This paper studies a field case where post grouting technology failed to greatly improve the bearing capacityof the single pile, and concludes that the main cause for the insufficient bearing capacity of the single pile is the poor control of the grouting process. Strict quality control and installation of the grouting pipe, jet grouting pipe and check valve as well as the grouting process control are the key factors for successful grouting, hence improvementof the bearing capacity. In the above field case, the area with insufficient bearing capacity was treated by supplementary piles. During construction of the supplementary piles, the production of the grouting device was standardized and the control of the grouting process was strengthened. After the supplementary piles reached the agingperiod, the static load test was carried out, and the results showed that the bearing capacity of the supplementarypiles met the design requirements.