Abstract: Compaction grouting and fracture grouting are studied by use of a grouting simulation test device dereloped by the authors.The generation and development of grout balls and fractures in clay are performed by laboratory tests.The results show that pressure filtration effect exists throughout the grouting process.That is to say,free water in the grout penetrates through the clay under high pressure,so that the water-cement ratio of the grout increases.Pressure filtration effect increases with the increase of the permeability of soil,water-cement ratio of the grout and grouting pressure.So it must be taken into account when grouting in the soil has good permeability.The increase of grouting pressure will lead to the decrease of the diameter of grout balls;it also will cause the increase of the number of grout fractures and result in the decrease of the width of the grout fracture.Under certain grouting pressure,the diffusion of grout will gradually turn from the compaction grouting to the fracture grouting as the water-cement ratio increases.The fracture grouting in the clay can be divided into three stages: grout ball stage,first fracture surface stage,and following fracture surface stage.Every fracture occurs in the weakest surface.By use of the scanning electron microscope,differences among the natural clay,consolidated clay and grouting clay are analyzed.Under the effect of pressure filtration,Ca2+ ions can travel into the clay and improve the strength of the soil.The results are important to both the theoretical researches and the application of grouting.