5. IN-PLACE DENSITY. In-place soil density can be measured on the surface
by displacement methods to obtain volume and weight, and by nuclear density
meters. Density at depth can be measured only in certain soils by the drive
cylinder (sampling tube) method.
a. Displacement Methods. Direct methods of measuring include sand
displacement and water balloon methods. See Reference 19, Evaluation of
Relative Density and its Role in Geotechnical Projects Involving
Cohesionless Soils, ASTM STP 523. The sand displacement and water balloon
methods are the most widely used methods because of their applicability to a
wide range of material types and good performance. The sand displacement
method (ASTM Standard Dl556, Density of Soil in Place by the Sand Cone
Method) is the most frequently used surface test and is the reference test
for all other methods. A procedure for the water or rubber balloon method
is given in ASTM Standard D2167, Density of Soil in Place by the Rubber
Balloon Method.
b. Drive-Cylinder Method. The drive cylinder (ASTM Standard D2937,
Density of Soil in Place by the Drive-Cylinder Method) is useful for
obtaining subsurface samples from which the density can be ascertained, but
it is limited to moist, cohesive soils containing little or no gravel and
moist, fine sands that exhibit apparent cohesion.
c. Nuclear Moisture-Density Method. Use ASTM Standard D2922, Density
of Soil and Soil-Aggregate in Place by Nuclear Methods (Shallow Depth).
Before nuclear density methods are used on the job, results must be compared
with density and water contents determined by displacement methods. Based
on this comparison, corrections may be required to the factory calibration
curves or a new calibration curve may have to be developed. Safety
regulations pertaining to the use of nuclear gages are contained in
Reference 20, Radiological Safety, U.S. Corps of Engineers ER 385-1-80.
6. DETECTION OF COMBUSTIBLE GASES. Methane and other combustible gases may
be present in areas near sanitary landfills, or at sites near or over peat
bogs, marshes and swamp deposits. Commercially available indicators are
used to detect combustible gases or vapors and sample air in borings above
the water table. The detector indicates the concentration of gases as a
percentage of the lower explosive limit from 0 to 100 on the gage. The
lower explosive limit represents the leanest mixture which will explode when
ignited. The gage scale between 60% and 100% is colored red to indicate
very dangerous concentrations. If concentrations are judged to be serious,
all possibilities of spark generation (e.g., pile driving, especially
mandrel driven shells) should be precluded, and a venting system or vented
crawl space should be considered. The system could be constructed as
follows:
(a) Place a 6-inch layer of crushed stone (3/4-inch size) below the
floor slab; the crushed stone should be overlain by a polyethylene vapor
barrier.
(b) Install 4-inch diameter perforated pipe in the stone layer below
the slab; the top of the pipe should be immediately below the bottom of the
slab.
7.1-109
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