Cristina de Hollanda Cavalcanti Tsuha

Serviceability Performance Evaluation of Helical Piles under Uplift Loading

AbstractThe use of helical pile foundations has widely expanded in recent years. However, the prediction of the load-displacement performance of helical piles under uplift loading for service-level displacements is still a challenge for designers. In this work, a probabilistic power law model is used to characterize the random nonlinear uplift load-displacement relationship of deep helical piles. Model parameters are found from a database of 30 uplift loading tests conducted on helical piles in Brazil. Using structural reliability concepts, a practical example of a transmission tower foundation is provided to illustrate the use of the developed probabilistic model for serviceability limit state (SLS) evaluation of helical piles. The investigation takes into account the uncertainties of three parameters: one parameter of the load-displacement curve, the empirical correlation between pile capacity and installation torque, and the wind loading on foundations of transmission lines. For the example evaluated, ...

The occurrence of residual stresses in helical piles

Residual stresses generated during the pile installation are usually observed and considered in the interpretation of conventional pile load tests. However, in the case of helical piles, residual stresses can be generated by previous loadings. At the end of the unloading stage in tension tests, the shaft resistance is reversed, and the helices are maintained forced against the soil. The occurrence of residual stresses measured at the lower portion of the shaft, close to the helices, was observed in four tension tests performed on instrumented helical piles in Brazil. This paper discusses the outcomes of these tests on helical piles installed in two different sites of tropical residual soil. The results illustrate that the amount of residual stresses is greater for piles with less contribution of shaft resistance in the pile uplift capacity.

Inlet and Outlet Pipe Heat Interaction in a Contiguous Flight Auger (CFA) Pile

The use of energy loop(s), fitted into the structural foundation elements dualizes the role of the pile in meeting the structural performance and the thermal comfort demand of the overlying structure. Heat carrier fluid (HCF) is circulated through the loops, to extract or reject heat energy into the ground, during the space heating or cooling operation. However, this results in thermal interaction between the inlet and outlet leg of the loop especially in contiguous flight auger (CFA) piles where the loops are bunched together to a central steel bar. This paper presents a numerical study to investigate the heat flow characteristics between the inlet and outlet loops installed in a CFA pile. It was found that the central steel bar, used in a CFA pile, contributes towards higher thermal interaction. Similarly, it was found that the use of plastic bar of adequate strength, to substitute the use of steel bar, has both economic advantage and positive significance on the performance of the CFA pile.

Parametric analysis for the estimation of the installation power for large helical piles in dry cohesionless soils

Abstract A parametric analysis was conducted to estimate the installation power for two helical piles with helix diameters of 0.6 and 0.45 m and a shaft of 0.3 m in dry cohesionless soils. After having compared three models with the installation power of eight helical piles in laboratory, 520 simulations have been performed with different friction angle values, unit weight of soils, different rounds per minute and constant helix thickness. The results show that the power increases with increasing unit weight of soil, friction angle and increasing RPM. Besides, by increasing the helix-to-shaft ratio from 1.5 to 2 the power increases of about 40%. Because the geometrical characteristics of the pile play alsoa role, by changing the helix-to-shaft ratio until 5, it was shown that if the helix diameter is kept constant, the installation power values decrease, whereas if the shaft diameter is kept constant, the installation power increases.

Determination of SPT End Bearing and Side Friction Resistances Using Static Uplift Tests

This note described a simplified procedure and an interpretation method introduced to the standardized standard penetration test (SPT) routine to obtain new parameters for the design of foundations. For the application of this new methodology, the following two additional measurements were necessary: the energy delivered to the rod during hammer impact and the sampler shaft resistance determined from static uplift. In this investigation, nine uplift tests were conducted on the SPT sampler at three different depths for the determination of the SPT side friction resistance. The SPT end bearing resistance was calculated from the difference between the shaft resistance and the static resistance of the SPT sampler during driving, which was estimated using Hamiltons Principle. The preliminary results of this research are promising and show that the SPT could provide measurements of end bearing and side friction resistances along the soil profile investigated. From these parameters, an SPT normalized friction ratio was determined for the classification of soil in a way that is similar to the normally used method based on CPT data. In addition, the results indicated that the direct correlation between SPT and CPT end bearing resistances appears to be more reliable than the commonly used correlation between CPT end bearing and SPT N-value.