From my experience, the design team always wants the geotechnical report as cheap as they can get. I try to help clients understand that the geotechnical investigation is not the design phase where you want to cut corners.
I suspect the $100 million is total design fees for the entire project, and the geotechnical investigation was kept to a minimum bare bones service.
Time will tell. We pre-drilled for a drilled shaft once to determine the depth at which rock excavation would start. When the drilled shaft contractor cored into the bedrock for the rock socket, they discovered a void just off from our 4-inch test bore. The ground does indeed contain surprises.
Bang on Tiger... my experience, too. I recall a cartoon from 40 years back... a 'builder/engineer' holding up the drawings for the Tower of Pisa, commenting to the owner, "...and we can save another 1000 lira if we skip the soils report."
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So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
Maybe this gives some perspective. At the rates listed, and making a whole lot of assumptions, if they test bored the entire 27km of tunnel for geo survey, the cost would have been $26 million USD or $40 million AUD.
The cost per for goes up linearly with length. Drilling multiple smaller bores linked together would have been substantially less.
The cost per foot varies greatly with the depth of boring. You can knock out a 25-foot boring, faster than you can drill the next 15 feet. Things only slow down more with increased depth.
With average drilling depths of over 3,000 feet, the average cost per foot is significantly higher than if they were drilling 100-foot borings along the alignment. There are many factors that come into play with a geotechnical investigation cost estimate. Distance of mobilization, terrain (track-mounted rigs typically charge out at an increased rate), type of drilling (auger vs mud-rotary vs rock coring), and daily travel from the hotel to the drilling site are just a few.
I've never drilled for gas and oil, so I can't speak to what all the steps are for drilling a new well. I presume their drilling has more involved and costs more per foot than geotechnical.
