Axial behavior of drilled shafts in soft rock

Drilled shafts, rock sockets, soft rocks, load-transfer function, settlement analysis, axial resistance, LRFD resistance factors

Asem, Pouyan

关键词: Drilled shafts, rock sockets, soft rocks, load-transfer function, settlement analysis, axial resistance, LRFD resistance factors;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101251/ASEM-DISSERTATION-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Shallow foundations are commonly not suitable to support the load of heavy structures such as tall buildings or bridges because the resulting contact pressures far exceeds the allowable pressure of the near-surface soils leading to bearing capacity failure or excessive settlements. Therefore, deep foundations are normally used to support heavy structures where loads are transferred to the more competent strata. Drilled shaft foundations are among the most commonly used types of deep foundations. Drilled shafts are often socketed into soft rock formations between near-surface residual soils and the unweathered bedrock that is commonly encountered at greater depths. Socketing drilled shaft foundations into soft rocks has increased in the recent years because it leads to safer and more economical designs. Therefore, a better understanding of the axial behavior of drilled shafts in soft rock is necessary.Field evidence is collected for study of the axial resistance and deformational properties of rock sockets in soft rock masses. These include six databases: i) back-calculated side and tip resistances from axial load tests on drilled shafts and back-calculated base resistances from plate load tests, all in soft rocks, ii) a database for shear strength and deformational properties of rock/concrete interfaces that are tested in the laboratory, iii) a database for in situ shear strength and deformational properties of soft rock masses, iv) a database for near-surface measurements of in situ horizontal stresses in soft rock masses, v) a database for the mode of failure for side and tip of drilled shafts in soft rock, and vi) a database of measured in situ values of socket wall roughness height.A predictive model is proposed for the peak shear strength for the side resistance of drilled shafts in soft rock. The back-calculated shear stress-shear displacement (t-z) relationships from drilled shaft load tests are used to develop a framework for prediction of t-z relationships for rock sockets in soft rock masses. The tip resistance database is used to develop design equations for prediction of the yield and fracture initiation pressures and a framework for prediction of the tip pressure-displacement (q-z) behavior of rock sockets in soft rocks. A probabilistic Limit State Design (LSD) framework is adopted. Two limit states are evaluated, namely axial resistance (strength limit state) and settlement (serviceability limit state). The theory of probability is used to calibrate the Load and Resistance Factor Design (LRFD) resistance factors for the proposed models for prediction of peak side resistance and the fracture initiation pressure. The strength limit state may be evaluated using the proposed design equations (peak shear strength and fracture initiation pressure) and the corresponding LRFD resistance factors. The serviceability limit state is assessed using the proposed q-z and t-z relationships in combination with the load-transfer approach and tolerable values of settlement from the structural engineering literature.