Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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| 6.7.6 Sloping Ground Surface Sloping ground is characterized by the angle, θ, which is the deviation of the ground surface from horizontal as indicated in Figure 6-15. Fora ground surface that slopes down in the direction of the applied load, the resistance to the applied load will be decreased and the p-y curves must be modified. Modifications to the p-y curves for sloping ground conditions are generally only applied to the equations for the wedge-type failures near the ground surface, under the assumption that the flow-around failure that occurs at depth is not influenced by the sloping ground condition (Isenhower and Wang 2015). It is noted that the ground slope must be assumed to be uniform. If the slope is not uniform or continuous, an approximation of a uniform slope must be made for analysis. For cases with variable slope angles or heights, it is recommended that multiple uniform slope approximations be made to assess the sensitivity of the results to the approximated input geometry. 92 Figure 6-15: Effects of sloping ground. 6.7.7 Deep Foundations Socketed in Rock Limited p-y curves are available for rock within the available computer software programs. Available p-y curves are based on a limited number of experiments and based on correlations. There have been a number of recent full scale lateral load test research programs performed with the intent of developing additional p-y curves in rock or weak or decomposed rock (examples include Robinson et al. 2005 and Boeckmann et al. 2014). Some of these test programs are applicable to a particular rock type or geology. There are generally two types of rock addressed in available software programs, weak rock and strong rock. P-y curves for these materials are discussed in Appendix A. In general, the shape of the weak rock p-y curve is similar to that for cohesive soils, although different input parameters are used. The p-y curve for strong rock is a bi-linear envelope based on tests in vuggy limestone of south Florida. This curve is recommended for rock with intact strengths greater than 1000 psi (Isenhower and Wang 2015). The tests that this p-y curve was developed from were run to only limited displacements. It was therefore assumed that brittle fracture may occur at higher displacements because the tested rock formation was known to be brittle in shear. As a result, the resistance of rock in the model drops to zero within the area of assumed brittle shear. Although conservative, this is a condition that is unlikely to occur in actual rock formations. This assumed brittle failure can result in misleading results, including the possibility of the strong rock analysis giving a weaker response than a weak rock analysis for the same input parameters (Brown et al. 2010). The weak rock and strong rock models both include inherent assumptions and conditions based on limited experimental results in particular formations. Care must be exercised when using these models for other applications. For example, the strong rock model assumes that cyclic loading results in a loss of resistance, and the assumptions for the development of the weak rock curve appear to be valid only for static loading (Ensoft 2016). Isenhower and Wang (2015) recommends performing lateral load proof tests if the deflection of a rock and foundation using the strong rock model are greater than b, where bis the width of the foundation element. Brown et al. (2010) recommend that the weak rock models be used for cases where shear failure of the rock mass is considered possible. 93 Another consideration regarding the design of rock socketed deep foundations are the shear and moments that occur internal to the foundation element at the top of rock. For strong, high quality rock, the top of rock boundary will result in high shear forces in the foundation element. For weak rock, especially if overlain by relatively stiff or dense soil, this transition will not be as abrupt and therefore the shear forces in the foundation may not be as high. The designer should consider this when evaluating which rock model is appropriate for use. Depending on the rock type and degree of fracturing, it maybe appropriate to evaluate the embedment depth or deflections with one p-y model (strong or weak rock) and evaluate the shear forces for the structural design of the foundation using the other p-y model. The large shear force computed at the top of rock can also be addressed by reducing the design shear force to the average value along a length equivalent to one shaft diameter below the top of rock, as proposed by Brown et al. (2010). It is clear based on the considerations above that the designer must use caution and judgment when analyzing deep foundations socketed in rock. Local practice or research program results, minimum socket depths required by AASHTO, and lateral proof load tests should be considered in the development of rock socketed deep foundation designs. Download 6.03 Mb. Share with your friends:
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