Abstract: To prevent the accident of fleeing bore during manual digging pile construction, the instability mechanism of soil between manual digging piles is discussed; then the feature of displacement in instability is analyzed; a stability analysis mechanical model is established; formulas for the stability coefficient and critical value of pile spacing are derived; and some suggestions for fleeing bore prevention are provided. The results show that during the instability of soil between piles, the sliding soil displacement parallels to the line through the two pile centers and its width is equal to minimum of diameters of the two piles. While the most disadvantageous value of the thickness of the sliding soil could be determined by repeated calculation, the stability coefficient of soil between piles is inversely proportional to the width and the depth of the sliding soil, is proportional to the pile spacing, and rises with increasing of the shear strength and residual shear strength of the sliding soil. Controlling the concrete height difference between two pile holes, increasing the pile spacing, strengthening the lining structures, and reducing disturbance of soil between piles, are the most effective measures for fleeing bore prevention. Engineering practice has proved that the stability analysis method of soil between manual digging piles is reasonable and practical; so as to provide reference for manual digging pile construction in cohesive soil-based site.

Key words: manual digging pile, soil between piles, stability coefficient, critical value of pile spacing