Âm thanh chia sẻ

UNIT 17: ROADS AND PAVEMENTS Page 81 INTRODUCTION Engineers do not only build buildings. They also build roads, highways, and parking lots. Roads must be strong, smooth, and safe. They must handle heavy trucks and rain. STORY 1: Layers of a Road A road is made of several layers. The bottom layer is the subgrade. This is the natural soil, compacted. On top of that is the sub-base. This is crushed stone. Next is the base course, made of larger stone. The top layer is the pavement. This is wh