Quality control of double fluid jet grouting below groundwater table: Case history

A jet grout block created by overlapping columns aimed to convert the saturated silty sand soils into a safe space for breaking down four underground parking uplift piles along the tunnel alignment. Since the grouting platform was located on the mat foundation in the basement of underground parking and was under the groundwater table, a series of dewatering wells were installed before the 177 columns were constructed to prevent intrusion of the groundwater flowing upward from sandy soil to basement through grouting hole during wash boring and grouting stages from happening. Several quality control measures were undertaken prior to quality assurance testing. Two measurements of spoil return and spoil flow rate for each column were implemented. From a back analysis from the mean values of spoil density and spoil flow rate, the column diameter was estimated at around 1.56 m, slightly smaller than the design diameter by 2.5%. In addition, a control chart with upper and lower control limits, established from a large dataset of flow rate of spoil return, was a means to recognize the likelihood of sand boiling and to serve as an early warning indicator of abnormal conditions for the jet grouting works below the groundwater table. As the mean spoil density of the infill column was larger than perimeter column, an optimal grouting sequence for clusters was suggested in this paper. The jet grout block exposed during the breakdown process of the piles showed safe working conditions as required and a shield machine then passed through as planned.