An Experimental investigation on shaft resistance cast in situ bored piles in sound rock
Dr. L.I.N. de Silva
Department of Civil Engineering
Faculty of Engineering
UNIVERSITY OF MORATUWA-SRI LANKA
Table of Contents
Problem Statement 1
Significance of the research. 2
Scope of the study. 2
Aims and Objectives of the research. 2
Hypothesis (if any) 3
Review of the literature. 3
Proposed Methodology. 3
Time schedule/work plan. 4
Budget/estimated cost built up. 4
Nowadays in the construction industry the pile foundation socketed into the rock plays a key role in the high rise buildings due to the high bearing capacity and minimum settlement. The bearing capacity of the rock socketed piles contains two basic components, base resistance and shaft resistance or skin friction. Shaft resistance is more significant when calculating the bearing capacity of the pile because shaft resistance is induced earlier than the base resistance for a very small movement. And the base resistance needs the pile base free from debris. It is difficult to achieve this condition in the practical situation. Therefore shaft resistance is considered as an important part of designing the pile foundation. First shaft resistance of the pile should be estimated to evaluate the bearing capacity of the pile foundation. This research focuses on the experimental investigation on shaft resistance of cast in situ bored piles in sound rock.
Many researches have been carried out in this area and from those studies it is found that the shaft resistance of the rock socketed pile is governed by the several parameters like mechanical properties of the surrounding rock, RQD value of the surrounded rock, pile diameter, roughness of the rock – concrete interface, discontinuities of the rock, the radial force induced by the load and the rock type. The shaft resistance should be evaluated correctly for the safety of the structure. The underestimation of the shaft resistance is not economical but it is conservative. Therefore the designers must pay attention to this.
The investigation of the shaft resistance of the pile – rock interface is a very complex problem because many factors should be considered and the lack of knowledge in shaft resistance. The knowledge of shaft resistance is necessary for the designing of the structure safely and economically. Empirical methods are used in the calculation process of shaft resistance. Those empirical methods may not be reliable in some cases.
From the literature survey it is noticed that several studies have been carried out on the shaft resistance of the cast in situ bored piles socketed in sound rock all over the world. Even though there is no proper investigation into the shaft resistance of rock socketed piles. The main problem of the design engineers is the value of the shaft resistance of the underlying rock. This study is to evaluate the shaft resistance of the rock socketed piles for different types of rock.
Significance of the research
Pile foundation is used in the heavy loaded structures like bridges, high rise building complex. The piles are socketed into the rock to increase bearing capacity. The bearing capacity of the pile foundation can be increased by introducing the rock socketed piles. The applied load is transferred to the bedrock in the rock socketed piles. The bearing capacity of the rock socketed pile foundation depends on the shear strength of the rock- concrete interface. The shaft resistance is mainly governed by the roughness of the concrete- rock interface.
For the safety of the structure the shaft resistance should be estimated correctly. In the normal practice the shaft resistance is calculated using empirical methods. It is not economical. In the Engineering practice all the design and construction works should be done to optimize the cost. Mostly the geotechnical engineers need the shaft resistance between the piles – rock interface in their design process.
The shaft resistance varies with the type of rock type. Most of the methods which are available to calculate the shaft resistance are empirical. So it may not be correct for other types. The significance of this study is to provide more understanding of the shaft resistance. From this study the ultimate shaft resistance of the rock socketed can be evaluated which is helpful in the design process of the pile.
Scope of the study
The scope of this study is to develop a fundamental knowledge of shaft resistance of the rock – concrete interface. This is a full laboratory experimental study. All the tests will be conducted in the laboratory. The shaft resistance is calculated using the rock sample.
Aims and Objectives of the research
The aim of this study is to investigate the shaft resistance of cast in situ bored piles in sound rock through laboratory experiment.
Review of the literature
Many researchers have carried out their researches on the shaft resistance of the bored piles in rock. Most of the researches have been conducted to find the parameters which are affecting the shaft resistance or shear strength of the intermediate surface between concrete and rock. From those studies it is concluded that the factors affecting the shaft resistance are the strength of rock material, rock mass, RQD, the roughness of pile-rock interface, the diameter of the pile, fracturing of the rock etc. The studies have been done according to different theories like cavity expansion theory, fractal theory etc. The shaft resistance correlations are different to the country to country and they have proposed different equation. Many empirical relationships have been proposed to calculate the shaft resistance.
Literature survey would be carried out to identify the findings on this topic up to now. The methods which the researchers were followed, factors affecting the shaft resistance, problems in finding the shaft resistance, the existing methods to estimate the shaft resistance of the rock socketed piles will be studied.
The intact rock samples would be collected from the field and brought to the laboratory. The rock sample will be cut at an angle about 60° using the rock cutter. The concrete will be placed on the top of the sample to get the concrete – rock interface. Then the sample will be tested in the triaxial apparatus. The shear strength of the concrete – rock interface will be calculated.
The above mentioned procedure will be proceeded by changing the rock type. The results obtained from the test will be analyzed.
Time schedule/work plan
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