Date of Award

Spring 2018

Document Type


Degree Name

Doctor of Philosophy (PhD)


Civil & Environmental Engineering

Committee Director

Isao Ishibashi

Committee Member

Duc Nguyen

Committee Member

Julie Z. Hao


Current foundation design practice considers conventional pile group foundation when simpler and rather economical raft foundations do not meet one of the design criteria. However, design requirements could be met by incorporating a reduced number of piles with the raft. Piled Raft foundation (PRF) is a composite foundation in which, the piles and the raft contribute to the total resistance of the foundation.

The objective of this dissertation is to study the behavior of PRF in fully and partially saturated soils. The primary purpose of this study is to develop models capable of estimating the resistance of different types of foundations in fully and partially saturated soils. In this investigation, an experimental program was first proposed to characterize the shear strength of partially saturated soils. Interaction factors between the raft and the piles at different suctions were explored. The effect of the suction on skin friction and end bearing resistances was also studied.

The outputs of the experimental program were discussed and analyzed. The analysis results produced models for predicting the capacity of single pile and 3x3 group foundations in soils at different suctions.

In addition, three-dimensional finite element models were developed using Abaqus software. Numerical analyses simulating the performance of different types of foundations in various suctions were conducted. Load-settlement curves and distribution of the suction resulted from these analyses were verified against the experimental models.

The numerical model was also utilized to study the effect of some key parameters on the behavior of PRF. The influence of raft thickness, settlement amount, pile spacing, and pile length to diameter ratios were examined. The outputs of the study were analyzed statistically and numerical models for predicting the capacity of PRF in different suctions were developed.

In this study, it was found that the shear strength was increased when the suction in the soil went up. The increment reached the peak at the Air Entry Value (AEV, 105 kPa) and then leveled off at 600 kPa suctions. The resistance of the single pile increased with suction. The increment in Skin Friction Resistance (SFR) attained the maximum while the increment in End Bearing Resistance (EBR) extended to 2AEV suction and then leveled off.

As expected, Piled Raft Foundation (PRF) showed higher resistance than Pile Group Foundation (PGF) due to the contribution of the raft in PRF. However, PRF resistance was less than the resistance of PGF and Unpiled Raft Foundation (URF) combined due to the interaction between the foundation components. The raft contribution increased with suction from 12 % for zero suction to 45 % at 600 kPa suction. In PGF and PRF, SFR was highly influenced by interaction effects while those effects were negligible in EBR.

It has shown that when piles with large spacing (7d) were added to 24x24 m raft, the resistance of PRF was doubled in comparison with raft foundation only. PRF resistance increased linearly with increasing L/d ratio of the piles. Selected raft thickness ranged from 0.8 to 1.6 m had an insignificant effect on PRF capacity.


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