01 Engineering Geological Overview

The case study (Dezhou to Jinan section, Shandong) mainly consists of Quaternary alluvial, alluvial-proluvial, and marine deposits, generally silty clay, clay, silt, fine sand, and similar soils. Rivers are densely distributed along the route, with numerous water conservancy facilities and abundant groundwater. The groundwater type is pore phreatic water, locally confined, with a depth generally between 0.4 and 6.0 m.

02 Key Points and Requirements of Bored Pile Construction Technology

1. Based on the geological conditions of this section, a steel casing of 5 m height, 10 mm thickness, and 1.2 m diameter was specially customized. The extra-high casing can effectively prevent leakage and collapse at the hole mouth and the impact of surrounding environmental vibration and shock on the pile hole. The larger inner diameter of the casing can store sufficient mud; when the drill pipe is lifted out of the pile hole, it ensures water pressure inside the casing and maintains the stability of the mud cake on the hole wall. At the same time, it effectively avoids collision and scraping of the casing by the drill bit during lifting and lowering, protecting the stability of the hole mouth. The casing is kept about 0.3 m above the ground, which not only protects the hole mouth from collapse but also prevents surface water, ground mud leakage, and debris from falling into the hole.

2. High-quality bentonite is used to prepare mud. To increase mud viscosity and colloid fraction, appropriate amounts of sodium carbonate, caustic soda, etc., can be added to the mud according to actual site conditions.

3 Drilling Construction

During drilling, the drill bit type and penetration rate are selected according to geological conditions. During the lifting of the drill bit, the bottom door of the rotary bucket must be kept closed to prevent sand or clay in the bucket from falling into the wall-protecting mud and disrupting the mud mix ratio. Each working cycle strictly controls the drilling depth to avoid buried drill accidents. At the same time, the lifting speed of the rotary bucket should be properly controlled. If the lifting speed is too fast, the mud flows at high speed between the rotary bucket and the hole wall, scouring the hole wall and destroying the mud cake, which is detrimental to hole wall stability and easily causes collapse. Especially in soft soil areas prone to hole shrinkage or collapse, the drilling should be controlled at about 30 cm per stroke, and the drill bit should be lifted slowly, which can effectively control hole shrinkage and prevent collapse.

Before drilling, an appropriate amount of mud is required to be stored in the casing, and sufficient mud should be prepared as reserve. During drilling, if mud is lost or leaks, new mud must be replenished into the hole promptly and directly.

4. Hole Cleaning

First hole cleaning: After the hole is completed, the slurry is allowed to settle for half an hour. The drilling rig is used for initial hole cleaning. After the supervising engineer checks that the sediment thickness, hole diameter, hole depth, and verticality are qualified, the drilling rig is removed, the reinforcement cage is installed, the sealing conduit is installed, and then the second hole cleaning is carried out.

The second hole cleaning adopts the mud replacement method, using the conduit for underwater concrete pouring as the pipe, and a mud pump is used to inject high-quality mud into the hole until all indicators of the returned fresh mud meet the standards. After measuring that the sediment thickness at the bottom of the hole meets the design and specification requirements (friction pile ≤20 cm) and obtaining the supervising engineer's approval, the large hopper is installed, and preparations for underwater concrete pouring are made.

During the hole cleaning process, the original water head height in the hole must always be maintained.

5. Installation of Reinforcement Cage

The reinforcement cage is centrally fabricated in the processing shed. After fabrication, it is transported to each pile position by flatbed truck and lifted into place by truck crane. To prevent deformation of the reinforcement cage during lifting and transportation, anti-deformation supports are welded with reinforcing bars at each end and at the inner ring stiffening hoops; these supports are cut off when the cage is lifted to the hole mouth.

The reinforcement cage is lowered after the first hole cleaning. The cage must be accurately positioned when lifted into the hole. To prevent the reinforcement skeleton from floating during concrete pouring, fixing rods are evenly arranged at the upper end of the cage, and the support system should be aligned with the centerline to prevent tilting and displacement of the reinforcement skeleton.

6. Underwater Concrete Pouring

After hole inspection is completed, underwater concrete should be poured within 4 hours using a Φ300 mm steel conduit. The concrete is centrally mixed at a batching plant and transported to the site by tanker. Underwater concrete must be poured continuously without interruption. During pouring, the concrete surface height should be measured at any time, and the depth of conduit embedded in concrete should be correctly calculated and kept within 2-6 m.

