The jet grouting method is commonly used for ground improvement. However, little is known about its actual improvement structure in the ground. The extent of improvement, such as the size of effective grout diameter and the degree of overlapping is still in question, especially for complicated ground conditions such as grounds with different soil strata. This paper presents the actual findings of the condition of improved ground by jet grouting method, from a tunneling site. It was observed that preceding grout column had a near circular shape, and that they interlocked with the adjacent grout columns, hence meeting the ground improvement requirements.