Is a new Geotech Investigation needed. 1

[655321]

We have a couple of 30-year transformers with pads that are 4.5' deep, 10'long and x 20' wide. We are replacing the transformers, but want to reuse the pads. The new transformers are about 20% heavier, and has its center or gravity shifted off the center-line in the short direction of the pad. We calculated the max pressure on one side of the foundation doubles from the increased load and eccentricity.

Back when the earthwork occurred, we know the soils were tested, and were a well controlled engineered fill. We have an old document that prescribed an allowable bearing of 8.5 KSF (for the whole site). The underlying soils are deep highly Compacted medium sand.

I'd appreciate any input on this you have to offer; do those numbers seem unreasonable for compacted sand, does getting a geotech in on this seem reasonable; drilling holes near these devices is terrifying for us, is there an other means to establish the grounds adequacy without drilling?

Thanks.

 

[FixedEarth]

Yes, you need a new geotechnical evaluation report. We don't know how much safety factor was used on the original design. Adding 20% additional pressure will cause new settlement. You don't have to drill right next to the existing pads-you could be 20 ft away without much compromise.

Also consider having a new structural engineer evaluate the structural adequacy of the existing pad, once you have the new soils report.

 

[hokie66]

I would tend the opposite way to FixedEarth in this instance. What is the new maximum pressure? The pad itself only exerts 4.5 x 150 = 675 psf. Even if the tranformer weighs twice as much as the pad, which I doubt, the pressure would only be about 2000 psf average, maybe 2500 psf maximum, compared to the stated allowable of 8500 psf. Seems an easy call to me.

 

[Ron]

Agree with hokie66. Bearing capacity shouldn't be an issue. Unless the soil is highly stratified (which by the description it is not), then settlement parameters won't change much.

 

[FixedEarth]

The OP states, "We calculated the max pressure on one side of the foundation doubles from the increased load and eccentricity", you don't think this will not cause tilt and some differential settlement? Further, how do you know, we don't have uplift on one of the corners when bearing pressure doubles on the other side?

Do you know, if this site has high ground water table? Have you tried supporting equipment on saturated SAND, when imposed bearing pressure is 8.5 ksf? I wouldn't put my stamp on it.

If cost of a small soils report is a concern, compare that to undepinning and replacing damaged equipments!

 

[mtu1972]

Now score 3 for the structurals vs 1 for the geothechs.


I was recently thrown under the bus when my boss insisted on new soil borings, at the 11th hour, for two additions we were putting on buildings previously designed by our firm 20 & 30 years ago. Soil reports were long gone due to records retention, but I had back calculated values from each building and they were totally in sync.

We finalized our design with presumptive Code bearing capacities, issued the drawings, and then revised them during bidding when the new soils report pretty much concurred with the values I had originally used.

The Owner's PM had me thrown off of the job. I was still doing the work behind the scenes - but never in direct communication with him.

Not being an Owner I can't fully appreciate my boss's caution maybe due to liability concerns or whatever.

Engineering certainly isn't the profession it once was.

gjc

 

[hokie66]

I suppose it depends on how sensitive the transformers are to a bit of settlement. My answer might be different if the pads were supporting something very sensitive.

 

[BigH]

To be honest, I am not sure that a new soils report will tell you anything new. If a standard investigation with SPTs, then you will have inherent variations of parameters (based on correlations). The permissible bearing originally given is quite high indicative of well-graded crushed rock fill, likely near bedrock (?). Why not first do a estimate of the new static bearing pressures (and, determine if there is an uplift on the backside - as suggested). If not, I think that you will be fine from a bearing point of view.

The pad is pretty massive (weight-wise). However, you should probably do a check on any new vibration amplitudes that might be developed and confirm that they are acceptable.

 

[655321]

The new units are pretty massive, they weight about 550 Kips a piece. I've come up with 7 ksf max contact pressure applied on one side of the foundation, and less than 1 ksf on the other. Its about 3.5 ksf with the current arrangement (evenly distributed).

 

[Ron]

That being the case, you don't need a new geotechnical report you need a new foundation design.

 

[fattdad]

I'm with Ron, BigH and the structural engineers too.

f-d

¡papá gordo ain’t no madre flaca!

 

[molerat2210]

Certainly sounds more like a settlement question than a bearing capacity question. You have the benefit of some pre-load from the previous transformer (assuming same footprint). A few hand augers and DCP tests could probably get the job done. This is how I like to build arm strength (and win arm wrestling contests against structural engineers).