Engineering and Technology Journal

This paper presents an experimental and numerical study to investigate the load carrying capacity of piled raft foundation embedded within partially saturated sandy soil. The effect of matric suction on the bearing capacity of the foundation system was investigated. The experimental work consists of two models of foundation, circular raft foundation and circular piled raft foundation. The circular raft foundation has dimensions of 10cm in diameter, and 2.5cm thickness, while the piled raft foundation has the same dimensions of the circular raft model but with a single pile of 2.0cm in diameter and 40.0cm in length fixed at the center of the raft. Both models are loaded and tested under both fully saturated condition and unsaturated conditions, which are achieved by, predetermined lowering of water table. The lowering of water table below the soil surface was achieved in to two different depths to get different values of matric suction and the relationship between matric suction and depth of ground water table was measured in suction profile set by using three Tensiometers (IRROMETER). The soil water characteristic curve (SWCC) estimated by applying fitting methods through the software (SoilVision). A validation process then was carried out for the case of circular piled raft foundation with lowering the water table 45cm bellow soil surface in the aid of a sufficient finite element computer program ABAQUS 6.12. An eight-node axisymmetric quadrilateral element CAX8RP and CAX8R were used to simulate the soil continuum and piled raft respectively. The interaction method used to simulate the intersect surfaces of the system (pile-raft-soil) is a surface-to-surface discretization method under the concept of master and slave theory. The behavior of piled raft material is simulated by using a linear elastic model while the behavior of soil is simulated by an elasto-plastic model by the use of the Mohr-Coulomb failure criterion. The results of the experimental work demonstrate that the matric suction has a significant role on the bearing capacity of all tested models. It shows that the ultimate bearing capacity of circular raft foundation under a partially saturated condition is increases by about (7.0-8.0) times than the ultimate bearing capacity of fully saturated condition when lowering the water table 45 cm below the soil surface. While the ultimate bearing of circular piled raft foundation under partially saturated condition increases by about (8.0-9.0) times than the ultimate bearing capacity of fully saturated condition when lowering the water table 45 cm below the soil surface. The results of the ultimate bearing capacity of piled raft foundation that obtained from the experimental model and from the numerical modelling for the same soil condition and same matric suction indicate that a successful validation is achieved for the simulation process.

This paper presents an experimental study to investigate the change of plug length for pipe pile under different state of saturation and investigate the effect of number of pipe piles on plug length. The influence of matric suction (i.e., capillary stresses) in unsaturated zone is typically considered on the plug length of pipe piles.
The experimental work consist of testing 20 models of pipe piles, these models divided into 4 different configuration of pipe piles; single pipe pile, group of double pipe piles, group of triple pipe piles and group of six pipe piles. All these models are loaded and tested under three different states; dry condition, fully saturated condition (i.e., matric suction equals to 0 kPa) and unsaturated conditions with three different matric suction values (6, 8 and 10 kPa), which are achieved by predetermined lowering of water table. The relationship between matric suction and depth of ground water table was measured in suction profile set by using three Tensiometers (IRROMETER). The soil, water characteristic curve (SWCC) was estimated by applying fitting methods through the program (Soil Vision).
The results of experimental work demonstrate that the values of plug length decreased with increase in value of matric suction for the same configuration of pipe piles, and the values of plug length decreased with increase in number of pipe piles.

The mechanical behavior of partially saturated soils can be different from that of fully saturated soils. It has been found that for such soils, changes in suction do not have the same effect as changes in the applied stresses, and consequently the effective stress principles is not applicable.
A finite element analysis was carried out on a single pile with a diameter (0.6 m) and (12) m length embedded in fully and partially saturated clayey Iraqi soils within Baghdad city. The partially saturated parameters were calculated using laboratory methods; the filter paper method was utilized to estimate the soil water characteristic curve (SWCC) from which the H-Modulus function was obtained. The program (SoilVision) was used to make a fit of the SWCC. The finite element programs SIGMA/W and SEEP/W are then used in the analysis. A parametric study is carried out and different parameters are changed to study their effects on the behavior of partially saturated clay. These parameters include the degree of saturation, depth of water table and shear strength of clay. The study reveals that when the soil becomes partially saturated by dropping water table at different depths with different degrees of saturation, the pile capacity increases. It is concluded that the change in the water table level and the degree of saturation has a great effect on the behavior of partially saturated clay. In this work, it is found that due to dropping of water table and contribution of matric suction (i.e. negative pore water pressure), the pile capacity in partially saturated soil is approximately (3-5) times higher than the capacity of piles in the same soil under saturated conditions