probabilistic design of massive jet grouted water sealing barriers

A probabilistic design procedure for jet grouted bottom sealing barriers of open excavations under ground water level is shown on the basis of geometrical considerations on the true shape of the columns, which are far from being vertical perfect cylinders. By considering a square grid disposition of the columns axes, and taking into account the possible defects of the columns (verticality and diameter) assuming typical statistical parameters of their distributions taken from field trial measurements carried out by the authors, a Monte Carlo procedure is used to get some simple design charts for a given risk level. The analyses have been carried out for different values of column mean diameters Dm and length, as well as for different spacing between columns s. It is shown that columns overlapping decreases with depth because of defects, the deviation form verticality being the most critical one. The true minimum thickness of the barrier is smaller than the ideal one, resulting into lower uplift safety factors. For a given value of the ratio s/Dm, larger columns are more effective. In the paper, it is argued that a cost effective design may be obtained by associating safety and economical considerations via the proposed probabilistic charts.