INTRODUCTION:

Soil compaction is the densifi cation—reduction in void ratio—of a soil through the expulsion of air. This is normally achieved by using mechanical compactors, rollers, and rammers with the addition of water. We will discuss the fundamentals of soil compaction, compaction tests, fi eld compaction, and quality control in the fi eld. When you complete this chapter, you should be able to:

•  Understand the importance of soil compaction.

•  Determine maximum dry unit weight and optimum water content.

•  Specify soil compaction criteria for fi eld applications.

•  Identify suitable equipment for fi eld compaction.

• Specify soil compaction quality control tests.

Importance:

Soil compaction is perhaps the least expensive method of improving soils. It is a common practice in all types of building systems on and within soils. As a sample practical situation, we consider a levee. A levee—a long earthen embankment, sometimes called a dike or dyke, used to retain and/or regulate water levels in rivers—is to be constructed from soil trucked from a borrow pit. You are required to (a) determine the suitability of the soil for the levee, (b) specify compaction criteria, and (c) specify quality control methods.

On Monday morning, August 29, 2005, Hurricane Katrina slammed into New Orleans, Louisiana, USA, as a Category 3 storm. Over 1800 persons lost their lives, and massive property damage occurred. Much of the damage was due to fl oods from the catastrophic failure of the levee system (Figure ). Hurricane Katrina is claimed to be the largest natural disaster in the history of the United States, with estimated damages exceeding $100 billion. Several investigations were conducted on the design and construction of the levee systems.

The reports of many of these investigations are available on the World Wide Web . One of the lessons from failures such as from Hurricane Katrina is that in designing any geotechnical structure you ought to consider a systems approach. You must consider how your part of the problem fi ts into the whole project and the implications if anything goes wrong with your part—that is, you should take a holistic approach.

BASIC CONCEPT:

Let’s reexamine Equation  for dry unit weight, that is,

The extreme-right-hand term was obtained by replacing e by e 5 wGs/S. How can we increase the dry unit weight? Examination of Equation  reveals that we have to reduce the void ratio; that is, w/S must be reduced since Gs is constant. The theoretical maximum dry unit weight is obtained when S 5 1 (S 5 100%); that is, emin 5 wGs.

A plot of the theoretical dry unit weight versus water content for different degrees of compaction is shown in Figure  The theoretical dry unit weight decreases as the water content increases because the soil solids are heavier than water for the same volume occupied. Recall that the specifi c gravity of the soil solids is on average 2.7. That is, the mass of the soil solids is 2.7 times the mass of water for the same volume occupied. The theoretical dry unit weight decreases as the degree of saturation decreases.

The mass of air is negligible, so as air replaces water in the void space, the volume of soil remains constant but its mass decreases. Thus, the dry unit weight decreases. The curve corresponding to S 5 100% is the saturation line, sometimes called the zero air voids curve. The achievement of zero air voids by compaction is rare.