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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 8 Examinations 2009/
Module Title: Foundation 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): Dr. M. Richardson Mr. J. O’Mahony
Internal Examiner(s): Mr. D. Cadogan Mr. G. Hayes
Instructions: Answer two (2) out of three (3) questions from Section A Answer two (2) out of three (3) questions from Section B Use separate answer books for Section A and Section B Both sections carry equal marks
Duration: 2 hours
Sitting: Autumn 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.
Section A
QA1. (a) A square foundation is to have a load of 1925kN applied to it. With a side dimension of 2.8m, it is to be constructed in a sandy soil at a depth of 3. 1 m below ground level. This soil has a bulk weight of 20. 2 kN/m^3.
A CPT test programme was undertaken and the average qc results for each layer are presented in Table A1 below.
Calculate the immediate settlement of the pad, as well as its predicted settlement after a period of two years, assuming that the water table is well below the level of the foundation base. ( 10 marks)
Table A
(b) Describe the significance of the efficiency in relation to a pile group, and explain why the efficiency varies with the density of sandy soil in the case of driven piles. (3 marks)
(c) 24 concrete bored piles are to be installed as a group in a 6 x 4 configuration beneath a 11.9m by 10.1m pile cap. Each pile is designed to be of a 610mm diameter and 28.7 metres in length, with the pile group expected to support a 30.2MN working load. A clay layer with a bulk weight of 16.7kN/m^3 is present, where the strength increases with depth according to the relationship cu = 69 + 2.6z , where z is the depth below the pile cap (in metres). Calculate the factor of safety of the pile group, as well as the efficiency factor. (8 marks)
(d) In relation to the reaction of clayey and silty soils under load beneath a foundation, explain the differences between undrained and drained conditions and outline the effect of any variation in the permeability values for such a soil. (4 marks)
z (m) qc ave 0.0 – 0.5 2. 0.5 – 1.0 2. 1.0 – 1.5 4. 1.5 – 2.0 6. 2.0 – 2.5 6. 2.5 – 3.0 8. 3.0 – 3.5 10. 3.5 – 4.0 7. 4.0 – 4.5 8. 4.5 – 5.0 11. 5.0 – 5.5 14. 5.5 – 6.0 13.
QA3. (a) A 2. 4 m by 2. 4 m shallow foundation is to be constructed on a sandy material at a depth of
- 3 m below the ground level. This soil is found to have a bulk weight of 1 9. 1 kN/m^3 , while the water table is at a level of 0.3m below the foundation base. The SPT N profile is presented in Figure A 3 and Table A 3 below. A tolerable settlement of 22mm is specified by the designer. Calculate the allowable load that can be supported by the foundation, using the Terzaghi and Peck method, the two Meyerhof methods and the Burland and Burbidge method. ( 10 marks)
Figure A3 Table A
(b) In relation to the design of a shallow foundation, describe the principal modes of shear failure. Explain the main differences between the terms safe bearing capacity and allowable bearing capacity. (3 marks)
(c) A retaining wall is subject to an eccentric load, such that the vertical and horizontal components are to be 301kN/m and 123kN/m respectively, with the eccentricity of the base reaction at 0.41m. The wall is constructed on a sandy soil and has a base width of 3. 1 m at
- 2 m below the ground level. With shear strength parameters of the soil found to be c’ = 0, an internal friction angle ’ of 32.7° and a bulk weight of 18.1 kN/m^3 , calculate the factor of safety against shear failure for the base – it is assumed that the water table is well below the base level. ( 8 marks)
(d) In regard to the design and construction of basement foundations in the range of soil types, explain how these two phases of such a project are reliant on each other, and describe the factors which are critical to this. ( 4 marks)
z (metres) SPT (N) 0.5 8 1.0 10 1.5 13 2.0 15 2.5 12 3.0 16 3.5 17 4.0 14 4.5 16 5.0 18 5.5 14 6.0 17
0 1 2 3 4 5 6 7
0 10 15 20
SPT (N)
z (m)
5
Section B
QB1. (a) Briefly state the two principle requirements of foundation design. (2 marks)
(b) A rectangular footing 10m wide and 40m long was constructed at a depth 3m below the existing ground level. The loading on the foundation was 75kN/m^2.
The ground conditions below the foundation were such that a sandy CLAY was identified to a depth 10m below the foundation, having an undrained elastic modulus E 1 of 15MN/m^2. Below this, a layer of sandy gravelly CLAY with a modulus E 2 of 40MN/m^2 was encountered and found to be 15m thick.
Calculate the settlement of the foundation using the appropriate design charts provided. (14 marks)
(c) Detail the advantages and disadvantages of both shallow and deep (> 6.0m) foundation construction within granular soils, making reference to foundation type and groundwater. (9 marks)
QB2. (a) A 5-storey apartment complex, with two basement levels, is to be constructed on a site in an urban environment. The site had previously been used for warehousing. The adjacent buildings, two-storey office blocks with existing basement parking, were recently constructed using a buoyant raft with tension anchors and the ground conditions have been characterised in detail. Bedrock was proved to be 18m deep. Both piling and a buoyant raft foundation are being reviewed.
Define an appropriate preliminary Geotechnical Category for the proposed works, outlining in detail your reasoning – make direct reference to the possible effects on the adjacent building. (5 marks)
(b) In accordance with EC7 (Geotechnical Design – Part 2), detail a proposal for a ground investigation for the proposed building 40m by 60m plan area making use of a diagram, outlining the following: zone of influence of the structure number of exploratory locations type of investigation techniques depths of the proposed investigation locations (12 marks)
(c) State the principles of the geotechnical design process in accordance with EC7, making use of bullet points or flow chart to outline the process expanding on the basic elements where applicable. (8 marks)
Useful Formulae and Charts – Foundation Engineering Autumn 2010
1. CN = 0.77 log (2000/σv) 8. si = C 1 x C 2 x qn ∑([ Iz / E ] x Δz) 2. CR = 1 at depths > 10m 9. E = qc x 2.5 for square foundation = 0.95 at 6-10m = 0.85 at 4-6m 10. Iz = [0.5] + [0.1 x √(qn / σv'ave)] = 0.75 at <4m 3. qs = ∑ cu 11. C 1 = [1] - [0.5][( σ'base / qn)], C 2 = 1 + 0.2 log (10 t ) 4. qsav = cu 12. Nc = 5.14 x [1 + 0.2(B/L)] x [1 + (0.053 x [D/B])0.5] 5. = [(0.5) / (cu/v)0.5)] 13. i = (1 – α/)^2 6. Ic = (1.71 / N1.4) 14. ic = iq = (1 – α/90°)^2 7. si = q’ x B^0 .7^ x Ic 15. qf = (½ B' ' N s i) + (c Nc sc ic) + (' D Nq sq iq)
