Experimental Research On Engineering Problems Caused By Construction Of Pressure Pumped CFG Pile By Continuous Auger Drilling

CFG pile is the abbreviation of Cement Fly-ash Gravel pile. Because CFG pile has some characteristics of that engineering build cost is low, that the construction speed is quick and that the quality is easily controlled, CFG pile has aroused extensive concern of each section, moreover, it has brought about notably economic and social benefit. Pressure pumped CFG pile by continuous auger drilling is a new technology developed in recent years, which is widely applied in nationwide.But there appear to be various problems in the actual process of engineering construction, along with the development of compound foundation technology of pressure pumped CFG pile by continuous auger drilling in recent years. Actually, CFG pile oneself comes to the problems of flee-bore, clog-pipe, reducing-path and so on. When this technology is applied in the stratum of saturated clay,powder and thin sand. in Zhengzhou and peripheral regions, there comes forth a series of engineering problems of ground settlement,soil around pile and building dehiscence,underground pipeline break etc., which brings much inconvenience to residents surroundings in living and working, and also results in huge of economic loss of construction unit and production unit, so it seriously prevents the technology of CFG pile compound foundation form further developing in Zhengzhou and peripheral regions. One building site of a certain housing estate in Zhengzhou is originally designed to CFG pile. However, later the developer heard about that the CFG construction will result in ground settlement and was afraid of that the settlement would influence the surrounding building built, eventually he don't hesitate to spend more one million Yuan on changing the pile form, changing CFG pile to high pressure jet grouting pile. So government has to intervene to promote the application of this technology. The Zhengdong New District management committee once issued that the using of CFG pile can not be recommended, especially there are buildings and important underground pipe lines around construction area, if CFG pile is adopted special approval should be authorized. So the mechanism study of engineering environmental problems caused by long spiral CFG pile construction and treatment is imminent.At present, there mainly exist the following problems in mechanism research process of engineering problems caused by pressure pumped CFG pile by continuous auger drilling: (1) Pore water pressure changing rule is unclear during construction;(2)the scope of construction influence is unclear(3) influence of dynamic characteristics of saturated silt for engineering problems caused by pressure pumped CFG pile by continuous auger drilling is not mastered. Some experts and scholars carry on some research, and put forward their views, but lack of theoretical and experimental support.Based on Henan Province Key Programs for Science and Technology Development "the research about adaptability of pressure pumped CFG pile by continuous auger drilling in saturated silt", in instituting detailed experiment scheme, we carry on a thorough research about environmental geotechnical engineering problems caused by pressure pumped CFG pile by continuous auger drilling, its goal is to obtain mechanism and influence factors of the engineering problems, and provides methods to cope with them. So the research has important practical significance to enhance the construction level of pressure pumped CFG pile by continuous auger drilling and promote long CFG pile technology application in Zhengdong New District,and it is use for reference that pressure pumped CFG pile by continuous auger drilling compound foundation is applied to similar area, meanwhile, it has theoretical significance to study dynamic characteristics of saturated silt.The contents of textual research work as follows:1.Fetching saturated clay in Zhengzhou and peripheral regions, and carrying on dynamic tri-axial test, to understand dynamic characteristics of saturate silt.2.Carrying on pore water pressure experiment on the spot. We bury pore water pressure instrument in the side of pile, to test the changing rule of pore water pressure in the construction of CFG pile of long twist auger pump pressure in pipe, and then analyze change circs of soil around the pile.Main conclusions as following:1.For saturated silt, on cyclic stress, initially, pore water pressure steep rise, later, increase rate slower, finally reached a steady. No matter vibrating how much times , when saturated silt reached destruction standard, pore water pressure do not develop to confining pressure,this is to say that it did not reach criteria for liquefaction.2.On different confining pressure, failure load increases followed by confining pressure increasing. Saturate silt is not at some certain stress. In the construction of CFG pile, if we can minimize the force to soil around the pile, and reduce to the lower point of a certain and particular value, that too big strain and displacement will not once take place, and saturate silt around pile develop to vibration density. Therefore, reducing cyclic stress of helical vane to soil around pile, then to reduce disturbance to soil around pile.3.In the process of drilling, when drill in the place of the pore water pressure instrument buried nearby, pore water pressure gradually increases. Pore water pressure gradually minimizes when drill below the place of the pore water pressure instrument buried, In the process of lifting the drill and pumping concrete, when drill drill into hole bottom, at the transient of lifting drill -pumping concrete, pore water pressure has a sharp decrease; with the drill pipe lifting, pore water pressure gradually ascends; when drill in the of place the pore water pressure instrument buried nearby, pore water pressure basically resume to level before lifting the drill.4.Whether in the drilling or in the lifting aiguille, as long as empty place forms between bore wall and drill pipe, pumping will take place. The forming mode of pumping during excrete soil in drilling and lifting aiguille is different .Pumping is main reason for reducing pore water pressure, and pumping bears a close relationship with fleeing bore. The result of pumping is that to cause negative pressure at pile hole and around pile, on the function of pressure difference ,make the week soil and water continuously flowing into bore.5. We can educe the influence scope of horizontal direction of CFG pile construction in different stratum by pore water pressure experiment. Thereinto, silt is farthest,powder sand take second place,thin sand is the last one.6. It can be concluded by three process about mechanism of engineering problems caused by CFG pile construction. (1) the soil around pile is disturbed by twist lamina soil becomes current-formed.(2)on pumping , the week soil and water continuously flowing into bore.(3) flowing soil layermake can not load weight of overlay soil , then cause ground to engender different settlement , and make ground appear crack.It is can be concluded that engineering problems caused by the construction of pressure pumped CFG pile by continuous auger drilling could be caused by two reasons:(1)it exists saturated clay,powder and thin sand,and saturated soft clay etc during the construction of CFG pile, after disturbance they easily become current-formed. this kind of engineering characteristic provides material condition for environmental geotechnical engineering problems. (2)construction technology of CFG pile of long twist auger pump pressure in pipe.7. According to the mechanism of engineering problems during construction and influence factors, put forward that homologous control measure. The measures are to increase pile distance or kipping to beat pile, to select reasonable drilling speed and lifting speed , to select reasonable lifting mode and height of pumping concrete, adapting small diameter spiral twist etc. Moreover, some assistance measures such as establishing water well, building obstruct and so on should be adapted to reduce further environmental geotechnical engineering problems during construction of CFG pile.