How to deal with the verticality deviation of bored pile with rotary excavator?
Original title How to deal with the verticality deviation mining drill bit of bored pile with rotary excavator [Abstract] Various rotary drilling rigs are widely used in the field of pile foundation construction at home and abroad but in the process of construction of bored piles by rotary drilling rigs the verticality deviation of pile body is often large which can not meet the design requirements Therefore the control of pile verticality is of great significance in this process This paper analyzes the reasons for the verticality deviation of bored piles with rotary excavator and puts forward the corresponding control measures [Key words] Rotary pile driver; Verticality; Bored pile; Control; 1 Project overview 11 Project overview Taking Fochen Road Rapid Reconstruction Tunnel Project as an example the project adopts open excavation construction the depth of foundation pit is more than 3 meters and less than 5 meters and the supporting structure adopts φ 07m@05m cement-soil mixing pile gravity retaining wall support; If the depth of the foundation pit is more than 5 meters and less than 11 meters it shall be supported by φ 10m@12m bored piles and single row of φ 07m@05m cement-soil mixing piles If the depth of the foundation pit is greater than 11 meters it shall be supported by φ 12m@14m bored piles and single row of φ 07m@05m cement-soil mixing piles 12 Engineering hydrogeological conditions According to the lithology revealed by the borehole the rock and soil are divided into the following layers (1) artificial fill layer with an average thickness of 455 m; (2) fluvial facies impact clay layer with an average thickness of 160 m; (3) marine and continental facies sedimentary layer with a average thickness of 657 m; (4) alluvium with a thickness of 540 m; and (5) residual soil layer average thickness 968 m; and (6) bedrock The measured water level is the mixed water level of pore water and bedrock fissure water in the fourth quarter and the buried depth of the water level is 179 ~ 550 m 13 Test pile condition According to the design requirements the bored piles constructed by the rotary pile driver shall be tested and the rotary single-door bottom bucket without air holes shall be used for the test pile It is specified that the verticality deviation of the pile is 1%  The verticality deviation of 3 out of 6 completed test piles is greater than 1% and the failure rate reaches 50%
2 Importance of verticality control The verticality control of piles is of great significance to the subsequent construction of foundation pitIf the verticality deviation of bored piles around the foundation pit is large it bra tape measure will lead to uneven stress of the retaining structure around the foundation pit which will bring great hidden dangers to the safety of the foundation pit At the same time if the verticality deviation of the bored cast-in-place pile is large it will have a greater impact on the construction and use of the main structure in the later period Because of the large verticality deviation of the bored cast-in-place pile around the main structure the stress around the main structure will be uneven which will lead to cracks in the main structure and bring hidden dangers to the subsequent use of the main structures 3 Reasons for deviation of verticality The verticality deviation of the test pile is large Through the analysis of the actual engineering project the following reasons are summarized from the mechanical selection to the final hole formation 1 Selection of drill bit During the drilling process of the rotary pile driver the geological hardness is uneven and the selection of drill bit can not meet the needs of different geological conditions resulting in the deviation of the drill bit and then the vertical deviation of the pile does not meet the requirements of the specification 2 Deviation of casing embedment Expand the full text 3 Displacement of drill pipe occurs during drilling 4 The positioning of the reinforcement cage is deviated due to the improper setting of the cushion block for controlling the reinforcement cage the deviation caused by the lack of centering review after the reinforcement cage is in place the deviation caused by the rapid pouring of concrete or the deviation of the steel cage caused by the hanging of the conduit 4 Control measures for verticality deviation 41 Selection of drill bit Select the drill bit according to the formation conditions 1 Clay the rotary drilling bucket with single-layer bottom shall be selected If the diameter is too small the two-part bucket or the drilling bucket with unloading plate can be used 2 Silt Tapered Rock Bit soil layer with weak viscosity sandy soil and pebble layer with poor cementation and small particle size select the drilling bucket with double bottom 3 Hard daub select rotary drilling bucket with single soil inlet (single and double bottoms) or straight screw with bucket teeth
