What is a Borehole?
A borehole is deep vertical hole with small diameter drilled into the ground to obtain soil samples for soil investigation required for the construction of suitable foundation for the planned structure. It is required to carefully plan boreholes so as to obtain required information with minimum possible cost. In this article, layout of boreholes and different factor that influence borehole distribution for different structures are discussed.
Fig.1: Borehole Layout for Different Foundations
How to Set Up Borehole Layout for Site Investigation?
Following are the procedure for setting up borehole layout for site investigation:- Selecting location and layout of boreholes
- Deciding number of boreholes
- Depth of boreholes
Selecting Location and Layout of Boreholes
The location of boreholes should be determined carefully and positioned properly to have test results that adequately represent the nature of the soil at project site. If the layout of the structure has not been established when site investigation is conducted, then it is recommended to consider uniformly spaced gird boreholes. Otherwise, boreholes should be positioned close to the suggested foundation especially in the case where the depth of bearing stratum is varied. Frequently, the spaces between borings are filled with static or dynamic probing as can be observed in Figure 1. If trial pits are used, they should be positioned away from the foundation location because it would weaken and disturb the ground. As per Eurocode 7 specifications, the spacing of exploration stations creating a grid ranges from 20 to 40m. The geological condition of the project site should be considered while spacing is specified for boreholes. If the geological formation varies substantially, then it is required to adopt small spacing and vice versa.Deciding Number of Boreholes for Site Investigation
The number of boreholes required for a thorough investigation of the given site is difficult to determine because it is affected by several factors such as time allocated for investigations, cost, the structure for which the investigation is conducted and sometimes the availability of necessary equipment and individuals with adequate skills and experience. As the numbers of boreholes are increased, greater information regarding soil condition is achieved and hence the foundation design would be more cost effective. The possibility of encountering unexpected soil condition is reduced. This is considerably advantageous because the increase of foundation construction cost due to unanticipated soil condition is prevented. Borehole numbers are assumed to be economical unless their cost would not surpass the amount of savings in foundation cost. Otherwise it will increase the total cost of the project which is not desirable. Finally, apart from small structures, the position of each structure need to be explored with 3 boreholes to determine strata dip accurately. Yet, there is still room for incorrect assumptions regarding stratifications of the soil.Determining Depth of Boreholes
The depth of boreholes is controlled by the depth of the soil that is under the influence of bearing pressure of the foundation. The recommended borehole depth is equal to one to three times the width of the loaded area. This is because it is suggested that bearing pressure below this depth would cause detrimental effect on the structure. If the shallow strip foundation is considered with considerable spacing, then deep boreholes would not be required as shown in Figure 2.
Fig.2: Strip Foundation Widely Spaced
However, it will be required to dig deep boreholes in the case of large raft foundations as shown in Figure 3 except in the case of encountering rock within the specified depth.
Fig.3: Large Raft Foundation with Required Borehole
If strip or pad foundation effects are overlapping, then their behavior would be similar to that of raft foundation and eventually deep boreholes are needed to be sunk as shown in Figure 4.
Fig.4: Influence of footing on soil overlapped and hence it is assumed to be like raft foundation
For pile foundation, Eurocode 7 specifies that borehole depth should extend 5m or five times shaft diameter below pile toe. Usually, it is considered that the response of pile in consistent soil is similar to that of raft foundation, and the equivalent raft foundation is at 2/3 of the pile length as explained in Figure 5. Finally, as a general rule, the depth of boreholes can be assumed to one and half times the width of the foundation, but this should be considered cautiously since risks are highly likely.
Fig.5: Pile Foundation