Page 118 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

3.12 THE WORK

A soil that contains too much moisture is likely to become rubbery under a roller, pushing in
waves ahead of and behind it, and springing back into its original position when it has passed. This
is a very common condition in highway work.
Soil that is too dry may become loose or powdery under pressure, or may be firm but not as dense
as it should be. It is standard practice to add water to each such layer by means of sprinkler trucks
or trailers. Extra payment may be made for watering.
Stability. If a road or runway fill is not compacted, it is likely to shrink and settle, injuring or
destroying pavements and any other structures on it.
Clay soils provide poor embankment material at best, as they will usually absorb moisture in
wet seasons and swell. High, but not excessive, compaction reduces this tendency. However, clay with
a high content of the montmorillonite variety is very expansive when exposed to water. It will
heave highway pavements and building foundations, even breaking pipes that extend through a
wall raised by expansive clay.
Variable compaction may cause the greatest damage. A whole road may settle a few inches with
little noticeable harm. But subsidence of a narrow strip of trench fill across a firm embankment
will ruin the road surface, as will also subsidence of a poorly compacted embankment on each side
of a well-compacted trench fill.
Unpaved road shoulders are more exposed to moisture than the pavement subgrade. If they absorb
it, shoulder surfaces may rise noticeably above the pavement, interfering with drainage.

Layers. Many compaction specifications stress the thickness of layers to be rolled. The best a
field crew can hope to do is to come somewhere near the standard. There are some grading bosses
who can keep exact control over spreading thickness and areas of a fleet of scrapers, and of the
work patterns of rollers. But it is more usual for this work to be somewhat hit-or-miss. The important
thing is that the layers should not be thicker than the compaction equipment on the job can handle.

Rock Fills. Layer specifications reach a point of absurdity in rock fills. Often they are subject
to the same maximum thickness regulation of 6 to 12 inches as the soil. In practice, the thickness
of rock lifts is often determined by the maximum size rock that the shovel can load.
A rock fill is usually stable if the rock is angular and is well mixed as to size. Such conditions
are characteristic of both blasted and ripped rock.
Steel wheel and sheepsfoot rollers are useless on coarse, hard rock and will be damaged. Very
heavy (50 tons and more) rubber tire rollers and big hauling equipment provide about as much
compaction as is possible or necessary.
Earth Dams. Discussion so far has been about problems of highway and similar fills. Dams do
not ordinarily carry structures, but they must resist penetration by water, changes of shape, and any
tendency to slump. There is more about earth dams in Chapter 6.
Fill used in dams is carefully selected for quality, with fine-grained soils used in the core and sand
and gravels on the slopes. Methods of compaction are about the same as for highways, but methods
and results are very carefully specified and checked.

Specifications. There are four basic ways in which compaction may be specified:
Method only
Method and result
Suggested method and result
Result only (performance specification)
Specifying the method only is usually the fairest to the contractor, who is told that a certain
number of passes with a specified type and weight of roller, on layers of a particular thickness,
constitute acceptable compaction. The contractor knows exactly what to do, and the equipment
that will be needed.