Bridges located in hilly terrains and resting on pile foundations are often subjected to significant scouring on account of fast flowing river water. Such phenomenon can be detrimental and can substantially lower the lateral carrying capacity of the piles due to the removal of soil support. Based on the permissible deflection of the pile at its cut-off level, this article reports the estimation of lateral load capacity of a single free-headed pile embedded in stratified deposit.
Oasys Alp has been used to carry out the analysis. Extreme scouring depths (in the tune of 5-7 m has been accounted) portray vivid degradation of the lateral load capacity of pile (in the tune of 20 – 40%).
How Oasys proved invaluable
Non Linear P-Y Analysis
Oasys Alp has been utilized to carry out the lateral load capacity analysis of a single vertical pile embedded in stratified deposit.
Apart from being capable of considering the presence of water table, the software also facilitates the determination of the flexural response of the pile through automatic load increment to reach the failure load in progressive steps. Alp also considers the passive resistance of the soil and indicates whenever the passive resistance of the soil is exceeded. The methodology taking into account of the generated P-Y curves has been considered in the present study. To carry out the analysis, the pile has been discretized into nodes with a uniform spacing of 0.5 m, with required refinements being made in the intersection with the water table and soil strata interface. A typical Alp output is shown below:
Results and Discussion
Load-Deformation Response and Lateral Load Capacity of Pile
The feature of automatic load increment available in Oasys Alp has been used to estimate the lateral load capacity of the pile. For the present problem, the load acting at the pile head has been set to a reasonable value (e.g. 250 kN) and the number of increments to reach the load has been provided (13 increments). This enabled the researchers to obtain the cumulative flexural response of the pile with the load increments. The following figure shows a typical load increment procedure in terms of the deflection response of the pile (1000 mm diameter):
From the above observation, considering the deflection criterion as 10% of the pile diameter at cut-off level, the lateral load capacity of the pile has been estimated. The following figure depicts the lateral load capacity as estimated from the Oasys Alp software. It can be observed from the plots that the load-deflection curves reveal nonlinear behaviour for higher diameter piles.
The researchers in IIT go on to show the P-Y curves that are generated at the top and bottom of each soil layer. It is noticeable that the clayey layers (which are more plastic than the sandy layers) show more non-linear behaviour, and reaches the plastic limit condition. The sandy layers show mostly bilinear behaviour and, as indicated in the figure, needs to be subjected to large load values to reach their ultimate state. Hence, the linear/nonlinear flexural behaviour of the pile will be governed by the thickness and location of the clayey or sandy stratum.
Based on the conducted study, the following important conclusions can be stated:
- As observed from the results of Oasys Alp analyses, an increase in the diameter of the embedded piles results in the increase in the maximums of its flexural responses.
- Oasys Alp results revealed that the length of fixity is not significantly affected by the change in the pile diameter. This is attributed to the fact that the length of fixity is not governed alone by the flexural rigidity of the pile, but by the relative flexural rigidity of the pile and the surrounding soil. This feature is portrayed by the software Oasys Alp and hence, length of fixity remains primarily invariant with the change in the pile diameter.
- Increase in the scouring significantly decreases the lateral load capacity of the soil (in the tune of 20 – 40%). The degradation is more prominent when scouring takes place in the softer soil stratum.
- The presence of water table, thus reducing the effective unit weight of the submerged soil, also decreases the lateral load capacity of the embedded pile.
- P-Y curves generated reveal that the clay layers show prominence in nonlinear behavior and reaches the plastic limit under failure load condition, while sandy layers, commonly behaving as elastic, show at most bilinear behavior and indicates that this kind of soil require high loads to reach into their plastic limit.
A. Dey, Assistant Professor, IIT Guwahati
A. K. Yadav, Resident Engineer, Jayprakash Associates