Assign_6_2011_solution

Assign_6_2011_solution - University of California, Davis...

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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
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2 specified load with the required factor of safety (#piles = total load times FS, divided by ultimate capacity per single pile). The total pile length (length per pile times the number of piles) is also computed. For this site, the total length of piles does not vary that much for a range of reasonable pile lengths, but in more general cases, longer piles are often more efficient when they pass from weaker soils into much stronger soils. 0 5 10 15 20 25 30 35 40 45 50 0 1 02 03 04 05 06 0 N 60 Depth (m) There is no one "best" choice for an efficient pile group design, so a wide range of possible answers are acceptable. However, the
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This note was uploaded on 03/24/2011 for the course ECI 173 taught by Professor Dejong during the Spring '11 term at UC Davis.

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Assign_6_2011_solution - University of California, Davis...

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