Borehole layout is decided based on the area to be covered and the type of the foundation be constructed. In addition, the importance of the structure is also considered in selecting the number of boreholes to be done.
For example, when a high rise building is designed, knowing the conditions of the ground preciously is very very important to the designer. Thus, the planning of the borehole layouts is done.
In low rise buildings, we may plan the number boreholes based on the condition of the ground and cover the whole footprint of the building. For small buildings such as two 2-4 story, constructed in a weak ground, one might select one borehole considering the spread of the structure.
Buildings such as high rise buildings, it is required to plan the borehole layout properly and accurately to cover the area covered by the structure.
The number of boreholes is depended on the area of the structure and based on the condition of the ground predicted based on the surrounding condition of the ground.
For a considerably larger building, three or five boreholes could be selected to examine the ground profile.
If there are five boreholes, four drilled around the building area and one drilled close to the middle of the building area, variation of the soil layers, a variation of rock profile, etc can be examined clearly.
The information of this nature is very useful to assess the ground condition in recommending suitable types of foundations.