“Response of Large Diameter Bored Piles in Clayey Soils under Axial Compression Loads”

I.       Introduction

The precise prediction of maximum load carrying capacity of large diameter bored piles is a complex problem because the load is a function of a large number of factors.

Axial pile loading test on single pile may offer the justification of the pile design load and the installation procedure. Codes of deep foundations stipulate the acceptance criteria for piles tested in compression based on specified limits for pile settlement at specified load levels.

Brinch-Hansen (1963), Chin-Kondner (1971), Chin (1980), and Decourt (1999)proposed variable methods of interpretation of the load test results rely on predicting the failure load or limit load by applying mathematical or graphical techniques, and applying a proper factor of safety to get the pile working load.. In addition a large variance appears between the predicted results obtained using the previously vious mentioned methods for the same pile and same soil parameters. Hence it is important to study the response of the large diameter bored piles to under axial loading.

Figure (1), shows the ultimate load for three different large diameter piles were calculated with previously mentioned methods. The difference between their results is up to (8-200%) of ultimate load capacity of the pile.

For large diameter bored piles which are embedded in cohesive soils, skin friction resistance is affected by the undrained shear strength, Cu, of clayey layer.

The Egyptian Code of Deep Foundations (Egyptian Code, 2005) employs certain criteria governing the acceptance of tested piles based on specified limits for pile settlement at specified load levels. A pile is considered acceptable if the observed settlement of pile head is within these limits.

Egyptian Code, compared with other international codes and widely accepted methods, namely the German Code (DIN 4014, 1990), AASHTO (1998), the French Code(1993), and the American Petroleum Institute Code (API, 1980).

A Comparison between Egyptian code and international codes shows large Conflicts in estimation of ultimate skin friction resistance in cohesive soils, which clarifies in figure(2).

The principal benefit of the full-scale field test is that they provide more precise information on the capacity of piles at a specific site. This allows designers to use lower factors of safety and which translate to reduce the construction costs. While there are benefits to performing full-scale field load tests on piles, the tests can be both time consuming and expensive to perform.

Finally, with the increase in the establishment of high-rise buildings, piles are attributed to be the most powerful and famous topic for research work. In spite of all former studies of this point, it still preside the degree of confidence in utilizing their results that achieves mutual consent among researchers and executors in the point of predicting large diameter pile capacity and displacement.

II.     Research Objectives

-        The main objective of this research is; to perform loading tests on large diameter bored piles in order to study the response of piles in various clayey soils under axial compression loads.

-        Numerical study will be investigated and verified using field loading tests data.

-        The effect of various pile-soil parameters affecting the interaction response will be investigated analytically.

III.  Research Plan

The proposed research plan for this study will involve the following:

i.) Review of the literature related to Bored piles, large diameter Piles,construction techniques, piles soil modeling using finite element method, clay soil properties, laboratory tests and review of vary pile loading case studies.

ii.) Perform well instrumented field loading tests on large diameter bored piles in order to study the response of piles in various clayey soils under axial compression loads.

iii.) Analysis phase: Analytical study of shear strength transfer formulations, soil parameters verification using laboratory tests for clay soil.

iv.) Calibration phase (I): By establishing finite element models for big diameter bored pile under compression axial load in clayey soils and compare its results with in-situ monitoring data of pile loading tests in order to calibrate finite element models results.

v.) Conducting a parametric study to discuss factor affecting shear stress transfer along pile length and influence factors affecting soil interface around pile shaft.

vi.) Set out the different conclusions extracted from the results of the numerical analysis presented in the form of tables and charts.

vii.) Set out recommendations for further studies.