Experimental And Theoretical Studies On The Construction Effects For Jacked Open-ended Concrete Pipe Piles

The construction effects of the jacked open-ended concrete pipe pile consist of plugging effect, compaction effect, time effect on bearing capacity, and residual load. All of these effects interact with each other, controlling the characteristics on pile bearing capacity. A comprehensive research program, involving in-situ and laboratory test, numerical solution and statistical analysis, was conducted to reveal the influences of construction effects on mechanical characteristics for open-ended concrete pipe pile. This study includes the topics as follows:Plugging effect means the influences of the soil column squeezed into pile on the. A series of in-situ and laboratory tests reveal the physical, mechanical characteristics and the layer sequence of soil plug, as well as the changes on plug length during pile penetration in different soils. A linear relationship between the plug resistance and the plug incremental fill ratio is established. The test results indicate that the engineering properties of soil plug in the range of 4-5 pile diameters above the pile tip is better than the corresponding in-situ deposit. The CPT cone tip resistance for the soil plug near the pile tip is 67% larger than that for the in-situ deposit. Considering the plugging effect, a pile-in-pile transfer model is proposed. The research indicates that the mobilized plug friction is concentrated within the range of 2 diameters above the pile tip. The plug friction on the pile tip is 3.5 times as large as the pile shaft resistance at same level. A CPT-based design method for jacked open-ended concrete pipe pile, named ZJU method, is proposed in this study.Compaction effect means the influences of displacement behavior on pile-soil system. Full-scale tests in silty deposits reveal the developments of the total radial stress, pore pressure, movements in surrounding soils during and after pile installation. The test results indicate that the affecting range of single pile jacking on surrounding soil is approximately 15 pile diameters from pile wall. Based on test results, a computation model is proposed for researching the compaction effects of open-ended pipe piles. This model simulates the pile penetration as the expansions of a series of spherical cavities in semi-infinite space. Moreover, a in-situ research indicate that the compaction effects of pile group reduce the single pile capacity by 35%-75% in clayey soil, and several suggestions that could reduce the influence of compaction effects are proposed in this part. A series of tests using an invented constant normal stiffness direct shear system indicate an exponential decay trend of unit shaft resistance with increasing shear cycles. Time effect is defined as an increase in pile capacity over time after installation. A three-phase conceptual model is suggested in this paper. In addition, a computation model is proposed based on consolidation theory for evaluating the soil set-up on pile capacity. The research indicates that the relative setup rate for friction-type open-ended pile under full coring condition is 10% larger than that for corresponding closed-ended pile. A series of researches using re-pressed tests and static load tests reveal an approximately logarithmic set-up on bearing capacity with time after installation. The average increase rate for total capacity is in the range of 15%-29% per log cycle. In addition, an optimum design method of jacked pile based on re-pressed test is proposed taking advantage of the setup effect.Residual forces are compressive forces in a pile caused during pile penetration due to elastic compression of the pile material. A series of full-scale tests using optical fiber sensors was conducted on jacked open-ended concrete pipe pile to reflect the post-installation residual loads. Based on the test results, an expression of residual stress for jacked pile is resolved according to energy balance formulation. Then the influences of characters for pile and soil as well as installation methods on post-installation residual stress are researched. The researches indicate that the toe resistance and shaft resistance under neutral point would be under-interpreted and the shaft resistance above neutral point would be over-interpreted in pile load tests if the residual force was ignored.All the above mentioned studies on construction effects reveal the significant influences of plugging effect, compaction effect, time effect on bearing capacity and residual load on the characteristics on pile bearing capacity. A database including about 2000 static load tests for open-ended concrete piles is established to reveal the relationship between the ultimate capacity for single pile and terminative. The statistical analysis reveals that the ultimate capacity is always less than the terminative jacking force for short pile. With the increase of pile length, the ultimate capacity will exceed the terminative jacking force for some extent.The ratio between the ultimate capacity and terminative jacking force increases hyperbolically with the increase in the ratio between pile length and diameter, and a largerst ratio is corresponding to the bearing stratum of cohesive soil and surrounding soil of silty soil.