Refer to the Environmental & Engineering Geoscience (Nov. 2012) study of settlement of shallow foundations on cohesive soil, Exercise 7.50. Recall that the researchers sampled 13 structures built on a shallow foundation and measured both the actual settlement value (measured in millimeters) and predicted settlement value (based on a formula) for each structure. How many more structures must be sampled in order to estimate the true mean difference between actual and predicted settlement value to within 2 millimeters with 99% confidence?

Data from Exercise 7.50

Structures built on a shallow foundation (e.g., a concrete slab-on-grade foundation) are susceptible to settlement. Consequently, accurate settlement prediction is essential in the design of the foundation. Several methods for predicting settlement of shallow foundations on cohesive soil were com-pared in Environmental & Engineering Geoscience (Nov., 2012). Settlement data for a sample of 13 structures built on a shallow foundation were collected. (These structures included office buildings, bridge piers, and concrete test plates.) The actual settlement values (measured in millimeters) for each structure were compared to settlement predictions made using a formula that accounts for dimension, rigidity, and embedment depth of the foundation.

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SHALLOW Structure Actual Predicted 1 11 11 2 11 11 3 10 12 4 8 6. 11 9 6 9 10 9 8 39 51 23 24 10 269 252 11 4 3 12 82 68 13 250 264 7.