This paper describes an approach for the design of piles to reinforce slopes, involving three main steps: (1) evaluating the shear force needed to increase the safety factor to the desired value; (2) evaluating the maximum shear force that each pile can provide to resist sliding of the potentially unstable portion of the slope; and (3) selection of the type and number of piles, and the most suitable location of these piles within the slope. For step 1, stability analyses can be used to assess the required additional shear force for stability. Step 2 involves the use of a computer analysis for the response of a pile to laterally moving soil. This analysis can be implemented via a computer program ERCAP, and enables the resisting shear force developed by the piles to be evaluated as a function of pile diameter and flexibility and the relative depth of the soil movement in relation to the pile length. Step (3) involves the use of engineering judgement in conjunction with the analysis results from steps 1 and 2. The paper describes the ERCAP analysis and the characteristics of pile behaviour it reveals. The application of the approach to a highway bypass problem in Newcastle, Australia, is described in detail. In the final design, a total of 64 bored piles 1.2 m in diameter were used over a total length of slope cutting of about 250 m. The pile lengths ranged between 6 and 12 m, with the spacings varying between 3.2 and 6.0 m. Key words : analysis, boundary element, piles, soil–pile interaction, slope stabilization, soil mechanics.
In situations where a raft foundation alone does not satisfy the design requirements, it may be possible to enhance the performance of the raft by the addition of piles. The use of a limited number of piles, strategically located, may improve both the ultimate load capacity and the settlement and differential settlement performance of the raft. This paper discusses the philosophy of using piles as settlement reducers and the conditions under which such an approach may be successful. Some of the characteristics of piled raft behaviour are described. The design process for a piled raft can be considered as a three-stage process. The first is a preliminary stage in which the effects of the number of piles on load capacity and settlement are assessed via an approximate analysis. The second is a more detailed examination to assess where piles are required and to obtain some indication of the piling requirements. The third is a detailed design phase in which a more refined analysis is employed to confirm the optimum number and location of the piles, and to obtain essential information for the structural design of the foundation system. The selection of design geotechnical parameters is an essential component of both design stages, and some of the procedures for estimating the necessary parameters are described. Some typical applications of piled rafts are described, including comparisons between computed and measured foundation behaviour.
Synopsis An analysis is made of the settlement interaction between two identical piles in an elastic mass and the increase in settlement of each pile due to interaction is expressed in terms of an interaction factor α. It is then shown that, for symmetrical pile groups (those in which the piles settle equally and are also equally loaded), the increase in settlement due to interaction may be obtained by superposition of the values of α for the individual piles in the group. On the assumption that superposition also holds for any general free standing pile group, the behaviour of pile groups is analysed for the case of a rigid pile cap (equal settlement of all piles) and a flexible pile cap (equal load in all piles). For the case of a rigid pile cap, values are obtained for the ratio of the settlement of the group to the settlement of a single pile carrying the same total load (the group reduction factor R G ), and the load distribution within the group. For the case of a flexible pile cap, values of the maximum settlement and maximum differential settlement are given. The influence of pile spacing, pile length, type of group, depth of layer and Poisson's ratio of the layer on the settlement behaviour of pile groups is examined. For a pile group in an ideal elastic twophase soil, it is shown that, as with a single pile, the major proportion of the total final settlement generally occurs as immediate settlement. Comparisons are made between reported observations on the behaviour of pile groups from model and field tests and the behaviour predicted by the theory. It is found that, as well as predicting the correct trends, the theory gives quantitative values which are in reasonable agreement with the observed values. On fait d'abord une analyse de l'interaction de tassement entre deux pieux identiques dans une masse élastique et l'accroissement du tassement de chaque pieu dû à l'interaction est exprimé en fonc-tion d'un facteur d'interaction α. On montre en-suite que pour des groupes de pieux symetriques (ceux dans lesquels les pieux se tassent d'une facon egale et supportent aussi des charges égales), l'accroissement de tassement dû à l'interaction peut etre obtenu par une superposition des valeurs de α pour les pieux individuels du groupe. En supposant que la superposition vaut aussi pour n'importe quel groupe general de pieux se tenant librement, le comportement des groupes de pieux est analysé dans le cas d'une longrine rigide (tassement 6gal de tous les pieux) et d'une longrine flexible (charge égale supporté par tous les pieux). Dans le cas d'une longrine rigide, des valeurs sont obtenues pour le rapport du tassement du groupe au tassement d'un seul pieu supportant la meme charge totale (le facteur de reduction du groupe RG), et la repartition de la charge a l'interieur du groupe. Dans le cas d'une longrine flexible, on donne des valeurs du tassement maximum et du tassement différentiel maximum. L'influence de l'espacement des pieux, de la longueur des pieux, le type du groupe, la profon-deur de la couche et le coefficient de Poisson pour la couche sur le comportement de tassement des groupes de pieux est examiné. Pour un groupe de pieux dans un sol idéal élastique à deux stades, on montre que, comme dans le cas d'un seul pieu, la plus grande proportion du tassement final total a lieu généalement sous forme d'un tassement im-médiat. Des comparaisons sont faites entre les observations ayant fait l'objet de rapports sur le comportement de groupes de pieux é partir d'essais sur maquette et sur les lieux et le comportement prévu par la théorie. On s'aperçoit que, outre une prevision des tendances exactes, la the'orie donne des valeurs quantitatives qui s'accordent de façon raisonnable avec les valeurs observéers.
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