CN324 Geotechnical Design
University of Brighton
School of the Environment and Technology
Division of the Build Environment and Civil Engineering
Semester 2 Examinations 2012-2013
CN324
GEOTECHNICAL DESIGN
Instructions to Candidates:
Time allowed: TWO hours
Attempt ALL questions
You can use Eurocode 7 (BS EN 1997-1:2004), National
Annex (NA BS EN 1997-1:2004) documents and your
lecture notes (FOUNDATION NOTES ONLY).
Formulae sheet and Design charts provided
Graph paper provided
28 May-7 June
Page 1 of 5
CN324 Geotechnical Design
Question 1
Figure 1 shows a reinforced concrete square footing of dimensions
1.8 m x 1.8 m carrying a vertical characteristic permanent load of 600 kN and
a variable characteristic load of 110 kN. Owing 2 an error on site, the imposed
actions of the footing act at distances eB=52 mm and eL=64 mm from the
centre of the footing as shown in Figure 1. The top of the footing is at ground
surface and its base is at a depth of 0.6 m below the ground surface. The
footing is founded on dry sand with characteristic values of unit weight
γs,k=18 kN/m3 and effective strength parameters φk=34° and ck=0 kPa. The
water table is at the ground surface.
Assuming that the unit weights of water and reinforced concrete are equal to
10 kN/m3 and 25 kN/m3 respectively, determine the following in accordance
with Eurocode 7 (BS EN 1997-1:2004) design approach 1 and the National
Annex (NA BS EN 1997-1:2004):
i)
The design bearing pressure.
(9 marks)
ii)
The ultimate bearing capacity.
(12 marks)
iii) The design bearing resistance.
(2 marks)
iv) The utilisation factors.
(2 marks)
v) Is this design solution economic? Justify your answer.
(2 marks)
600+110 kN
eB
γw= 10 kN/m3
eL
e
γw= 10 kN/m
0.6 m
3
1.8 m
1.8 m
Gravely sand
γs,k = 18 kN/m3
φ'k=34°
c'k=0 kPa
Figure 1
Gravely sand
γs,k = 18 kN/m3
φ'k=32°
c'k=0 kPa
Page 2 of 5
CN324 Geotechnical Design
Question 2
Figure 2 shows a reinforced concrete pile of diameter D = 1.0 m which is
driven to a depth L = 15 m into sand of medium density overlying clay. The
pile carries a vertical permanent characteristic load of 520 kN and a variable
characteristic load of 140 kN. The soil profile at the site and the characteristic
values of certain soil parameters are given in the table below.
Soil type
Sand
Clay
Depth
(m)
0–5
>5
γs,k
(kN/m3)
20
19
φk
(°)
36
-
ck
(kPa)
0
-
cuk
(kPa)
50
The sand has an angle of shearing resistance at constant volume φcv,k =32°.
The water table is at the ground surface. The unit weights of water and
reinforced concrete are equal to 10 kN/m3 and 25 kN/m3 respectively.
Determine the following in accordance with Eurocode 7 (BS EN 1997-1:2004)
design approach 1 and the National Annex (NA BS EN 1997-1:2004):
i)
The design vertical actions on the pile.
(4 marks)
ii)
The design base resistance of the pile.
(5 marks)
iii) The design shaft resistance of the pile.
(18 marks)
iv) The total design resistance of the pile.
(3 marks)
v) The utilisation factors. Is this design solution acceptable?
Justify your answer.
(3 marks)
γw =10 kN/m3
SAND
6m
γs,k = 20 kN/m3
φ'k = 36° c'k = 0 kPa
φ'cv,k = 32°
1m
9m
CLAY
3
γs,k = 19 kN/m
cuk =50 kPa
Figure 2
Page 3 of 5
CN324 Geotechnical Design
CHARTS AND TABLES
Adhesion factor α for driven piles in clay.
Value and remarks
α=1 for cu ≤ 25 kN/m2
α=0.5 for cu ≥70 kN/m2
Linear variation in between
α=1 for cu ≤ 35 kN/m2
α=0.5 for cu ≥80 kN/m2
Linear variation in between
Length factor applies for L/D>50
Reference
API (9184)
Semple and Rigden (1984)
Values of Nq against the angle of shearing resistance for the pile formulae
after Berezantzev et al. (1961).
Page 4 of 5
CN324 Geotechnical Design
Skempton’s chart for Nc
For circular/square piles with L/B>5 → Nc = 9
Page 5 of 5