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Determine Factor of Safety - Geotechnical Engineering - Old Exam Paper, Exams of Materials science

Main points of this past exam are: Determine Factor of Safety, Disturbing Shear Stress, Taylor’s Chart, Residual Shear Strength, Coulomb’s Analytical Method, Components of Maximum Thrust, Angle of Wall Friction, Bearing Capacity

Typology: Exams

2012/2013

Uploaded on 03/27/2013

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Cork Institute of Technology
Bachelor of Engineering (Honours) in Structural Engineering – Award
(Bachelor of Engineering in Structural Engineering – Award)
(NFQ – Level 8)
Summer 2005
Geotechnical and Foundation Engineering
(Time: 3 Hours)
Answer five questions Examiners: Mr T. Corcoran.
Prof. P. O’Donoghue
Mr J P Murphy
1. (a) A road embankment is 22 m high and its side slopes are formed at an angle of 30° to the
horizontal. The soil from which the embankment is constructed has the following
properties:
γ
= 18 kN/m3,
φ
= 25° and c = 10 kN/m2.
The depth to any firm stratum (DH on Fig Q1) is very great and its effect can be ignored.
Using Taylor’s chart (attached), determine the factor of safety F for the slope, given
τ
φ
σ
tanc
F+
=, where
σ
= normal stress and
τ
= disturbing shear stress.
(15 marks)
Fig Q1
(b) What is residual shear strength and what is its relevance to slopes that have previously
failed?
(5 marks)
pf3
pf4

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Cork Institute of Technology

Bachelor of Engineering (Honours) in Structural Engineering – Award

(Bachelor of Engineering in Structural Engineering – Award)

(NFQ – Level 8)

Summer 2005

Geotechnical and Foundation Engineering

(Time: 3 Hours)

Answer five questions Examiners: Mr T. Corcoran. Prof. P. O’Donoghue Mr J P Murphy

  1. (a) A road embankment is 22 m high and its side slopes are formed at an angle of 30° to the horizontal. The soil from which the embankment is constructed has the following properties: γ = 18 kN/m^3 , φ = 25° and c = 10 kN/m^2. The depth to any firm stratum (DH on Fig Q1) is very great and its effect can be ignored. Using Taylor’s chart (attached), determine the factor of safety F for the slope, given

τ

F = c +σ^ tan φ, where σ = normal stress and τ = disturbing shear stress.

(15 marks)

Fig Q (b) What is residual shear strength and what is its relevance to slopes that have previously failed? (5 marks)

  1. The back of a 10 m high retaining wall slopes away from the soil it retains at an angle of 10 ° to the vertical. The surface of the soil slopes up from the top of the wall at an angle of 15 °. The soil is cohesionless with a unit weight of 18.5 kN/m^3 and φ′= 30°. If the angle of wall friction, δ′ = 16°, using Coulomb’s analytical method, determine the maximum thrust on the wall. Determine the horizontal and vertical components of the maximum thrust. (20 marks)

(Useful formula: Ka =

sin( ) sin( ) sin( )sin( )

cosec sin( - )

  1. The proposed design of a cantilever retaining wall is shown in Fig Q3 below. The unit

weight of concrete is 24kN/m^3. The soil parameters are: γ′ = 18 kN/m^3 , φ′ = 38° and c ′ = 0.

The safe bearing capacity of the soil is 250 kN/m^2. The soil behind the wall carries a uniform surcharge of 10kN/m^2. Check the proposed design against failure in sliding, overturning and bearing capacity in accordance with BS 8002 (use a factor of 1.2 on the shear strength of the soil).

Fig Q3 (20 marks)

  1. A 6 m thick layer of medium sand overlies a deep deposit of dense gravel. A series of standard penetration tests carried out through the depth of the sand has established that the average blow count N = 18. Further tests show that the gravel has a standard penetration value of N = 42 in the region of the interface with the sand. A precast pile of square section 0.3 m x 0.3 m is to be driven down through the sand and to penetrate sufficiently into the gravel to give good end bearing. Use a factor of safety of 2 overall, or 3 for end bearing and 1.5 for skin friction, to determine the allowable load the pile will be able to carry. (20 marks) (Useful formulae: fs =N kN/m^2 ; qb ≈ 40 N B

D (^) kN/m (^2) or ≈ 400 N kN/m (^2) )

Bachelor in Engineering in Structural Engineering Geotechnical Engineering Summer 2005

Taylor’s Stability chart

Fadum’ Influence Factors