Highway Subgrade (1) Introduction *Definition Highway subgrade or basement soil may be defined as the supporting structure on which pavement and its special undercourses rest. 2. *Types of Highway Subgrade 1) In cut sections, the subgrade (defined as cut or excavation) is the original soil lying below the special layers designated as base and subbase materials. 2) In fill sections, the subgrade (defined as embankment or embankment fill) is constructed over the native ground and consists of imported material from nearby roadway cuts or from borrow pit. 1 Monday, April 24, 2017

Highway Subgrade (1) Introduction *Cross-section of subgrade (depending on the type of surfacing) 1) On earth roads, the subgrade, which is also the surface course, is shaped to the standard road cross section; 2) For surfaced roads, the subgrade is graded to the same slope as the finished surface; 3) In trench method, the earth excavated to form a trench is pushed to the sides of the road to form retaining shoulders. Modern traffic demanding for deeper cuts and higher fills as well as higher standards of alignment and grade, which in turn means stronger subgrade. 2 Monday, April 24, 2017

Highway Subgrade (1) Introduction * Factors determining the strength and stability of subgrade Strength and stability are the most important two indexes of highway subgrade. Stability includes both temperature stability and moisture content stability of the soils for subgrade. The following factors most significantly influence the stability and strength of subgrade Types of soils Moisture Content (Water aids as a lubricant up to the optimum moisture content) Compaction methods and devices(kneading or rolling, vibrating, and heavy weight dropping) 3 Monday, April 24, 2017

Highway Subgrade (1) Introduction 6. Compactive devices 1) tamping or sheep’s foot roller; 2) pneumatic-tire rollers; 3) smooth-steel wheeled rollers; 4) segmented plate compactors; 5) grid rollers; 6) vibratory compactors in the form of vibrating pads, steel wheels or pneumatic tires; 7) combinations of the above (e.g. smooth steel wheels and pneumatic tires); 8) hauling and spreading equipment; 4 Monday, April 24, 2017 赵永平：Lesson Eight