The magnitude and distribution of sidewall friction of open caisson are very important to caisson structure design and the selection of sinking-aid methods. However

the computation of sidewall friction is still quite empirical

which often leads to significant deviations from real situation. In order to further understand and master the distribution rule of the sidewall friction of open caisson during the process of sinking

a new type of micro-friction-sensor is invented to measure the sidewall friction directly; and its accuracy

stability and reliability are verified. Then

it is used in model test along with static strain test system. The result shows that the variation characteristics of sidewall friction can be divided into three phases with the increase of sinking depth. In phase one

the increase of sidewall friction and depth is close to linear relation; in phase two

the sidewall friction keeps increasing with depth

yet the rate of increase drops and the value reaches the peak at a particular depth; in phase three

the sidewall friction drops gradually with the increase of depth. The mechanism of this phenomenon is analyzed with theory of limit analysis. The test result can be treated as a complement of in-situ monitoring

and both provides fundamental data for analytical and numerical computation for sidewall friction of open caisson.