# Statistics for Environmental Engineers

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happens, the permeability will probably exceed the target of 10 cm/sec.

As a first step it was decided to study the importance of water content (%), compaction effort (psi), and reaction time (min) before compaction. These three factors were each investigated at two levels. This is a two-level, three-factor experimental design. Three factors at two levels gives a total of eight experimental conditions. The eight conditions are given in Table 27.1, where W denotes water content (4% or 10%), C denotes compaction effort (60 psi or 260 psi), and T denotes reaction time (5 or 20 min). Also given are the measured densities, in lb/ft3. The permeability of each test specimen was also measured. The data are not presented, but permeability was inversely proportional to density. The eight test specimens were made at the same time and the eight permeability tests started simultaneously (Edil et al., 1987).

The results of the experiment are presented as a cube plot in Figure 27.1. Each corner of the cube represents one experimental condition. The plus (+) and minus (_) signs indicate the levels of the factors. The top of the cube represents the four tests at high compression, whereas the bottom represents the four tests at low pressure. The front of the cube shows the four tests at low reaction time, while the back shows long reaction time.

It is apparent without any calculations that each of the three factors has some effect on density. Of the investigated conditions, the best is run 4 with high water content, high compaction effort, and short

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