Dive summary:

• Somewhere in a construction budget is usually money for a geotechnical consultant, either one hired directly by the contractor or one whose work is part of the design process.
• Costs for the work can mount up, but taking too few borings can lead to an inaccurate – perhaps seriously inaccurate – understanding of the construction site, somewhat like saying you can draw a line between two points but you cannot tell if it is curved or straight without more points.
• One example of the surprises possible is finding that new utilities conflict with existing ones, requiring redesign or relocation, and another is encountering wetter-than-expected soils that will change at least schedules and perhaps budgets.

From the article:

Either way, the number of borings the geotechnical consultant will take is often dependent on the funding available. ...