CE6611 – Structural Engineering 2 and Geotechnical Engineering 2

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Foundation Design Coursework

Assume that you work in a small firm of consulting engineers and that you have been tasked to
design a foundation for one of the internal columns of a storeyed reinforced concrete building of
category GC2 (as per Eurocode 7 – EC7). The client requires two alternative foundation designs: (1) a
rectangular concrete pad footing and (2) a driven closed-ended steel pipe pile. A structural engineer
in the design team has analysed the superstructure in two stages: (i) Permanent loads and (ii)
variable loads and furnished you with the column loads, moments and shears acting as shown in Fig.
1 and Fig. 2 below. Each student will be assigned a UNIQUE set of values of {V 1 , F 1 , M 1 , V 2 , F 2 , M 2 }.
Please see the Study space for your values. Failure to use the values assigned to you will lead to
ZERO marks! All calculation steps must be shown together with relevant sketches, any
assumptions made and citations to support the assumption. Do NOT use a computer program
apart from your own purpose-written spreadsheet for iterative calculations. Even if you use a
spreadsheet you MUST still submit a clear write-up of the final design calculations to allow
another person to check and understand your design steps, without referring to your spread-
sheet. A borehole log from the ground investigation carried out at the site of the building is given in
Fig. 3. The ground water level is variable and can be up to the ground surface. Note that the
column lies centrally on the foundation.
Permanent verticalforce = V1(kN)Permanent Moment= M1(kNm)Depth=3.2 mVariable verticalforce = V2(kN)Variable moment= M2(kNm)Depth=3.2 mFig. 1 Permanent structural actionsFig. 2 Variable structural actionsPermanent Shear = F1(kN)Variable Shear = F2(kN)

The pad footing is to be installed at 3.2m depth, which is within the medium dense silty sand and
gravel stratum. Use the borehole information to deduce an average SPT (standard penetration test)
Figure 3 – Borehole log

blow count for the bearing stratum. Then estimate the angle of shearing resistance   using the
chart by Peck et. al. (1967) in your lecture notes. Adopt the result as the characteristic value  K in
your design. Also make the following assumptions:
(a) Characteristic effective cohesion of layer c K = 0 kN/m 2 .
(b) Characteristic weight density of layer  K = 18 kN/m 3
(c) Characteristic weight density of reinforced concrete  cK = 24 kN/m 3
(d) Thickness of footing = 475 mm.

Task No. 1
Using Eurocode 7 (EC7) design approach No. 1, carry out calculations in a methodical manner to
determine the minimum size of a rectangular pad foundation to carry the full set of permanent and
variables actions shown. The design must comply with EC7 requirements for GEO limit state for
ground strength. Use the EC7 bearing capacity formulae partial safety factors given in the lecture
notes.
Task No. 2
Considering the design vertical forces only (determined using partial safety factors for actions for
piles) and ignoring the column shear forces and bending moments, determine the depth to which a
closed-ended tubular steel pile having internal diameter of 0.3m and wall thickness of 10 mm should
be driven to achieve a design pile head capacity under the GEO limit state as per EC7 design
approach 1. Assume that the characteristic undrained cohesion for any cohesive layer is c u,k =5N
kN/m 2 (where N= SPT blow count for that layer). Check that, under the design load, the pile head
settlement does not exceed 50 mm. Use the Weltman and Healy chart in the lecture notes to
estimate the adhesion factor, . If you find that the pile length required exceeds the depth extent of
the borehole then re-calculate the required internal diameter for a pile (keep the wall thickness =10
mm) that just slightly penetrates into the bottom stratum shown in the borehole log. Utilise any
relevant assumptions from the previous steps regarding the pad foundation design and obtain
relevant pile design formulae/charts/ EC7 pile safety factors from the lecture notes.

End of Coursework Descriptor

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