作業現場における地耐力確認の方法: 移動式クレーンを含めた建設機械等の転倒防止対策

Abstract

本技術資料は，移動式クレーンや基礎工事用機械などを含めた建設機械等が，地耐力不足のために転倒する災害を防止するために，設置する作業現場の地耐力を確認する方法を示したものである．これまでの研究から明らかになった動的転倒の危険を指摘したうえで，安定確保には地盤の「強さ」と「沈下量」に加えて降伏後の「沈下速度」の条件の考慮が必要なことを述べた．そして地耐力確認では，載荷圧力p(sub a) に対する極限支持力q(sub d)の比である支持力安全率F(sub s) を求めて閾値SR と照査することとし，SR の値は急激な沈下による転倒危険を考慮して，3.0 を標準とすることを提案した．地盤調査の方法については，代表的な手法を整理して留意点を述べるとともに，新たな調査法を検討した．そして，作業現場における仮設の地耐力確認の観点から，効率的かつ簡便な「現場地耐力試験」の新たな方法も提案した．法令等では「堅固」な面へ設置や「軟弱」な地盤上での使用禁止を述べているものの，その表現が定性的であることから現場での評価や判断は異なる現状も認められる．本研究では，必要地耐力を定量的に示すとともに，その確認の方法を示したうえで，安全レベルの向上に必要な対策を述べている．

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 p(sub a) should be less than a value of the ultimate bearing capacity of ground q(sub d) 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 q(sub d). 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 p(sub a) and q(sub d) is considered as the margin of safety that is also identified by a safety factor of bearing capacity F(sub s), which is defined as the ratio q(sub d) divided by p(sub a). Bearing safety of the supporting ground is ensured by verification of F(sub s) 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 R(sub E), which is ratio of an initial tangent modulus K(sub 0) divided by a tangent modulus after the yield K(sub d) 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 q(sub d). 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 q(sub d) and R(sub E) 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.