Abstract: There were many accidents in which the workers were killed and buried under collapsed trench when the construction machinery such as a drag shovel fell or toppled over the edge of the trench during excavation.According to the results of centrifuge model tests simulating the process of trench excavation,the failure mode of trench wall and its velocity field were established.Upper-bound solution of the excavation depth could be given by the upper-bound theory.According to the characteristics of failure shape of trench walls,the expression for the upper-bound excavation depth was simplified.The simplified expression for the upper-bound excavation depth was verified by the results of centrifuge model tests.The division of influence area of machinery load was given by the expression width of for sliding soil body,and the influence area and non-influence area were visually represented.When the machinery load acted on the non-influence area,there was no influence on excavation depth,and the excavation depth given by the simplified expression for the upper-bound solution equals to that of without machinery load.When the machinery load acted on the influence area,the excavation depth was affected by machinery load,and the ratio of the excavation depth to that of without machinery load could be given by the simplified expression for the upper-bound solution.