Experiment Study On The Negative Skin Friction On Piles In The Field Of Loess Collapsible Under Overburden Pressure Of Guanzhong Area

With the rapid development of engineering construction in collapsible loess region recently, especially widespread application with the big diameter piles in this region, many theories now, especially the one about negative skin friction (NSF) on piles, are not perfect enough to fit the requirements of engineering practice. So the field tests for piles are quite significant to perfect the design theories and satisfy the application for engineering practices.The dissertation, based on the results from the field immersion test at large test pit for piles in collapsible loess region and the data tested by the sliding micrometers, discusses the changes of the vertical bearing capacity of single pile in two states, original state and waterlogged state. It also researches the facts that NSF is changed in the process when the test pit is watered and the results that the vertical bearing capacity influenced by water. The field results indicated as follows:1. According to indoor test results to appraise the engineering properties of the loess has great error. The field immersion test at large test pit could measure the amount of collapse deformation and the outer extreme depth of the loess collapsed under overburden pressure and so on precisely. Then the engineering properties of the loess could be appraised exactly, and the NSF researches which depend on the field results will be more reliable.2. The ultimate bearing capacity of the piles has distinct difference because of the immersion. The ultimate bearing capacity of single pile which is not submerse is more lager than the one immerged. The pile which load the work load first then water it ,after that load it to failure compare with the pile which load to failure directly after watering, they have the very same ultimate bearing capacity. But the extra settlement of the pile begotten by immersion is 6.3 times than the one which is caused by working load. The pile design in collapsible loess region must consider the deformation more important than the bearing capacity.3. Stress test indicates that friction is unimodal distribution in natural case, the peak value position moves downwards with the load increases. In watering case, the NSF generated by the collapsible loess around piles has a peak value and the peak value is not catch the peak at same time along the pile length. The distributing and evolution of the NSF are controlled by the length of pile, thickness of the collapsible loess layer and the collapsible course of the loess.4. Once the immersion occurs, there will be NSF at once. The evolution of the NSF is coherent with the collapsible course. The NSF has remarkable time effect and its peak value appears twice in immersion process. The test educes a formula which is used to calculate the NSF changes with time:5. The existence of the NSF is foreign to the collapsible style and NSF is sensitive to the relative displacement between pile and soil. Moreover, it can not ignore the NSF engendered by pimping displacement. Neutral point which has a process: increase fast early days first, then keep stabilization and ascend along with the continuous load finally. Moreover, the depth of the neutral point is greater than the measured outer extreme depth of the loess collapsed under overburden pressure.