Solving Drilling Sampling Errors: Techniques for Backfill & Soft Soil

The core of geological exploration sampling is obtaining "authentic, complete, and undisturbed" core/soil samples, which directly impacts stratigraphic analysis and engineering decisions. However, in actual construction, problems such as interference from backfill debris, borehole wall collapse in soft soil, and sample loss from sand layers often lead to inaccurate sampling. The core solution is to "adapt operations to stratigraphic characteristics, controlling accuracy throughout the entire process from obstacle removal, borehole protection, and sampling." The following practical techniques for different stratigraphic layers can be implemented on-site.

I. Backfilled Soil Strata: Remove obstacles first, then sample to avoid contamination

Sampling Challenges

Contains bricks, gravel, rebar, and other debris, which can easily damage the drill bit and contaminate the sample; loose borehole walls are prone to collapse, leading to discontinuous sampling.

Accurate Sampling Techniques

Preliminary Obstacle Removal: Before starting the drill, use a shovel to probe for shallow obstacles (0-3 meters), manually removing large pieces of construction waste and rebar; for deep obstacles, use the "casing isolation method," inserting a casing with a diameter 20-30mm larger than the drill bit to protect the borehole wall and isolate debris.

Drill bit compatibility

Select a wide-pitch, high-slag-removal drill bit. Control drilling pressure at 8-12 kN and rotation speed at 60-80 r/min. Proceed slowly at low speed to avoid debris impacting the drill bit.

Sampling procedure

Clean the drill bit every 2-3 meters to remove slag. Use high-pressure air to blow out loose debris from the hole. Use a semi-closed core sampler to ensure the sample is not contaminated with debris. Immediately after sampling, screen and remove impurities, marking the clean sample section.

Accuracy assurance: After clearing obstacles, verify the borehole wall verticality (error ≤ 0.3°) to avoid sampling deviations caused by borehole inclination.

II. Soft Soil Strata: Focus on Hole Protection, Minimize Disturbance to Prevent Sample Deformation

Sampling Challenges

Soft soil and silt are loose in texture, making the borehole wall prone to collapse; disturbance from the drill bit can easily lead to sample compression and deformation, causing loss of original characteristics.

Precise Sampling Techniques

Borehole Protection Measures: Use mud slurry for wall protection (concentration 1.1-1.2 g/cm³), or pre-support with thin-walled casing to prevent borehole wall collapse; avoid prolonged drilling stoppages (soft soil easily consolidates and buries the drill bit, disturbing the sample).

Parameter Control

Drilling pressure 5-8 kN, rotation speed 40-60 r/min, feed rate 2-5 mm/s; low speed and low pressure reduce disturbance to soft soil; use a spiral core extractor to reduce friction and compression between the sample and the drill bit.

Sampling Protection

When lifting the core sampler, the speed should be ≤0.2m/min to avoid violent vibrations that could cause sample detachment. After removal, gently rinse the surface mud with clean water, immediately place the sample in a sealed sample box, and label it with the sampling depth and formation name. Avoid direct sunlight exposure (soft soil is prone to cracking).

Accuracy Guarantee: Complete the sealing within 2 hours of sample removal. Do not press or rub the sample by hand to ensure the integrity of its original structure.

III. Other Common Formations: Targeted Operating Techniques

Sand Formations

Use a double-layer borehole protection system of "mud + casing" (mud concentration 1.2-1.3g/cm³), select a bag-type core sampler (to prevent sand sample loss), and increase the mud flow rate (30-40L/min) during drilling to carry sand particles out.

Hard rock formations

Drill pressure 12-18 kN, rotation speed 80-100 r/min, using a diamond drill bit, clean the hole every 5-6 meters; use a diamond coring tube for core sampling, with a uniform lifting speed (0.3-0.5 m/min) to avoid core breakage.

Fractured zones

Reduce drill pressure (6-10 kN), use segmented coring (coring every 1-2 meters), and fix the fractured samples with a binder (such as epoxy resin) to prevent scattering.

Conclusion

The core issue of inaccurate geological exploration sampling is "mismatch with formation characteristics": backfill soil emphasizes "obstacle removal and isolation," soft soil emphasizes "hole protection and disturbance prevention," sand layers emphasize "loss prevention," and hard rock emphasizes "fracture prevention." During field operations, it is necessary to first investigate the formation type, then adjust the drilling tools, parameters, and sampling methods according to the above techniques, while ensuring proper sample sealing and labeling. Remember: The key to sampling is to "respect the characteristics of the strata and reduce human disturbance" in order to obtain true and reliable exploration data.