When pouring the first batch of concrete, the distance from the bottom of the conduit to the hole bottom should be controlled at about 30 cm. Pile top elevation control: after concrete pouring, the pile top elevation should be about 0.5-1.0 m higher than the design pile top elevation.

03 Key and Difficult Points Control in Construction

1. Control of sediment thickness at pile tip: To ensure that the sediment thickness at the pile bottom does not exceed the standard, in addition to requiring good mud performance, the drilling speed in different strata must also be controlled during construction. Especially when entering the fine sand layer, each rotary excavation should not be too much to prevent sand from emerging from the top of the drill bit into the mud. In special cases where concrete cannot be poured in time, causing excessive sediment, a crane can be used to lift the reinforcement cage, and then a rotary drill can be used to remove the sediment before pouring.

2. Hole formation measures in special strata: For strata prone to diameter shrinkage, the drilling speed should be slowed down during drilling, with each stroke controlled at about 30-40 cm, and the hole should be repeatedly swept until it meets the standard.

04 Common Quality Defects in Construction and Prevention

1. Hole Collapse

Hole collapse during drilling: If it is slight and does not affect normal drilling, it can be left untreated; if the collapse is severe and drilling cannot continue, backfill with soil and wait until natural settlement is mature before drilling again.

Hole collapse during pouring: While maintaining the water head in the hole, a mud suction machine should be used to remove the soil that has collapsed into the hole. If the collapse stops, pouring can resume; if the collapse continues, the conduit and reinforcement cage need to be pulled out, and the hole backfilled with soil to wait for natural settlement before drilling again.

When rotary drilling is carried out in areas prone to hole collapse, first ensure that all indicators of the prepared mud meet the requirements. Second, during drilling, try to achieve slow penetration and avoid collision of the drill bit with the hole wall that would damage the wall-protecting mud cake. At the same time, reduce the amount of drill cuttings falling from the drill bit into the hole to avoid disrupting the mud mix ratio.

2. Water Ingress into Conduit

Water ingress into the conduit can be caused by failure of the first batch of concrete to seal the bottom, unsealed conduit joints during pouring, or the conduit being pulled out of the concrete during pouring. If bottom sealing fails, the conduit and reinforcement cage should be pulled out promptly, the concrete at the hole bottom removed with a rotary drill, the reinforcement cage and conduit reinstalled, and pouring resumed after qualified hole cleaning. If conduit joints are not sealed, the conduit must be replaced with a qualified one. Both conduit replacement and conduit pull-out require that before resuming pouring, the mud in the newly installed conduit be cleaned out with a mud bucket or mud pump. Carefully calculate the conduit embedment depth to ensure smooth re-pouring.

Before pouring, check the sealing of the conduit. When installing the conduit, check whether the sealing ring is properly placed to ensure good sealing. Strictly calculate the concrete volume for the first batch to ensure sufficient concrete for bottom sealing. During pouring, accurately measure the conduit embedment depth to avoid pulling the conduit out.

3. Floating Cage

If the reinforcement cage is found to be floating during pouring, immediately slow down the pouring speed. While ensuring sufficient conduit embedment depth, quickly lift the conduit. After the reinforcement cage returns to the design elevation, remove the conduit. If the conduit embedment depth is insufficient and the conduit cannot be removed, quickly lift the conduit (be careful not to pull it out), then slowly lower it, repeating this process until the reinforcement cage returns to the design elevation.

To prevent the reinforcement cage from floating, fix it on the steel casing after installation. Second, during pouring, accurately measure the concrete top elevation. When the concrete is about to enter the reinforcement cage, slow down the pouring speed and strictly control the conduit embedment depth.

4. Pipe Blockage

During concrete pouring, if the concrete cannot go down in the conduit, first use the crane's wire rope to shake the conduit, or install an attached vibrator to make the concrete fall. If these methods do not work, pull out the conduit, clean the blocked concrete inside, reinstall the conduit, clean the mud inside the conduit, and resume pouring.

During pouring, strictly control the workability and slump of the concrete to avoid pipe blockage.