Rotary drilling rigs have become indispensable in modern foundation engineering, with bit selection being the critical factor determining project success. Today we will talk about how to choose a suitable rotary drilling rig drill bit based on the geological conditions of the construction site.
What Are The Types of Drill Bits?
Soil-Layer Double - Bottom Sand - Grabbing Drilling Buckets
These are the most commonly used bits in the industry. They are suitable for various soil conditions, including silt, fine sand, silty clay, and some soft rock formations, such as completely and strongly weathered mudstone and mudstone sandstone.
Rock-Socketed Double - Bottom Sand - Grabbing Drilling Buckets
Ideal for cobble and gravel layers, moderately weathered soft rock like moderately weathered mudstone sandstone and mudstone conglomerate. For extensively weathered hard rock, such as thoroughly and strongly weathered granite, it is often necessary to use barrel drills and spiral bits in combination when drilling hard rock.
Barrel Drills
Barrel drills are effective for cored drilling in moderately weathered sandstone with distinct stratification. They can also be used for circumferential cutting in hard rock layers. In cases where friction drill pipes slip in densely compacted soil layers or some soft rock strata, barrel drills can be a viable solution.
Rock-Socketed Spiral Bits
These bits are designed for breaking up boulders, isolated rocks, and hard rock layers in the borehole. For sticky clay strata and moderately compacted cobble-soil layers where soil - layer double - bottom sand - grabbing drilling buckets fail to penetrate, spiral bits can be tried.
Double-Layer Barrel Drills
Specifically designed for drilling in cobble and boulder layers with particle sizes ranging from 200 to 500mm. The double-layer barrel drill uses a unique “squeezing” drilling concept, which squeezes and densifies the cobbles and boulders of various sizes inside the barrel before bringing them out of the borehole.
Soil-Layer Single - Bottom Sand - Grabbing Drilling Buckets
Similar in function to double-bottom sand-grabbing drilling buckets, single-bottom sand-grabbing drilling buckets feature side soil-entry ports and do not require reverse drilling to close the bucket door, resulting in higher drilling efficiency in soil layers. However, their lighter weight may cause slipping and poor penetration in some hard soil layers, especially when used with friction drill pipes.
Soil Spiral Bits
Suitable only for small-diameter piles in clay layers with high cohesion. The advantage of soil spiral bits lies in their maximum height of up to 4m, allowing for large single-pass penetration. They can effectively avoid the problem of difficult slag discharge encountered by other bits in such projects.
Double-Bottom Single-Door Drilling Tools
Typically used for drilling tools with a diameter ≤ 1.2m. Compared to double-bottom double-door drilling tools, they have a larger single-side soil-entry port, which is more conducive to drilling in certain strata, such as cobble strata with particle sizes below 200mm.
Selecting Bits Based on Geological Conditions
The main function of rotary drilling rigs is to create holes and trenches on the surface.
Clay
For clay layers, single-bottom rotary drilling buckets are recommended. For smaller diameters, double-blade drilling buckets or drilling buckets with soil-discharge plates can be used.
Silt, Weakly Cohesive Soil, Sand, and Cobble Layers
In layers with poor cementation and small particle sizes such as silt layers, weakly cohesive soil layers, sand layers, and cobble layers, double-bottom drilling buckets are a good choice.
Hard Clay
For hard clay, rotary drilling buckets with single-layer soil-entry ports (either single-bottom or double-bottom) or rotary drilling buckets with bucket-tooth straight spirals can be selected.
Frozen Soil
B bucket-tooth straight spiral and rotary drilling buckets can be used for frozen soil layers with low ice content. For layers with high ice content, conical spiral bits are more suitable. It should be noted that spiral bits are effective for soil layers (except for silty sand), but they must be used in the absence of groundwater to avoid water absorption and soil jamming.
Well-Cemented Gravel and Strongly Weathered Rock
Conical spiral bits and double-bottom rotary drilling buckets (single-port for larger particle sizes and double-port for smaller particle sizes) are required for well-cemented gravel and strongly weathered rock.
Medium-Grade Bedrock
For medium-grade bedrock, rotary drilling buckets equipped with pick-tooth core bits—conical spiral bits—double-bottom, or rotary drilling buckets equipped with pick-tooth straight spiral bits—double-bottom can be used.
Slightly Weathered Bedrock
For slightly weathered bedrock, conical core bits, conical spiral bits, and double-bottom rotary drilling buckets are needed. For overly large diameters, a staged drilling process must be adopted.
Selecting Bits Based on the Functions of Rotary Drilling Rigs
Continuous Construction Function
If the rotary drilling rig has a continuous construction function, liquid grab buckets or double-wheel milling machines can be selected for continuous construction.
Automatic Rope-Release Function of Auxiliary Winches
If the auxiliary winch has an automatic rope-release function, impact bits and impact grab bits can be used to drill large-diameter cobbles or boulders.
Gas-Liquid Drilling
If the rotary drilling rig is equipped with an air chamber and a swivel, gas-liquid drilling can be carried out.
Casing Drilling
If the rig is equipped with a double-power head or a pipe-winding machine, casing drilling can be performed. This is an effective solution for dealing with problems such as quicksand, broken cobble-gravel, and silt layers.
By carefully considering the geological conditions of the construction site and the functions of the rotary drilling rig and selecting the appropriate drilling bits, we can significantly improve the efficiency and quality of pile foundation construction projects.
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