that a drag-shovel becomes unstable condition due to dynamic operation such as driving forward or pivoting in a slope and that a drag-shovel and worker are working together within the maximum excavation radius of the drag-shovel. As the countermeasure for the safety, we recommend the use of a drag-shovel with ROPS (Roll-Over Protective Structure) and fastening a seat belt, to set the maximum stable grade considered with dynamic operation, the system alerting both an operator and worker to their approach. Language: ja

CENTRIFUGE MODEL TESTS ON INSTABILITY OF AUTOMOTIVE PILE DRIVERS

The automotive pile driver is a piece of large-scale construction machinery. This machinery is used for ground improvement and foundation work. In recent years, there have been some reports of accidents in which the machinery overturned. In this study, a 1/25 scale model of an automotive pile driver was made to clarify the mechanism making the pile driver unstable, and centrifuge tests were performed. The load of each axle and the behavior of the model were measured in a centrifuge test. The grounds used with the model are two types of urethane foam with different strengths, with conditions imitating the uniform ground and non-uniform ground. As a result, the response acceleration did not have a significant difference due to differences in ground conditions. On the other hand, the effect of the ground condition was different in the axle load, and it was possible to evaluable the instability of the automotive pile driver when self propelling.

A Case Study on the Overturning of Drill Rigs on Construction Sites

Drill rigs are large pieces of construction machinery used to build pile foundations and to improve the ground stability. Crawlers comprising the lower base enable the machines to propel themselves. A tall leader given in the upper structure provides top heavy weight distribution. Sufficient bearing capacity and flatness are required in the bearing ground to keep the machinery stable without any tilts while it is propelling itself. Nevertheless, overturning often occurs at constructions. This study focuses on the phenomenon of ground instability causing machines to overturn. A drill rig weighing 372.5kN and with a leader 15.9m tall overturned on a building construction site. This machinery moved within the site to the positions required for building the diaphragm walls. This paper, firstly, summarizes operations prior to the accident. Secondly, the equilibrium condition of the machinery was calculated to clarify the stability, and the pressure acting on the ground through the crawlers was estimated. Ground properties were also investigated to assess the potential risk of failure in bearing ground. Finally, problems with the stability of drill rigs are discussed.