Setting up a borehole layout is an essential part of site investigation for construction projects. The layout consists of multiple boreholes arranged in different plans according to the requirements. This arrangement helps in collecting data about the subsurface conditions of the site.
Various methods can be used to set up a borehole layout. One commonly used method is a grid layout, where the boreholes are arranged in a rectangular or square pattern at equal distances. This method is suitable for sites where the subsurface conditions are relatively uniform.
Another method is the triangular layout, where the boreholes are arranged in a triangular pattern. This layout is useful in sites with irregular subsurface conditions, as it provides better coverage and more data points.
In some cases, a radial layout is used, where boreholes are arranged in a circular pattern around a central point. This layout is suitable for sites where the subsurface conditions change significantly with distance from the center.
Regardless of the method used, the borehole layout should be carefully planned to ensure that it adequately covers the entire site and provides accurate data. It is crucial to consider factors such as the site’s size and shape, the depth of the boreholes, and the equipment needed for drilling. Proper planning and execution of the borehole layout can help ensure a successful site investigation and, ultimately, a successful construction project.
What is a Borehole?
Boreholes are drilled into the ground to obtain soil samples for soil investigation purposes that are necessary for constructing a suitable foundation for a planned structure. Careful planning of borehole placement is crucial to ensure that the required information is obtained with the least possible cost. This article focuses on the layout of boreholes and various factors that influence the distribution of boreholes for different types of structures.
Borehole placement is essential to obtain reliable soil information, and it is crucial to avoid inaccuracies caused by poorly placed boreholes. A thorough understanding of the underlying soil conditions is necessary to determine the appropriate number and location of boreholes. Factors such as soil type, geological formations, topography, and the size of the structure to be constructed should be considered when planning borehole placement.
The layout of boreholes for a structure depends on the type of foundation being used. For instance, in shallow foundations, boreholes should be placed around the perimeter of the foundation. In contrast, for deep foundations, boreholes are placed at the center of the foundation. The spacing between the boreholes is also dependent on the size and shape of the structure.
In summary, boreholes are an essential part of the soil investigation process for the construction of any structure. The proper planning of borehole placement, including the number and location of boreholes, is necessary to obtain accurate soil information. Various factors such as soil type, geological formations, topography, and the type of foundation used should be considered when planning the layout of boreholes.
Fig.1: Borehole Layout for Different Foundations
How to Set Up Borehole Layout for Site Investigation?
To set up a borehole layout for site investigation, the first step is to carefully select the location and layout of the boreholes. This involves identifying suitable areas within the site where the soil or rock conditions are likely to vary, and where the boreholes can be safely and easily drilled.
Once suitable locations have been identified, the next step is to decide on the number of boreholes required. This will depend on a range of factors, including the size and complexity of the site, the nature of the subsurface conditions, and the specific goals of the investigation.
Finally, the depth of the boreholes must be determined. This will typically be based on the specific objectives of the investigation, and may involve drilling to different depths in different boreholes to obtain a comprehensive understanding of the subsurface conditions.
Selecting Location and Layout of Boreholes
Borehole locations play a critical role in determining the nature of soil at a project site, and their positioning should be carefully determined. In cases where the structure’s layout has not been established, it is recommended to use uniformly spaced grid boreholes during site investigation. However, if the foundation location has been suggested, boreholes should be positioned closer to it, particularly when the depth of the bearing stratum varies. To fill the gaps between borings, static or dynamic probing is commonly used.
When using trial pits, it’s important to position them away from the foundation location to avoid weakening and disturbing the ground. Eurocode 7 provides specifications for exploration station grid spacing, ranging from 20 to 40 meters. The spacing of boreholes should take into account the geological conditions of the project site. For instance, if the geological formation varies significantly, a smaller spacing is required, while a larger spacing can be used if the variation is minimal.
Deciding Number of Boreholes for Site Investigation
The determination of the appropriate number of boreholes needed for a comprehensive site investigation can be a complex process due to various factors. These factors include the allotted time for the investigations, the associated cost, the nature of the structure being investigated, and the availability of essential equipment and skilled personnel. Increasing the number of boreholes can provide more detailed information about the soil conditions, leading to a more cost-effective foundation design. Additionally, it can reduce the chances of encountering unexpected soil conditions, which can significantly increase construction costs. Therefore, borehole numbers should be selected to be economically advantageous, where their cost does not exceed the savings in foundation costs. However, for small structures, only three boreholes are typically needed to determine the position of each structure accurately. Nonetheless, there is still a possibility of incorrect assumptions regarding the soil stratifications.
Determining Depth of Boreholes
Borehole depth is determined by the soil depth that is impacted by the foundation’s bearing pressure. The suggested depth for boreholes ranges from one to three times the width of the loaded area. Going deeper than this recommended depth is not advisable, as it may have a harmful impact on the structure. However, if the strip foundation is shallow and the spacing between the foundations is significant, then there may not be a need for deep boreholes, as depicted in Figure 2.
Fig.2: Strip Foundation Widely Spaced
In the case of large raft foundations, it may be necessary to dig deep boreholes, as illustrated in Figure 3. However, if rock is encountered at a depth that is within the specified range, then it may not be necessary to dig such deep boreholes. The exact depth at which rock is encountered will depend on various factors, including the type of soil and the size of the foundation. Nonetheless, it is important to ensure that the foundation is built on a solid and stable base, which may require additional measures such as deep boreholes. Ultimately, the decision on the depth of the boreholes will depend on a thorough assessment of the soil and geological conditions at the construction site.
Fig.3: Large Raft Foundation with Required Borehole
When strip or pad foundations are constructed too close to each other and their effects start to overlap, their behavior becomes similar to that of a raft foundation. This can be problematic as the load distribution on the soil may not be uniform, which can result in uneven settlement and structural damage.
To mitigate this issue, deep boreholes may need to be sunk. This involves drilling deep into the soil to create a stable foundation for the structure to be built on. Deep boreholes can help distribute the weight of the building more evenly, reducing the risk of settlement and damage.
It’s important to note that overlapping foundations can be avoided by proper planning and design. Engineers should consider the load requirements of the structure and the characteristics of the soil before deciding on the type and placement of foundations. By doing so, the need for deep boreholes can be minimized or eliminated altogether, saving time and cost in construction.
Fig.4: Influence of footing on soil overlapped and hence it is assumed to be like raft foundation
As per Eurocode 7 guidelines for pile foundations, the borehole depth should extend to at least 5 meters or five times the diameter of the pile below the pile toe. In cases where the soil conditions are consistent, the response of the pile is assumed to be similar to that of a raft foundation. Therefore, an equivalent raft foundation is considered to be located at 2/3 of the pile length, as shown in Figure 5. However, it should be noted that the depth of boreholes should be cautiously determined, as risks are likely to arise. As a general rule, the depth of boreholes can be assumed to be one and a half times the width of the foundation, but this should be carefully considered due to potential risks.
Fig.5: Pile Foundation