Recommended boring depth on a 10ft x 20ft slab 4

[pelelo]

Engineers,

I am in involved in a proposal related to a geotechnical investigation for a proposed generator which will be supported on a 10 ft by 20 ft slab on grade. Strata is sand in the top 60 ft. Water table is around 10 ft below grade.

Using the reference of Stress distribution by Boussinesq, the engineer in charge recommended 1 boring 40 ft deep (2 x 20 ft= 40 ft)(assuming 0.1q is at 2B for square footings, he picked B as the largest side to be safe).

Other engineers understand that a boring is not needed, instead 1 or 2 hand augers, 5 to 10 ft deep are enough. The reason for this is because the load of the generator is too low (~200 psf), so they undertand that 200 psf is nothing therefore by hand augering is more that enough.

What do you think?

In my opinion, the stress distribution is independent of the load of the structure. What drives the stress distribution is the dimension of the footing or the slab. I still think that the approach of using 1 boring is appropriate.

 

[oldestguy]

My rule of thumb is to drill to a depth where added stress is 10% of existing. Take into account stress distribution.

 

[GeoEnvGuy]

Your honour in 2020 a

generator which will be supported on a 10 ft by 20 ft slab on grade

was constructed.

We completed a limited investigation because

the load of the generator is too low (~200 psf)

In 2021 additional loads were added to the slab on grade but remained within the recommended bearing capacity. In 2022 following winter cracks developed in the slab on grade and settlement caused damage to the buried utilities.

If you are going to perform geotechnical investigations then investigate the site to full depth. In my experience owners change the location and size of structures on a site following a geotechnical report frequently. If you don't want to waste time in dispute management later on don't offer the cheapest and lowest scoped geotech investigation to win the job.

 

[Mad Mike]

I would excavate hand auger holes to intersect and confirm the level of the ground water table. I would drive in deep probes to establish the density of the sands. I would not drill a borehole unless I was concerned there might be compressible clays in the saturated profile, or for some other reason needed to sample the subsoils at depth.

In my part of the world, I would never get any work if I priced for Rolls Royce investigations on every site. I would like to, but I can't.

Best,
Mike