example for exam

Practice Examination A Subject Code(s): HES2155 Full Subject Title: Geomechanics STUDENT’S NAME: ______________________________ ID No: ________________ Duration: 3 hours Reading Time: 15 minutes

INSTRUCTIONS TO CANDIDATES

Materials Allowed • Calculators and Drawing Instruments are permitted. • Alpha numeric and programmable calculators are permitted but must be cleared of all

information. • No Notes or Reference books are allowed. Answering Requirements • This paper consists of seven questions. • Candidates should attempt all questions. • Candidates should note that each question carries different marks. • Graph paper used in questions four and six must be submitted with the examination answer

booklet. • Selected formulas and charts are available on pages six to ten. • Page eleven of this exam question booklet must to be submitted with the examination

answer booklet as part of the answering requirements for question three.

Note: This practice exam paper is identical to the actual exam used in Semester 2, 2003.

SWINBURNE UNIVERSITY OF TECHNOLOGY

PRACTICE EXAMINATION A HES2155 - GEOMECHANICS

Question One:

(a) Physically describe the igneous rocks: Vesicular Basalt and Granite. In your description, be sure to discuss features such as colour, crystal size, texture, level of acidity, type of igneous rock and likely geological origin.

(b) List all the physical properties that are used to help identify rock forming minerals.

(c) If given any clastic sedimentary rock, list all the possible types of metamorphic rocks that could be formed as well as the type of metamorphism required for each.

(d) In drawing a geological cross-section, briefly discuss the meaning of the following terms:

• Exaggerated Apparent Dip • Anticline

(6 + 4 + 6 + 5 = 21 marks)

Question Two:

A 672 gram disturbed sample of sandy clay was brought into the soils laboratory. Its density and void ratio were measured in the field at 1.82 t/m3 and 0.68 respectively. If the sample was placed in the oven to dry and later weighed 574 grams, calculate: (i) the moisture content, (ii) the dry density, (iii) the degree of saturation, and (iv) the specific gravity of the soil particles.

(10 marks)

Practice Examination A HES2155 Geomechanics Page No. 1 of 11

Question Three:

Classify soils A and B using the particle size determination charts (on page 11) and the soil classification sheets on pages 9 and 10.

Make sure that you submit the particle size determination charts (page 11) with your exam booklet.

Soil A Soil B Sieve Size (mm) Mass Retained Percent Passing

37.5 - 100 19.0 - 99 13.2 - 97 9.5 - 95

6.70 22 g 91 4.75 44 g 86 3.35 66 g 80 2.36 132 g 75 2.00 88 g 72 1.18 352 g 66 0.850 220 g 63 0.600 286 g 60 0.425 286 g 58 0.300 242 g 56 0.212 132 g 54 0.150 66 g 52 0.075 44 g 50 Pan 220 g 0

Atterberg Limits

Liquid Limit 50 60 Plastic Limit 40 25

(10 + 5 = 15 marks)


Question Four: In Figure 1 below, a thin sheet pile wall has been installed and one side has been excavated down to 6m.

(a) Using a scale of 1cm = 1m (in your exam booklets or on the graph paper), draw the sheet pile retaining wall structure and boundary conditions shown in Figure 1. Then draw an appropriate flownet diagram for constant water flow from water level A to B. In your diagram, label a typical flowline, flowtube, equipotential line and equipotential drop to show your understanding.

(b) Using the flownet generated in (a), calculate the discharge beneath the sheet pile retaining wall (in litres per minute per meter length of wall), if the permeability of the soil is 0.3 cm/sec.

8m

3m

Impermeable Bedrock

6m

water level A Sheet Pile Retaining Wall

Ground and water level B

Not to Scale

1m

Figure 1

(6 + 4 = 10 marks)


Question Five:

In Figure 2 below, a railway embankment is to be constructed above natural ground surface from a crushed rock material that has a unit weight of 20 kN/m3.

(a) Determine the increase in vertical stress within the soil at point A due to the construction of the railway embankment.

(b) Determine the increase in vertical stress within the soil at point A due to a freight train travelling on Rails 1 and 2 if the freight train had an overall length of 600 m and a total mass of 8,400 tonnes (assume the mass of the train is distributed evenly along the rails).

Railway Embankment

Rail 4 Rail 3 Rail 2 Rail 1

Point A

1m 2m 0.5m 2m 0.5m

Natural ground surface level – before construction of elevated railway structure.

∆σ

∆σ

2m

2m

3m 4m

2m

6m

Figure 2 (9 + 6 = 15 marks)

Question Six:

The results of two consolidated undrained triaxial tests with pore pressure measurements on two identical samples of clay are shown in Table 1.

