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Building Design and Construction Handbook - Merritt - Ventech!

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SOIL MECHANICS AND FOUNDATIONS 6.53<br />

leads to a compression of the underlying porous soil or rock structure. Since subsidence<br />

is due to a secondary influence (extraction of oil or groundwater), its effect<br />

on the structure would be included in the second basic type of settlement.<br />

6.5.3 Foundation <strong>Design</strong> Parameters<br />

Determining the settlement of the structure is one of the primary obligations of the<br />

geotechnical engineer. In general, three parameters are required: maximum total<br />

settlement (� max), maximum differential settlement (�), <strong>and</strong> rate of settlement. Another<br />

parameter that may be useful in the design of the foundation is the maximum<br />

angular distortion (�/L), defined as the differential settlement between two points<br />

divided by the distance between them. Figure 6.19 illustrates the maximum total<br />

settlement (� max), maximum differential settlement (�), <strong>and</strong> maximum angular distortion<br />

(�/L) of a foundation. Note in Fig. 6.19 that the maximum angular distortion<br />

(�/L) does not necessarily occur at the location of maximum differential settlement<br />

(�).<br />

6.5.4 Allowable Settlement<br />

The allowable settlement is defined as the acceptable amount of settlement of the<br />

structure <strong>and</strong> it usually includes a factor of safety. The allowable settlement depends<br />

on many factors, including the following (D. P. Coduto, ‘‘Foundation <strong>Design</strong>, Principles<br />

<strong>and</strong> Practices,’’ Prentice-Hall, Inc., Englewood Cliffs, N.J.):<br />

The Type of <strong>Construction</strong>. For example, wood-frame buildings with wood siding<br />

would be much more tolerant than unreinforced brick buildings.<br />

FIGURE 6.19 Diagram illustrating the definitions of maximum total settlement<br />

( � max), maximum differential settlement (�), <strong>and</strong> maximum angular<br />

distortion (�/L).

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