Chapter 9, Problem 9.7P

### Principles of Geotechnical Enginee...

9th Edition
Braja M. Das + 1 other
ISBN: 9781305970939

Chapter
Section

### Principles of Geotechnical Enginee...

9th Edition
Braja M. Das + 1 other
ISBN: 9781305970939
Textbook Problem

# An exploratory drill hole was made in a stiff saturated clay having a moisture content of 29% and Gs = 2.68 (Figure 9.27). The sand layer underlying the clay was observed to be under artesian pressure. Water in the drill hole rose to a height of 6 m above the top of the sand layer. If an open excavation is to be made in the clay, determine the safe depth of excavation before the bottom heaves.Figure 9.27

To determine

Find the safe depth of excavation before the bottom heaves.

Explanation

Given information:

The moisture content (w) of stiff saturated clay is 29 %.

The specific gravity Gs of the stiff saturated clay is 2.68.

The depth h1 of saturated clay is 10 m.

The depth h2 of sand is 4.5 m.

The depth h3 of water table from the bottom of the saturated clay is 6 m.

Calculation:

Determine the saturated unit weight γsat(clay) of the clay using the relation.

γsat(clay)=(1+w)Gsγw1+wGs

Here, γw is the unit weight of the water.

Take the unit weight of the water as 9.81kN/m3.

Substitute 29 % for w, 2.68 for Gs, and 9.81kN/m3 for γw.

γsat(clay)=(1+29100)(2.68×9.81)1+(29100×2

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