Final Exam 2011, Questions and answers PDF

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University of Alberta Faculty of Engineering Department of Civil and Environmental Engineering

CIVE 381 SOIL MECHANICS FINAL EXAMINATION

STUDENT NAME:

ID#:

Time: 2 Hours 9:00 am – 11:00 am

Question

Mark

1

/20

2

/25

3

/20

4

/15

5

/20

TOTAL

/100

No Notes or texts permitted

All answers must be contained on the examination sheets. Three extra pages are provided. Use the back of the page if necessary. When answering all the questions, it is very important to provide all the steps leading to the answer. Correct answers without showing calculations will not be given full marks. Formula sheets are provided.

NAME: ID #: QUESTION 1 - GENERAL

Define clearly and unambiguously (using equations or diagrams where possible), the following terms:

(2)



Effective Stress

(2)



Overconsolidation Ratio

(2)



Pore Pressure Parameter A

(2)



Zero Air Voids Line



Mohr-Coulomb Failure Criterion

(2)



Excess Pore Pressure

(2)



Coefficient of Volume Compressibility

(2)



Coefficient of Consolidation

(2)



Coefficient of Active Earth Pressure



Preconsolidation pressure of clay

(2)

(2)

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NAME: ID #: QUESTION 2 - CONSOLIDATION

A site having the subsurface profile shown below is to serve as the foundation for a series of rectangular footings of dimension 10 m x 8 m. The instantaneous column load on each footing will be 28.0 MN. The results of a consolidation test on the clay stratum are also shown below. (20) (5)

a) Estimate the primary consolidation settlement of the clay layer under the center of the footing? b) Determine the time required for 70% consolidation to take place if Cv = 7.0 m2/year?

b = 9.80 kN/m3  = 19.7 kN/m3 Cc = 0.31 Cr = 0.02 b = 9.6 kN/m3

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NAME: ID #: QUESTION 3 – SHEAR STRENGTH (6)

a) What are the three major factors that control the strength of a mass of sand? Briefly outline the influence of each factor.

(9)

b) Indicate the behavior of normally consolidated and over-consolidated clays by showing the following typical diagrams: Volume Change vs. Axial Strain

V/Vo

1 - 3

Deviator Stress vs. Axial Strain

a

a Pore Pressure Response vs. Axial Strain

Effective Strength Envelope

 u

a ’

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NAME: ID #: QUESTION 3 – SHEAR STRENGTH (cont’d)

c) Consolidated-Undrained experiments on a saturated normally consolidated clay gave the following results: Sample

(5)

i.

(5)

ii.

1

Void Ratio after Consolidation .985

Cell Pressure (kPa) 172.8

Deviator Stress at Failure (kPa) 103.7

Pore Pressure at Failure (kPa) 103.7

2

.910

343.3

202.5

202.5

3

.840

518.5

306.2

306.2

Determine the cohesion and friction angle in terms of total stress. Determine the cohesion and friction angle in terms of effective stress

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Page 5

NAME: ID #:

QUESTION 4 – SEEPAGE

In order to establish the flow net to analyze water flow through a porous media, the flow boundary conditions must be known. For each of the structures described below, IDENTIFY (SHOW) on a diagram the LOCATION of the following:  

The known boundary conditions, such as equipotential (constant head); and The boundary conditions which must be determined in order to complete a flow net for the structure.

(5)

1. Concrete gravity dam on sand stratum underlain by impervious rock

(5)

2. Earth dam on an impervious rock foundation with a toe drain

(5)

3. A square coffer dam used to build a pier in the center of a river. The dam consists of an impervious steel sheet pile wall driven 4 m into the sand sediments. The sand sediments are 10 m thick and are underlain by impervious clay shale.

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NAME: ID #:

QUESTION 2 – Sh ear Strength and Consolidation (5)

a. A structure will impose a normal effective stress of 100 kPa and a shear stress of 30 kPa on a

plane inclined at 58o to the horizontal. The friction angle of the soil is 25o. Will the soil fail? If not, what is the factor of safety?

(5)

b. A CU test on a stiff, overconsolidated clay at a cell pressure at 150 kPa fails at a deviatoric

stress of 448 kPa. The change in pore water pressure at failure due to shear was –60 kPa. Determine: 1. The effective friction angle, ’ (assume c’ = 0) and 2. Skempton’s parameter, Af.

(5)

c. A consolidation test in an oedometer on a sample of clay indicated that a 25 mm thick sample

consolidated 50% in 2 minutes and 20 seconds. In the field, for a 6 m thick clay layer, singly drained, what would be the time, in DAYS, to reach 90% consolidation?

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NAME: ID #: QUESTION 3 – SHEAR STRENGTH

CU tests were carried out on two samples of normally consolidated clay. Each sample was isotropically consolidated before the axial stress was increased. The following results were obtained: Sample No. 1 2 (5)

3f (kPa)

(1 - 3)f (kPa)

uf (kPa)

420 690

320 365

205 350

a. Draw Mohr’s circles (total AND effective stresses) for each test on the same graph. (If you do not have a protractor, etc. just draw these as NEATLY as possible freehand but clearly

(5)

b. Why are the total stress circles not the same size?

(5)

c. Determine the effective friction angle, ’, for each test.

(5)

d. Determine the undrained shear strength at a cell pressure, (3)f of 690 kPa.

(5)

e. Determine the shear stress on the failure plane for each sample.

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NAME: ID #: QUESTION 3 – SHEAR STRENGTH (cont’d)

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NAME: ID #: QUESTION 4 – Effective Stress

A soil profile consists of a clay layer underlain by a sand layer as shown in the figure. Ignore any capillary pore pressure rise in the clay layer. A standpipe inserted into the bottom sand layer shows that the water rises to 1.5 m above the ground surface.

1.5 m t= 18.5 kN/m3

CLAY

sat= 17.0 kN/m3

v

(kPa)

2.0 m

C

0

1

Depth, m

2

3

4

5

6 6.5

0

h

(kPa)

0

1.5 m

SAND

 If Ko = 0.6, determine the effective and total horizontal stresses at A, B and C.

(5)

2.0 m

sat= 19.0 kN/m3

A

 Determine the vertical total stress, the pore pressure and the vertical effective stress at points A, B and C.

(10)

1.0 m

u (kPa)

0

 'v

(kPa)

0

'h

(kPa)

0

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NAME: ID #:

FORMULA SHEET

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NAME: ID #:

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NAME: ID #:

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