Analysis of Laterally Loaded Long or Intermediate Drilled Shafts of Small or Large Diameter in Layered Soil
Strain wedge (SW) model formulation has been used for evaluating the response of a single pile or a group of piles in layered soils to lateral loading. The SW model approach provides appropriate prediction for the behavior of an isolated pile and pile group under lateral static loading in layered soil (sand and/or clay). In this study, the authors extend the capability of the SW model to predict the response of laterally loaded large diameter shafts considering 1) the influence of shaft type on the lateral shaft response; 2) the nonlinear behavior of shaft material and its effect on the soilshaft-interaction; 3) developing partial or complete liquefaction in the surrounding soil profile based on far and near-field induced porewater pressure; and 4) vertical side shear resistance along the shaft wall that has a significant contribution to the lateral shaft response.
LIQUEFACTION AND RESIDUAL STRENGTH OF SANDS FROM DRAINED TRIAXIAL TESTS
The state of knowledge with regard to static and cyclic liquefaction has progressed tremendously since the mid-1960s. In fact, it is generally accepted, based on the steady-state concepts of Casagrande, Castro, and Poulos that the end or steady-state condition of a liquefied loose sand is the same whether due to monotonic, cyclic, or dynamic undrained loading. Even so, a straightforward and fundamental explanation of the initiation and end-state liquefaction behavior is still needed. Toward that end, an effective stress approach that allows the assessment of the whole undrained stress-strain curve (including the peak and residual strength values) and the undrained effective stress path of a monotonic test from drained triaxial tests is presented. Results from three series of tests are shown to demonstrate the validity of the method. This method makes it possible for a geotechnical firm with the capability of performing the traditional drained test with volume change measurements to participate in static liquefaction and residual strength assessment. More importantly, it provides the conditions and logic for the development of complete versus limited liquefaction (and even dilative) response.
LOADING SYSTEMS, SAMPLE PREPARATION, AND LIQUEFACTION
The liquefaction behavior of a clean, fine, uniform sand designated F-70 Ottawa banding sand is examined. Specifically, an investigation into the effects of loading systems and sample preparation methods on the stress-strain behavior is made. A comparison between the results obtained with a dead-load apparatus and a hydraulic loading system on samples prepared by dry pluviation, moist tamping, and moist vibration indicates differences in the soil’s response during undrained shear. The former loading system generates and maintains over a greater strain range lower residual deviator stresses and peak pore-water pressures during steady-state flow. Furthermore, the steady-state velocities of the samples tested in the dead-load apparatus are two–three times higher than the velocities of those loaded hydraulically. These differences can be attributed to the dead-load system’s ability to provide a constant driving force fast enough to achieve a sample’s maximum rate of strain once liquefaction commences. The relative position of the steady-state curves is unaffected by the type of loading system. However, the method of sample preparation dictates their position. Samples prepared by dry pluviation exhibit residual effective minor principal stresses that are approximately one-quarter to one-half the value of samples prepared by either moist method.