BEHAVIOUR OF SHALLOW PLATE ANCHORS IN SOILS

Abstract

The investigation deals with shallow horizontal plate anchors subjected to vertical pullout loads. Such types of anchor foundations are commonly used as foundations for trans mission line towers, retaining structures, suspension bridges etc. Taking into account the nonlinear constitutive relation ship an analytical analysis has been develooed to predict load displacement behaviour of shallow horizontal strip anchors subjected to vertical loads. The analysis is based on the equilibrium of forces on the linear soil wQdge resisting the uplifting forces using the shear parameters, cohesion(c) and angle of internal friction (0) of the soil. Expressions have been derived for critical load, breakout load and stresses in the soil mass within the soil wedge. Displacements have also been computed by numerical integration of strains in the soil mass. The analysis for strip anchors has been extended to the cases of square and circular plate anchors. Further, an analysis has also been developed to predict the time dependent behaviour of horizontal strip anchors in clays. The analysis is based on constitutive relationships established by framing suitable rheological model. The parameters of the rheological model may be evaluated by performing conven tional triaxial test. This analysis has also been extended for square and circular anchors. Model tests in laboratory were conducted under controlled conditions in soft cohesive soil. Strip, square and circular anchors with width of anchor plates of 5 cm and 10 cm and the depth to width ratios of 2, 4 and 6 have been tested. The anchors were subjected to short and also long duration loads and sustai ned loading conditions. The experimental data is analysed to bring out the influence of various parameters and to study the time dependent behaviour. Experimental results are compared with the values predicted by proposed analysis. .A good agreement between the predicted breakout load and experimental breakout load has been noted. The nondimen sional breakout load factors obtained by proposed theory have been found to agree well with most of the established uolift capacity theories. Large amount of data available on breakout loads reported by various investigators has been compared with the theoretical values obtained from the proposed analysis. A very good agreement has been noted. Very limited data is available on the load-displacement behaviour. However, the available data on load displacement behaviour is also noted to agree well with the predicted data. Load-time-displacemGnt behaviour predicted by proposed theory, by using the rheological constants obtained from triaxial tests, matched well with observed load-di spl acement-time data on model anchors under sustained loads. Depending upon the depth to width ratio, the behaviour of vi an anchor changes from shallow to deep. The analysis of data indicates that for soft cohesive soils the plate anchors behave as shallow anchors if the depth to width ratio is less than 4 for strip anchors and less than 2 for circular or square anchor s. Suggestions for future research have also been made.