Page 261 - Civil Engineering Formulas

SOIL AND EARTHWORK FORMULAS 195

California Bearing Ratio
The California bearing ratio (CBR) is often used as a measure of the quality of
strength of a soil that underlies a pavement, for determining the thickness of the
pavement, its base, and other layers.
F
CBR (8.39)
F 0
where F force per unit area required to penetrate a soil mass with a 3-in 2
2
(1935.6-mm ) circular piston (about 2 in (50.8 mm) in diameter) at the rate of
0.05 in/min (1.27 mm/min); and F force per unit area required for corre-
0
sponding penetration of a standard material.
Typically, the ratio is determined at 0.10-in (2.54-mm) penetration, although
other penetrations sometimes are used. An excellent base course has a CBR of
100 percent. A compacted soil may have a CBR of 50 percent, whereas a
weaker soil may have a CBR of 10.

Soil Permeability

The coefficient of permeability k is a measure of the rate of flow of water
through saturated soil under a given hydraulic gradient i, cm/cm, and is defined
in accordance with Darcy’s law as

V kiA (8.40)
3
where V rate of flow, cm /s; and A cross-sectional area of soil conveying
2
flow, cm .
Coefficient k is dependent on the grain-size distribution, void ratio, and soil
fabric and typically may vary from as much as 10 cm/s for gravel to less than
10 –7 for clays. For typical soil deposits, k for horizontal flow is greater than k
for vertical flow, often by an order of magnitude.

COMPACTION EQUIPMENT

A wide variety of equipment is used to obtain compaction in the field. Sheeps-
foot rollers generally are used on soils that contain high percentages of clay.
Vibrating rollers are used on more granular soils.
To determine maximum depth of lift, make a test fill. In the process, the
2
most suitable equipment and pressure to be applied, lb/in (kPa), for ground
contact also can be determined. Equipment selected should be able to produce
desired compaction with four to eight passes. Desirable speed of rolling also can
be determined. Average speeds, mi/h (km/h), under normal conditions are given
in Table 8.3.