Abstract: The subgrade fill of some reaches of Qin-Shen Special Passenger Line consists of clean, round and poorly graded fine sand. For satisfied with “Detailed Rules and Regulations of Qin-Shen Line Railway Subgrade Construction Technology”, on the basis of a vast amount of comparative test, an economical and rational technical plan for improvement of fine sand is determined. According to the material proportion and construction technology, several millions of cubic meters of subgrade construction have been completed. A vast amount of technology data and rich experience for improving fine sand has been accumulated. Through analysis and study for improvement of fine sand, making further improvement on much respects has been found. On the basis of integration of theory with practice, this paper raises a calling in question for necessity of improvement of fine sand fill and control standard with a double targets; beside, reveals and sets forth essence of K30 value of poorly graded fine sand being on the low side and change regularity of fill strength as construction height.

Key words: poorly graded fine sand, round gravel, improvement of fill, K30 , porosity, relative density