Journal of Scientific and Industrial Studies, Volume 6, Number 3, 2008 Printed in Nigeria. All Rights Reserved ISSN: I 1 1 7 - 1 693 Cop>rightC2008 Duncan Scicnce Company ' BEARING CAPACITY AND SETTLEMENT ANALYSIS OF A SHALLOW FOUNDATION ON RECLAIMED SAND OVERLYING SOFT CLAY S.B. Akpila, ThankGod O. and Igwc A. Department of Civil Engineering Rivers State University of Science and Technology, Port Harcourt, Nigeria. ASTRACT A study on the performance of a shallow foundation placed on reclaimed sand overlying soft clay has been carried out. The study area is geomorphologically within the zone of fresh water swamps and alluvial plains (FWS), and geologically within the Benin formation. Results of soil classification, stratigraphy, standard penetration tests, shear strength parameters and consolidation tests were obtained from field and laboratory investigation. The possibility of shear failure surfaces lying within the sandfill layer and into the soft clay layer was considered. Also the values of induced vertical stresses were only about 19% and 6% of the foundation bearing pressure at their respective depths of 3m and 4.5m below ground level. A total settlement of about 12mm was obtained and both allowable bearing capacity and total settlement were within tolerable limits. Keywords: Bearing capacity, Settlement, Vertical stress INTRODUCTION The analysis and design of foundations hold on two important criteria, notably bearing capacity and settlement requirements been satisfied. While foundation stability assesses the bearing capacity of the soil to loading, deformation evaluation focuses on the requirement of vertical displacement of the soil resulting from the induced vertical stress on loading. Generally, the requirements for a satisfactc»y foundation demands that the bearing pressure resulting from loading should be lesser than the bearing c ipacity of the soil, while soil deformation should be within the tolerance limit of the super structure. The increasing demand for land resulting from rapid urbanization has culminated in reclamation of the mangrove and fresh water swamps and alluvial plains with hydraulically or manually placed sandfill on the naturally occurring soft clay. Most often, construction works commences without sufficient time for consolidation of the underlying soft clay induced by the sandfill surcharge. Besides, low surcharge height can also create insignificant induced vertical stress at the sandfill-soft clay interface necessary to accelerate the consolidation rate of the soft clay. Three modes of shear failure under foundation have been described in literature as general shear failure, local shear failure and punching shear failure (Terzaghi, 19-13, DeBcer and Vesic 1958, Vesic 1973). In cases where foundation is placed on sand layer overlying soft clay, the failure surface may extend into the soft clay layer if the thickness of the sand layer beneath the foundation is relatively small compared to the foundation width; a punching shear failure will occur in the top soil layer followed by a general shear failure in the bottom soil layer. However, the failure surface will lie entirely in the sand layer if its layer is large compared to the foundation width. This paper reports on bearing capacity and settlement analysis of a shallow foundation on reclaimed sand overlying soft clay within the south most Niger Delta. * * MATERIALS AND METHODS Field / Laboratory Investigation Field investigation involved ground borings using the percussion drilling rig with all accessories and the standard penetration test (SPT). Subsurface conditions at the site were studied from five geotochnical holes bored to 24m each. Soil samples were retrieved at depth intervals of 1.0m and at points where a conspicuous change in soil lithology was observed for visual examination, laboratory test and classification. Undisturbed cohesive soils samples were also recovered with the aid of U-tubes. As boring advanced, SPT were conducted to determine the penetration resistance of cohesionless soils at varying depths within the boreholes. Laboratory investigation included the use of conventional soil test equipment: triaxial test, oedometer test, sieves, Cassagrandc liquid limit apparatus, weighting balance and oven in carrying out relevant test.