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The application of rotary drilling rig in the construction of Bored Piles in backfill soil

the application of rotary drilling rig in the construction of Bored Piles in backfill soil

China engineering machinery information

1. Project overview

a total of 40 cast-in-place piles for dry bridges in a mountainous city, with pile diameters of 1.8 meters and pile lengths ranging from 18 to 35 meters. The pier foundation adopts rock socketed pile foundation, and the pile foundation should be embedded into the weakly weathered rock surface with the rock socketed depth greater than 3 times the pile diameter. Geological description: the top layer is 8 ~ 25 meters of backfill, under which there is 5 meters of undisturbed soil, and then rock. Hydrological Description: groundwater is mainly Quaternary loose rock pore water and bedrock weathered fissure water. There is no obvious water supply on the surface, and it is mainly supplied by atmospheric precipitation. The artificial fill layer has a large thickness, continuous distribution in local sections, poor water carrying capacity and good water permeability. The terrain of the site is gully ridge terrain, which is conducive to the discharge of groundwater. Bedrock weathered fissure water in the site is poor

2. Comparison and selection of construction schemes

(1) because the cast-in-place pile is located in the original mountain ditch, the artificial fill is relatively thick, the self stability of the backfill is insufficient after excavation, and the pile is relatively deep. If the manual hole digging pile is used for construction, there is a great safety risk

(2) according to the geological and hydrological data, the backfill of the project is relatively thick, and the soil layer has good water permeability, so it is not suitable for drilling with mud retaining wall, and it is not suitable for drilling with wet operation machinery

(3) according to the fact that the auger is generally suitable for drilling piles with small pile diameter, because the pile diameter of this project is 1800mm, it is not suitable for auger drilling

(4) according to the geological and hydrological conditions, the rotary excavator is the most suitable hole forming machine for the project. It has the advantages of fast hole forming speed, safe excavation, and is suitable for strata without groundwater; Its disadvantage is that the hole is easy to collapse in the backfill, and the drilling in the rock is slow. XCMG xrs680 rotary drilling rig is used in this project. The maximum drilling diameter of this machine is 2500mm, the maximum drilling depth is 105m, and the maximum torque is 390kn m. The maximum ROP is 7 ~ 18r/min, and the rated power is 298kw

3. Construction process and technical parameters

3.1 construction preparation

before construction, the site shall be leveled, sundries shall be removed, and the surface cultivated soft soil shall be replaced to ensure that the filling of the rig base is dense, so as to avoid uneven settlement; At the same time, do a good job in the drainage of the operation site, excavate and set up temporary drainage ditches within the construction scope to ensure that the construction site does not accumulate water

3.2 setting out of pile position

before formal drilling, the center of pier and abutment pile foundation shall be accurately set out with a total station according to the design drawings and calculation coordinates and recorded. After passing the self inspection, it shall be reported to the supervising engineer for inspection in time

3.3 bury the pile casing

before drilling, set a solid and watertight steel pile casing, which is 1.5m high, made of 1cm thick steel plate, with a diameter 20cm larger than the design pile diameter, and the top surface is about 30cm higher than the construction platform. When excavating and burying the pile casing, the bottom of the pit shall be leveled, and then the central position of the hole shall be marked on the bottom of the pit through the setting out of the positioned control pile, and then the pile casing shall be hoisted into the pit to find out the central position of the pile casing, which shall be fixed on the top or bottom of the pile casing with a cross line, and then the pile casing shall be moved to make the center of the pile casing coincide with the center position of the borehole, and at the same time, a level ruler or hammer ball shall be used to check to make the pile casing vertical. After that, backfill the clay symmetrically and evenly around the pile casing, and tamp it in layers to prevent the deflection of the pile casing during tamping

the deviation between the top center of the pile casing and the design pile position shall not be greater than 5cm, and the inclination shall not be greater than 1%

3.4 drilling machine drilling

the drilling process of rotary drilling machine adopts the static dry soil direct borrowing process, which is a drilling method without flushing medium circulation. When the rotary drilling rig works, it can do the overall rotary movement in situ. When the rotary drilling rig is drilling for soil, it cuts into the soil layer by the self weight of the drill pipe and the drill bit, and the oblique bucket tooth cuts the soil block and pushes it into the bucket when the bucket rotates to complete the drilling for soil; In case of hard soil, the self gravity is not enough to make the bucket teeth cut into the soil layer. At this time, the drill pipe can be pressurized by the pressure cylinder, and the bucket teeth can be forcibly cut into the soil to complete the drilling and borrowing. After the drill bucket is filled with soil, the crane lifts the drill pipe and the drill bucket to the ground, pulls the switch on the drill bucket to open the bottom door, and the soil in the drill bucket is automatically discharged by its own weight. The drill pipe is placed downward, the bucket door is closed, and then it rotates to the hole to excavate the next bucket. Hole forming should be carried out continuously without interruption. According to the 2018 deeds report released by leading paper enterprises recently

