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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 7 Examinations 2010/
Module Title: Geotechnical Engineering
Module Code: CIVL
School: School of Building and Civil Engineering
Programme Title: Bachelor of Engineering (Honours) in Structural Engineering
Programme Code: CSTRU_8_Y
External Examiner(s): Mr. J. O Mahony, Dr. M. Richardson Internal Examiner(s): Mr. D. Cadogan
Instructions: Answer four (4) out of five (5) questions
Duration: 2 hours
Sitting: Winter 2010
Requirements for this examination: Graph paper
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt please contact an Invigilator.
Q1. (a) You are required, as part of the design process for a industrial warehouse scheme, to calculate the immediate settlement likely to take place under the centre of a concrete raft foundation, where the underlying saturated clay is due to be subjected to a uniform contact pressure of 215 kN/m^2.
The raft foundation, which is 19 metres long and 9 metres across and which is assumed to be flexible, is to be located at a depth of 1.7m below the existing surface level. The undrained elastic modulus of the soil, Eu, is expected to be 45 MN/m^2 , while the bulk weight of the soil has been determined to be 20.5 kN/m^3. (6 marks)
(b) You are required to calculate the factor of safety against shear failure for a 12.5m high embankment, which has a slope angle of 30°. This process is to be carried in regard to the effective stress along a potential slip surface, using the Fellenius Method of Slices – the centre of the trial slip circle is taken at 20m above the toe of the slope and 4m to the right of the toe, in the direction of the slope. It has been calculated that the breakout point of the given slip circle on the lower horizontal surface will be 3m in front of the toe.
The properties of the soil forming the slope are such that the cohesion factor c´ is 9.2 kN/m^2 , the internal friction angle ´ is 31.4 and the bulk weight is 19.4 kN/m^3 , while the average pore pressure ratio ru is taken as 0.27. (19 marks) (use a minimum of five slices for your calculations)
Q2. (a) A concrete retaining wall as shown in the diagram below is to be designed on the basis that the soil surface lies at an angle of 18.5 to the horizontal and that there is no surcharge applied in that area. The water table has been found to be well below the level of the base of the wall, while the soil behind the wall is a slightly clayey gravelly sand, with soil properties of c = 0, = 32.1and = 20.2 kN/m^3. The unit weight of concrete is taken to be 24 kN/m^3.
Check the stability of the wall in terms of sliding , overturning and bearing capacity. (22 marks)
(b) Explain the difference between compaction and consolidation , and describe some of the causes of each. (3 marks)
6.7m
5.6m^ 1.1m
8.2m
Q5. (a) A retaining wall with a smooth vertical back retains soil for a depth of 11 metres, with the water table measured to be 4.4 metres above the base of the wall. The soil consists of two horizontal layers:
upper layer – c' = 0; ' = 27 ; = 18 kN/m^3 ; thickness = 4.7m
lower layer – c' = 0; ' = 32 ; sat = 21.5 kN/m^3 ; = 19 kN/m^3
A uniform surcharge load of 42 kN/m^2 acts on the whole surface of the ground.
Using the simplified Rankine theory method, determine the magnitude and position of the resultant active thrust. (7 marks)
(b) As part of the design process for a foundation scheme to be placed over a 3.7m layer of clay, a core sample was taken from the soil and an oedometer test was carried out on a specimen of this – the readings obtained are presented in the table below. Each load increment was held for a 24-hour period, prior to the subsequent load increment being applied. The load was removed once the required load cycle was completed, and the sample was allowed to expand over a further 24-hour period. The thickness of the sample was recorded at 18.34mm at this point – in addition, the specific gravity of the soil was found to be 2.49, while the water content was calculated to be 32.1%.
Applied Stress (kN/m^2 ) 0 25 50 100 200 400 800 0
Thickness (mm) 20.02 19.64 19.37 19.02 18.53 18.10 17.64 18.
(i) Plot the e/ curve and calculate the coefficient of volume compressibility ( mv ) for the material, for an effective stress range of 200 to 350 kN/m^2. (8 marks)
(ii) Plot the e/log curve and calculate the compressibility index ( Cc ). (4 marks)
(iii) Calculate and compare the values for consolidation settlement ( sc ) using data from parts (i) and (ii) above for the 3.1 metre thick layer of clay, when the expected average stress to be applied changes from 200 to 350 kN/m^2 , and comment on the result in terms of acceptable settlement values. (6 marks)
Useful Formulae and Charts Geotechnical Engineering Winter 2010
1. F =
2. F =
W sin
c l tan W(cos rusec )
3. Ka =
Sin
Sin
= tan^2 (45 - /2) 4. Kp =
Sin
Sin
= tan^2 (45 + /2)
5. qA = 2
B
Ve
B
V
6. Nq = exp^ ^ tan^ ^ tan^2 (45+ /2)
7. N = 1.8 (Nq-1) tan 8. s = 1 - 0.4
L
B
9. qULT = (½) x (B) x ( ) x (N ) x (s ) 10.
11. e = ( e )
h
h 0 0
12. mv = ( 1 )
e 0
e
13. Cc =
log
e =
0
1
0 1
log
e e
14. sc = mv Δ H 0
tan
tan 1
z
wh
p u
i I E
qB( 1 ) s
^2
