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Building Design and Construction Handbook - Merritt - Ventech!

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SOIL MECHANICS AND FOUNDATIONS 6.59<br />

is known as the consolidation curve <strong>and</strong> consists of two important segments, the<br />

recompression curve (defined by the recompression index C r) <strong>and</strong> the virgin consolidation<br />

curve (defined by the compression index C c). Figure 6.22 also illustrates<br />

the Casagr<strong>and</strong>e construction technique, which can be used to determine the maxi-<br />

mum past pressure (� ).<br />

�vm<br />

Using the calculated values of C r <strong>and</strong> C c, the primary consolidation settlement<br />

(s c) due to an increase in load (�� v) can be determined from the following equations:<br />

1. For underconsolidated soil (OCR � 1):<br />

Ho ��vo � ���v � ��v<br />

sc � Cc log (6.19)<br />

1 � e ��<br />

o vo<br />

2. For normally consolidated soil (OCR � 1):<br />

3. For overconsolidated soil (OCR � 1):<br />

Case I: �� � �� � ��<br />

vo v vm<br />

Case II: ��vo���v � � vm �<br />

Ho ��vo � ��v<br />

sc � Cc log (6.20)<br />

1 � e ��<br />

o vo<br />

Ho ��vo � ��v<br />

sc � Cr log (6.21)<br />

1 � e ��<br />

o vo<br />

Ho ��vm Ho ��vo � ��v<br />

sc � Cr log � Cc log (6.22)<br />

1 � e �� 1 � e ��<br />

o vo o vm<br />

where s c � settlement due to primary consolidation caused by an increase in<br />

load<br />

C c � compression index, obtained from the virgin consolidation curve<br />

(Fig. 6.22)<br />

C r � recompression index, obtained from the recompression portion of<br />

the consolidation curve (Fig. 6.22)<br />

H o � initial thickness of the in-situ saturated cohesive soil layer<br />

e o � initial void ratio of the in-situ saturated cohesive soil layer<br />

��vo � initial vertical effective stress of the in-situ soil (see Art. 6.4.1)<br />

���v � for an underconsolidated soil (this represents the increase in vertical<br />

effective stress that will occur as the cohesive soil consolidates under<br />

its own weight)<br />

��vm � maximum past pressure, also known as the preconsolidation pressure<br />

(� �p ), which is obtained from the consolidation curve using the Casagr<strong>and</strong>e<br />

construction technique (see Fig. 6.22)<br />

��v � increase in load, typically due to the construction of a building or<br />

the construction of a fill layer at ground surface<br />

The value of �� v (also known as � z) can be obtained from stress distribution<br />

theory as discussed in Art. 6.4.2. Note that a drop in the groundwater table or a<br />

reduction in pore water pressure can also result in an increase in load on the cohesive<br />

soil.

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