Reply to the discussion by Zhang et al. on “Field study of residual forces developed in pre-stressed high-strength concrete (PHC) pipe piles”

Abstract

We thank the Discussers for their interest in our paper (Kou et al. 2016). We agree that the jacking of adjacent piles could cause a tensile effect when the center-to-center distance between two adjacent piles is small. However, as this is not the case for our study as pointed out by the Discussers (Zhang et al. 2017), this question is irrelevant. As explained in the paper, residual forces are usually caused by pile compression, rebound of the soil at the pile base, and the soil strength recovery after installation. If the distance between adjacent piles is small, the tensile effect caused by the installation of adjacent piles will push the pile upward and reduce the residual forces in the pile.We were well aware of the temperature effect on the wavelength of fiber Brag grating (FBG) strain sensors during monitoring. It was for this reason that the field tests presented in the paper were conducted in June and July where the temperature variation was small during the day.In the paper, residual force refers to the compressive force caused by the elastic compression of the pile. This elastic compression is mainly caused by jacking force, shaft resistance, and base resistance. The jacking force plays a key role during the process. After installation, the pile recovers from the elastic deformation. There are two types of forces preventing the pile recovering from the elastic deformation after installation. One is the shear force between the soil and pile, which is enhanced due to the dissipation of excess pore-water pressures. The other is the additional downward force caused by the settlement of the ground, which is also caused by the dissipation of pore-water pressures. Note that the additional downward force can prevent the pile from recovering from the elastic deformation. If the additional downward force causes the residual forces to increase, it means that the deformation caused by the additional downward force is larger than the initial elastic deformation. However, this is almost impossible because the force that caused the initial elastic deformation is larger than the additional downward force. Therefore, for a pile after installation, residual forces would decrease until the recovering force, shear force, and additional downward force reach an equilibrium state.The measured base resistance presented in the paper included both annulus resistance of the pile base and soil plug resistance. Figure 4 in the paper shows the arrangement of strain sensors along the test pile. Note that the inner diameter of the test pile was only 250 mm and thus it was difficult to install strain gauges inside the pile. To measure the annulus resistance of the pile base and the soil plug resistance separately, double-walled pile systems have been used for open-ended steel pipe piles in calibration chambers (e.g., Lehane and Gavin 2001;Paik et al. 2003) and in the field (e.g., Ko and Jeong 2015). However, the double-walled pile method is not applicable to concrete piles as the wall thickness is significantly larger than steel pipe ...