Page 142 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
Soil Minerals 137

not be allowed to air-dry and should be stored in sealed plastic bags or other
containers.

Whenever soils are air-dried prior to testing, this and the procedure used to
rehydrate should be stated in the test report.

6.5 LIME STABILIZATION OF EXPANSIVE CLAY

6.5.1 Overview

Mixing hydrated lime, Ca(OH) 2 , with soil to make it stronger dates from
prehistory, and lime mortar was used for masonry walls well into the twentieth
century. In modern times lime is mixed with clay soil and compacted for road
bases. A more recent adaptation of this procedure is to simply drill holes into an
expansive clay layer and either mix the soil with hydrated lime or fill the open
borings with quicklime. It therefore is important to recognize the chemical
reactions that take place.

6.5.2 Flocculation with Lime
An immediate reaction between expansive clay and lime will be observed as the
clay appears to dry up and becomes crumbly, even though there is no change in
the water content. The reason for this is that the clay particles become stuck
together, and the clay is said to be flocculated. This stabilizes the expansive nature
and greatly increases the clay strength. A simple method for ‘‘drying’’ expansive
clay in a mudhole without really drying it is to dump in a bag or two of hydrated
lime and mix it in.

This change occurs even though the clay already is calcium-saturated, so one

hypothesis to explain the sudden flocculation is that OH ions from dissolved
þ
lime pull H ions from within the clay crystals to make water, and in doing so
increase the negative charge on the clay. This is called a ‘‘pH-dependent charge.’’
The negative charge is balanced by the Ca 2þ ions that remain outside of the clay
layers and link clay particles together.

6.5.3 Why Use Quicklime?
For safety reasons hydrated lime is preferred for mixing with soil for road bases,
but quicklime is preferred for filling borings in a procedure called ‘‘drilled lime.’’
There are several reasons for this.

First, the quicklime quickly extracts water from the soil, hydrates, and approx-
imately doubles in volume. The drying action can quickly stop a landslide, and
as the lime hydrates and expands, the surrounding expansive clay shrinks and

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.