Test No. 1 2 Cell Pressure 50 kPa 122 kPa Deviator Stress at failure 80 kPa 107 kPa Pore Pressure at failure 30 kPa 80 kPa

Table 1

On the graph paper provided, plot the relevant Mohr circles (using a scale of 1 cm = 10 kPa) to determine:

(a) the effective strength parameters of the soil, (b) the approximate angle of failure (θ) within the soil sample – show this on your Mohr

circle, and (c) the total strength parameters of the soil.

(7 + 3 + 4 = 14 marks)


Question Seven:

(a) When calculating immediate elastic settlements in soil, discuss the difference between Janbu’s elastic settlement formula and the standard elastic formula.

(b) In Figure 3, the natural soil profile consists of sand overlying clay overlying sand then bedrock, with a water table present 1m beneath the surface of the natural ground. If 4m of consistent fill material (γfill = 15 kN/m3) was placed on top of this soil profile that covered a large area, determine:

• the consolidation settlement of the clay layer (using 4 equal layers in your calculation), and

• the time (in days) for this consolidation to be 90% complete given the appropriate time factor values in Table 2.

U (%) 0 10 20 30 40 50 60 70 80 90 100

Tv 0 0.008 0.031 0.071 0.126 0.197 0.287 0.403 0.567 0.848 ∞

Table 2

1m

1m

Impermeable BEDROCK

2m

4m

2m

Dense SAND: γ sat = 20.5 kN/m3

Over Consolidated CLAY: γ sat = 19.5 kN/m3 E clay = 4.6 MPa cs = 0.013 cv = 0.6 m2/yr eo = 0.82

Loose SAND: γ dry = 16 kN/m3 γ sat = 19 kN/m3

Figure 3

(4 + 11 = 15 marks)


Appendix – Some Basic Formulas and Charts

Soil Classification: 6010

230

DDD

Cc = 10

60

DD

Cu =

Soil Hydraulics: kiAvAQ == d

f

NNhkQ ∆=

Elastic Settlement: IEvqB

i)1( 2−

=δ MH

iσδ ∆

=

Janbu’s Elastic Settlement: E

qB Oi

1µµδ =

Consolidation Settlement: σδ ∆= Hmvc

∆+

+=

''log

1 σσσδ

o

cc e

Hc

Time Rate of Consolidation: tdT

c vv

2

=

Stress Increase in Soil due to a Point Load:

r

∆σy

∆σx

∆σz

Load, P (kN)

L z

y

x

y

z

x

( ) ( )

+

+−

×−−=∆ 23

2

2

22

5

2

)( 2132 rL

zyzLLr

yxLzxP

xhorizontal νπ

σ

( ) ( )

+

+−

×−−=∆ 23

2

2

22

5

2

)( 2132 rL

zxzLLr

xyLzyP

yhorizontal νπ

σ

( )5

3

25

22

3

)( 23

23

LPz

zr

zPzvertical ππ

σ =+

=∆

where: 22 yxr += , 22222 zrzyxL +=++= and ν = Poisson’s ratio


Stress Increase in Soil due to a Uniform Line Load:

y

Load, w (kN/m)

z

∆σhorizontal(x)

∆σvertical(z)

x

∆σvertical(z)

∆σhorizontal(x)

z

x

( )222

3

)(2

zx

wzzvertical

+=∆

πσ and

( )222

2

)(2

zxzwx

xlhorxizonta+

=∆π

σ

Stress Increase in Soil due to a Uniform Strip Load:

Centre Line

θ

b b

Uniform Pressure, q (kPa)

α β

∆σhorizontal(x)

∆σhorizontal(x)

∆σvertical(z)

∆σvertical(z) x

z

y z

x

( )( )βαββπ

σ ++=∆ 2cossin)(q

zvertical and ( )( )βαββπ

σ +−=∆ 2cossin)(q

xhorizontal


Stress Increase in Soil due to a Triangular Strip Load:

Centre Line

c / 2 c / 2

Pressure, q (kPa)

α β

∆σhorizontal(x)

∆σhorizontal(x)

∆σvertical(z)

∆σvertical(z) x

z

y z

x

−=∆ αβ

πσ 2sin

21

)( cxq

zvertical and

−−

−−+=∆ 222

22

)( ln2sin21

zcxzx

cz

cxq

xhorizontal ββπ

σ


Soil Classification Chart: Coarse Grained Soils


Soil Classification Chart: Fine Grained Soils


This page must be submitted with your examination booklet

Student Name: …………………………………… Student I.D …….…………………………. For Soil A


For Soil B

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