OCR Text |
Show SUBGRADE MUST BE PROPERLY PREPARED Subgrade soils which absorb and retain' re-tain' a large amount of moisture give the most trouble In the construction of roads. These are the various forms of clay and a few of the heavier clnylike loams. They are difficult because they cannot be drained by side ditches or tile. When frozen, the wet soil expands ex-pands and lifts the pavement. When It dries out it often shrinks so materially mate-rially that wide cracks form, setting up tensile stresses in the slab above which may prove disastrous. When the dried soil again absorbs moisture It expands, lifting the slab. Unless laboratory tests have located the good nnd bad soils In the project the Inspector must depend upon personal per-sonal observation alone. Ask those who live along the road where the worst spots appear each spring. A-company A-company the roller and note any spongy places. Keep your eyes open for seepage water. Bog holes, or short, deep, spongy spots, are usually caused by a pocket In an Impervious underlying stratum. Water from some higher point "Is caught In this pocket and then absorbed ab-sorbed by the earth above. There are several methods of remedying rem-edying bog holes. If not too deep they may be drained by cutting through the Impervious stratum and laying tile to the bottom of the pocket. When the bog hole Is formed by a rock pocket' It will probably be necessary to Intercept Inter-cept the seepage water before it reaches the road or to remove the spongy material and replace it with cinders, sand, gravel or crushed stone. If the spongy soil extends for some distance along the roadway its removal re-moval would be too expensive. If the other suggested courses seem impracticable imprac-ticable and It is impossible or Inadvisable Inadvis-able to relocate the road, the plastic material can be greatly improved by harrowing sand into it until a 6-Inch crust Is formed. This crust should be 8-10 to 9-10 sand. Sometimes a 1-inch sand cushion is placed under the slab. With such treatment there is little danger that the pavement will be damaged. Hydraulic lime or light road oil harrowed into the soil has proved beneficial In stabilizing adobe and heavy clay. The light soil encountered In past bogs will support a pavement much as water supports a boat'and need not be feared. It will, however, settle under the superimposed load and at approaches ap-proaches of bridges should be removed and replaced with stable soil near the bridge. The solidity of the fill should be gradually reduced as the distance from the bridge increases so that the change from firm to yielding foundation founda-tion will not be too abrupt. |