:Drill rigs and piling equipments are large pieces of construction machinery used to build pile foundations and to improve the ground stability. Crawlers comprised in the lower base enable the machines to propel themselves. A tall leader given in the upper superstructure provides top heavy weight distribution. Sufficient bearing capacity and flatness are required in the supporting ground to keep the machinery stable without any tilts while propelling between the positions to drill. Nevertheless, overturning occurs in connection with these kinds of construction works. This study focuses on the phenomenon of ground instability causing machines, so called drill rig in this study, to overturn.
Two sets of crawlers support the drill rig to restrict pitching during operations of self propelling. Nevertheless, the drill rig may become quite unstable if the crawlers should happen to sink into the bearing ground. Overturning huge machinery gives quite large damages to facilities in the sites as well as surrounding societies. Accordingly, it is important to prevent the overturning of drill rigs at works.
Two potential causes concerned in drill rigs becoming unstable. One is a problem caused by structural instability so that the body is composed of a top heavy weight distribution. An overturning moment much increases with an increase in tilt due to ground penetration. In addition, a pitching at the self propelling causes to increase inertia to overturn. The other is concerned with supporting grounds. An excavation in sites and an overlaying by reclaimed soil are performed in the processes of constructions. These works produce an uneven surface as well as a distribution of different stiffness on the supporting grounds. Then, the drill rigs pitch by differential settlement in the crawlers during the self propelling. Therefore, risk of the overturning in drill rigs arises in aspect of the self propelling in the work sites.
In this paper, firstly, investigations on safety code and regulations in Japan as well as in Europe and analyses of the accidents in the past are conducted to make sure the problems in terms of keeping stability in the drill rigs.
Secondary, prototype model tests were carried out to investigate the basic characteristics of pitching during the self propelling. A huge drill rig composing 1MN of self weight moved on a test yard that was prepared in the developing site of the Tokyo International Airport. A series of leveling, ground density tests and bearing capacity tests were performed to clarify the condition of supporting ground to be propelled by the drill rig in advance. Predominant frequency is distributed in around 1Hz so that the drill rig pitches by long period during propelling. As a value of horizontal acceleration in the upper leader shows the maximum, it is ascertained that the drill rig pitches around fulcrum of the front drivers supporting crawlers.
Thirdly, a series of experimental simulations using a newly developed 1/30 scaled model of the drill rig was performed in a centrifuge in order to classify the hazards causing the drill rigs unstable. It was made sure that distribution of differential stiffness in supporting grounds induces the drill rigs to pitch at the propelling as well as uneven surface. An acting pressure through the crawlers shows the highest value at the front drivers. It was also confirmed that pitching during the self propelling causes to oscillate the values of acting pressure. And, a difference in characteristics of pitching, while amplitude and a frequency of the pitching, varies both the maximum value and standard deviation of the acting pressure. Coefficients of variations decrease in the flat and stiff supporting grounds. An average inclination of supporting ground(θbv) is introduced by frequency analyses on plofile of surface. Several sets of experiments were conducted on model grounds which consist of a different θbv and its variation.
This study concludes that the stability angle (θa) in drill rigs should be set higher values to keep stable against happening of settlement in crawlers, existence of uneven surface and occurrence of pitching. A number of safety factors on bearing capacity(Fs) also should be set higher by penetration measures on supporting ground to prevent the tilt in the drill rigs. It is also important to reduce the ranges of distribution on bearing capacity of supporting ground so as to decrease a differential settlement in crawlers. Accordingly, this study proposes to set the drill rigs by Fs > 3, θa> 10 and θbv < 1.25 to keep stable while propelling.