Water oozing out of casing
Water oozing out of the outer wall of the casing can cause the foundation to sink, the casing to tilt and shift, causing the pile hole to deflect, and even make construction impossible.
Cause analysis When burying the casing, the surrounding soil is not dense, or the water level difference of the casing is too large, or the drill bit collides when it rises and falls.
Prevention and control measures When burying the casing, clay with the best water content should be used for layered compaction at the bottom and around the pit; holes should be opened at an appropriate height of the casing to keep a water head height of 1~1.5m in the casing; prevent collision with the casing when raising and lowering the drill bit; when water oozing out of the casing is first discovered, clay can be used to fill and reinforce it around. If the casing sinks or shifts seriously, it should be reworked and reburied.
Extremely slow drilling or no drilling
Foundation Drilling in hard plastic clay layers is extremely slow, generally 8~10h, accounting for 60%~70% of the drilling time of a single pile.
Cause analysis Improper drill bit selection, improper installation angle of alloy cutter, too shallow soil cutting by cutter, too light counterweight of drill bit, and the drill bit is covered with clay.
Prevention and control measures Replace or modify the drill bit, rearrange the angle, shape, and arrangement direction of the cutter, increase the counterweight, strengthen slag removal, reduce the mud specific gravity or change the drilling method, and adopt the reverse circulation drilling method.
Collapse of pile hole wall
During or after drilling, the hole wall collapses to varying degrees. Bubbles continue to appear in the mud discharged from the hole, and sometimes the water level in the casing drops suddenly, which are all signs of hole collapse.
Cause analysis: The main reasons are loose soil, poor mud wall protection, poor casing burial, low water level in the casing, lifting the drill bit during drilling, too fast drilling speed or too long idling time, which can easily cause the bottom of the borehole to collapse, and too long waiting time and grouting time after the hole is formed.
Preventive measures: Bury casings appropriately deep in loose and easily collapsed soil layers, backfill soil densely, use high-quality mud, increase mud specific gravity and viscosity, raise casings, replenish mud after hole completion, maintain required head height, ensure quality of steel cage production, and prevent deformation; align with hole position when hanging, hang straight and steady, slowly sink to prevent collision with hole wall; waiting time for pouring after hole formation is generally not more than 3h, and speed up pouring speed and shorten pouring time as much as possible; when steel cage is not in the hole, fill mortar sand and clay mixture to 1~2m above the collapsed hole depth, or backfill the whole hole and compact it, then use the original drill bit and high-quality mud to sweep the hole; when the steel cage hits the hole wall and causes slight collapse, use a drill bit with a diameter smaller than the inner diameter of the steel cage to sweep the hole with high-quality mud or use a catheter to clean the hole.
Local necking of pile hole
Local necking refers to the local hole diameter being smaller than the designed hole diameter.
Cause analysis: Poor mud performance and large water loss. Cause plasticity, soil layer absorbs water and expands, or forms loose, honeycomb-like thick mud skin; improper spacing between adjacent piles, stress in soil layer has not yet dissipated, soft soil rheology of new hole wall; excessive wear of drill bit diameter.
Prevention and control measures Use high-quality mud, control mud specific gravity and viscosity, and reduce water loss; when the designed pile spacing is less than 4D, 1~2 piles should be skipped for construction; new piles should be drilled 36 hours after adjacent piles are piled as much as possible; select cage drill bits with double guide and environmental protection diameter; use mud and full-size drill bits to sweep the hole; pour concrete as soon as possible after clearing the hole.
Pile hole deviation and inclination
After drilling, the pile hole has a large vertical deviation or bending.
Cause analysis The drilling rig is not installed flat or there is loose soil under the drilling platform, resulting in uneven settlement; the pile frame is unstable, the drill rod guide frame is vertical, the drilling rig is worn, and the parts are loose; the casing is buried obliquely, the drill rod is bent, and the active drill rod is tilted; when encountering underground obstacles such as old foundations or large stones, the soil layer is uneven or the bedrock is tilted.
Preventive measures: The drilling rig is installed straight, horizontal, stable, and has no leading edge tangent point. The three points of the turntable center and the casing center are in a straight line. The casing is not skewed, the drill rod is not bent, the active drill rod is kept vertical, a guide frame is added, the lifting and guiding faucet is controlled, and the drill brace is used as much as possible to increase pressure; clear underground obstacles; in addition to the technical parameters of light pressure and slow rotation for soft and hard interlayers, when entering the hard plastic clay layer from the soft plastic clay layer, especially the fluid plastic clay layer and sand layer, or entering the bedrock from the clay layer, the conical guide small drill bit at the lower end of the cage drill needs to be replaced with a flat Use a small bottom guide drill bit, or directly drill with a flat bottom drill bit without a small guide drill bit; use caissons, controlled hole piles and other methods to remove underground obstacles; when the hard plastic clay layer deflects, use a mixture of sand and material soil to backfill 1~2m above the deflection, and use a flat alloy drill bit to gently press and slowly tilt after compaction; when the bedrock surface deflects, 20~40mm particle size crushed stone can be put in, slightly higher than the deflection, and after impact compaction, use a flat bottom alloy drill bit, a roller cutter drill or a flat bottom steel grain drill bit to correct the deflection.
Too much sediment at the bottom of the hole
The bottom of the hole is silted, the residual mud and sand are too thick, or the soil on the hole wall collapses at the bottom of the hole.
Cause analysis The hole is not clean, the specific gravity of the mud in the hole is too small or replaced with clean water; the steel cage is not vertically centered, and the soil on the hole wall collapses at the bottom of the hole; the time for filling after cleaning the hole is too long, and the mud settles; the bottom elevation of the hole measured by the sediment thickness is not uniform.
Preventive and control measures: After the hole is completed, raise the drill bit by 10-20cm from the bottom of the hole, keep it idling at a slow speed, and maintain a circulating hole cleaning time of not less than 30min; use high-quality mud for hole cleaning, control the mud density and viscosity, do not directly replace it with clean water, and slowly put the steel cage vertically into the hole; when using a flat-bottom drill bit, the sediment thickness is calculated from the hole bottom plane reached by the bottom of the drill bit; when using a cage drill bit with a round hammer at the bottom, the sediment thickness is calculated from the hole bottom plane reached by the bottom of the drill bit; or use a catheter for secondary water cleaning, and the punching time shall be based on the sediment thickness at the bottom of the hole measured in the catheter meeting the specification requirements; increase the impact force on the bottom of the hole when the concrete is initially poured, and the bottom end of the catheter shall be controlled at 30-40cm from the bottom of the hole. The amount of initial concrete pouring must meet the requirement that the bottom end of the catheter can be buried in the concrete for more than 1.0m, and use water-blocking plugs and concrete to flush residual sediment.
