consolidation settlement examples 4_solutions-2

8/21/2019 Consolidation Settlement Examples 4_Solutions-2

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Consolidation Settlement – Examples 4

1. An airfeld pavement is to be constructed on a large area o sot clay,

8.0m thick, overlying a heavily fssured sandstone. In order to provide

a stronger ormation or the runways, current ground level is to be

raised by .0m o granular fll, compacted to a bulk unit weight o

18k!"m#. $he phreatic surace is at current ground level% the

saturated unit weight o the clay is 0k!"m#.

$he ollowing results were obtained rom an oedometer test carried

out on a sample o the clay over the pressure range &0'100k!"m.

(sing these data, determine the coe)cient o compressibility,

consolidation and permeability. By considering the stress changes at the centre

of the clay layer, calculate the total consolidation settlement of the layer and the time

required to achieve 90% consolidation. (0.897m2/!" 0.8#$m2/yr" 7.#$&0'# m/yr"

28mm" &.7 years).

$ime *mins+

ample $hickness

*mm+

0 18.0&

0.& 1-.

1 1-.0/

.& 1-.81

/ 1-.81#

1-.-

1 1-.

& 1-.&/&

# 1-.&08

/ 1-./

/ 1-.//

/ hours 1-./

&


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. $he results o a consolidation test on a sample o clay or the

pressure range 00'#00 k!"m are shown below. rom these data

determine,

*a+ determine the coe)cients o permeability, compressibility and

consolidation

*#./210'# m"yr% 0./1m"3!% 0.81&m"yr+.

*b+ determine the time re4uired or an 8m thick layer o this clay

to reach 805 primary consolidation i it is drained rom both

upper and lower boundaries and sub6ected to the same stress

changes *10.- years+.

$ime *mins+ ample

$hickness*mm+

0 1-.0

0.& 1-.1#

1 1-.8/

.& 1-.81

/ 1-.--#

1-.8#

1 1-.&0

& 1-.&0

# 1-./

/ 1-./#

/ 1-./10

/ hours 1-.0/

2


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#. * #.2m thic+ clay layer is su-ected to a uniform increase in effective vertical stress

from 80+!/m2 to &$0+!/m2. rainage of the clay layer can ta+e lace vertically

uards and donards. 1areful monitoring of the consolidation settlement of the

layer gave the folloing results

3ime (years) 0.04 0.2$ 0.$ 0.94 &.0 2.&8 #.02 $.#& 4.$ 9.0

5ettlement (mm) & #0 $ 4& 74 9& &07 &22 &$4

(a) *ssuming &'dimensional conditions revail, calculate,

(i) the coefficient of consolidation, c6 (in m2/year).

(ii) the coefficient of volume comressiility, m6 (in m2/!).

() * 2.m layer of similar clay is su-ected to a change in effective stress from

70+!/m2 to &20+!/m2. f this layer has drainage from the to surface only, calculate the time

required to achieve 47mm consolidation.

*ns (a)(i) 0.##m2/yr" (a)(ii) 0.78m2/!" () 7.#4 years

/. A clay layer o thickness &.8m is underlain by an impermeable layer

o hard shale and overlain by a moderately permeable granular layer.

$he loading will be such that the e7ective stress in the clay layer will

be increased uniormly over its whole thickness over a wide area.

In a laboratory oedometer test a specimen o the clay o thickness

0mm, sub6ect to the same e7ective stress, showed a change in void

ratio rom 0.8- to 0.80. It was observed that ater #0 minutes hadelapsed, &5 o the primary consolidation had taken place.

#


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*a+ calculate the epected consolidation settlement o the layer

*Ans9 -mm+%

*b+ plot the primary settlement versus time curve or the layer

*time at which 105, 05, #05 etc. consolidation settlement is

achieved+% *continued over+

*c+ obtain the ecess pore water pressure *as a percentage o the

original ecess pore water pressure at the beginning o

consolidation+ at the centre o the layer at &05 and -&5

consolidation *Ans9 &&5% -5+.

. * layer of clay eneath a uilding of large lan area has consolidated and careful

monitoring has shon that a settlement of &mm has resulted in the initial #00 days

since the structure ecame oerational. *ccording to the results from laoratory

consolidation tests, this corresonds to aroimately #% consolidation of the

ultimate consolidation settlement of the layer. ou are required to rovide the

settlement versus time curve for the initial 7'year eriod *time at which &5,

105, 1&5, 05, #05 etc. consolidation settlement is achieved+.

*ssume that drainage of the layer can occur in one direction only (i.e. single'drainage

conditions) and neglect any settlement during construction.

$


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SOLUTIONS


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consolidation settlement examples 4_solutions-2

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