Study On Hole-forming Quality Detection Of Bored Piles Based On DTS

Bored piles have been widely used in engineering constructions.However,bored piles are concrete structures concealed in the ground,and various quality problems such as necking,expansion,mud trapping,concrete segregation and slagging at the bottom of the pile may occur,which threaten the stability of upper buildings and the safety of people’s lives.Therefore,it is of great importance to detect the quality of pile formation and ensure the quality of piles.In this paper,the current status of research on traditional pile-forming quality inspection technologies and distributed fiber optic sensing(DFOS)technologies for foundation pile inspection was summarized.Based on the measurement principle of distribured temperature sensing(DTS)technology and temperature sensing fiber optic(FO)cables developed by the group,the principle of pile-forming quality detection of bored piles based on the DTS was proposed;common temperature sensing FO cables and their installation methods,type of interrogators and their application conditions were introduced.The principle of calculating the concrete casting thickness of piles based on hydration heat of concrete was clarified;the differences in the temperature rise of cement hydration of different concrete casting thicknesses and ambient temperature are analysed by conducting model tests;combined with engineering example,back analysis of pile radius was conducted using the finite element(FE)software COMSOL,and the numerical simulation results were compared with the actual hole measurement results of mechanical pore-forming quality detector.The main results of this paper are as follows:(1)Structural characteristics and installation methods of temperature sensing FO cables applicable in the quality detection of grouted pile were introduced;the steps of hole-forming quality detection of piles were given;the calculation method of concrete pouring thickness based on the temperature rise of cement hydration heat with FEM was proposed.(2)The feasibility test of concrete casting defect detection under constant ambient temperature was carried out.The process of concrete hydration heat temperature change measured by DTS is consistent with the theoretical cement hydration exothermic process,which verifies the feasibility of using DTS to monitor the concrete hydration exothermic process.The hydration temperature rise value,rise rate and the slope value of the fitted straight line of the temperature rise-time curve increase with the increase of casting thickness,which verified the feasibility of using the hydration temperature rise of concrete to judge casting defects.(3)An outdoor concret casting thickness simulation experiment was carried out to investigate the effect of surface ambient temperature on the hydration heat temperature rise of concrete.The hydration temperature rise in the middle of the concrete is higher than that at the top and bottom,the hydration temperature rise value,rise rate increases with the increase of casting thickness;the greater the depth of concrete burial,the smaller the slope of the fitted straight line of the temperature rise-time curve is.(4)Combined with the piling quality inspection of a bored pile in Suzhou Industrial Park Headquarters Base,DTS was used to obtain the pile body temperature change after pouring concrete and the pile body radius of the test pile was obtained by FEM back analysis.The pile diameter results of FEM back analysis shows great agreement with the results of mechanical pore-forming quality detector,which verified the reliability of DTS-based hole-forming quality inspection.Compared to clay layers,the wall of the hole near the intersection of sand or sand and clay is prone to slumping and falling blocks,which can lead to pile flare-ups.