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Homework _5 - Homework#5 CEE 452 FALL 2005 FOUNDATION...

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Homework #5 CEE 452 FALL 2005 FOUNDATION ENGINEERING PROBLEM SET # 7 11.3 What is predrilling and when might it be used? What might happen if the predrill diameter or length was excessive ? (2.0/2.0) Predrilling is drilling a vertical hole prior to driving the pile into the ground to reduce the heave, lateral movement, vibrations associated with pile driving, especially for hard ground conditions. Excessive predrill diameter would result in reduced side frictional resistance. Excessive predrill length would result in reduced end bearing resistance. 11.10 In what circumstances would you expect caving or squeezing conditions to be a problem? What construction method could a contractor use to overcome these problems? (2.0/2.0) Caving is likely to be a problem in clean sands below groundwater table. Squeezing is mostly likely in soft clays and silts and highly inorganic soils. The two most common techniques for dealing with these problems are the use of casing and the use of drilling fluid. 11.16 A proposed ten-story office building is to be supported on a series of deep foundations embedded 60 ft below the ground surface. The soils at this site are loose to medium dense well-graded sands (SW) and silty sands (SM), and the groundwater table is at a depth of 12 ft. What type or types of deep foundations would be most appropriate for this project? What type or types of deep foundations would probably not be appropriate? Explain the reasons for your selection . (2.0/2.0) Driven prestressed concrete piles would be the most appropriate for this project because of large structural loads, required depth of embedment, and the subsoil conditions. These piles would drive well through these soils. However, an assessment should be conducted to see if the loose sand layers would liquefy due to pile driving. Driven steel pipe piles may also be appropriate but piles may be subject to corrosion. Timber piles may not be feasible as the required embedment depth is 60 ft. Because some portion of the pile would be within the vadose zone, the portion of the timber pile above groundwater table may deteriorate with time. Drilled shafts would be difficult to construct as most of the length of the foundation is below groundwater table and caving might be a problem. Although auger-cast piles will work well in caving soils, they may not provide the required load capacity. 12.1 An HP 13x60 pile is made of steel with F y =50 ksi and is to be loaded in compression only (i.e., no moment or shear loads). Using F a =0.35 F y , compute the maximum allowable compressive load on this pile . (2.0/2.0) F a =0.35 F y Cross-Sectional Area, A =17.5 in = 0.35 x 50 ksi = 17.5 ksi 2 Allowable compressive load, P (Table 12.1 of Coduto, 2001) a = F a A = 17.5 ksi x 17.5 in 2 = 306.2 305 kips
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