Lecture19-Pile-Groups

Lecture19-Pile-Groups - Foundation Engineering Foundation...

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Unformatted text preview: Foundation Engineering Foundation Engineering Lecture #19 Lecture #19 Pile Groups- Group Settlement- Group Bearing Capacity- Group Uplift, Moments and Lateral Loads- Pile Group Efficiency- Pile Cap Design L. Prieto-Portar 2009 Cutting off the top of the pre-cast concrete piles using a hydraulic cutter. Cutting off pre-cast concrete piles for a marina dock. A hydraulic cutter being used on a pre-cast concrete pile. A hydraulic cutter being used on auger-cast piles. A hydraulic cutter cutting a 4.5-foot diameter steel-casing and concrete filled pile. The finished cut of the 4.5-foot diameter steel-concrete composite pile. Analysis of Pile Groups. There are three methods commonly used to analyze a pile group: 1. The Simple Static analysis. In this method, the presence of the soil is ignored and the pile group is regarded as an isolated structural system. It also assumes a zero moment at the head of each pile. 2. The Equivalent Bent analysis. This method considers the reaction of the soil sub- grade on the equivalent free standing length of the piles. The pile cap is assumed to be rigid, and the piles are assumed to behave elastically. 3. The Elastic Continuum Analysis. The soil is considered to be an elastic material that is consistent through out its mass. An increase of the spacing between the piles increases the load bearing efficiency of the group. In comparing the various methods of group pile analysis, the vertical loads are similar, but the elastic continuum method predicts a higher maximum load. The zero moment is assumed for the static method, but there is considerable difference in the magnitude of the moment between the other two methods. The equivalent-bent method predicts larger rotation than the elastic method and a larger vertical deflection of the leading pile, but smaller horizontal deflections. The Settlement of Pile Groups. A. Elastic Settlement of pile groups. The elastic settlement is the immediate settlement of the pile group due to the deformation of the soil mass. The elastic settlement of pile group under service working loads per pile increases with the width of the group B g , ) g e e B H H = where & H g(e) is the elastic settlement of the entire pile group, B g is the width of the pile group section, s is the spacing of the piles to each other (center-to-center), d is the width or diameter of each pile in the group, & H e is the elastic settlement of each pile at their service load. B) Elastic Settlement of pile groups (after Meyerhof, 1976). ( ) g e e s d 2 [1 ( )] 0.5 8 f g D d I H p where I d N = =- In granular soils the settlement increases with the pile group width B g and the center-to- center distance d (from Meyerhof, 1961). C. Consolidation settlement of Pile Groups using the 2:1 method. The stress increase & p i at the middle of layer i in the soil mass due to the group load Q g is, and the settlement at that level i can be given as a function of the voids ratio, ( )( ) i i g g g i Q p B z L z =...
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This note was uploaded on 09/07/2011 for the course CEG 4012 taught by Professor Staff during the Spring '10 term at FIU.

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Lecture19-Pile-Groups - Foundation Engineering Foundation...

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