Geotechnical Soil Investigation Techniques

Understanding Soil Exploration: Test Pits & Boreholes

Test Pits and Trial Pits

Test pits, also known as trial pits, are excavations made in the soil to observe and analyze the subsurface layers. These pits are typically dug to a depth of less than 5 meters and have a width of no more than 0.75 to 1 meter. Key requirements for test pit excavation include:

• Wall stability to ensure safety and accurate observation.
• Excavation above the Groundwater Level (GWL) where possible.
• Soil excavation primarily by mechanical means.

Test pits are utilized to explore soil conditions and the sequence of strata up to a depth of 4-5 meters. They also allow for the collection of soil samples for subsequent laboratory testing.

In-Situ Soil Testing Tools

Several tools are used for in-situ (on-site) testing to determine immediate soil properties:

• Pocket Penetrometer: This tool features a spring that compresses when pressed into the soil. It is used to obtain unconfined compression strength and undrained shear strength.
• Hand Vane Test: A rod-like instrument that is torqued until soil failure occurs. This test provides the undrained shear strength of the soil.

Borings and Boreholes for Subsurface Investigation

Borings, or boreholes, are drilled holes designed to explore and define the soil profile at greater depths. They typically have a diameter between 65-140mm. Boreholes facilitate the extraction of cores and samples essential for soil identification and laboratory analysis. In-situ tests can also be performed within boreholes at various depths to acquire specific soil properties.

Other significant characteristics of boreholes include:

• Unlimited depth capability, allowing investigation through very deep strata.
• Ability to penetrate hard layers.
• Feasibility of drilling below the groundwater level.
• Use of slurry or casings for wall stability, preventing collapse.

Types of Drilling Methods

Rotary Drilling with Continuous Core

In rotary drilling, a drill bit creates an angular hole into the soil. As drilling progresses, the soil core enters the core barrel. Once the barrel is filled, the core is retrieved. To obtain additional cores from the same borehole, a rod is used to drive the drill bit deeper into the soil. Water is sometimes used to lubricate the cutters and aid in drilling.

Two primary types of core barrels are used:

• Simple Tube Core Barrel: The core is not protected, and water can penetrate the soil, potentially disturbing the sample.
• Double Tube Core Barrel: The core is protected by an extra tube, ensuring the sample remains undisturbed. Samples obtained with this method are suitable for advanced laboratory tests like triaxial or direct shear tests.

Auger Drilling

Auger drilling utilizes an auger to penetrate the soil. This method is primarily suitable for cohesive soils. The auger features a blade at its end that cuts and advances the rod deeper into the soil.

Common auger types include:

• Short-Flight Auger: Requires periodic stopping to empty the instrument before continuing drilling.
• Continuous-Flight Auger: Allows for continuous drilling without interruption.

While solid augers do not allow for core retrieval, a hollow stem auger can be used for this purpose. Once the soil is bored to the desired depth, a sample can be introduced into the hollow part of the auger, allowing for core extraction.

Percussion Drilling

Percussion drilling is employed in hard, well-cemented soils, as well as in boulder and gravel layers. This technique typically does not yield intact core samples.

Types of percussion drilling tools include:

• Bottom-hole hammer
• Tricone (featuring conical rollers that bore the soil)
• Tetraleta