Stress Paths on Displacement Piles During Monotonic and Cyclic Penetration

Abstract

In this contribution, a study on the behavior of instrumented model piles in slow, cyclic penetration tests using a cylindrical full model test set-up is presented. The tests are performed under 1g-conditions in a uniform medium sand. A hydraulic driving system enables a displacement controlled penetration similar to the pile motion during vibro-driving at strongly reduced frequency. The pile instrumentation allows the measurement of shaft and tip force during the driving process. Systematic variation of soil density and displacement amplitude reveals the occurrence of typical stress paths of vibratory pile penetration. By comparison with results from monotonic and vibratory penetration tests, the influence of the penetration mode is deduced. Results from FE simulations applying a hypoplastic soil model help to illustrate the strong requirements and the considerable challenges to obtain realistic simulations of cyclic pile penetration processes. Some hints towards a further numerical modeling of the tests are given.

A Details of the FE Model

Details concerning the setup of the numerical model can also be found in [19]. Axisymmetric CAX4 elements are used. The horizontal length of the elements near the symmetry axis is greater than the height in order to reduce mesh distortion problems. An impression of the FE mesh near the pile tip can be received in Fig. 14. In order to ensure a better numerical stability, the first 0.05 m of soil are replaced by a uniformly distributed pressure equivalent to the soil weight. The initial conditions are assumed to be geostatic with $K_\mathrm{0}=0.37$. The initial void ratio is chosen according to the corresponding experiment (Table 1). The pile penetration begins in 0.1 m depth (position of the pile shoulder with respect to the sand surface). After an initial phase of 30 mm monotonic displacement the cyclic pile motion is prescribed. The increment size corresponds to a pile displacement of about 0.06 mm.

The material parameters used for the simulations are given in Table 2. Note that these differ from the parameters used in [2]. The calibration procedure is described by Vogelsang [19]. The slight differences of the current test sand compared to older charges (see Sect. 2.3) have not been considered during the calibration.

The Coulomb friction model is used to model the interaction between soil and pile resp. soil and side walls. A friction angle of 12$^\circ$ is chosen for the pile-soil and 22$^\circ$ ($\approx 2/3\varphi _\mathrm{c}$) for the soil-wall interface.

Table 2. (a) Applied constitutive parameters of the test sand and (b) additional constitutive parameters of the extended hypoplastic model with intergranular strain