Pile is a structural element constructed to overcome heavy loads from super structure, when proper bearing strata is not available at shallow depth. The prediction of bearing capacity of a bored cast in-situ pile is a complex problem, as it depends on installation method, concrete quality, ground condition and pile geometry. It is considered that the reliable method for finding bearing capacity is pile load test, which is time consuming and costly. The bearing capacity can also be analysed by empirical and analytical methods using soil data and SPT data.
In this paper, empirical methods and graphical methods are used to evaluate beating capacity of cast in-situ bored piles. For empirical methods - IS code method, Meyerhof method, Bazaraa and Kurkur method are employed and for graphical methods - Hansen’s method, Chin-Kondner’s method, Decourt’s Extrapolation method are employed.
All these methods are summarized for comparison with the pile load test values. A database of 3 bored piles is collected from different sites in Kerala. The above chosen SPT methods are calibrated by trial and error method to propose a new formula. A pile structural design also proposed.
Now a days the use of bored cast in-situ piles has multiplied around the world. Bored cast in-situ piles have moderate bearing capacity, low cost, reduced vibration during installation and allow easy length adjustments. The prediction of bearing capacity of a bored cast in-situ pile is a complex problem. It is necessary to consider factors such as method of boring, installation process, quality of concrete, ground conditions and experienced expertise while designing piles. The method of installation has a great impact on pile foundation i.e., drilling can cause vibration and disturbs the surrounding soil. Even after installation of pile, changes may occur in the soil nature with time. The appropriate pile capacity can be obtained only by conducting a pile load test. The conduction of pile load test for small projects is not economical. In such cases, other methods can be adopted for prediction of pile bearing capacity.
Various methods have been developed for predicting the pile bearing capacity, considering soil-pile interaction, soil stratigraphy and soil resistance along the pile.
The static method in the IS 2911- Part 1/Sec 2 to determine the bearing capacity of bored cast in-situ pile contain many parameters that need to be evaluated using trigonometric functions or graphs and tables.IS code method uses the concept of critical depth for cohesive and cohesionless soil to find angle of internal friction. Recently using the results of penetrometer tests like Standard Penetration test to estimate the bearing capacity of piles had been the subject of considerable number of researchers and several approaches have been proposed [2,8]. Pile capacity by SPT is one the easiest and earliest applications is used.
To conduct extensive study on the methods to determine ultimate pile capacity in cohesive and non-cohesive soil.
To collect required bore log details and pile load test details.
To conduct a comparative study of all the static analysis methods selected for determining pile capacity by comparing it with the pile load test values.
To suggest best suitable existing method for prediction of ultimate pile capacity for cohesive and non-cohesive soil.
To derive new equations to find pile capacity by combining parameters considered in various methods.
To suggest a structural design of pile.
The main goal of this project is to formulate new equation to find the bearing capacity of the soil in Kerala region and also propose a structural design for the pile. Also, critical evaluation of existing equation will be done to know which is the best existing method to find the bearing capacity of soil.
A. Data Collection
Pile load test data and corresponding soil investigation report of three bored cast in-situ piles are collected. The sites in these areas are covered by fine sand, laterite, gravel, silty sand and clayey sand. The soil report shows that the sites contained weak bearing strata at shallow depth, leading to the construction of pile foundation. The diameter of piles varies from 0.5 m to 1 m and embedment length varies from 9 m to 12 m. Out of three sites, two sites have cohesive soil and other area is covered by cohesionless soil. The summary of pile data is given in Table 1.
The bearing capacity determination of pile is always a complex problem faced by engineers and researchers. Among all the methods for capacity calculation, SPT method is found to be flexible in terms of estimation, cost and time. In some cases, unpredictable values are obtained. The geological changes that may occur in soil and the surrounding resources with period of time is one the reasons causing failure of pile.
In empirical method it is found that Bazaraa and Kurkur method shows more comparable value with the test load. Value of cohesion is influencing the bearing capacity of pile very much. And for graphical method Chin-Kondner’s method shows more comparable value with the test load. The proposed method shows very close values to the test load. And also, the error percentage is coming under 5% which shows that the proposed method has better precision performance compared to other chosen methods. Due to its precision, we can consider it as a better method. Also, the structural design of pile at different location also designed in this project.
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