Page 162 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
Particle Size and Gradation 157

Compaction grouting is a relatively new procedure that normally is intended to
laterally compact and densify loose soil to reduce settlement under a foundation
load.

Regardless of the grouting procedure the maximum grouting pressure is limited by
the overburden pressure of the soil, or lateral planar injections can lift the soil.
When this occurs, pumping pressure should decrease while the pumping rate
increases, referred to as the grout ‘‘take.’’

If the lateral stress existing in the soil is lower than the vertical pressure from
overburden, the pumping pressure at which the ‘‘take’’ occurs is that which causes
vertical radial cracking and is used as an approximate measure of lateral stress in
the soil. This is called ‘‘hydraulic fracturing.’’ It was first developed in the
petroleum production industry to increase the flow of oil into oil wells.

Grout Materials
The most common grout materials for rocks and soils are aqueous suspensions of
Portland cement and/or fly ash. Sand-cement mortar may be used for grouting
rubble that has large voids. Bentonite sometimes is used as a sealing grout, but has
the disadvantage that it will shrink and crack when it dries out.

The first injection grout was developed by Joosten in Germany and uses chemical
solutions of sodium silicates and calcium chloride, which react to make insoluble
calcium silicate and sodium chloride. Some more recent chemical solution
grouts have been removed from the market because of potentially toxic effects
on groundwater. Emulsions of asphalt in water are sometimes used as grout for
sealing cracks and joints in basements.

Soil Groutability
For injection grouting the particle size ratio is reversed from that used design-
ing filters, D 15 for the soil and G 85 for the grout. To ensure success, the ratio
should be substantially higher than the corresponding ratio of 5 used for
filters. Tests by the U.S. Army Corps of Engineers suggest that the ratio of soil
D 15 to cement G 85 should be a minimum of 20. G 85 for Portland cement typi-
cally is about 0.040 to 0.050 mm. The smaller figure represents high-early strength
cement, and also is fairly representative of fly ashes. Specially ground cements
may have G 85 of only 0.005 mm. Bentonite is composed of montmorillonite
particles that expand on wetting, with an effective hydrated G 85 of about
0.030 mm.

Example 7.6
Can any of the soils of Fig. 7.2 be injection grouted with cement grout?
Answer: The soil with the largest D 15 is the sand, with D 15 ¼ 0.12 mm. For cement, assume
G 85 ¼ 0.050 mm. Then D 15 /G 85 ¼ 2.4 20, so this sand cannot be injected with cement

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