Abstract: The dry-jet-mixing columns have been widely used in the soft clay improvement of highway engineering,but the installation effect has not been understood deeply.A model machine was established to study the installation effect.Before the laboratory model tests,the block soils were sampled by the large diameter PVC tubes,and the samples were preserved from disturbance.After the lab oratory model tests,it was found that the physical properties of the surrounding soils were the same as those before installation,but their strength decreased.It was also emphasized that the strength of the surrounding soils decreased with the increase of jet pressure and the decrease of distance between the test point and the column border.