Page 379 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

ROADWAYS 8.41

Sand and loose, clean gravel have excellent bearing power but afford poor traction, are hard to
compact, and must be held in by other materials.
The most desirable fills are mixtures of two or more simple types. Varying proportions of clay,
silt, sand, gravel, and stones are found in loams, boulder clay, and glacial till. Sand and gravel are
most desirable when mixed with enough clay or silt to bind them together. Various soil mixtures
are described in Chap. 3.
Granular soils with a high percentage of sand or gravel are desirable when work must be done
in rainy places or seasons. They absorb and drain off large quantities of water, and do not get slip-
pery easily.

Moisture Content. The water content of soils largely determines their behavior in a fill. Each soil
has a best (optimal) water content which favors compaction. Less water will allow the grains too
free motion in relation to each other, and more will permit soil to bend or creep away from pressure.
A soil which contains too much moisture will develop a rubbery quality. It will move away from
the roller, and when its weight has passed, spring back into nearly its original position.
A loose soil may hold too much moisture for best compaction and still appear fairly dry. When the
grains are squeezed together, water films between them are displaced and tend to work up toward
the surface, rendering it wet. This condition may be cumulative through a number of layers of fill.
Some compaction is accomplished by rolling a rubbery soil, and the operation warms the
ground and brings moisture to the top so that drying is speeded up.
The problem caused by soil that is too wet for specified compaction is discussed in Chap. 3.
If the soil is too dry, it is watered by sprinkler-equipped tank trucks or trailers while being
spread and rolled.

Swell and Compaction. Undisturbed soil has generally been in the same position for long periods.
The particles are well settled against each other, leaving little space. Natural cements may bind
them together.
When such a soil is dug or disturbed, it breaks up into chunks or grains which are thrown against
each other in a disorderly arrangement, leaving airspaces or voids between. This increases the bulk
of the soil, and increases its ability to absorb and conduct water. Such a loosened soil will turn to
a very soft mud if soaked.
The process of soaking and then drying will settle the grains together somewhat, reducing the
voids. Repeated wetting and drying will cause it to shrink to about its original bulk. Freezing and
thawing will accelerate this settlement, as will also the weight of traffic or additional fill.
Compaction by Hauling Units. Considerable packing down of fill can be done by hauling and
grading equipment. Ground pressure under loaded scraper tires may be 30 to 40 pounds per square
inch, and the kneading effect of these tires and/or the vibration of crawler tracks are quite effective.
However, compaction tends to decrease with distance from the cut, as all the fill material must
pass over the near portion, and only a small fraction over the far end. In addition it is difficult,
sometimes impossible, to get the operators to vary their routes enough to give systematic rolling
to the full width. Routes may have to be shifted by stationing one or more operators along the way
to tell or signal the operators where to go, or by the use of movable obstacles.
It is usually inadvisable to have a heavily loaded unit break a new path in soft fill, as the power
requirement and strain on the machine are excessive. Trail breaking should be done on the empty
return trip, and loaded units then turned into those tracks.
These difficulties can be avoided by compacting the fill as it is placed. Rollers of various types
are used on thin layers of fill to squeeze the grains into even closer contact than they had in the
bank. They are aided by the weight of grading and hauling units. Loam soils may be reduced to
90 percent of their bank volume by thorough compaction.
A properly compacted fill should not shrink on exposure to time and weather, so that it is the-
oretically possible to put a permanent surface on it immediately. In addition, it has the highest
bearing power possible to its particular soil type, so that wheels and tracks will not sink into it
much, and speed and capacity of hauling equipment on it are increased.