Open Excavation Method of Soil Exploration

Open excavation, often referred to as the trial pit or test pit method, is one of the a direct exploration method performed to study the subsurface conditions of a site. This soil exploration method involves digging pits at specific locations to examine the soil layers first-hand. 

Soil exploration is a critical step in any construction project, as it provides essential information about the ground's composition, strength, and water conditions. This information is vital for designing appropriate foundations and structures.

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This article will delve into the details of open excavation, explaining its purpose, process, advantages, and limitations. We'll also explore when it's the most suitable method and provide a quick guide to conducting a successful open excavation.

What is Open Excavation?

Open excavation are pits that are excavated at required intervals or locations near the area of construction, to study the sub-surface strata and the strength of the soil. We perform this exploration to collect soil samples in areas where a building structure and foundation needs to be planned.

Note: 
Trial Pit are intrusive method of soil exploration. Here, Intrusive means physically disturbing the ground to gather information. This involves techniques like drilling boreholes, excavating trial pits, or probing the soil to obtain direct samples and measurements.

Trial pits are excavated at the site such that the subsoil is exposed thoroughly. We will see various levels of strata, and soil samples are collected from each stratum.

In some situations, while excavating, groundwater is encountered. This is seen in pervious soils. In such cases, we provide additional lateral support and ground water-lowering methods, to avoid collapse of the soil layers.

When depth is greater than 1.2 m, in some sites, lateral support is provided to prevent structural collapse.

Objectives of Open Excavation Method

Open excavation, or the trial pit method, serves several critical objectives in soil exploration:

1. Subsurface Profile Characterization: To provide a detailed understanding of the soil layers, their composition, thickness, and arrangement beneath the ground surface.
2. Soil Bearing Capacity Assessment: To determine the soil's ability to support the proposed structure's loads by evaluating its strength and compressibility characteristics.
3. Groundwater Level Determination: To identify the depth of the water table, which is essential for foundation design and construction planning.
4. Excavation Feasibility Assessment: To evaluate the potential for soil instability or collapse during excavation for the foundation.
5. Site Grading and Preparation: To identify areas of varying soil conditions that may require adjustments, such as trimming, ramping, or leveling.
6. Planning Home Extensions: They provide crucial information about the ground conditions, foundation depth and strength, which is vital for accurate structural design and cost estimation.

Location of Trial Pits

• The location of the trial pits must not be made too close or not too far away from the foundation location proposed.
• Around 4 to 5 trial pits are excavated near the building periphery.
• The minimum dimension of the trial pits excavated is 1.2 x1.2 m considering a plan size to provide access for the operators or workers.
• The maximum economic depth of the trial pit is 4 m, and the minimum depth is 1.5 m or the depth of the foundation proposed.
• Trial pits are observed for a few days to observe the entry of water. This helps to understand the source of water passage and helps to close or divert the water channel.

A Quick Guide to Open Excavation

Conducting a successful open excavation requires careful planning and execution. Here’s a basic guide:

Planning and Preparation 

• Site selection: Choose locations that represent the site's conditions and avoid areas with potential hazards.
• Equipment: Assemble necessary tools, including shovels, picks, buckets, and safety gear.
• Permits: Obtain any required permits or approvals from local authorities.
• Safety precautions: Implement measures to protect workers from cave-ins, equipment hazards, and other risks.

Excavation and Sampling

• Digging: Excavate the pit to the desired depth, ensuring stable slopes. Excavation methods can be either machine-based or hand-dug. 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.
• Soil description: Observe and record soil color, texture, structure, and moisture content.
• Soil sampling: Both disturbed and undisturbed samples can be collected, depending on the required tests. Disturbed samples are suitable for general soil properties, while undisturbed samples are crucial for determining the soil's engineering characteristics.
• Groundwater observation: Measure water levels and infiltration rates if necessary.

Data Recording and Analysis

• Documentation: Maintain detailed records of pit locations, depths, soil descriptions, and sample details.
• Photographs: Take photos of the pit and soil profiles for reference.
• Laboratory testing: Conduct appropriate tests on soil samples to determine properties like grain size, plasticity, and strength.
• Data interpretation: Analyze the collected data to assess soil conditions and make engineering recommendations.

Advantages of Open Excavation

• Direct observation: Allows for direct visual inspection of soil profiles and layers.
• Cost-effective: Generally less expensive than other exploration methods for shallow depths.
• Versatility: Suitable for various soil conditions and project types.
• Detailed sampling: Enables collection of soil samples for thorough analysis.
• Groundwater assessment: Can help determine groundwater levels and infiltration rates.

Limitations of Open Excavation

• Depth restrictions: Not suitable for deep excavations.
• Safety hazards: Potential for cave-ins, especially in unstable soils.
• Site accessibility: May be difficult in restricted spaces.
• Weather dependence: Can be affected by adverse weather conditions.
• Environmental impact: Potential for soil disturbance and erosion.

Safety Measures for Trial Pits

• Site Security: To prevent accidents and injuries, trial pits should be adequately fenced off to alert and restrict access for unauthorized personnel.
• Pit Closure: Once soil investigations and sampling are completed, the trial pit must be promptly filled and covered to eliminate hazards and restore the site to its original condition.

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