# Parametric Study on the Interaction Factor of Piled Raft

## Summary (2 min read)

### INTRODUCTION

- The design of foundation system to support structures that are either heavily loaded or sensitive for settlements was always tradition bound.
- The presence of the raft and its contribution in sharing the load is ignored and the design does not weigh the transfer of the load directly below the raft or the pile cap as the case may be.
- However it is interesting to note that every structure has got certain amount of permissible settlement and this fact has come to recognition recently.
- The piled raft foundation system transfers the load by a three dimensional interaction among the constituent elements, namely the raft, pile and the soil.

### THE STUDY

- The study was carried out by conducting a series of 1g model tests on circular, square and rectangular shaped piled raft.
- The tests included tests on un-piled raft, free standing pile group and the piled raft.
- The tests were carried out in steel tank by preparing the bed adopting sand raining process in pre-calibrated manner so that the required densities could be achieved.
- The 8mm thick raft diameter was kept as 200mm; the piles were 10mm dia 160mm long and the pile –raft area ratio was kept as 5.2% and the 21 piles were placed in radial grid pattern.
- The bed material was poorly graded palar sand.

### RESULTS ANALYSES AND DISCUSSIONS

- Figure 1 presents the load settlement response of free standing pile group and pile group of piled raft.
- In the case of free standing pile group the raft was not in contact with the soil and was kept 40mm above the bed level.
- Load was applied in small increments till the settlement reached 20mm.
- In the case of pile group of piled raft till the settlement level reached 2mm, the response was very stiff and the load at this level was 1.8kN which is nearly 50% more than the load taken by the free standing pile group.
- The characterized load settlement response presented in Figure 2 clearly shows that the freestanding pile group has a two phase response with the initial stage showing a stiff response till the friction is overcome.

### QUALITATIVE ASSESSMENT

- The interaction between the raft pile and the soil can be explained based on the interesting hypothesis proposed by Chin [6].
- It may be visualized that when the soil mass is under a compressive stress the load is transmitted by internal columnar grain structures and that as they reach the limiting loads more and more columns begin to support the load each having approximately same yield load.
- It is seen that as the pressure applied from the raft increases, the assembly of columnar structure become denser creating higher confining pressure.
- Once the applied stress increases beyond the limiting value , the grain structure tends to collapse or does not resist the applied stress but pile soil system stiffness remains at the enhanced value, helping the raft to take higher load.
- The loss of stiffness is gradual till the settlement reaches 6mm and at the final settlement of 20mm the.

### QUANTITATIVE ASSESSMENT

- Quantification has to be done through a suitable parameter in non-dimensional form mainly because the interaction is a measure of varying relative stiffness.
- Randolph [7,8] in his studies had expressed the interaction between the pile and the cap considering the average displacements beneath the cap due to the loading on the pile, and down the center line of the pile due to the cap.
- Clancy [9] had later established the raft pile interaction factor pr and based on this, more rigorous analyses were conducted to show that the value is constant as the number of piles increases.
- All measured at the same settlement level.

### Effect of pile length

- Figure 3a presents the bilinear variation of the interaction factor with the pile length at different settlement level.
- As the settlement level increases the interaction factor reduces and at 20mm the value is the least.
- When the pile raft area ratio is very high (9.25% with 37 piles) the interaction factor varies from 0.8 to 1.2 indicating that the piled raft has a tendency to behave as a fully piled system.
- The interaction factor varies from 0.75 to 1.2 ; at 2mm settlement the pile group has very high stiffness and tends to function as fully piled.
- However under certain optimum conditions the variation is similar to what has been obtained by Randolph [7].

### CONCLUSIONS

- The interaction behaviour of piled raft was studied through a series of 1g model tests and the nondimensional interaction factor which is a function of the pile group, raft and the piled raft stiffness was evaluated under various parametric conditions.
- It was established that that the columnar grains contribute to the enhancement of the confining stress leading to the higher pile group stiffness and to the increased capacity of the whole system.
- The interaction factor was computed from the model tests and were compared with the values predicted by Randolph.
- The variation was found to be somewhat wider but perhaps within an acceptable level as the method of study was different.
- Which is in close agreement with predicted value with numerical analyses by Randolph.

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379 citations

### "Parametric Study on the Interaction..." refers background in this paper

...Extensive research has been done so far on the behaviour of piled raft and various aspects of behaviour relating to the load settlement and the load sharing response have been studied in detail by various researchers like Reul [1],Poulos[2] and so on....

[...]

296 citations

### "Parametric Study on the Interaction..." refers background in this paper

...Randolph [7,8] in his studies had expressed the interaction between the pile and the cap considering the average displacements beneath the cap due to the loading on the pile, and down the center line of the pile due to the cap....

[...]

64 citations

### "Parametric Study on the Interaction..." refers background in this paper

...As pointed out by Lee and Chung [5], the complex interaction can become favorable like increase in the group capacity or unfavorable like causing additional settlement....

[...]

8 citations

### "Parametric Study on the Interaction..." refers background in this paper

...Extensive research has been done so far on the behaviour of piled raft and various aspects of behaviour relating to the load settlement and the load sharing response have been studied in detail by various researchers like Reul [1],Poulos[2] and so on....

[...]

...ISBN: 978-981-07-4948-4 :: doi:10.3850/978-981-07-4948-4 259 1 Extensive research has been done so far on the behaviour of piled raft and various aspects of behaviour relating to the load settlement and the load sharing response have been studied in detail by various researchers like Reul [1], Poulos [2] and so on....

[...]

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##### Frequently Asked Questions (2)

###### Q2. What have the authors stated for future works in "Parametric study on the interaction factor of piled raft author" ?

Even though the pile group of piled raft started losing its stiffness rapidly it continued to take further load which is similar to elastic work hardening behaviour.