Reasons and solutions for the inability to effectively remove the sediment at the bottom of the pile under cast-in-place piles construction

As an important form of modern foundation engineering, the construction quality of bored piles is directly related to the safety and durability of buildings. Excessive sediment at the bottom of the pile is a typical quality problem of this process, which often leads to serious consequences such as reduced bearing capacity at the pile end and increased settlement. This article will analyze the reasons and solutions for the inability to effectively remove sediment at the bottom of the pile from the perspective of the product used.

I. Causes of sediment at the bottom of the pile

1. Drill tool selection and structural defects

(1) Insufficient drill bit slag removal efficiency: The bottom opening area of ​​the traditional rotary drilling bucket does not match the slag space. When drilling into the sand layer or silt layer, the debris generated by the drill teeth cannot be discharged in time and a large amount remains at the bottom of the hole.

(2) Poor sealing of the drill bucket: The valve of the ordinary drill bucket is not closed tightly, and mud backflows during the drilling process, causing the cut soil to fall back to the bottom of the hole.

(3) Unreasonable drill tooth arrangement: When the in-line drill teeth are arranged, the overlapping rate of the cutting track is low, which is easy to form a "furrow effect", resulting in uneven bottom of the hole and accumulation of sediment in local areas.

2. Improper control of mud properties

(1) Low mud viscosity: When the mud viscosity is lower than 19s (funnel viscosity), the suspension capacity is insufficient and the sand particles settle quickly in a static state.

(2) Colloid rate does not meet the standard: The colloid rate of the mud prepared with inferior bentonite is less than 85%, and a stable colloid structure cannot be formed, resulting in the segregation of suspended particles.

(3) Defects in the circulation system: Insufficient mud pump displacement or unreasonable pipeline layout leads to a mud flow rate in the hole lower than 0.3m/s, which cannot maintain effective slag carrying capacity.

3. Deviation of construction process parameters

(1) Too long stagnation time after hole completion: The time from hole completion to concrete pouring exceeds 4 hours, and the mud is left standing, resulting in a significant increase in the thickness of the sediment.

(2) Insufficient hole cleaning times: In order to meet the construction deadline, some construction units only use a single positive cycle hole cleaning, which fails to completely remove suspended particles.

(3) Improper drilling speed control: In formations prone to hole collapse, when the drilling tool lifting speed exceeds 1.2m/s, negative pressure suction collapse occurs in the hole, bringing in additional sediment.

II. Solutions

1. Optimization and improvement of the drilling tool system

(1) Application of a special bottom cleaning bucket

As a key equipment for sediment treatment, the bottom cleaning bucket must meet the following requirements:

Double-valve bottom cover structure: A double-layer bottom cover with spring-assisted closing is used, and the closing gap is less than 2mm to prevent mud backflow when lifting the drill.

Side wall slag discharge hole design: A 50mm diameter round hole is added to the side wall of the bucket body, with a hole spacing of 150mm to form a multi-directional slag discharge channel.

Bottom reinforced cutting teeth: 8 groups of staggered pick-type alloy teeth are configured, and the cutting angle is controlled at 45° to ensure a flat bottom of the hole.

Operation points:

After the hole is completed, lower the bucket to the bottom of the hole, maintain the drill pipe speed at 15-20r/min, and run it idle for 3-5 cycles to disturb the sediment.

When slowly lifting the drilling tool, the bottom cover is sealed by the bucket body's own weight, forming a negative pressure adsorption effect.

The cleaning time of each bucket is controlled within 8-10 minutes, and the operation is repeated until the sand content of the return slurry is less than 2%.

(2) Use of combined drilling tools

Establish a two-stage operation mode of "rotary drilling bucket + cleaning bucket":

Use a short spiral drill bit with a diameter enlarged by 20mm to form a guide hole in the initial drilling

Use a drilling bucket to perform fine slag cleaning in the final drilling stage

Use an impact drill bit to break large-size stones in the pebble layer

2. Precise control of mud properties

Establish a "three-level mud control system":

Construction stage	Viscosity control (s)	Specific gravity control	Sand content	Colloid rate
Drilling stage	22-26	1.15-1.25	≤4%	≥90%
Cleaning stage	19-21	1.08-1.12	≤2%	≥88%
Casting stage	18-20	1.05-1.10	≤1%	≥85%

3. Innovation and improvement of construction methods

(1) Step-by-step hole cleaning process

First hole cleaning: Pump suction reverse circulation hole cleaning is used immediately after the hole is completed, and the cycle time is ≥30min

Second hole cleaning: Use a drilling bucket with positive circulation hole cleaning before lowering the steel cage, and the time is ≥20min

Final hole detection: Use a measuring rope with a flat-bottomed hammer for three-point measurement, and the sediment thickness is controlled at ≤50mm

(2) Dynamic drilling parameter control

In fine sand layer: The drilling speed is controlled at 0.8-1.2m/min, and the rotation speed is 18-22r/min

Before entering the bearing layer: reserve 0.5m and use slow drilling (0.3-0.5m/min)

Grade control of drilling speed: ≤0.8m/s within 5m of the bottom of the hole, ≤1.2m/s in the upper hole section

III. Key points of quality control

Drilling tool inspection: measure the closing gap of the bottom cover of the bucket before each drilling, and replace the sealing strip immediately if it exceeds 3mm.

Process monitoring: check the sand content of the return slurry every 2 buckets after cleaning the hole, and adjust the mud parameters if it exceeds 3%.

Final hole acceptance: use a cross-shaped measuring hammer to detect the sediment thickness, and the distance between the measuring points is not more than 1m.

Then how Drillmaster can support you for upon questions?

1) Drillmaster Multifunctional Ultrasonic Drilling Monitor - Multifunctional probe help you to test the thickness of sediment.

2) Drillmaster drilling bucket and cleaning bucket - Design for cleaning the sediment.

3) Drillmaster polymer mud - After dissolving, it is a polymer chain structure with high viscosity.

• Chain-shaped high-viscosity mud penetrates into the hole wall to achieve the wall protection effect

• Accelerate the sedimentation of residual drill cuttings at the bottom, and use the drill bit to salvage them so that there is no sediment at the bottom of the hole.