# Experimental Analysis of the Behaviour of Piled Raft Foundations in Loose Sand
## 1 Introduction
### Introduction  
In geotechnical engineering, a shallow foundation structure is not always applicable due to deformability of soil. One of the most significant aspects in foundation design is bearing capacity [1]. Accordingly, alternative foundation structure solutions are used, suchas deep foundations. Calculation of the pile group is based on the assumption that the entire load from the aboveground part of the structure is taken over by the piles, without taking into account the load bearing capacity of the raft. This approach to calculation is very conservative; the basic principle of calculation should be based on the fact that the foundation structure contains as many piles as would be required to reduce the settlement to an acceptable level. That way, the load from the aboveground structure is transferred to the ground partly through the raft and partly through piles. This calculation approach aims to contribute to optimization of the foundation structure, thus reducing the number of piles. The mechanism and design theory considering utilization of soil below the raft to bear a part of the total applied load in piled raft foundation has been a research hotspot for more than half a century [2; 3; 4]. Assuming that the load is evenly distributed over the area of the raft, a deflection will occur in the middle of it, as schematically shown in Figure 1a. By adding a specific number of piles under the raft, settlement and differential settlement will be reduced (Figure 1b). [PERSON] et al. [5] have conducted tests with a limited number of piles beneath the central part of the raft, using piles as settlement reducers. Using piles to reduce settlement is an idea that originated earlier, in the 1970s [6]. There are many studies that deal with behaviour of raft foundations and piled raft foundations [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19] and settlement of piled raft foundations [20; 21; 22; 23]. In recent research [24; 25; 26; 27; 28; 29; 30; 31], the response of piled raft foundations to static and cyclic loading was studied experimentally and numerically. [PERSON] [32] investigated piled raft foundations in multilayer soil with different stiffness, exposed to different loads, by using software package Plaxis 2D. [PERSON] and [PERSON] [33] dealt with the influences of certain parameters on the load distribution between the raft and the piles, such as the number and arrangement of piles, the piles' length, and the raft stiffness. They concluded that, when using longer piles, there may be less settlement than when using a larger number of piles, and bending moments in the raft cannot be reduced by supporting it with piles. [PERSON] [34] concluded that the conventional practice of designing piled raft foundations is based on the assumption that external load is carried by piles and any raft contribution is neglected. Therefore, this approach is too conservative because the raft can carry a significant part of the load due to direct contact with the ground. [PERSON] and others [35] concluded that, in tests conducted on piled raft foundations \(2\times 2\), \(3\times 3\), and \(4\times 4\), when the raft settlement is 10 mm, which was the smallest lateral dimension of the raft, 60% of the total load was taken over by the pile group. Each pile group behaves as a raft with piles except those cases in which there is no contact between the raft and the soil [36]. [PERSON] [37] states that, when designing piled raft foundations, rafts have certain load bearing capacity. Due to not considering the contribution of the load bearing capacity of the raft, when designing piled raft foundations, more piles are required than actually  
Figure 1: Piles required to reduce differential settlement. needed [38]. [PERSON] et al. [39] compared designs of piled raft foundations by applying Eurocode 7 and Kazakhstan regulations. [PERSON] concluded that Eurocode 7 shows more conservative results, with an increased level of safety [40; 41; 42].