CIV3247 Past exam 2010 PDF

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04129336

Monash University Semester Two Examination Period 2010 Faculty of Engineering

EXAM CODES:

CIV3247

TITLE OF PAPER:

GEOENGINEERING

DURATION:

180 minutes

READING TIME:

10 minutes

THIS PAPER IS FOR STUDENTS STUDYING AT: ( tick where applicable)  Berwick  Clayton  Malaysia  Distance Education  Open Learning  Caulfield  Gippsland  Peninsula  Enhancement Studies  Other (specify) During an exam, you must not have in your possession, a book, notes, paper, calculator, pencil case, mobile phone or other material/item which has not been authorised for the exam or specifically permitted as noted below. Any material or item on your desk, chair or person will be deemed to be in your possession. You are reminded that possession of unauthorised materials in an exam is a discipline offence under Monash Statute 4.1.

AUTHORISED MATERIALS CALCULATORS

YES



NO 

OPEN BOOK

YES 

NO 

SPECIFICALLY PERMITTED ITEMS if yes, items permitted are:

YES 

NO 

Items supplied: Tracing paper, drawing pin, multiple-choice answer sheet.

Page 1 of 24

Use the Unit weight of water as 9.8 kN/m3 for all calculations. QUESTION 1

(30 minutes) – All part of equal value

Circle the most correct answer to the following multiple choice questions in the multiple-choice answer sheet.

5-storey building

2m

Sand  = 18 kN/m3

Water table 2m X

4m

Clay  = 17 kN/m3

Gravel

Figure A

(i)

Figure A shows current ground conditions for an existing old five-storey building. The water table is located 2 m below the ground surface. The building has applied a gross foundation pressure of 100 kPa on the ground level. What will be the current vertical effective stress on a soil element (referred to as the soil element X herein) at the centre of clay layer? Ignore any reduction in stress due to foundation pressure, with depth. (a) (b) (c) (d)

(ii)

70.0 kPa 50.4 kPa 120.4 kPa 150.4 kPa.

With reference to Figure A, what will be the current horizontal total stress at the element X, if Ko may be approximated as 0.5? (a) (b) (c) (d)

85.0 kPa 75.2 kPa 94.8 kPa 60.2 kPa. Page 2 of 24

(iii)

The owner of the building has decided to demolish this old building and construct a 10storey building at the site. Immediately after demolishing and removal of the current building, what will be the vertical effective stress at the element X? (a) (b) (c) (d)

70.0 kPa 50.4 kPa 120.4 kPa 150.4 kPa.

(iv) With reference to Figure A, what will be the likely vertical effective stress on the soil element X after the effect of stress change due to building removal has stabilised? (a) (b) (c) (d)

(v)

With reference to Figure A, what will be the OCR of the soil element X after the stress effects of building removal has stabilised and the soil was in a state of normal consolidation when the building was standing? (a) (b) (c) (d)

(vi)

70.0 kPa 50.4 kPa 120.4 kPa 150.4 kPa.

2.98 1.0 0.33 < 1.0.

With reference to Figure A, what will be the most likely danger for building the 10-storey building immediately after demolishing of the existing building? (a) (b) (c) (d)

Heaving of the ground causing damage to foundations of the new building Continuing consolidation of clay causing damage to foundations of the new building Clay may not be able to take pressures from foundations of the new building in excess of the pressures exerted by the old building Possible tilting of the new building due to uneven settlement.

(vii) With reference to the cone penetration test (CPT), which of the following statements is correct? (a) (b) (c) (d)

The sleeve resistance is normally higher than the cone resistance Continual increase of cone resistance with depth may indicate that the soil is an overconsolidated clay For clay materials friction ratio is relatively high For sands, sleeve resistance is relatively high.

Page 3 of 24

(viii) For which of the following conditions is the drained condition in clay soils applicable? (a) (b) (c) (d)

The soil is drying due to water drainage The soil is saturated with water as found below the water table The soil has water pressure in excess of static water pressure The soil has an inclined phreatic surface with steady sate seepage.

 1  250kPa  3  100kPa

Figure B

(ix)

Figure B shows a triaxial cylindrical specimen during a consolidated undrained triaxial with pore pressure measurement (CUPP) test at failure. What is the deviator stress applied to this specimen? (a) (b) (c) (d)

(x)

With reference to Figure B, the specimen was first consolidated to 100 kPa cell pressure with a back pressure of 50 kPa, prior to application of deviator stress. If the volume change equation for soil is given by e 1.5  0.2 log  , where   is the effective consolidation stress given in kPa. What is the void ratio of the soil after consolidation? (a) (b) (c) (d)

(xi)

250 kPa 100 kPa 150 kPa 0

1.16 1.1 0.3 The void ratio cannot be calculated since Poisson’s ratio is not given.

