The process of determining the adequate number and sufficient depth of boreholes is a critical aspect of gathering necessary data for the design and construction of various geostructures, including bridges, cut slopes, embankments, foundations, and retaining walls. There is no fixed rule for determining the number and depth of boreholes, and several factors come into play, such as the significance of the structure, the geological character of the ground, and the availability of equipment.
To determine the minimum number and depth of boreholes, it is essential to make use of engineering experience and judgment, while also adhering to the guidelines provided in applicable codes. The number and depth of boreholes should be sufficient to cover the soil zone that will be impacted by the load of the geostructure. This will ensure that the necessary information is gathered to facilitate proper design and construction.
How to determine Number and Depths of Boreholes for Geostructures?
When conducting soil investigations at a construction site, borehole numbers can provide valuable information about soil-layer variations. It is recommended to drill at least one borehole at the position of the heaviest load if the load is within the boundary of the structure. The minimum number of boreholes required can be determined based on the area of the proposed buildings and due diligence in subdivisions, as shown in Table-1.
For most cases, the minimum depth of boreholes should be 6m, unless the soil is very dense or there is bedrock present. In such cases, the borehole should extend at least 3m into the bedrock. In situations where the soil is dense, coarse-grained, or very stiff clays, it may be necessary to increase the boring depth to a range of 5-6m to verify the thickness of the soil layers.
Boreholes must also penetrate very soft deposits or fills below the proposed geostructure. In compressible soils like clay, the boring depth should be determined based on the greater value between the foundation width multiplied by a factor between 1 and 3, or the depth at which the stress increment caused by the heaviest foundation load is smaller than 10%.
Table-2 provides guidelines for the minimum number and depth of boreholes for common geostructures. By following these recommendations, a comprehensive understanding of the soil conditions at the construction site can be obtained, aiding in the design and construction of safe and stable structures.
| Building area, m2 | Number of boreholes | Subdivision area, m2 | Number of boreholes |
| 100 | 2 | 4000 | 2 |
| 250 | 3 | 8000 | 3 |
| 500 | 4 | 20000 | 4 |
| 1000 | 5 | 40000 | 5 |
| 2000 | 6 | 80000 | 7 |
| 5000 | 7 | 40000 | 15 |
| 6000 | 8 | – | – |
| 8000 | 9 | – | – |
| 10000 | 10 | – | – |
| Geostructure types | Minimum borehole depth | Minimum number of boreholes |
| Shallow foundation | 5m, or foundation width time a value ranging from 1 to 3 | One but place boreholes at node points along grids of sizes from 15mX15m to 40mX40m |
| Deep foundation | 25-30m except for bedrock which is 3m | One but place boreholes at node points along grids of sizes from 15mX15m to 40mX40m |
| Retaining wall | 1 to 2 times wall height, 3m for walls on bedrock | One borehole for wall-length smaller than 30m, For wall-length longer than 30m; 1 for every 30m or wall height times a value between 1 to 2 |
| Bridge | 25-30m except for bedrock which is 3m | Two boreholes for piers, Two boreholes for abutments |
| Embankments including roadway | Either 6m or two times embankment height, whichever is greater | 1 for every 60m If soil does not vary crucially, otherwise 1 for every 120m |
| Cut slopes | 6m below the bottom of the cut slope | On slope: 3 boreholes Along the length of the slope: 1 for every 60m If soil does not vary crucially, otherwise 1 for every 120m |
FAQs
What is the purpose of borehole?
Boreholes play a crucial role in construction site assessments, as they allow engineers to gather essential information about the site’s soil composition. By drilling into the ground and extracting soil samples, engineers can perform a variety of tests to determine the soil’s characteristics, such as its density, porosity, and stability. This information is vital for developing safe and effective construction designs and processes.
Without boreholes, engineers would have to rely on less accurate methods of assessing soil conditions, such as visual inspections or surface-level testing. These methods often provide limited information, which can lead to design flaws or construction errors. By using boreholes to collect comprehensive soil data, engineers can make informed decisions about the site’s suitability for construction and design structures that can withstand the soil’s specific characteristics.
Overall, boreholes are an essential tool for any construction project, providing critical information that can help ensure the safety and success of the project. By using borehole data to inform construction processes, engineers can minimize risks, avoid costly errors, and create structures that are built to last.
What should be the depth of borehole in bedrock?
The requirement is to have a borehole that penetrates bedrock with a depth of 3 meters.
How many boreholes are required for bridge construction?
To ensure the stability and safety of a bridge, a minimum of two boreholes are required for each abutment and two boreholes for each pier. These boreholes provide important information about the soil and rock conditions at the site, such as its bearing capacity and stability, which are critical factors in determining the type and size of foundation needed to support the bridge structure.
Without this information, it would be difficult to design a foundation that could withstand the loads imposed by the bridge and the surrounding environment. By conducting borehole investigations, engineers can make informed decisions about the foundation design and construction, which ultimately contribute to the overall safety and longevity of the bridge. Therefore, it is essential to conduct thorough borehole investigations before constructing any bridge abutment or pier.
How many boreholes are needed for adequate soil report?
Determining the number and depth of boreholes is not governed by a specific rule. As a result, there are a variety of factors that can influence this decision. These factors may include the structural significance of the site, the geological composition of the ground, and the availability of equipment.
When considering the number and depth of boreholes, the importance of the structure being built or investigated must be taken into account. This can help determine how many boreholes are required and at what depths they should be drilled.
Another key factor to consider is the geological character of the ground. The type of soil and rock formations in the area can affect how deep and how many boreholes are necessary. Understanding the geology of the area can also help identify potential challenges and hazards that may arise during the drilling process.
Equipment availability is also an important consideration. The type of equipment available, and the resources required to operate it, can impact the number and depth of boreholes that can be drilled. In some cases, the availability of equipment may limit the number or depth of boreholes that can be drilled, which can impact the quality of data collected during the investigation.
What is the minimum depth of boreholes for shallow foundation?
According to the guidelines for constructing shallow foundations, the minimum depth of a borehole must be 5 meters. Alternatively, the depth of the foundation can be determined by multiplying the foundation width by a factor ranging from 1 to 3. This information is important for those involved in the planning and implementation of shallow foundation construction projects, as it provides a standard for determining the appropriate depth of boreholes. Adhering to these guidelines can help ensure the stability and safety of the foundation, as well as the structure it supports. It is therefore crucial to take these guidelines into account during the planning and construction phases of any shallow foundation project.