Treatment Plan for Buried Drilling Tools in Backfill Strata with Large Boulders

Construction in backfill strata is often accompanied by hole collapse, and occasionally by buried drilling tools. Therefore, before drilling, it is necessary to accurately investigate the backfill conditions of the pile hole and to properly master suitable drilling methods and techniques.

▲ Backfill strata containing large boulders

This article introduces a treatment method for hole collapse and buried drilling tools in backfill strata mixed with large boulders and cobbles, hoping to provide some reference for construction in backfill strata. This incident occurred on Yunan Avenue, Banan District, Chongqing. The geology consists mainly of backfill soil: from 1 to 16 m, the backfill soil is mixed with cobbles and boulders; from 16 to 22 m, the backfill soil is mixed with relatively large isolated rocks and boulders; from 22 to 25 m is native soil; and from 25 to 35 m is bedrock, with rock hardness between 30 and 40 MPa. The pile diameters for this project are mainly 1.4 m and 1.6 m, and the pile depth is about 32 m.

▲ Distribution of backfill strata

When the borehole reached a depth of 32 m, hole collapse occurred. The operator then tried rotating the power head left and right and slowly lifting the main winch in an attempt to raise the drilling bucket. After half an hour of effort, the bucket was finally lifted by 1 m. Unfortunately, hole collapse occurred again, causing the drilling bucket and drill rod to be unable to rotate any further, and the main winch could no longer lift. So what should be done?

Option 1

Continuing to try rotating the power head left and right and slowly lifting the main winch resulted in only 20 cm of lifting over 5 to 6 hours. Therefore, this option was abandoned.

Option 2

By analyzing the causes of the stratum, this formation contains large isolated rocks and boulders; hole collapse may have caused a boulder to form a protruding rock, that is, half embedded in the soil and half protruding into the pile hole, firmly compacting the drilling tool so that the main winch could not lift it. The situation is shown in the figure below:

▲ Protruding rock pressing down on the drill

Solution: Use a drilling rig to drill downward along the circumference of the hole wall, aiming to break the protruding rock in the middle, thereby reducing resistance in the pile hole, and then try lifting the main winch to raise the drill rod and bucket.

▲ Using a core drill to drill around the hole wall and cut off the protruding rock

Facts proved that during drilling, the drilling rig could clearly feel rock at the bottom, and core samples could be retrieved, which confirmed the correctness of the above scheme. Therefore, the project department persevered, and after 5 days, the protruding rock was finally cut off and the drilling bucket and drill rod were lifted.

In addition, during the lifting of the main winch, a high-pressure water gun was also used to wash away the rock and soil above the drilling bucket to reduce the load on the main winch.

▲ Core drilling construction and retrieved cores

▲ Normal construction after the drilling bucket was retrieved