1
University of California, Davis
Department of Civil and Environmental Engineering
ECI-173 Foundation Design
MEMORANDUM
TO:
ECI-173 Students
Date: February 28, 2011
FR:
Jason T. DeJong
SUBJECT:
Assignment No. 6 (Due by 5 pm Wednesday, March 9)
Question 1:
A pile foundation is needed to support a vertical column load of 20,000 kN (dead plus live load) with a factor of safety of 3 against
bearing failure. The subsurface conditions consist of a deep deposit of sand and silty sand having SPT N
60
values as shown on the
Figure below. Bedrock is at a depth of 80 m, and the water table is at a depth of 4
m. Use driven 0.46-m-square prestressed concrete
piles. Plot the number of piles needed versus embedded length, considering piles lengths of 10, 15, 20, 25, 30, and 35 m.
Compute
the pile capacities using the procedures in section 13.5 (of Salgado 2006) that are attributed to the following group of authors:
Randolph (2003), Salgado et al. (2004), Foye et al., and Salgado and Prezzi (2006). Recommend an efficient design, including the
number of piles, their spacing, and their layout (arrangement in plan view).
0
2
04
06
08
0
SPT N
60
Values
60
40
20
0
Depth (m)
Boring 1
Boring 2
Boring 3
The computation of ultimate capacities, using the prescribed design relationships, for different pile lengths is given on the attached
spreadsheet.
I used a wider range of pile lengths than were required by the question, just to illustrate the trends better.
Representative SPT N
60
values for different depth intervals were selected, as shown in the above plot:
Some people would choose to
reduce the values that are in excess of 15 according to the procedure by Burland & Burbidge (1985), but it is not required for these
pile design methods and so many will not make such a correction; You would not be faulted for making either choice.
I chose
values that I would consider representative – not the lowest, and not greater than an average.
For each pile length, the spreadsheet computes an ultimate capacity and the number of piles that would be required to carry the