Chapter 11, Problem 11.1CTP

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9th Edition

Braja M. Das + 1 other

ISBN: 9781305970939

Textbook Problem

Foundation engineers are often challenged by the existence of soft compressible soils at the construction site. Figure 11.49 shows a soil profile with a silty sand
^{3}, will be placed on top of the silty sand layer. The plan area of the fill is 8 m × 8 m. The fill load will be left in place for 18 months, after which construction will begin with the fill becoming part of the permanent foundation. Undisturbed samples collected from the clay and organic layers had the following properties:

**a.**Estimate the total consolidation settlement under the action of the fill load. Consider both the clay and peat layers to be normally consolidated.**b.**Estimate the time for 99% primary consolidation in each layer. Are the layers singly or doubly drained? Explain.**c.**Estimate the secondary compression in each layer up to end of 18 months.**d.**What will be the total settlement after 18 months?**e.**What is the remaining excess pore water pressure at point*A*two months after the application of the fill load?

**f.**Determine the effective stress at point*A*two months after the application of the fill load.**g.**A piezometer was installed at point*A*to monitor the pore water pressure. What should be the piezometer reading (*u*_{piezometer}) two months after the fill load was applied?

(a)

To determine

Calculate the total consolidation settlement under the action of fill load.

Explanation

**Given information:**

The thickness of fill material

The compacted unit weight of fill material

The length of the foundation

The breadth of the foundation

The depth of fill

The height of the layer silty sand

The height of the clay layer

The height of the peat layer

The dry unit weight of sand

The saturated unit weight of sand

The saturated unit weight of clay

The saturated unit weight of peat

The time

The properties of clay and organic layers are given in the Table.

**Calculation:**

Consider the unit weight of water

Calculate the distributed load

Substitute *t* and

Calculate the increase in vertical stress

Here,

For clay layer:

For the depth

Calculate the width

Substitute *B*.

Calculate the ratio

Substitute *L* and *B*.

Calculate the ratio

Substitute *b* and *z*.

Similarly calculate the remaining values and tabulate as in Table 1.

Refer Table 10.11 “Variation of

Take the value of

Take the value of

Calculate the value of

Similarly calculate the remaining values and tabulate as in Table 1.

Calculate the increase in vertical stress

Substitute

Similarly calculate the increase in vertical stress values and tabulate as in Table 1.

Show the increase in vertical stress for each depth below the center of the loaded area as in Table 1.

Depth, | |||||

1 | 4 | 3 | 0.75 | 0.823 | 28.82 |

1 | 4 | 5 | 1.25 | 0.599 | 20.98 |

1 | 4 | 7 | 1.75 | 0.403 | 14.11 |

Table 1

Refer to table 1.

Calculate the stress increase in the clay layer

Here,

Substitute

Calculate the average effective stress at the middle of the clay layer

Substitute

(b)

To determine

Calculate the time for

(c)

To determine

Calculate the secondary compression in each layer up to end of

(d)

To determine

Calculate the total settlement after 18 months.

(e)

To determine

Calculate the excess pore water pressure at point *A* two months after the application of the fill load.

(f)

To determine

Calculate the effective stress at point *A* two months after the application of the fill load.

(g)

To determine

Calculate the piezometer reading at point *A* two months after the application of the fill load.

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