Common Quality Problems of Bored Pile Construction

Due to the complexity of construction technology, the final pile quality may be affected by various visible or invisible factors. In order to ensure the quality of the whole pile foundation engineering meets the design requirements, it is very important to prevent and deal with the common quality problems of bored pile.

• Preparation for construction
  

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The drilling site should be based on the terrain, geology, hydrological data and pile top elevation and other conditions combined with the requirements of construction technology, the following preparations should be made:

Site formation and island building:

    If the drilling site is in dry land and the groundwater level is more than 1m below the original ground during construction, the site should be levelled, debris removed, soft soil replaced, and compacted. The rig seat should not be placed directly on the weak fill to avoid uneven settlement. To build the road for the location of dry land, to facilitate the transportation of construction machinery and materials.

    When the drilling site is on a steep slope, it should be dug into a flat slope. If there is difficulty, the work platform can be built with shelving or sleepers.

    When the drilling site is in shallow water, the island building method should be adopted. The top of the island is usually 0.75 ~ 1.0m higher than the construction water level. The area of the island is determined by the drilling method and the size of the equipment.

Drilling pile positioning measurement:

    First determine the drill pile position: according to the control network and pier design coordinates, use the total station to precisely release the pile position. Use long wooden piles (fixed with iron nails) to accurately mark the center of each pile, bury the protection cylinder after accurate positioning, and set the protection piles around. Piling principle: does not affect drilling machinery, lifting steel cage and concrete pouring construction; If it is found that the bollards are loose or abnormal, let the surveyors check in time. The bollard is about 50 cm higher than the original site and is fixed with concrete for anytime detection of the center of the pile, elevation and installation of the steel cage and positioning.

Buried casing:

    Install a strong, watertight orifice guard before drilling. Steel casing is generally made of steel plate not less than 8mm thick in ordinary working occasions and under small and medium aperture conditions; In deep water, complex geology and large aperture conditions, the thickness of the steel plate is not less than 12mm, in order to increase the stiffness, the steel plate is not less than 10mm, and the upper and lower ends of the protection cylinder and the outer side of the joint are welded. The inner diameter of the protection cylinder is about 20cm larger than the drill diameter, the top surface of the protection cylinder is 2m higher than the construction water level or the groundwater level, and the height of the mud surface in the hole must be met, and it is also 0.5m higher than the construction ground when the dry land or the island is built. The buried depth of the casing meets the following requirements:

    On the beach, the viscous soil is not less than 1m, and the sandy soil is not less than 2m. When the topsoil is soft, bury the casing in a hard and dense soil layer of at least 0.5m. The protective cylinder is buried on the beach, and the clay is backfilled around the protective cylinder and rammed by layers. The deviation between the center of the top surface and the design pile position is not more than 5cm, and the inclination is not more than 1%.

    In water, the protective cylinder is sunk by means of hammer, pressure and vibration. The protection cylinder is buried 1m below the riverbed surface; On the underwater platform, the burial depth is determined according to the highest construction water level, flow rate, erosion and geological conditions, and the impervious layer is driven into if necessary.

    The guard is sunk on the underwater platform, and the position of the guard is controlled by the guiding equipment.

• Construction technology

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Mud preparation:

    The hole is drilled in the interlayer of sandy soil, crushed (egg) stone or clay, and the wall is protected by bentonite mud. When the plastic index is greater than 15, the original mud in the hole can be used to protect the wall.

    With the increase of the hole depth, grouting is timely and continuous in the hole to maintain the water head in the protection cylinder and prevent the hole wall from collapsing.

    When concrete is poured into the pile hole, the overflow mud in the hole is diverted to the mud pool and used in the wall protection of the next foundation pile hole.

    The specific gravity of the mud into the hole is 1.1 ~ 1.3, the specific gravity of the mud at the bottom of the hole should not be greater than 1.3, the large drifter and pebble layer should not be greater than 1.4, and the rock layer should not be greater than 1.2.

Rig placement and drilling:

    ① Before the drilling rig is in place, all preparations for drilling should be checked. The base and top of the rig after installation should be stable, and should not cause displacement or subsidence during drilling. After being in place, the construction team will self-check the rig in place.

    ② Before drilling, draw the geological profile according to the geological and hydrological data provided by the construction design and hang it on the drill floor. Select different bits, drilling pressure, drilling speed and appropriate mud weight for different geological layers.

    ③Drilling operations should be carried out continuously in different shifts, fill in drilling construction records, and inform the drilling situation and precautions for the next shift when changing shifts. Drilling mud and drilling rig alignment should be regularly tested, and it should be corrected in time if it does not meet the requirements. Attention should always be paid to the formation changes, and the sample slag should be saved at the formation changes.

    ④ Drilling operations shall be carried out continuously without interruption.

    ⑤ When the drilling depth meets the design requirements, check the hole depth, aperture, hole position and inclination, and confirm that the design requirements are met, immediately fill in the final hole inspection certificate, and be approved by the resident supervision engineer, before the hole bottom cleaning and underwater concrete pouring preparation work.

Rotary drilling rig into holes:

    Drilling rig positioning: the rotary drilling rig uses the walking system to position itself. After positioning, the center of the drill bit and drill pipe should be aligned with the center of the pile, and then the drilling rig should be levelled to make the drilling frame vertical. The leveling and positioning of the drilling rig are completed by the microcomputer automatic control system.

    Injection of mud: After the rig is precisely in place, the mud pump is used to inject mud into the guard barrel, and the mud is injected until the rig does not spill when it is spinning. In the process of rotary digging, each bucket is injected into the hole once, so that the hole always maintains a certain water head and the mud quality is stable.

    Rotary drilling: When drilling in general soil formation or soft rock formation, according to the geological conditions, the appropriate bit is used. Start rotary digging at a slow speed to prevent disturbing the casing. Special attention should be paid to monitoring the verticality through the control panel in the process of rotary digging in the hole section from 5 to 8m, and timely correction should be made if there is any deviation. In the process of drilling, the slag sample should be collected at 2m per foot and the formation change, the drilling record table should be filled in and checked with the geological column chart, and the mud index and rotary digging speed should be adjusted according to different strata. Always maintain a certain head of water in the hole, and replenish mud in the hole in time for each bucket dug. During the drilling operation, the ground where the main engine is located and the ground changes at the support of the leg (or track) should be observed at any time, and the sinking phenomenon should be stopped in time.

    External transport of abandoned slag: The drilling slag excavated in the drilling process is temporarily stacked near the pile site, transported to the designated location by the dump truck in time, and treated according to relevant regulations to reduce environmental pollution.

Percussion drilling holes:

    When there are hard rock, isolated stone and large particle size pebble layer in the pile site, hammer drill is used. When the hole is opened, the mud or direct water is injected into the hole, the clay is put into the hole, and the slurry is made by repeated impact of the impact cone with a small stroke. The water level in the hole is always 1.5-2.0m higher than the groundwater during the opening and the whole drilling process, and the overflow is prevented, and the water is replenished in time after the cutting. The 2 ~ 4m range below the foot of the protection cylinder is generally loose, and the thick mud (or clay and small pieces of stone are poured 1:1), small stroke and high frequency are repeatedly punched to promote the formation of a "hard shell" at the bottom of the protection cylinder. Avoid slurry leakage at the bottom of the drum. During the impact drilling, if the inclined rock face is encountered, the clay and small pieces of stone are backfilled and smashed with small strokes. During the punching process, the stone is squeezed to build the wall, and the inclined rock face is cut on the one side until the whole section enters the rock after normal drilling.

This is the sharing of the rotary drilling rig

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