1,Characteristics of soft soil and other easily collapsible strata and risks of borehole formation

In soft soil, newly deposited backfill, and recently deposited loose fine sand layers, if the groundwater level is high, even with mud slurry wall protection, the borehole wall is difficult to stabilize after the rotary drilling rig excavates the soil, easily leading to borehole collapse or necking. Since these strata are mostly distributed in the shallow surface, as the collapse volume gradually increases, the ground around the borehole will subside, often causing the casing to sink. In severe cases, this can also threaten drilling equipment and personnel. When encountering such situations, especially if the casing at the borehole opening is found to be suspended or sunken during construction, drilling should not be continued with the hope of avoiding the problem. Instead, the borehole should be backfilled as soon as possible, the casing re-secured, and drilling resumed.

2,Rotary drilling construction method

In water-bearing strata such as soft soil and recently backfilled soil, where mud slurry wall protection is ineffective, the most efficient and safest rotary drilling method is to use long casing walls. Each rotary drilling rig is equipped with about 3 steel casings, 1 vibratory hammer, and 1 crane with sufficient lifting capacity, depending on the difficulty of the drilling. The length of the steel casing can be lowered to the bottom of the soft layer, and the diameter of the steel casing should be about 100mm larger than the pile diameter (too large will waste concrete; too small will easily cause the drill bit to get stuck in the casing, making it difficult to ensure smooth construction).

3,When using long casing for rotary drilling, the following points should be noted:

(1) It should be basically confirmed that there are no large-diameter stones larger than 100mm within the range of the casing's lowering depth; select a vibratory hammer with appropriate power according to the diameter of the long casing and the length of each lowering.

(2) When constructing small-diameter drill pipes, the outer diameter of the drill pipe and the diameter of the casing should be considered together. The outer diameter of the drill pipe should be at least 200mm smaller than the diameter of the casing. When the casing length is large, the difference should be even greater to facilitate construction.

(3) The verticality of the steel casing should be ensured, especially for smaller diameter bored piles.

(4) The casing should be removed promptly after the concrete is poured.

(5) The placement of the steel casing and rotary drilling should be closely coordinated. After the steel casing is pulled out, it should be able to be directly placed into the next borehole to be drilled. This forms a continuous operation, which will greatly improve construction efficiency.