4 Cemented gravel and strongly weathered rocks conical auger bits and rotary drilling buckets with double bottoms shall be provided (single port for larger particle size and double port for smaller particle size) 5 Stroke bedrock rotary drilling bucket equipped with cutter barrel coring bit conical auger bit double bottom or cutter straight auger bit double bottom 6 Slightly weathered bedrock If the diameter of the rotary drilling bucket equipped with cone barrel coring bit and conical auger bit with double bottom is too large the grading drilling process shall be adopted The single-layer bottom drilling bucket without air holes is used for the test pile Since it is only applicable to the soil layer with strong viscosity the soft and hard geology in the project is uneven and there are slightly weathered and moderately weathered bedrock in the deeper part which leads to the deviation of the drill bit during the construction of the rotary pile driver and then leads to the deviation of the verticality of the pile body 42 Pile casing embedment Before the steel casing is buried accurate measurement and setting out shall be carried out to ensure that the deviation of the top surface of the steel casing is not greater than 5cm and the inclination of the steel casing shall not be greater than 1% during the burial; the drilling rig shall be in place and the drill bit shall be accurately aligned with the pile position Before the steel casings are buried the drill bit with big bore shall be used to pre-drill to the elevation of the casing bottom and then the auxiliary winch of the drilling rig shall be used to lift the casing into the hole The drilling bucket is lifted out and the steel casing is pressed into a predetermined position with the drilling bucket In order to maintain the verticality of the pile casing when the pile casing is buried the intersection control shall be carried out with different distances from the pile to the pile center until the top of the pile casing reaches the specified elevation After the pile casing is buried restore the position of pile center according to the distance and the previously determined direction and check whether the pile casing center coincides with the pile center and control it within ± 5 cm At the same time tamp the surrounding of the pile casing to make it stable without deviation and collapse in the drilling process 43 Drilling process Bored piles shall be drilled slowly after opening so as to form a good and stable retaining wall and ensure the correct hole position During the drilling process the drill pipe position shall be regularly checked with the distance intersection and the deviation shall be adjusted immediately until the hole position is finalized Before the drilling operation the mast needs to be positioned and set and during the operation the mast also needs to be adjusted vertically Vertical adjustment can be divided into manual vertical adjustment and automatic vertical adjustment The automatic verticality adjustment can only be carried out through the automatic verticality adjustment button on the display within the ± 5 ° range of the relative zero position of the mast; when the mast exceeds the ± 5 ° range of the relative zero position the verticality can only be adjusted manually through the inching button on the display or the electrical handle on the left operation box In the process of vertical adjustment the position status of the mast can be monitored in real time through the working interface of the mast on the display so that the mast can finally reach the set position for hole forming  44 Positioning of reinforcement cage Pile verticality deviation detection is determined by the deviation between the center of the reinforcement cage and the design pile center so the positioning of the reinforcement cage is an important item in the control of pile deviation (1) When lowering the reinforcement cage two hanging bars shall be used to ensure the verticality of the reinforcement cage after lifting (2) Add protective cushion blocks according to the specification requirements especially add more protective cushion blocks at the top of the pile And (3) aft that reinforcement cage is lowered into the hole pul a cross line on the framework reinforcement of the reinforcement cage to determine a center point performing distance intersection to restore the center of the pile position through the guide pile and the set direction compare a hanging vertical line with the center of the reinforcement cage performing adjustment by slightly moving the reinforcement cage through a crane to ensure that the two centers are superposed welding a positioning bar to enable the positioning bar to abut against the wall of a protective cylinder O as to achieve the purpose of stabilizing the reinforcement cage If the pile top is far away from the top of the pile casing the false cage shall be welded to the top of the pile casing and the above method shall be used for positioning It shall be noted that the false cage shall have certain rigidity (4) When the poured concrete is close to the reinforcement cage slow down the concrete pouring speed and the normal pouring speed can be restored when the bottom of the conduit is raised to more than 2 m above the reinforcement cage so as to ensure that the reinforcement cage will not float up and deviate due to the recoil force of the concrete and at the same time the conduit is located in the center of the hole to avoid deviating to one side In order to avoid the displacement of the skeleton caused by the uneven reaction of the concrete during the pouring process 5 Conclusion Because of the many advantages of rotary drilling it is more and more widely used in the field of pile foundation construction and constantly summarizes the measures to control the verticality of pile position in the construction process which is of great significance to the construction technology of bored pile with rotary drilling machine
References  Standard for Quality Inspection and Evaluation of Highway Engineering (JTGF80/1-2004)  Luo Yu Application of Rotary Pile Digging Machine in the Construction of Foundation Pit Supporting Pile Guangdong building materials ，2008（6） Source Architecture and Culture Academic Edition 2013 (8) 114-114 By Li Zhihong Return to Sohu to see more Responsible Editor wt-dthtools.com