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Building Design and Construction Handbook - Merritt - Ventech!

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6.58 SECTION SIX<br />

effective stress greater than the existing vertical effective stress. An example of<br />

a situation that creates an overconsolidated soil is where a thick overburden<br />

layer of soil has been removed by erosion over time. Other mechanisms, such<br />

as changes in groundwater elevation <strong>and</strong> changes in soil structure, can create an<br />

overconsolidated soil.<br />

For structures constructed on top of saturated cohesive soil, determining the<br />

overconsolidation ratio of the soil is very important in the settlement analysis. For<br />

example, if the cohesive soil is underconsolidated, then considerable settlement due<br />

to continued consolidation owing to the soil’s own weight as well as the applied<br />

structural load would be expected. On the other h<strong>and</strong>, if the cohesive soil is highly<br />

overconsolidated, then a load can often be applied to the cohesive soil without<br />

significant settlement.<br />

The oedometer apparatus is used to determine the consolidation properties of<br />

saturated cohesive soil. The typical testing procedure consists of placing an undisturbed<br />

specimen within the apparatus, applying a vertical seating pressure to the<br />

laterally confined specimen, <strong>and</strong> then submerging the specimen in distilled water.<br />

The specimen is then subjected to an incremental increase in vertical stress, with<br />

each pressure remaining on the specimen for a period of 24 hr. Plotting the vertical<br />

stress versus void ratio of the soil often yields a plot similar to Fig. 6.22. The plot<br />

FIGURE 6.22 Example of a consolidation curve. The Casagr<strong>and</strong>e construction<br />

technique for determining the maximum past pressure is also shown on this figure.

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