The working frequency of the Rapid Impact Compaction is 30-80 times per minute, and the working gears are divided into three gears: low, medium and high. In order to study the influence of different gears on the reinforcement and compaction effect, plum blossom is used to arrange compaction points on the subgrade surface with qualified compaction at the junction of new and old subgrade, and the center spacing of compaction points is 150cm, that is, the edge spacing of adjacent compaction points is 500cm. Different points I, II and III are selected for 18 times of compaction. The settlement is observed with a level gauge every 3 times of compaction, and 3 points are taken under each standard to observe the settlement. The observation results show that the reinforcement and compaction effect of level III is the best, and the reinforcement and compaction effect of level II is the worst. According to the evaluation data, the effect of three times of compaction of gear III is higher than or close to that of gear I and gear II. Therefore, when using Rapid Impact Compaction to strengthen the compaction of loess subgrade, it should choose III to strengthen the work, with the highest efficiency.

In order to study the influence of compaction frequency of Rapid Impact Compaction on the compaction effect, after backfilling the Loess Subgrade on the back of buttress retaining wall, select III to carry out 6 times, 9 times, 12 times, 15 times, 18 times, 21 times and 24 times of compaction experiments at different points. In the experiment, observe the tamping settlement once every 3 hammers. After compaction, use a ring knife to take samples at different depths in the center of the compaction point to check the compactness of different compaction frequencies.

According to the tamping settlement caused by different frequencies, the program high-speed hydraulic tamping machine is applied to reinforce and tamp the loess subgrade. According to the different positions and quality requirements of reinforcement and compaction, the construction efficiency of reinforcement and compaction shall be considered. The tamping frequency of each point is 12 ~ 18 times, and the tamping settlement caused by the final 3 hammers shall not exceed 1 cm~1.5cm.

After reinforcement and compaction, samples shall be taken at different depths at the center of the compaction point with different frequencies to check the compactness. From the change of compactness caused by different frequencies of compaction, Rapid Impact Compaction is used to reinforce and compact the loess subgrade. According to the different positions and quality requirements of reinforcement and tamping, considering the construction efficiency of reinforcement and tamping, the thickness of reinforcement and tamping should not be too large. If each filling volume is 1m~1.5m, it should be strengthened once, and the tamping frequency of each point should be 12 ~ 18 times.

In order to study the influence of tamping point spacing on the tamping effect, after the loess subgrade is backfilled at the back of the wall, the tamping point spacing is 1.25m, 1.5m, 1.75m and 2.0m respectively, and the tamping experiment is carried out. The layout of tamping points is plum blossom. According to the previous experimental results, the compaction frequency of each point is 18 times, and the compaction deposition is observed every 3 times. After compaction, samples shall be taken at different depths of the center of the compaction point, the edge of the compaction point and the center between two points to check its compactness.