Some in situ seismic piezocone penetration tests (SCPTU) were performed in undisturbed soils and the inter-pile soils between T-shaped, bidirectional, soilcement deep mixing columns at lacustrine clay sites to quantitatively assess the effects of ground improvement. The difference of cone resistance and sleeve friction of soils among the columns after cement column construction was analysed. From pore pressure dissipation tests, the consolidation characteristics of the inter-pile soils were evaluated. The effect of the deep mixing column on the behaviour of the inter-pile soils is discussed. Preconsolidation pressures were estimated using some methods based solely on net cone tip resistance, excess pore pressure, effective cone resistance and a multiple regression based on cone tip resistance and shear wave velocity. Cement columns increase the lateral stress locally within the zone of influence of the column. Soils initially appear to be normally consolidated to slightly overconsolidated, and a significant increase in preconsolidation pressure was noticed in clay layers after treatment.