Naotaka Kikkawa

A Full-Scale Model Test for Predicting Collapse Time Using Displacement of Slope Surface During Slope Cutting Work

It is important to predict an onset of slope failures or rock falls for the occupational safety because about 15–20 workers were killed by these every year in Japan. Approximately half of the victims suffered from slope failure during slope excavation construction. In this research, in order to predict the time of slope failure during an experimental testing on a full-scale model slope was conducted, and the displacements of the slope surface were monitored during the slope excavation. The surface displacements rapidly increased with the elapsed time after the excavation, and the relationship between the displacements and the elapsed time included an exponential function just before the collapsed. We validated that the time of slope failure could be predicted by the relationship between the acceleration and the velocity of the obtained slope surface displacements. However, in order to predict the time of collapse, the data was required to compute only 2 s before the collapse. Therefore, we realised the importance of providing advisory and warning signal to give workers enough time to escape the slope failures. We have discovered that by computing the inverse of velocity of slope surface displacement, advisory and warning signals can be provided 2 min before the collapse.

Failure Mechanism of Anchored Retaining Wall Due to the Breakage of Anchor Head

In this research, a case history of temporary earth support collapse is first illustrated briefly and the mechanisms of accident occurrences are introduced, with the results showing that the shallow penetration of piles mainly caused the sequences of collapse. In order to understand these failure characteristics and mechanisms, centrifuge model tests using an in-flight excavator were carried out. The failure mechanism of the retaining wall in this labour accident was first demonstrated using centrifuge model tests by Toyosawa et al. Failure mechanism of anchored retaining wall, 667–672 (1996). In this paper, we added some viewpoints regarding the mechanism of the retaining wall and it was thus clarified that the active and passive earth pressures in the retaining wall increased during excavation and then the anchor head exceeded the capacity with respect to tensile stress. As a result, the retaining wall and ground behind the wall collapsed suddenly.

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.

Unstable behavior of segmental ring under various pressures and its discrete element simulation

In February 2012, a serious accident which resulted in five fatalities happened during a TBM-tunnel construction under the seabed in Japan. The cause of the accident appeared to be due to the Key-segment slipping out of the segment ring by the thrusting tailskin (wire brushes) of the TBM into the segment ring. This resulted in the collapse of the rings, causing the seabed ground and seawater to flow into the tunnel. We investigated how thin and thick segments without any circumferential joints behave under isotropic and anisotropic pressures using small-scale physical model. In the model tests, pressures were applied to the surroundings of the segment rings and the strains at each segment were measured in order to evaluate the damage. In addition, cases where lubrication on the contact area between the K- and B-segments was present or not were investigated and their discrete element simulations were also conducted.

Study on Mechanism of Rock Fall at Tunnel Cutting Face after Blasting

There are around 10 casualties due to rock-fall at cutting face annually in conventional tunnel construction in Japan. As from the analysis conducted on the cases involving such casualties, workers were either killed or seriously injured when they works in front of or near the cutting face. For the purpose of evaluating the mechanism of rock fall at tunnel cutting face, this paper performed experimental tests which involved blasting to excavate a model ground of tunnel cutting face, and then analyzed the stress state which is in the cutting face by using Discrete Element Method (DEM) simulation. Based on the results, the tensile stresses remained even when the action of gas expansion due to blasting has completed. Therefore, it is suggested that rock falls might be induced because of the residual tensile stresses. The tensile stresses would gradually open small cracks between rocks and then rocks may suddenly fall after sufficient crack opening due to 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