Page 293 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
288 Geotechnical Engineering

including the size of the contact area, the soil cohesion, and its angle of internal
friction.

Successive roller passes on clay compact the soil and increase its cohesion, which
increases the bearing capacity so that the roller walks out. Without the contri-
bution of cohesion, as in cohesionless sands, there is very little walk-out so the
projections only knead and stir the soil. Sands are compacted with smooth-wheel
rollers or plates, and can be aided by vibration.

13.4.5 Proof Rolling

After a soil is compacted it may be subjected to ‘‘proof rolling’’ with a heavy roller
or even a heavily loaded truck. Contrary to popular opinion, a track vehicle such
as a bulldozer is not appropriate for proof rolling because it is designed to spread
pressure and be supported on soft soils. The soil pressure during proof rolling
equals the tire pressure in the vehicle.

Proof rolling can reveal pockets of soil that are insufficiently compacted or are
compacted too wet, but the reliability of this procedure depends on the soil
moisture content. If the moisture content is on the dry side of the OMC the soil
will support a heavier load because of capillary tension or ‘‘apparent cohesion’’
that is destroyed when the soil becomes wet.

13.4.6 ‘‘Oreo Cookie Effect’’
Even with a firm base and compaction in layers, stress distribution is not uniform
through the depth of a single layer, which results in a nonuniformity that has been
called the ‘‘Oreo cookie effect’’ (H. Allender, personal communication). Density
measurements therefore should involve an entire layer or multiple layers in order
to obtain reliable averages.

13.5 OUT GOES THE AIR, NOT THE WATER

13.5.1 A Critical Distinction

Compaction means to squeeze air out of the soil, which is rapid and is in contrast
to consolidation, which means to squeeze out water. The viscosity of water is about
1 10 2 poise, or about 50 times that of air. Compaction therefore is rapid and
consolidation is at least 50 times slower. A saturated soil can be consolidated but
not compacted.

As compaction proceeds and air is squeezed out, the soil eventually will reach the
point where there is very little air left, and the soil is close to saturation as a result
of compaction. The remaining air is trapped in tiny pockets, and compaction stops

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