Abstract: Rectangular closed diaphragm wall foundation is a new type of bridge foundation.The mechanism for negative skin friction and the bearing behaviours of closed diaphragm walls after immersion were studied through model tests.It was observed that the neutral plane depth ratio of diaphragm walls was 0.34⁰.64,and the neutral plane depth of closed diaphragm walls was lower than that of single diaphragm wall.Distribution curves of negative skin friction were similar to parabola,as was also found in some field test studies.With good integrity and anti-seepage capacity,there was no effect of water on soil core,then,the inner shaft resistance and soil resistance under the cap were developed well.So the working load and downdrag load of closed diaphragm walls were shared by the inner shaft resistance,outer shaft resistance,toe resistance and soil resistance under the cap.Additional settlement of closed diaphragm walls was smller than that of single diaphragm wall in the same loess stratum under the same immersion condition.The good anti-settlement capacity of diaphragm walls could reduce the hazard of negative skin friction in bridge engineering.