With reference to Figure B, if the pore water pressure increased to 75 kPa (from 50 kPa) at failure, what is the Skempton’s A value at failure if the soil was fully saturated at consolidation? (a) (b) (c) (d)

0.17 0.33 0.4 0.5.

Page 4 of 24

(xii) In quick undrained test (UU), a specimen was sheared with a cell pressure of 100 kPa and a deviator stress of 120 kPa was measured at failure. Another identical specimen was sheared at a cell pressure 200 kPa. What is likely to be the maximum principle stress at failure for the second specimen? (e) (f) (g) (h)

100 kPa 320 kPa 260 kPa Cannot compute because pore pressures are not known.

(xii) Which statement is correct? (a) (b) (c) (d)

The soil coefficient of consolidation does not depend on soil stiffness The soil coefficient of consolidation is inversely proportional to soil permeability The soil coefficient of consolidation depends on whether one-way or two-way drainage is used The soil coefficient of consolidation can be dependant on the effective stress level the soil is subjected to.

Level after hea vy rainfall Past level Potential failure surface

Figure C

(xiii) Figure C shows a cross section of a clay dam. Due to the recent heavy rainfall, the water level in the dam has risen significantly as shown. Due to this change, the factor of safety against failure along the potential failure surface is expected to: (a) (b) (c) (d)

Increase Decrease Remain the same since the condition is predominantly undrained immediately after the change Can either increase or decrease depending on the water level rise.

Page 5 of 24

(xiv) Which statement is correct with respect to rock slope stability analysis? (a) (b) (c) (d)

Rock slope stability is mainly determined by intact rock strength For toppling failure to occur the joint dip vectors must be close to the centre in stereographic projection Discontinuity dip angle should be greater than the friction angle of the joint for plane sliding to occur Failure is unlikely at all to occur in a cut slope with a dip direction of 1800, if the slope contains a joint that strikes in east-west direction regardless of the values of slope and joint dip angles.

(xv) At rest soil pressure is applicable when: (a) (b) (c) (d)

The lateral strain is zero The soil is compressed until the lateral stress increases to a constant value The soil expands until the lateral stress increases to a constant value The soil expands until the lateral stress decreases to a constant value.

Page 6 of 24

QUESTION 2 (20 minutes) (a) Figure B shows results obtained from Cone Penetration Test (CPT) during a site investigation. Answer the following questions. 4

6

8

10

Sleeve Resistance, f s (kPa)

12

0

1

1

2

2

3

3

4 5 6

Depth (m)

Depth (m)

2

10

20

4 5 6

7

7

8

8

9

9

10

10

30

40 50

60

70

Soil Profile 0

0

Water Pressure

Depth (m)

Tip Resistance, qu (MPa) 0

Figure B

(i) Interpret the ground profile in the space provided above. Show any calculations you do in the space below. (10 minutes) …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………….................................................................................................... …………………………………….................................................................................................... …………………………………….................................................................................................... …………………………………….....................................................................................................

(ii) If a pore pressure measuring device was also attached to the cone tip (like in a piezocone), plot the likely profile of pore pressure measured. Assume water table is at 1 m depth below the ground level. Use the space provided above. Give reasons for your estimation below. (5 minutes) …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………….................................................................................................... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... Page 7 of 24

(b) Figure C below shows a view of Yarra river at the Dights Fall in Melbourne looking east. Answer the following questions with respect to this figure.

Yarra river

mudstone

Basalt

Figure C

(i) Give the likely sequence of key geological events that may have led to the current landform conditions seen in this figure. Note the formation of mudstone and basalt are similar that encountered in Task 1 – Geological model development. (5 minutes) ……………………………………………………………………………………………….......... …………………………………………………………………………………………………........ ……………………………………................................................................................................... …………………………………………………………………………………………………......... …………………………………………………………………………………………………........ …………………………………………………………………………………………………........ …………………………………………………………………………………………………......... ……………………………………................................................................................................... …………………………………………………………………………………………………........ …………………………………………………………………………………………………........ ……………………………………................................................................................………........ …………………………………………………………………………………………………......... Page 8 of 24

QUESTION 3 (20minutes) All parts are of equal value. Provide a one line definition/description of the following terms (i)

Overconsolidation ratio or OCR ……………………………………………………………………………………………….

