Construction Challenges in Pebble Strata
Pebble strata consist of loose particles and possess poor structural stability, making them prone to particle displacement during drilling operations. Although the high-frequency impact of a Down-the-Hole (DTH) hammer can effectively crush pebbles, the associated vibrations may exacerbate the loosening of the borehole walls. If there is significant variation in pebble size or if clay interlayers are present, the borehole walls become even more susceptible to instability. Under these conditions, drilling without the use of casing significantly increases the risk of borehole collapse.
Operational Characteristics of DTH Hammers
Concentrated Impact Force: Energy is concentrated at the face of the drill bit, resulting in minimal disturbance to the borehole walls.
High Debris Removal Efficiency: The airflow effectively and promptly carries away cuttings, thereby reducing the risk of secondary collapse.
Limited Adaptability: In completely loose geological formations, the impact shockwaves may disrupt the existing structural equilibrium.
Practical Strategies for Preventing Borehole Collapse
The use of casing is recommended when encountering the following conditions:
The pebble stratum exceeds 3 meters in thickness and exhibits high moisture content.
Groundwater flow is present and actively eroding the borehole walls.
The project requires maintaining the integrity of the borehole diameter for an extended period.
Alternative solutions include utilizing polymer-based drilling fluids to stabilize the borehole walls, or employing a casing-following drill string assembly to provide temporary support to the walls simultaneously with the drilling process.