Abstract of TD-No.3

National Institute of Occupational Safety and Health, Japan

A Study on Safety Practices of Investigation of Bearing Capacity of Supporting Ground for Prevention of Overturning of Heavy Machineries

Satoshi TAMATE and Tomohito HORI

Drill rigs and mobile cranes are heavy machinery used in construction sites. Sufficient bearing capacity is required in the supporting ground to keep the machinery horizontal. However, overturning accidents frequently occur because of ground penetration at the foundations of lower careers. A differential settlement ΔS makes machinery unstable because an overturning moment increases whereas a moment of stability decreases. Thus, the machinery overturns by reaching the limit equilibrium of moment. Therefore, a value of the acting pressure through the foundations pa should be less than a value of the ultimate bearing capacity of ground qd so that ΔS can be reduced to negligible small values. Therefore, careless investigation of merely observing the ground surface is an insufficient method for survey to confirm the safety. Ground investigations must be conducted to clarify a value of qd.
In addition, a tall super structure of the machinery takes high center of gravity. Then, the rotational energy induced by a quick tilt causes the machinery quite unstable by reaching the limitation of kinetic equilibrium earlier than that of static equilibrium. Accordingly, brittle failure is dangerous characteristics of ground for the stability rather than ductile failure introducing the slow penetration. Then, it is important to survey the potential risk of rapid penetration at ground investigations.
A value of difference between pa and qd is considered as the margin of safety that is also identified by a safety factor of bearing capacity Fs, which is defined as the ratio qd divided by pa. Bearing safety of the supporting ground is ensured by verification of Fs by a threshold value of SR. As this type of machinery is usually placed on construction grounds for a short period, 1.5 of SR is generally referred in accordance with a code for the temporary placements. However, a limiting depth of penetration decreases in dangerous ground that composes the potential risk of rapid penetration. Therefore, authors propose to use the higher value of 3.0 for safety unless nothing of the potential risk of rapid penetration is confirmed.
An index of rapid penetration RE, which is ratio of an initial tangent modulus K0 divided by a tangent modulus after the yield Kd those are obtained from curves of the relationship between an acting pressure through the loading plate q and its settlement S, is introduced to associate with the bearing characteristics of the kinetic overturning. Then, a plate loading test (PLT) is required to derive the value of RE in addition to qd. However, PLT has also problems of taking a long period to perform the test as well as doing many procedures to set up the instruments. Therefore, high speed bearing capacity testing (BCT) method was newly developed to derive values of qd and RE efficiently. q is loaded through the same loading plate of PLT though a penetration depth increases by constant velocity of 5mm/min in consideration of both a condition of 1mm/min in CBR testing and a ratio of diameters between PLT and CBR. In addition, a simple method to measure S was also introduced to decrease a number of the procedures at the preparation of test. Several sets of BCT and PLT were conducted in various conditions of grounds to compare the results. It was ascertained that an almost identical relationship between q and S are obtained from both the tests. Accordingly, bearing conditions in the supporting ground can be evaluated by BCT of simplicity method.
This paper summarizes safety requirement for prevention of the overturning for machinery. Ground investigations are needed to ensure the safety on bearing capacity of supporting ground. In addition, a new testing method BCT is also developed to measure the ultimate bearing capacity and rapid penetration characteristics. Authors finally propose to conduct the safety practices of quantitative evaluations in consideration of the results from ground investigations.

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