Direct Methods of Soil Exploration

Soil exploration is a critical phase in understanding the unique characteristics and properties of soil at a specific construction site. There are various methods by which soil exploration can be conducted, including direct, indirect, and semi-direct methods. 

Image Credits: Pearson Exploration

In this article, the direct methods of soil exploration are clearly explained. 

Direct soil exploration methods involve physically assessing the soil to directly observe samples or conduct tests. The main two direct methods are the trial pit method and the cone penetration test, where drilling, sampling, and testing of the soil occur simultaneously.

1. Trial Pits and Trenches

Trial pits, also known as test pits, are a type of subsurface ground investigation used to visually determine the ground conditions before construction begins. They are commonly employed to explore shallow ground conditions, especially when buried structures, variable conditions, or contamination are suspected.

Trial pits involve excavating pits or trenches either by hand or using a backhoe excavator, typically to depths ranging from 3.5 to 4.5 meters. If the pit is deeper than 1.2 meters and intended for human entry, precautions against structural collapse must be taken.

Fig.2. Trial Pits and Trenches in Soil Exploration

These pits are cost-effective compared to boreholes but have limitations in depth. Despite their quick excavation, trial pits can cause significant surface disturbance and pose challenges in filling them back properly once their purpose is served.

Excavation methods can be either machine-based or hand-dug, with each having its advantages and applications based on site-specific requirements. Hand-dug pits are often preferred for smaller-scale investigations or areas with known subsurface structures. If there are known subsurface structures e.g., utilities, water, electric cables, etc then the test pits would be dug by hand, while machine-excavated pits are suitable for greater depths over larger areas, albeit with potential ground disturbance.

Despite their limitations, trial pits offer a rapid and relatively inexpensive means to understand soil profiles and assess ground conditions, aiding in foundational designs, contamination analysis, and other sampling methods. Various methods, including hand shear vane, soak away, and CBR testing, are employed within trial pits to gather soil samples and assess ground characteristics. 

2. Cone Penetration Test (CPT)

A cone penetration test (CPT) is a direct soil exploration method performed directly on the ground surface. The test setup consists of a cone penetration rig that pushes a steel cone (about 32mm wide) into the ground to a depth of around 20m below the ground surface or until it touches a hard strata. The steel cone consists of an electronic measuring system that records the tip resistance and sleeve friction while penetrating the surface. The sensors in the cone tip measure the degree of resistance and the sensors in the friction sleeve measure the sleeve friction as shown in Figure -3. 

Fig.3. Cone Penetration Test (CPT)

When the cone is pushed into the ground, they come in contact with different layers of the soil that would respond with different degrees of resistance. This variation in resistance is recorded by the tip of the cone using force sensors. Simultaneously, the friction sleeve records the sleeve friction along a 100mm length. In certain cases, the cone possesses a pore water transducer that helps to record the groundwater responses as the cone is pushed through the soils. 

The CPT test takes between 30 minutes to 3 hours. During the penetration, the measurements are sent back to the rig and recorded on a computer for further analysis. The obtained data is called the trace. 

Geotechnical engineering specialists utilize cone penetration test results to comprehend soil properties, including relative density and soil behavior type. These properties are determined from the cone penetration test's cone tip resistance and sleeve friction. Additionally, the results help predict how the ground will respond to various levels of earthquake shaking.

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