THE UNIVERSITY OF HONG KONG
BACHELOR OF ENGINEERING: LEVEL 3 EXAMINATION
DEPARTMENT OF CIVIL ENGINEERING
EARTH RETAINING SYSTEMS CIVL3031
DATE: 7th May 2011 TIME: 09:30- 12:30 (3 hours)
Answer FOUR questions. At least one question should be from Section B.
All questions carry equal marks.
Use the GREEN BOOK for SECTION A, and the YELLOW BOOK for SECTION B.
MATERIAL PROVIDED: GRAPH PAPERS
Only approved calculators as announced by the Examinations Secretary can be used in this
examination. It is candidates' responsibility to ensure that their calculator operates
satisfactorily, and candidates must record the name and type of the calculator used on the
front page of the examination script.
SECTION A
QUESTION Al.
(a) Demonstrate that the active horizontal effective stress on a retaining wall built in soil of
cohesionc'andfrictionangle lj/is: rr.'= K.rr,'-2c'.Jif:, with K. = l-s~n~'.
l+sm~'
[7 marks)
(b) Determine the depth of cracks that are likely to develop behind retaining walls built in
cohesive soil.
[5 marks)
(c) Calculate the total active force and its point of application on the retaining wall shown in
Figure Al below. The clayey retained soil has a cohesion c', an angle of friction ~· and a
saturated unit weight Ysat·
[13 marks]
W.T
12m CLAY
c'=10kPa; $'=28°; r••1=19kNfm3
IMPERMEABLE
Figure Al
2
QUESTION A2.
A concrete retaining wall has to be designed based on the geometry shown in Figure A2. The
sandy retained soil has no cohesion (c'=OkPa), an angle of friction$' and a bulk unit weighty.
The coefficient of friction wall/soil is 8'. You will apply Coulomb's method using the wedge
shown in the figure, which forms an angle of 55° with the horizontal. You will assume that
the water table is beneath the foundation of the wall. The concrete has a unit weight of
24kN/m3 .
(a) Determine the vertical and horizontal components of the force acting on the wall.
[7 marks)
(b) Determine the width of the wall base if a factor of safety FS=2.0 rs used against
overturning
[9 marks)
(c) Determine the width of the wall base if a factor of safety FS=l.5 is used against sliding
[9 marks]
..........
.. . . . . ...
..• :: :····>SAND
3m c'=OkPa· "''=30°·
' 'I' '
11'=20°;
• y=20kNfm3
. . . . ............. .. .. .. .. . . . . . . . . . . . .
. . . . . . . . .
___:L..--......1....--.-.. - . .....~..:.;.:;,. .:.,:.
.. L:..;,. . . . . . . .
. . . . ... . .. .
FigureA2
3
QUESTION A3.
(a) Determine the depth of embedment required for the stability of the cantilever wall built in
dry sand shown in Figure A3 below. The sand has no cohesion (c'=OkPa), an angle of friction
~· and a bulk unit weighty. You will assume that there is no friction between the wall and the
soil.
[13 marks)
(b) Calculate the location and magnitude of the maximum bending moment in the wall.
[12 marks)
3m
SAND
c'=OkPa; ~'=30°; y=20kN/m 3
FigureA3
QUESTION A4.
Figure A4 below shows the flow net at the back of a retaining wall with a drainage blanket
along it. The figure including the flow net is drawn to scale. A possible failure plane 1r is
drawn on the figure.
(a) Determine the magnitude of pore water pressure at each point on the plane 1r(A, B, ... ,I).
[15 marks]
(b) Determine the total horizontal force acting on the wall.
[10 marks)
4
6.5m '
IMPERMEABLE
Figure A4
SECTIONB
QUESTION Bl.
(a) Differentiate between the following pairs of terms:
(i) active ground anchor and passive ground anchor;
(ii) temporary soil nail and permanent soil nail;
(iii) reinforced fill slope and reinforced fill structure.
[6 marks]
(b) Describe using a flowchart the typical installation procedure for a grouted anchor with
anchorage zone in jointed rock.
[10 marks]
(c) Highlight the concerns arising from the following situations and the actions you would
take to address them:
(i) the ground surrounding part of an anchor hole is liable to collapse;
(ii) water tests carried out in an anchor hole show that too much water is lost;
(iii) the result of a suitability test on a trial anchor shows that the full design anchorage bond
strength cannot be developed.
[9 marks]
5
QUESTION B2.
(a) Outline the major functions of geosynthetics.
[10 marks]
(b) Figure B2 below shows a typical cross section of a road embankment supported by a
reinforced fill (RF) structure. It has incorporated geosynthetic products (A), (B) and (C) in its
construction. For each of (A), (B) and (C), comment on the type of geosynthetics that is best
suited for the application.
[6 marks]
(c) Due to an increase in traffic surcharge, there is a risk that the factors of safety against
sliding and overturning of the RF structure may become marginally below the required
standard. Suggest with supporting reasons three options for enhancing the stability of the RF
structure.
[9 marks]
RF structure with full
height facing panels geosynthetic (A)
·/~'
/}""
/~./
~ /'' Rock
./.~·· .
.,..,.. 1..,~?'
/ ,/
/ /
,/ ,/
·-·-;·-:-·-·-·=.-:··([:_...-JI<....
,...._ .'4 .· ~· ~- .. .. / """
drainage blanket wrapped
' <>,;;!,:.:_ __ ·- :.::•.<:·/' in geosynthetic (B)
Rock '----I Rock
Figure B2
-END OF PAPER-
