Abstract: The loose and soft soils in the mountain area frequently contain random boulder sized rock blocks with various grain sizes. It is usually difficult to accurately characterizing the vertical and horizontal limits of boulderly soil strata by using the conventional boring spacing defined in the code. In accordance with the author’s experience of characterization of clayey, gravelly, and sandy soils strata in the valley area, the potential random boulderly soil lenses should be verified by using smaller bring spacing of 20 m to 40 m in the longitudinal direction. Further, in the transverse direction, at least two test borings should be arranged with a test boring spacing of 50 m to 100 m. Based on the proposed fill height and the boulderly soil distribution, various soil improvement and structural measures, such as, sheet pile with strong penetration; soil replacement and surcharging with large surcharging ratio, can be used to meet the settlement control requirement of high-speed rail projects.

Key words: Valley Area, High-Speed Rail, Loose and Soft Soils, Settlement Control, Soil Improvement