T-bar penetrometer is currently used to characterize seabed sediments mostly based on the full-flow mechanism around the probe in soft clays. However, open and trapped cavities may form above the T-bar during its penetration in stiffer clays, which can make the traditional interpretation method inaccurate and requires complex corrections. This paper proposes a novel T-bar test method by adding sufficient surface pressure on the soil surface to ensure the full-flow mechanism and no cavity. Thus the current formulas based on full flow soil mechanism can be directly used to interpret soil strength. To verify the effectiveness of the proposed method, both laboratory tests on Kaolin clay and Guangzhou Offshore clay and large deformation FE (LDFE) analyses with various surface pressures and soil strengths were conducted. The results show that, if the surface pressure is sufficient, no open cavity or trapped cavity was formed during a monotonic T-bar penetration and the first cycle of cyclic T-bar penetration tests. While without a surface pressure, an open cavity or trapped cavity was always formed. The cavity formation contributed to 7.9% ~ 18.6% lower T-bar resistance relative to that of T-bar with a sufficient surface pressure to maintain a full-flow mechanism. In the cyclic tests, with further number of T-bar loading cycles, the cavity effect diminished and the surface pressure showed minimal effect on the T-bar resistance. The critical surface pressure which ensures the full-flow mechanism was suggested for T-bar monotonic and cyclic tests in the box core sample.