Abstract: For the newly-dredged mud foundation improved by the no-sand-cushion vacuum preloading technology (NSVPT), the soil strength increases only a little, the effective improved depth is small and the ground bearing capacity is still low. To analyze the causes, more comprehensive researches are made on the engineering properties of newly-dredged mud from different areas, and the comparative single-well model experiments with 3 kinds of representative vertical drains are conducted in laboratory using NSVPT. It is concluded: (1) the severe clogging problem in vertical drains is induced by high content fine-grained soils (d0.005 mm), too small characteristic opening size of filter (O₉₅) and too fast loading rate of vacuum preloading; (2) the partial loss of vacuum degree is great, and the friction loss of vacuum degree is also greatly caused by serious curvature of vertical drains and large transfer resistance of vacuum degree in horizontal drains; (3) the drainage efficiency of drainage system is reduced and the vacuum pressure in soils is weakened so as to mitigate the improvement effect of soils owing to high content of strongly hydrophilic minerals, severe clogging in vertical drains and large loss of vacuum degree. Accordingly, some countermeasures are put forward.