Centrifuge modelling and numerical analysis of bearing capacity of ring foundations on sand

Zhu, Fanyu (1998) Centrifuge modelling and numerical analysis of bearing capacity of ring foundations on sand. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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    Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission.
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Abstract

Ring foundations are often adopted for large and tall structures to resist lateral loads and to increase the stability against overturning. They have been used worldwide under various structures, including telecommunication towers, liquid storage tanks, bridges and offshore structures. However, the behaviour of these foundations has not been well understood. An accepted method for estimating the bearing capacity is not available. In the design of ring foundations, very crude simplifications have to be made. Therefore, developing a rational and practical procedure for estimating the bearing capacity of ring foundations is of great importance. -- This thesis presents research on the bearing capacity of ring footings on a dense sand under vertical loads. The effects of footing size, ring radii ratio and load eccentricity have been investigated by means of centrifuge modelling, the method of characteristics and the finite element technique. To support the research, triaxial and oedometer compression tests have been conducted to determine the soil friction angles, in situ stress ratios, and plastic and elastic behaviour of the sand. The peak and critical state friction angles of the sand from triaxial tests are reduced by 4° to 5° with a log-cycle increase of confining pressure. -- Over 40 centrifuge tests of ring footings have been conducted at acceleration levels from 10 to 160 gravities. High quality sand test samples with density index of 90% were prepared using a developed raining technique. The aluminum model footings with a constant area of 15 cm² and with ring radii ratios from 0 to 0.9 were tested under load eccentricity ratios from 0 to 0.375. Test results indicate that the bearing capacity is significantly affected by footing size, ring radii ratio and load eccentricity. It is found that The bearing capacity of circular footings increases linearly with footing diameter in a double-log scale diagram. The bearing capacity of a vertically loaded ring footing can be expressed in terms of a bearing capacity ratio (Br), a reduction factor (Re) and the bearing capacity of an axially loaded circular footing with the same area. Test results show that the value of Br is related only to the ring radii ratio (n)T independent of footing size; when n is from 0 to 0.35, Br increases slightly with n. Further increase of n beyond 0.35 results in significant decrease of Sr. The value of Re decreases with load eccentricity and is independent of ring radii ratio. The procedure presented for evaluating the bearing capacity of ring foundations is very practical. -- Circular footings under axisymmetric conditions have been analyzed by the method of characteristics to further study the effect of footing size on bearing capacity. In the analysis, the soil friction angle can be variable or assumed to be constant. The bearing capacities calculated also increase linearly with footing diameter in a double-log scale diagram and are close to the centrifuge test results. The compatibility of bearing capacities obtained from both variable and constant friction angle analysis provides a basis for the FE analysis using an equivalent constant friction angle for each footing. -- The FE technique has been applied to circular and ring footings on the dense sand under axial vertical loads. In the analysis, a footing is represented by a rigid body consisting of rigid surface elements. Interface elements are used to model the interaction between the footing and soil. The elasto-plastic behaviour of soil is simulated by the Drucker-Prager/Cap constitutive model. Compared with centrifuge test data, the analytical results regarding the effect of footing size and ring radii ratio on bearing capacity are satisfactory. When calibrated with experimental data, the FE technique is very useful for analysis of very large foundations or for cases when experimental data are not available or difficult to obtain.

Item Type: Thesis (Doctoral (PhD))
URI: http://research.library.mun.ca/id/eprint/1164
Item ID: 1164
Additional Information: Bibliography: leaves 178-190.
Department(s): Engineering and Applied Science, Faculty of
Date: 1998
Date Type: Submission
Library of Congress Subject Heading: Foundations--Testing; Centrifuges; Sand--Mechanical properties--Testing

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