Subsurface investigations require the collection of soil samples for visual inspection or laboratory testing. To do this, boring techniques such as auger boring, rotary drilling, wash boring, percussion drilling, auger drilling, and test pits are employed. The choice of boring method depends on the soil type, the efficiency of the technique, the type of soil sample required (disturbed or undisturbed), and the availability of equipment and the accuracy needed to determine soil and groundwater variations.
Different techniques are used to obtain soil samples at practical depths. For example, auger boring is used to collect undisturbed soil samples at a depth of 35m, whereas percussion drilling, wash boring, and rotary drilling are used to collect soil samples at depths around 70m. The selection of the appropriate boring method is crucial to ensure that the collected samples are representative of the soil being investigated.
Boring Methods for Soil Sampling
1. Auger Boring
Auger boring is a budget-friendly and straightforward method that can be utilized on almost all soil types, except for rocks and gravelly soil. While this technique may face some difficulties in gravelly soil, it is still possible to use special drilling bits to overcome the challenge.
Auger boring is commonly utilized to collect disturbed soil samples. It is capable of gathering soil samples from depths of up to 35 meters, provided that there is enough time and the appropriate equipment is available.
Fig. 1: Auger Boring Method
2. Rotary Drilling
The rotary drilling method is a versatile technique that is suitable for boring through all types of soil, including rocks. This method can be used to extract both disturbed and undisturbed soil samples, making it particularly well-suited for stiff soil layers. The practical depth for soil sampling using this method is typically around 70 meters, although this may vary depending on the equipment being used.
To collect undisturbed soil samples, various thin-walled tube samplers and piston samplers are typically employed. These instruments are capable of extracting soil samples with a diameter of approximately 100 mm, although larger samples of between 150mm to 200mm may be taken for rock specimens. Overall, the rotary drilling method is a reliable and effective means of obtaining high-quality soil samples for analysis and testing purposes.
Fig. 2: Rotary Drilling Method
3. Wash Boring
The wash boring method is commonly employed to collect both disturbed and undisturbed soil samples in various soil types, with the exception of rocks. The technique involves the use of inexpensive and portable equipment, making it a favorable option. Additionally, the use of thin-walled tube samplers and piston samplers in the process enables the recovery of undisturbed soil samples with diameters ranging from at least 50 mm to a maximum of 100 mm. This allows for the efficient collection of soil samples while minimizing the degree of disturbance to the samples during the sampling process, similar to the rotary drilling method.
Fig. 3: Wash Boring Technique
4. Percussion Drilling
Percussion drilling is a method that can be used on various types of soils and rocks, including stiff ones. It is used to extract both disturbed and undisturbed specimens from the ground, although the quality of the undisturbed samples may not be as good due to the forceful impact of the chisel during the drilling process.
This method is comparable to other drilling techniques, such as rotary drilling and wash boring, as it allows for the extraction of soil specimens from depths of 70 meters or more, depending on the specific equipment being used. However, the diameter of the extracted soil samples is typically around 100mm or greater, and obtaining smaller diameter samples can be costly and impractical.
Overall, percussion drilling can be a useful tool for gathering soil and rock samples, although it may not be the best option for obtaining high-quality undisturbed samples or smaller diameter specimens.
Fig. 4: Percussion Drilling
5. Test Pits and Open Cut
A soil sample collection technique is employed for all types of soils, which involves the use of power equipment to extract both disturbed and undisturbed soil samples from the earth. The maximum practical depth for this technique is 6 meters. After extraction, the samples are manually trimmed for further use or analysis.