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Causes and Treatment Plans for the Collapse of Bored Pile
Jul 28, 2022

1. Cause analysis and treatment of common problems in the drilling process


1.1 Collapse hole


1.1.1 Analysis of the causes of collapsed holes


Hole collapse is one of the most common accidents, which can occur during drilling or after hole formation. The reasons are as follows.


(1) The mud consistency is small, the wall protection effect is poor, and water leakage occurs; or the casing is buried shallowly, and the surrounding sealing is not compact, resulting in water leakage; or the thickness of the soil layer at the bottom of the casing is insufficient, and water leaks at the bottom of the casing, resulting in the height of the mud head Insufficient, the pressure on the hole wall is small.


(2) The relative density of the mud is too small, and the pressure of the water head on the hole wall is small.


(3) In the soft sand layer, the footage is too fast, the formation of the mud retaining wall is slow, and the pore wall sees water.


   (4) During drilling, the drilling time is long in the middle, and the water head in the hole cannot be kept 2.0m above the water level outside the hole or the groundwater level, which reduces the pressure of the water head on the hole wall.


   (5) When the drill bit is lifted or the steel cage is dropped, it collides with the hole wall.


   (6) There is large equipment or vehicle vibration near the borehole.


   (7) Water loss in the hole causes insufficient water head height.


   (8) Concrete can not be poured in time after hole cleaning, and the storage time is too long.


1.1.2 Preventive measures for hole collapse


(1) According to the geological exploration data provided by the design department, for different geological conditions, select the appropriate mud proportion, mud viscosity and different drilling speed. For example, in the sand layer, a better slurry material should be selected, the consistency of the slurry should be increased, the viscosity of the slurry should be increased to strengthen the wall protection, and the footage rate should be appropriately reduced.


(2) When the casing is embedded on land, the bottom should be compacted and filled, and the surrounding of the casing should also be backfilled and compacted.


(3) When the water vibrates and sinks into the protective tube, according to the geological data, the protective tube is passed through the silt and permeable layer, and the protective tube is connected tightly and does not leak.


(4) When the water level changes greatly during the flood season or tidal period, the casing should be raised to increase the water head to ensure that the water head pressure is relatively stable.


(5) Drilling should be operated continuously as far as possible without special reasons.


(6) Lift the drill bit or drop the steel cage as vertical as possible, and do not collide with the hole wall.


(7) Try to avoid large equipment operations or vehicles passing through when drilling.


(8) When the perfusion work is not available, temporarily do not clear the hole to reduce the specific gravity of the mud.


1.1.3 Treatment of collapsed holes


(1) If the hole is slightly collapsed, immediately increase the specific gravity of the mud, increase the water head of the mud, and increase the water head pressure.


(2) When the collapsed hole is not deep, a deep-buried protective tube can be used instead, the surrounding of the protective tube is compacted, and the drilling is re-drilled.


(3) In case of serious hole collapse, it should be backfilled with schist or sandy soil immediately, or with clay mixed with not less than 5% cement mortar. If necessary, remove the drilling rig to prevent the drilling rig from being buried in the hole and wait for backfilling. Re-drilling after stabilization. When the rock surface of the schist after backfilling is inclined greatly, the drill bit is easy to swing, hitting the casing or the hole wall, resulting in the phenomenon of partial hole or collapsed hole, stuck drill, etc. At this time, use a small stroke to impact, wait for it After the floating soil and protruding part at the bottom of the hole are leveled, the platform appears. Then increase the stroke and transfer to the normal stroke. 1.2. Drilling deflection


1.2.1 Analysis of the cause of drilling deflection


(1) The drilling rig is not in a horizontal position, or the site is not leveled and compacted, and uneven settlement of the drilling rig occurs.


(2) The water drilling platform is unstable or not in a horizontal state.


(3) The soil layer is unevenly soft and hard, or encounters boulders, resulting in uneven stress on the hammer or drill.


(4) If it is a rotary drill, the drill pipe is bent and the joint is loose, resulting in a large range of drill bit shaking.


(5) Drill holes in the original building position, and squeeze the hammer head to one side when encountering obstacles.


 1.2.2 Drilling deflection precautions


 (1) Before the drilling rig is in place, level and compact the construction site. If necessary, place sleepers and adjust the drilling rig to a horizontal state. The water drilling platform is strictly inspected and accepted before the drilling rig is in place to ensure that the platform is firm, level and the frame is stable. During the drilling process, the adjustment should be checked frequently to keep the drilling rig level.


(2) Ensure that the pulley groove on the top of the drilling rig, the drill pipe chuck and the center of the pile position are on the same vertical line, and frequently check the displacement or swing of the drilling rig during the drilling process, and adjust it at any time.


(3) Check the drill pipe frequently, and adjust or discard the bent drill pipe in time.


(4) When drilling holes in the original old buildings, do a good job of detection in advance. If obstacles are found, use impact drills instead.


(5) When drilling in soft and hard uneven soil layers, reduce the stroke and drill at a slow speed. If necessary, backfill schist clay and drill again.


(6) When suddenly encountering hardened formations, control the footage and drill at low speed. During the drilling process, the pressure should be reduced to avoid the occurrence of inclined holes, curved holes, reaming, and even twisting and breaking of the drill pipe.


1.2.3 Drilling deviation treatment measures


(1) When encountering boulders or obstacles, use an impact drill to impact holes.


(2) When the deviation of the drilling hole exceeds the specification requirements, backfill with clay or schist, and drill again.


1.3 Shrinkage


1.3.1 Analysis of the causes of shrinkage


(1) There is a weak layer in the geological structure, and under the pressure of the soil, it squeezes into the hole to form a shrinkage cavity.


(2) There is a plastic soil layer in the stratum, which swells with water to form shrinkage pores.


(3) The drill hammer is worn and the repair welding is not timely, and the drilled hole diameter is often smaller than the designed pile diameter.


1.3.2 Preventive measures for shrinkage cavities


   (1) According to the drilling data of the design department, if there is a weak layer or plastic soil, pay attention to sweep the hole frequently.


(2) When the hammer head is seriously worn, repair welding should be done in time, and then the hole should be cleaned and the hole diameter should be designed.


1.3.3 Treatment of shrinkage holes After shrinkage holes appear, sweep the holes repeatedly with a drill until the designed pile diameter is met.


1.4 Reaming


1.4.1 Reasons for reaming


(1) When encountering extremely soft muddy or silty sandy soil layer, the hole wall collapses and the hole is expanded.


(2) The drill hammer swings too much during the drilling process.


1.4.2 Precautions for reaming


(1) Steady footage during drilling to prevent the hammer from shaking too much.


(2) Check the drill pipe frequently, and replace the unqualified drill pipe in time.


(3) Take decompression drilling.


1.4.3 Reaming measures


(1) When encountering extremely soft muddy or silty sandy soil layer, which causes the hole wall to collapse and the hole is expanded, the specific gravity of the mud should be increased to 1.3-1.4.

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