Page 120 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

3.14 THE WORK

In “method and result” the contractor is told what equipment to use and how, and also the
result in density that must be obtained. If the specified equipment will not produce the desired result,
there is trouble. The contractor will need a special arrangement to either change the equipment or
to get by with less compactions.
The usefulness of “suggested method and result” may depend on the wording and intent of the
specification. If the result must be obtained, there is no important difference from specifying result
only. However, if the contractor is not held absolutely liable for the result if he or she follows the
suggested method, this is a reasonable approach.
A performance specification (result only) allows the contractor to choose the method, but
requires a certain density. The state will have established by laboratory tests that this density is
possible, but it may develop that the density cannot be obtained in the field because of moisture
or other conditions, or can be obtained only by far greater effort than was contemplated by either
the state or the contractor. In many jobs it works out very well, but for reasons to be considered
below it may be unfair to the contractor unless softened by irregular practices on the part of the
state engineers and inspectors. It should include a provision for renegotiation.
Required density is usually specified as a percentage of maximum density produced in a par-
ticular test, such as 95 percent modified Proctor.

Laboratory Tests. Tests are required to determine the optimum moisture content of a soil, the
extent to which it can be compacted, and whether it is being sufficiently compacted on the job.
The standard Proctor or AASHO (American Association of State Highway Officials) Standard
Test T99 is made as follows:
A sample of soil is moistened and is then compacted in a standard mold 4 inches in diameter
1
with a volume of ⁄ 30 cu. ft. The soil is placed in three layers of approximately equal thickness, and
each layer is subjected to 25 blows of a rammer with a striking face of 2-inch diameter and a
1
weight of 5 ⁄ 2 pounds, falling freely a distance of 12 inches. This produces 12,400 foot-pounds of
energy per cubic foot of soil.
The sample, which contains a known volume, is then weighed and dried at 105°C for 20 minutes.
After that, it is weighed again. The moisture content is computed by the difference between the
wet and dry weights. The dry weight is recorded as well as the moisture content. The moisture
content is recorded in percentage of weight by the entire dry sample.
By plotting the results of a series of these tests, using the same soil but with different moisture
factors, a curve similar to that of Fig. 3.10 will be produced. This curve shows the resulting dry
weights obtained in a series of tests on a single sample compacted under a uniform method with
varying amounts of moisture.
There are a number of modifications of this basic test, so any reference to an AASHO modified
test should specify which one. In general, they employ a heavier hammer or plunger with a longer
stroke than in the basic test. The use of modifications is becoming more general because of the
higher densities required in construction.

Field Tests. The standard method of testing actual density of the embankment is to remove a
measured sample for laboratory comparison. A sample may be of bulk to leave a hole 8 inches
wide and 10 inches deep. Measurement is made by filling the hole with a measured quantity of
sand or water. The water is prevented from soaking away by lining the hole with a rubber balloon.
The Proctor needle penetrometer measures the resistance of soil to penetration by a heavy needle,
by means of a spring and gauge in the push handle. It is used to compare the density of the embank-
ment with a laboratory sample. It is very useful to check on how things are going, but should be
reinforced with regular volume tests. Results in gravel are likely to be misleading.
The most satisfactory and accurate, but also the most expensive, method is nuclear testing. A
device using radioactive material directs a beam of gamma or neutron rays into the ground, and
counts those that are reflected back into the instrument. This count is compared with a laboratory
sample. The higher the count, the greater the density.
This type of instrument is nondestructive, does not interfere with grading operations, does not
depend on individual judgment, permits a large number of tests in a short time, and can be used for
stony or frozen ground. On the other hand, the instruments are now both expensive and fragile.