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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2011/
Module Title: Geotechnical Engineering
Module Code: CIVL
School: Civil, Structural and Environmental Engineering
Programme Title: BEng in Civil Engineering – Year 3
Programme Code: CCIVL_7_Y
External Examiner(s): Ms Maria Kyne, Mr John Murphy Internal Examiner(s): Ms N. Hurley
Instructions: Answer FOUR questions, put units with each answer, where applicable
Duration: 2 Hours
Sitting: Summer 2012
Requirements for this examination:
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you are attempting the correct examination. If in doubt please contact an Invigilator.
(a) Fig Q1 shows an embankment made of cohesive soil with cu =21 kN/m^2 and u =
0 o. The unit weight of the soil is 19 kN/m^3. For the trial circle shown, determine the factor of safety against sliding. The weight of the sliding sector is 346 kN, acting at an eccentricity of 5 m from the centre of rotation. What would the factor of safety be if the shaded portion of the embankment were removed? In both cases, assume no tension crack develops and centre of gravity of circle remains the same. (11 marks) F = cu R^2 θ W d
70
e = 5
r = 9 m
1
Fig Q
(b) In terms of total stress, effective stress and pore pressure, cuttings and embankments reach final equilibrium in different ways over a period of time. With the aid of indicative plots, describe what happens in each case. (14 marks)
(a) Adopting a factor of safety of 1.4, determine the maximum permissible angle for
a slope in the following sandy soil, =19.5 kN/m^3 , c= 0; =32
i) When dry ii) When just waterlogged, with steady seepage parallel to the surface (8 marks)
(b) Calculate the factor of safety of the waterlogged slope against failure along a slip plane parallel to the surface at a depth of 4 m if at this depth there is a thin layer of cohesive soil with the following properties:
=18 kN/m^3 ; c = 18 kN/m^2 ; =25
(8 marks)
FOS =
tan 1 tan '
z wh for slip planes in cohesion-less soils
FOS =
(sin )(cos )
' ( )(cos^2 )(tan ')
z
c^ z wh slip planes on or in cohesive soil
(c) What three measures could be undertaken or put in place to stabilise a slope tending towards instability and failure. (9 marks)
Question 5
(a) Explain what is meant by settlement and consolidation, giving causative mechanisms for each. (9 marks)
(b) A clay layer 10m thick, with sand beneath is to be loaded with a layer of fill, 2.5m thick and with unit weight 20kN/m^3. The coefficient of compressibility of the clay decreases with depth z in the form mv = 0.24 – 0.02z. Determine the settlement due to consolidation of the clay layer. sc = mv Δσ’ H (10 marks)
(c) Describe two methods of accelerating the anticipated settlement of a soil (6 marks)