control the footage speed according to the geological conditions during drilling: when drilling from hard formation to soft formation, the drilling speed can be appropriately accelerated; When the soft stratum becomes hard stratum, slow down and advance slowly; In the formation prone to shrinkage, the number of hole sweeping should be appropriately increased to prevent shrinkage; Adopt fast speed drilling for hard plastic layer to improve drilling efficiency; In the sand layer, slow speed drilling is adopted, and the specific gravity and viscosity of mud are appropriately increased; If there is a pebble layer in the actual construction process, the following measures should be taken: for the pebble layer with small particle size, use bucket bit to drill slowly, and for the pebble layer with large particle size, use conical screw bit to drill and then replace the bucket bit to clean the slag, and so on until it passes through the pebble layer. The drilling slag shall be transported out of the construction site in time, and one excavator and three dump trucks shall be equipped to clear and transport the spoil at the extreme time. The waste shall be transported to the place designated by Party A to meet the requirements of environmental protection

when forming a hole in the soil layer, use the general conical bucket bucket teeth to take soil. After penetrating the soil layer, replace the drill bit with the excavator teeth for tunneling, and finish the hole until it reaches the depth required by the design

during hole forming, the footage speed shall be controlled according to the soil conditions. In order to ensure that the verticality of the hole meets the design requirements, the pile driver must be kept flat, and the inspection must be strengthened. It is necessary to attach a large scale extension device for inspection and correction

3.5 hole forming and hole forming inspection

(1) after the hole forming reaches the design elevation, check the hole depth, hole diameter, hole wall perpendicularity, sediment thickness, etc., and prepare testing tools before testing. The packaging with too high price such as testing rope has no practical significance

(2) the sediment thickness must be checked, and the detection tools, measuring ropes, hole detection tools, etc. must be prepared before the detection

In addition, the central height of the experimental host

3.6 core sampling

according to the pre excavation depth calculated in the geological survey report and in combination with the actual situation on site, after the depth reaches the moderately weathered layer, use a barrel drill to drill, take out the moderately weathered layer rock at the bottom of the hole, and after passing the inspection, drill downward according to the construction drawings to meet the requirements of the designed rock socketed depth. After reviewing the depth, the hole can be terminated

3.7 hole cleaning

after the drilling reaches the required depth, use a hole detector to inspect the hole. After the hole diameter, hole perpendicularity and hole depth are checked to be qualified, the final hole inspection certificate shall be filled in immediately and approved by the resident supervision engineer before the hole bottom can be cleaned, otherwise the hole shall be cleaned again

the hole cleaning bit is used for hole cleaning. After the drilling reaches the design elevation, stop the footage, raise the bit, use the hole cleaning bit to clean the hole bottom for many times, measure the thickness of the ballast at the hole bottom to meet the requirements of the technical specifications and design requirements, that is, stop the hole cleaning operation, and put it into the reinforcement cage that has passed the inspection of the supervising engineer

4. Treatment of hole collapse and drilling failure

4.1 hole collapse

due to the thickness of the backfill soil layer, the compactness is not high, and the earth pressure is high after the hole is formed, the hole below the steel casing is easy to collapse. In case of serious hole collapse, C20 concrete shall be used to backfill to the position 1m above the hole collapse position, and drilling shall be carried out after the initial setting of concrete; If the hole collapse occurs when the reinforcement cage installation has sunk into the hole, it should be lifted in time, and the collapsed sediment in the hole should be cleaned before placing the reinforcement cage

4.2 drill drop

there are many reasons for drill drop, including large-scale collapse of the hole wall, resulting in buried drill; The hole wall is easy to shrink and cause sticking; There is no gap between the outer wall of the drill barrel and the hole wall. If the drilling is too deep, it is easy to cause sticking; It is easy to drop the drill when coring at the bottom of the hole

the main methods to deal with the stuck drill are:

(1) direct lifting method, that is, lifting with a crane or hydraulic jack up directly

(2) dredging method around the drill bit, that is, using reverse circulation or underwater 4 cutting, clean the sediment around the drill barrel, and then lift it

(3) high pressure injection method, that is, two small holes are drilled symmetrically on both sides of the original drill hole (the center of the hole is about 0.5m away from the edge of the drill bit), and then the nozzle is lowered to aim at the stuck drill bit for high-pressure injection until the two holes are sprayed through, so that the sediment in the original hole falls into the small hole, and then the stuck drill bit can be rotated and lifted

(4) retaining wall excavation method, that is, when the drill is not deep, protect the wall with items such as casing and cement, and excavate and clean the sediment manually

in this project, there is a drill drop accident during coring at the hole bottom. If the drill bucket is not taken out, the pile bottom will have a large settlement after the drill bucket is rusted, which is very unfavorable to the stress of the beam. Due to the thick backfill soil, the drill bucket is stuck in the moderately weathered rock, which is difficult to take out by general methods. If it cannot be taken out in time, it may collapse and bury it. After careful study, it was decided to manually lower the well to trap the drilling bucket, and then lift the drilling bucket with a crane. In order to ensure the safety of personnel, first check whether there is toxic gas in the hole, then ventilate, lower the reinforcement cage, put the personnel into the protective tube, put the hole bottom and cover the drill bucket. Through practice, this method is safe, reliable and very practical

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