Behaviour of Adjacent Strip Footings on Unreinforced/Reinforced Granular Bed Overlying Clay with/without Void

Abstract

In many situations, due to rapid urbanisation, such as lack of construction sites, structural and architectural restrictions, buildings are placed close to each other. In such cases, the stress isobars or the failure zones of closely spaced foundations may interfere with each other leading to the phenomenon called ‘Interference’. It has an impact on the stresses in the subsoil due to overlapping of stresses, bearing capacities, settlements and tilts of footings due to the superstructure loads. Recognising the effects of interference and designing the footings accordingly ensures the safety and good performance of the structures. The first part of this doctoral research work studies the interference effects of two/three-strip footings placed adjacent to each other on unreinforced/reinforced granular soils, including some experimental studies. Effects on stresses in foundation soil; bearing capacities, settlements and tilts of footings are being investigated. Parameters varied in this study are (i) Number of footings (In the case of two footings loaded simultaneously, both experimental and numerical studies are conducted. In the case of two footings loaded sequentially and three footings, numerical studies are done) (ii) Loading conditions (iii) Clear spacing between the footings and (iv) Number of reinforcement layers in foundation soil. With two footings, two loading conditions are considered. In the first loading condition, both the footings are loaded simultaneously up to failure. In the second loading condition, one of the footings representing an already existing foundation is loaded with half of the estimated failure load of single strip footing and adjacent footing loaded up to failure. It is observed that in the case of simultaneous loading, there is a certain critical spacing (S=2B) at which the footing/s carry the maximum load. At S/B=2, the interference effect improves the bearing capacity of the 50mm and 100mm footings by 37% and 74%, respectively. The effect of providing the reinforcements in layers in the foundation soil, beneath the footings, is seen in the increased bearing capacities, reduced settlements, and reduced tilts of the footings. Tilts are also found to be influenced by the loading conditions. On unreinforced soil, increasing the distance from 1B to 4B between the footings results in a nearly 12% reduction in tilt in interfered footing. At S/B=2, introducing three reinforcing layers beneath simultaneously loaded interfering footings results in a 2.6 per cent tilt viii reduction. In the case of sequential loading of old and new footing, providing reinforcement beneath the new footing and loading it to maximum, causes a somewhat larger tilt (6.32% increment) of already existing strip footings. As the second part of this doctoral research work, numerical studies are undertaken on the behaviour of two adjacent strip footings on unreinforced (GB) and reinforced granular bed (RGB) overlying clay with/without voids. The influence of different parameters such as granular bed thickness, length of reinforcement/s, number of layers of reinforcement, presence of void/s beneath the footing/s in the weak soil etc., on the behaviour of footings are carried out. With two adjacent strip footings on GB overlying weak soil, the bearing resistance of each footing is more (14% for B=1m and 36% for B=2m) than a single independent strip footing on GB overlying weak soil. The voids could be formed in weak soil due to various reasons, and the presence of voids will affect the performance of footings. Such voids tend to reduce the load-carrying capacity of the footing/s and alter the failure pattern of foundation soil. In the case of a single void under two footings, the maximum reduction in the bearing capacity of new footing (53% reduction for B=1m, H/B=1) is reported when the void is formed directly below the new footing. When a void is formed anywhere beneath the footing/s in the weak soil, either directly beneath or nearby close to the centre line of the footing/s, failure surfaces developed from the nearest footings tend to move towards the void and are found to be narrower than the no void case. However, providing a reinforced granular bed (RGB) over weak soil can be used as an effective method to maintain the good performance of footings, even when voids could be formed in future. The interference effect in top granular soil combined with the reinforcement effect is seen to effectively nullify the void effect. This research work attempted to provide an analytical model to estimate the ultimate bearing capacity of two and three adjacent strip footings resting on granular bed overlying weak soil, with a fair and acceptable degree of accuracy. The accuracy of the proposed model has verified with finite element simulations and the percentage error is about 13%.