International Research Journal of Engineering and Technology (IRJET)
e-ISSN: 2395-0056
Volume: 08 Issue: 05 | May 2021
p-ISSN: 2395-0072
www.irjet.net
Settlement Behaviour of Disconnected Piled Raft Foundation under Vertical Loading Conditions Arun K.U1, Rubia Sulekha Basheer2 1Post
Graduate Student, Department of Civil Engineering, IES College of Engineering, Thrissur, Kerala, India Professor, Department of Civil Engineering, IES College of Engineering, Thrissur, Kerala, India ---------------------------------------------------------------------***--------------------------------------------------------------------2Assistant
Abstract - Disconnected piled raft (DPR) foundations have
structural failure load. The capacity of the piles is generally governed by geotechnical considerations rather than by the compressive strength of the pile material. In addition, the resistance of piles to horizontal forces through suitably designed connections is usually adequate due to the large number of piles used.
been widely adopted as an effective foundation system where the piles are separated from the raft by a cushion layer, which can limit the shear forces and moments transmitted between the raft and the piles. Thus, DPR foundations may avoid the problem of horizontal forces, such as those from an earthquake or dynamic loads, which damage the structural connection between the pile head and raft. DPR foundation is an economical and efficient system, but the studies based on DPR foundations are limited. A series of static vertical loading tests were carried out on DPR foundation on fine sand using a geotechnical model. Loading tests were carried out by plate load test apparatus. In this paper we discuss the behaviour of interposed cushion layer under vertical loading conditions. The effect of vertical loading on cushion thickness, pile spacing and pile length were presented and discussed. The result showed that most of the vertical forces were carried by raft or interposed layer friction in the DPR foundation. It is observed that with increase in pile spacing and pile depth, DPR foundation was able to bear more load.
1.1 Disconnected Piled Raft Foundation When settlement reducing piles are designed as structural components, the settlements are often relatively large such that the ultimate geotechnical capacity of the piles is fully mobilized. For an efficient design of rafts with settlement reducing piles, indicated that the geotechnical pile capacity could be assumed to be 80% mobilized under working load conditions. However, when these piles are structurally connected to the raft, as they are in traditional construction, a high axial stress may develop in the relatively small number of piles. Thus the load carrying capacity of these settlementreducing piles may be governed by their structural capacity rather than by their geotechnical capacity. A high safety factor will then have to be applied in order to avoid structural failure.
Key Words: Disconnected piled raft foundation, cushion layer, static vertical loading, plate load test.
In addition, these sparsely arranged structural piles beneath a raft may not provide adequate horizontal resistance to lateral loads. For structures resting on raft foundations in seismically active zones or areas with high wind loads, some building authorities therefore deter the use of settlement reducing piles. Thus the practical use of settlement reducing piles is restricted. As an example, designers in Jakarta, Indonesia, are reluctant to use these structurally connected settlement-reducing piles out of concern that the design would not be approved by the building authorities, in view of potential damage to the connections between the relatively few piles and the raft during an earthquake.
1. INTRODUCTION Raft foundations are generally used to support buildings and structures, with or without basements, in dry or high water table conditions. When the shallow subsoil conditions are unfavorable (unsafe bearing capacity or excessive settlements) then load bearing piles are used to transfer the entire load to more competent soil layers. In many cases, the maximum and differential settlements are the controlling factors for the selection of piled raft foundations. The piled raft foundation consists of three load-bearing elements; namely piles, raft and subsoil. According to their relative stiffness, the raft distributes the total load transferred from the structure to the top soil and the connected piles. In conventional design of piled foundations, it was usually postulated that the overall load is supported by the piles. In piled raft foundation systems, the contribution of the raft is taken into consideration to verify the ultimate bearing capacity and the serviceability of the overall system. In conventional piled raft design, the number of piles is normally large and the load carried by each individual pile is relatively small. There is a high safety margin before the piles reach their ultimate geotechnical bearing capacity or
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Since the main objective of adding piles to a raft is for settlement control, and thus to achieve an economical design of the foundation, one alternative in the design of these piles is to consider them as stiffeners for the base soil such that the above mentioned problems can be avoided. Hence an alternative method of disconnected piled raft foundation introduced as shown in figure 1.
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