Yongkyu Choi

Comparison of the bidirectional load test with the top-down load test

For the past decade, the Osterberg testing method (O-cell test) has been proved advantageous over the conventional pile load testing method in many aspects. However, because the O-cell test uses a loading mechanism entirely different from that of the conventional pile loading testing method, many investigators and practicing engineers have been concerned that the O-cell test would give inaccurate results, especially about the pile head settlement behavior. Therefore, a bidirectional load test using the Osterberg method and the conventional top-down load test were executed on 1.5-m diameter cast-in-place concrete piles at the same time and site. Strain gauges were placed on the piles. The two tests gave similar load transfer curves at various depth of piles. However, the top-down equivalent curve constructed from the bidirectional load test results predicted the pile head settlement under the pile design load to be approximately one half of that predicted by the conventional top-down load test. To improve the prediction accuracy of the top-down equivalent curve, a simple method that accounts for the pile compression was proposed. It was also shown that the strain gauge measurement data from the bidirectional load test could reproduce almost the same top-down curve.

Implementation of Noise-Free and Vibration-Free PHC Screw Piles on the Basis of Full-Scale Tests

AbstractNoise and vibration caused by pile driving may disturb surrounding areas to the extent that nearby residents and businesses may file civil complaints, which can bring construction to a halt. To overcome these issues, construction engineers have worked diligently to develop low-noise and low-vibration pile driving methods. One such method, involving noise-free and vibration-free pretensioned spun high-strength concrete (PHC) screw piles, is proposed in this paper. This pile driving method penetrates the ground by rotating and compressing the pile to minimize noise and vibration in addition to maximizing capacity. A noise-free and vibration-free PHC screw pile prototype, the first of its kind, was manufactured and subjected to two pilot tests to determine its production and construction requirements in addition to its performance. In pilot tests, the new noise-free and vibration-free PHC screw pile driving method was found to successfully reduce noise and vibration compared with pile driving involvi...

Determination of Loading Capacities for Bi-directional Pile Load Tests Based on Actual Load Test Results

A bi-directional pile load test (PLT) is regarded as the most reliable method for verifying the design capacity of a large-diameter drilled shaft. The loading capacity is often improperly set while conducting this test, leading to inadequate verification of appropriate design capacities for large-diameter drilled shafts. This problem necessitates a new, rational method for estimating the loading capacity for bi-directional PLTs. In this study, results of numerous bi-directional PLTs conducted by different researchers were analyzed for their failure patterns, load increasing ratios, loading capacity increasing ratios, and sufficiency ratios of the design load. The results indicate that most failure patterns involved a lack of loading capacity. In our assessment, the load increasing and loading capacity increasing ratios were less than 2, confirming that the maximum equivalent test load is not always twice the total bi-directional load. Hence, it is difficult to verify the design capacity using the current planned loading capacity. An analysis of the sufficiency ratios of the design load revealed that 18.6 % of the test results did not satisfy the requirement. To eliminate the uncertainties in verifying the design load, the one-directional loading capacity should be at least 2.5 times the design load.

Structural Resistance Factors for Shear Loading in Drilled Shafts with Minor Flaws Based on Experimental Study

An experimental study was performed to assess the reduction in shear capacity of model drilled shafts having minor flaws, in the form of voids or soil inclusions occupying 15 per cent of the cross-sectional area of the shaft, mispositioned reinforcing steel and corroded reinforcing steel. Such flaws are often produced in construction but either go undetected by NDE or appear as uncertain anomalies on NDE records. Resources did not permit performing large numbers of tests sufficient to investigate the effect of the probability distributions of these factors; however, selected tests were sufficient to develop a general understanding of the phenomena involved and to suggest provisional values of resistance factors to account for the effect on shearing resistance of unknown construction flaws.

