The potential for 'windows' in deeper jet grouting treatment becomes more significant when there are known variations in the soil characteristics over the length of the column. When changes in soil type or density occur, it is normal practice to vary the jet grouting parameters in an attempt to obtain regular column diameters. However, according to Soletanche Bachy, it is not possible to adjust jetting parameters for minor soil variations.
To compensate for these difficulties, it would seam appropriate to tighten up the drilling pattern. However, experience in Cairo reveals this increases the risk of secondary columns deviating into the body of a primary adjacent column. Since jet grouting energy is generally insufficient to erode a hardened primary column, the secondary column diameter is significantly reduced, leaving a masked zone of untreated soil. Accordingly, says Soletanche Bachy, drilling pattern, column diameter and jet grouting parameters must be carefully selected to minimise the potential for untreated 'windows' in the treated mass.