Research on visual model test of displacement distribute characteristics and load transfer model of root pile during uplifting

In order to explore the uplift bearing characteristics of the roots pile,the non-intrusive test was used to meas- ure the roots pile,the belled pile and the equal section pile which had same size with the deformation visualization sys- tem consisting of PIV(particle image velocimetry) technology and transparent soil. The test measured the Q-S curves of each pile and collected the images of change in the displacement field around the pile in real time. Based on the above images,the area of pixels with different color scales in the contour map of displacement field was calculated and compared to make the precise comparison of displacement field and plastic region of soil come true. At last,the test in- ferred the calculation model of soil side resistance of small-scale transparent soil model test pile with the support of the existing triaxial test results of transparent soil. The result shows that under the test condition,although the bearing ca- pacity of the belled pile with the same size to the roots pile is higher,the roots pile with multi-layer shows a better con- trol ability to the foundation displacement under the middle and low load,and it is more suitable for the building foun- dation with strict requirements on the displacement. There are optimization problems in the setting of root key. In order to ensure the full play of the bearing capacity of soil and root key,the space between two layers of roots should install at 4 ~ 6L(L is the length of roots). When the number of layers of roots is small,the effect of roots on load diffusion is not obvious,and the range of soil displacement field is even slightly lower than that of equal section pile. With the in- crease of the number of layers of roots,the range of soil displacement field around pile is also significantly expanded. Under the test conditions,the area of soil displacement field driven by the roots pile with 4 layers of roots is about 113% of that belled pile,while the area of plastic area is only 58% of belled pile. The improvement potential of the bearing capacity of the root key pile is better than that of the belled pile. Under the ultimate load,the shape of all dis- placement field of soil around pile is “inverted triangle”,and the angle between the displacement field and the hori- zontal plane is (45 °+φ / 2). The calculation model of pile-soil side resistance can reflect the law of foundation load transfer in certain extent supported by the existing research results of transparent soil. According to the calculation and analysis,the setting of roots and expansion head increase the ultimate bearing capacity of soil by 1. 27 and 1. 18 times respectively,and the setting of variable section makes the bearing capacity of soil to play a greater role. The depth effect on the normal resistance of roots is far less than that on the side frictional resistance.