Development and Implementation of a High-Pressure, Double-Acting, Bi-Directional Loading Cell for Drilled Shafts

Drilled shaft foundation elements provide a cost-effective foundation alternative for the support of building and bridge superstructure loads. Bi-directional pile loading tests (BDPLTs) to evaluate the capacity of drilled shafts have become popular owing to their capacity to save time and effort as compared to the use of top-down loading tests. However, the use of BDPLTs requires that production shafts be post-grouted following testing in order to assure appropriate in-service performance. Commonly used single-acting loading cells and/or loading cell construction details can pose the potential for the development of voids following post-grouting due to their monotonic jacking action and large footprint. This paper described the development and use of high pressure bi-directional loading cells intended to minimize the possibility of post-test construction defects. First, a comparison was made between the single-acting and double-acting loading cells. Second, the results of laboratory calibrations on the pressurized loading cells were performed, as were component testing of the pumps, hoses, and hydraulic fluid synchronization lines. Then, the use of the new high pressure double-acting loading cells in production testing of instrumented shafts was described, and the efficacy of the new loading cells was illustrated. The new loading cells provided the profession with a load cell alternative for conducting BDLTs and should serve to help reduce the risk of post-test grouting defects in drilled shaft foundations.

Characteristics of Pore Pressure and Volume Change during Undrained Loading of Unsaturated Compacted Granite Soil

A series of triaxial compression tests were performed on samples of compacted granite soils in a modified triaxial cell that can separately control pore air pressure and pore water pressure in order to examine the characteristics of pore pressure, volume change and stress-strain behavior under undrained loading condition. Unsaturated granite soil samples were prepared by compaction in a mould. These samples were tested at different suction and different confining stresses. The volume change of an unsaturated soil during shearing undrained is much sensitive to the confining pressure compared to the initial water content, and the matric suction. The volume expands during shearing, and the volumetric strain is much larger at the smaller confining pressure and at the higher matric suction. The variation of the internal frictional angle according to the initial water content and the matric suction is negligible, but the effective cohesion increases according to matric suction.

Field Evaluation of the Vertical Bearing Capacity of a Screw Pretensioned Spun High-Strength Concrete Pile

This study has evaluated the vertical bearing capacity by conducting static load tests for noise-free and vibration-free screw pretensioned spun high-strength concrete (PHC) piles installed using two different methods (end-squirting shoe and pre-boring methods). Vertical bearing capacity differences seem to occur due to the displacement of soils near the external circumference of a pile, depending on the installation method. A method by which to evaluate the bearing capacity of screw concrete piles is suggested by considering the equations that already have been used to calculate the bearing capacity of piles. Based on static load tests and analysis, the pile installed using the end-squirting shoe method was assumed to be a bored pile and it was reasonable to use the equation proposed by the Japanese Geotechnical Society. At the same time, the pile installed using the pre-boring method was deemed a low soil displacement pile and so it was reasonable to apply the equations proposed for calculating the bearing capacity of the driven pile suggested by the Architectural Institute of Japan.

A Study on the Load Bearing Characteristics Depending on Pile Construction Methods and Pile Load Test Methods Based on Case Analyses

In our country, in the case of traditional design of pile foundations, only a design depending on end bearing has been performed. However, through the load transfer measurement data that have been carried out for in-situ piles, it was known that skin frictional force was mobilized greatly. In this study, through the analysis of the load transfer test cases of driven steel pipe piles and large-diameter drilled shafts, load bearing aspects of pile foundation depending on pile construction methods and pile load test methods were established. The average sharing ratios of skin frictional force were independent of pile types, pile load test methods, relative pile lengths, pile diameters and soil types. Because the average sharing ratios were over 50%, the case pile foundations mostly behaved as a friction pile and the extremely partial case pile foundation behaved as a combined load bearing pile.

3-D NUMERICAL ANALYSIS OF A REPAIRED ROCK-SOCKETED BATTER PIPE PILE

Large diameter (D - 2,500 mm) batter steel pipe piles socketed into soft rock were constructed for the off-shore foundations of Kwangan grand bridge in Pusan, Korea. Concrete integrity tests so called cross-hole sonic logging tests were performed for mos of off-shore piles. Some of the rock-socketed piles were constructed during the initial stage of the project showed minor concrete defects, which were repaired with high pressure cement grouting. One pile had a severe defect near pile tip, which was repaired after three stages of grouting and non-destructive testing to check the repair. But three-dimensional numerical modeling and analysis was performed to check the quality of defect repairing and provide the reinforcement plan if the pile capacity was below the design load. As result of numerical analysis, the pile behaved primarily as a friction pile and most of applied external load was supported by friction resistance above the repaired rock-socket. Estimated bearing capacity (100 MN) of the pile was well larger than design load (24MM).