BEHAVIOUR OF LOOSELY SKIRTED SHALLOW FOUNDATION RESTING ON REINFORCED SAND

Abstract

The increase in design and construction practices in metropolitan areas has resulted in a substantial jump in land cost and a scarcity in suitable development sites in the recent years. The unavailability of proper land with adequate strength and stability creates a serious threat for the foundation. It has led to device a cost-effective ground engineering technique. The research interest regarding estimation of bearing capacity of shallow foundations is quite an old subject. An array of ground improvement methods have been engaged to improve the soil properties at the unsuitable working sites. The use of skirts, a consummate venture for the infrastructure industry has proved to be less expensive and more effective. Skirts are basically vertical or inclined members attached monolithically with the shallow foundation strictly confine the soil underneath. The skirt and the confined soil work together as a single unit to transfer loads to the soil at the skirt tip thus improves footing performance. Skirted foundations are used widely in offshore structures as discrete foundation units, at corners of jacket structures, tension leg platform and subsea foundations. The response of skirted foundations to combined vertical, horizontal and moment loading is important for the design of offshore installations. Ease of installation, short installation time and economic efficiency are some prime factors which allow the skirted foundation over deep foundation. Skirted foundations have been used successfully as an alternative to surface, pier and pile foundations for the offshore industry, oil platforms, wind turbines and jacket structures. The beneficial effect of skirt has been extended as an accompaniment with the conventional shapes of footing providing significant improvement in performance. In addition to offshore structures, conventional skirted foundation can be an alternative to the areas where water table is comparatively higher. Shallow foundations such as isolated and square footings are widely used in transmitting loads from super structures to the supporting soil. In addition to vertical load, foundations are often subjected to moment and shear caused by forces such as earth pressure, wind, earthquake and water may be replaced by eccentric or inclined load, results in a reduction in load bearing capacity of footing. Keeping economy in view, skirting technique is one of the suitably applicable methods that have been accepted to enhance the bearing capacity of footing and reduce settlement to the permissible limit. In the past few years different materials such as concrete walls, un-plasticized polyvinyl chloride (UPVC) cylinder, semi flexible vertical reinforcement, mild steel casing, plastic hollow cylinder, timber box have been used to confine the soil underneath the foundation and shown notable improvement in footing performance. Ground improvement of cohesive and granular soil using skirting technique has been a newest emerging innovation, grabbing attention of the researchers in last few years. In the present study, the response of loosely skirted shallow square footing under vertical, inclined and eccentric loading placed on reinforced sand has been investigated. Biaxial geogrid with high tensile strength and stiffness has been tried both as skirt and reinforcement. A thorough study on the efficiency of skirted footing under different loading conditions has been presented in this thesis based on laboratory investigations and numerical analyses. The laboratory model tests have been carried out on surface footing, footing with skirt, footing with horizontal reinforcements and footing with skirt and horizontal reinforcement configurations subjected to vertical, inclined and eccentric loading. The load was applied at an angle of 5°, 10° and 20° with the vertical whereas the eccentricity was varied as 0.1B, 0.15B and 0.2B. The effect of various parameters like depth and top surface dimension of skirt, spacing between horizontal reinforcements, length and number of geogrid layers have been investigated. The effects of angle of inclination and eccentricity on all described footing configurations have also been studied. In the present study the depth of skirt (d) was varied as 1.0B, 1.5B and 2.0B for the top surface dimension of skirt (D) 1.0B, 1.5B and 2.0B; where B is the width of the footing. The spacing between the horizontal reinforcements was kept 0.25B, 0.5B, 0.75B, 1.0B arranged to a depth of 1.0B, 1.5B, 2.0B individually. The depth of placement of horizontal reinforcements for footing with reinforcement configuration was chosen same as the depth of skirt to have a clear comparison with footing with skirt and footing with skirt and horizontal reinforcements for improvement in bearing capacity and settlement. The top layer of reinforcement was kept 0.1B (constant) under the footing followed by placement of rest of reinforcements according to the spacing. A comparison in improvement of bearing capacity and reduction in settlement for all described configuration has been presented. Finite element analyses have also been performed using finite element software, PLAXIS 3D to validate the experimental results. The nonlinearity of sand was modelled using hardening soil model, an elasto plastic second order hyperbolic isotropic hardening model. The footing was modelled as a plate element and the skirt as well as the horizontal reinforcements as elastic geogrid element. The results of experimental and numerical study in terms of load intensity – settlement behavior for all footing configurations have been presented. The load intensity corresponding to 25 mm settlement was considered as the ultimate load intensity. The improvement in bearing capacity and reduction in settlement of square footing with skirt and horizontal reinforcements is quantified using non-dimensional factors, bearing capacity ratio (BCR) and settlement reduction factor (SRF). The experimental model tests and numerical analyses results were found to be in good agreement. Results show that both skirt and horizontal reinforcement have more efficiently improved the footing performance under all loading conditions. The ultimate load carrying capacity of square footing was found to increase due to inclusion of skirt. It shows further enhancement with reinforcements placed horizontally and the optimum improvement was registered with skirt and horizontal reinforcements. A gradual improvement in ultimate load intensity was observed with increment in skirt depth as 1.0B, 1.5B and 2.0B. However, for i = 20° and e/B = 0.15 and 0.2, the increment in load intensity for skirt depth of 1.5B to 2.0B showed a marginal improvement. The load carrying capacity of footing was found to increase significantly with increase in skirt depth and the minimum vertical spacing between reinforcements provided the highest value. The load carrying capacity of footing was observed to decrease with increase in top surface dimension of skirt. It may be observed that the ultimate bearing capacity decreases drastically with increase in top surface dimension of skirt for the extended variation of load eccentricity and angle of inclination. The spacing between reinforcement layers was found to have vital role in ameliorating footing performance. The bearing capacity decreases resulting an increase in settlement of footing with increase in spacing. Minimum reinforcement spacing was noted to have strong impact to counteract the sliding and rotational failure in case of extended variation of angle of inclination and eccentricity. The ultimate load bearing capacity was observed to increase with increase in number of reinforcement layers. The load intensity of footing was found to increase with increase in length of horizontal reinforcement and the optimum increment has been observed at L/B = 2.0. The combined effect of skirt and reinforcement has significantly reduced the settlement of footing under all the loading conditions. The highest reduction in settlement was in the range of 39%-93% (i = 0°), 39%-90% (i = 5°), 36%-88% (i = 10°), 31%-80% (i = 20°), 37%-90% (e = 0.1B), 36%-87% (e = 0.15B), 33%-82% (e = 0.2B). This shows a significant iv performance of horizontal reinforcement even at varied eccentricity and angle of inclination. The bearing capacity ratio values for all footing configurations were found to follow the same trend of variation as the load intensity. The ultimate load intensity values obtained for vertical loading was highest in magnitude compared to inclined and eccentric loading for all parametric variations of skirt and reinforcements. Further analysis interprets that ultimate load intensity obtained at i = 5° and 10° are higher in magnitude than ultimate load intensity obtained at e/B = 0.1 and 0.15 respectively. However the ultimate load intensity obtained at 20° angle of inclination was lesser in magnitude than e/B = 0.2. The failure patterns for all footing configurations are discussed as obtained from the numerical study. Conclusively, the extensive study thus provides a viable alternative to the conventional skirts for better performance of footing subjected to different loadings.