(ii)

What is the basic difference between soil consolidation and compaction? ………………………………………………………………………………………………

(iii) Compression Index, Cc …………………………………………………………………………………….............. (iv) Deviator stress ………………………………………………………………………………….................... (v)

Why is it difficult to achieve a B value of 1.0 for materials like sandstone? …………………………………………………………………………………......................

(vi) Why don’t we perform undrained test for sand? …………………………………………………………………………………...................... (vii) Why does dense sand dilate when sheared? …………………………………………………………………………………...................... (viii) Why do sandy slopes tend to be shallow failures? …………………………………………………………………………………......................... (ix) Why do slopes tend to fail after heavy rainfall? …………………………………………………………………………………...................... (x)

What are the three main modes of failure you consider in rock slope stability assessments? ………………………………………………………………………………….........................

Page 9 of 24

QUESTION 4 (30 minutes) All parts are of equal value.

(i)

An undisturbed soil sample is taken out of the ground at 5 m depth (unit weight of soil is 20 kN/m3 ). If the water table was at the ground surface, what would be the water pressure of the soil sample when it is free standing on the laboratory bench table? …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... ………………………………………………………………………………………….........

(ii) Why is the friction angle determined as zero in quick undrained test of a saturated clay specimen? …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... (iii) Give three processes that can lead to soil over-consolidation. …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... ………………………………………………………………………………………….........

Page 10 of 24

(iv) Explain why you select a particular set of strength parameters (undrained or effective) to analyse: (a) The foundation stability of the leaning tower of Pisa as it stands now. …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... ………………………………………………………………………………………….........

(b) Stability of a retaining wall soon after its construction with clay backfill. …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... (v)

The diagram below (Figure D) shows a stereonet projection of a slope cut in rock mass. Also shown are the pole concentrations of three persistent joint sets (1, 2, and 3).

N Cut slope P1

P2

P3

Figure D (a) Which joints are likely to fail in toppling? …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... Page 11 of 24

(b) Which joints are likely to fail in plane sliding? …………………………………………………………………………………………......... …………………………………………………………………………………………......... …………………………………………………………………………………………......... (vi) Briefly explain how tension cracks influence the stability of retaining walls? …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... (vii) At a construction site, a 3m fill layer (with unit weight = 15 kN/m3) is to be placed on a 10 m clay deposit (with unit weight = 18 kN/m3) overlying rock. If the water table is at the current ground level, plot the pore water pressure profile before and after filling? Indicate the excess pore water pressure within the clay layer. …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... ………………………………………………………………………………………………….........

Page 12 of 24

(viii) Give three methods by which we could improve the stability of a soil slope. …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... ………………………………………………………………………………………………….........

(ix) In rock slopes, why does the stable slope height vary on a wide range for a given slope angle? Explain your answer with respect to persistent joint orientation within the slope. …………………………………………………………………………………………………......... ………………………………………………………………………………………………….......... ………………………………………………………………………………………………….......... …………………………………………………………………………………………………........... …………………………………………………………………………………………………............ …………………………………………………………………………………………………............ …………………………………………………………………………………………………............ …………………………………………………………………………………………………............ …………………………………………………………………………………………………............ …………………………………………………………………………………………………............

Page 13 of 24

(x)

Figure E shows a field condition where a tunnel has been constructed in fractured rock. During tunnel construction, the pressure within gravel has been lowered to facilitate tunnel construction. Explain how this can lead to surface settlement? Water table

Clay

Gravel Tunnel Fractured rock

Figure E …………………………………………………………………………………………………........... …………………………………………………………………………………………………............ …………………………………………………………………………………………………............ …………………………………………………………………………………………………............ …………………………………………………………………………………………………............ QUESTION 5 (i)

(40 minutes) All parts are of equal value

A 8 m thick layer of sand (the saturated unit weight = 18 kN/m3; the bulk unit weight above the water table = 16 kN/m3) overlies a 4m thick layer of clay (the unit weight = 20 kN/m3) which overlies an impermeable layer of rock. The water table is located at 3 m depth below the surface. The clay has a coefficient of consolidation of 1.6 m2/year, a compression index of 0.4 and a recompression index of 0.04. The initial in-situ void ratio of the clay is 0.8. A uniform surcharge of 32 kN/m3 is placed on the ground as part of a construction. Preconsolidation pressure of clay is 120 kPa. Using one layer for the clay determine the long term consolidation settlement at the surface.

…………………………………………………………………………………………...................... …………………………………………………………………………………………...................... …………………………………………………………………………………………...................... …………………………………………………………………………………………...................... …………………………………………………………………………………………...................... …………………………………………………………………………………………......................

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