Laboratory Testing of Construction-Induced Ground Displacements for Open Caisson Shafts in Sand

Abstract

The development of underground spaces inevitably poses significant risks to nearby infrastructure due to construction-induced ground displacements. While our understanding of tunnel-induced ground movements is now relatively mature, there is a distinct lack of literature on large-diameter open caisson shafts. This paper fills this gap by describing results from a small-scale laboratory study exploring soil deformation mechanisms during caisson construction in dry sand. Results from seven tests are analyzed to identify the influence of key caisson geometric properties as well as the effectiveness of external cofferdams in minimizing soil displacement. The results show that the primary mechanisms driving ground movements are a compressive ‘bearing’ front beneath the cutting face and a ‘frictional’ contribution above the cutting face. The normalized radial settlement profile is also shown to be insensitive to the normalized caisson embedment depth, and the settlement zone of influence extends up to 0.25 diameters below the caisson cutting edge. Furthermore, the presence of an external cofferdam is shown to be highly effective in reducing soil settlements. Quantitative analysis reveals a significant decrease in soil settlement with an increase in cofferdam depth from 0.25 to 0.5 of caisson depth, with good consistency between results for different soil elevations. In addition, larger cofferdam diameters provide maximum benefits in minimizing ground displacements.