rock pad above frost line

[haemon17]

Have a client in oil and gas that wants to place equipment on a gravel pad. The pad would be approx 12" thick with an underlay. Underlying soil would be compacted. Geotech report gave compaction guidelines for the underlying soil, but nothing about compacting the gravel or the allowable bearing capacity. Frost line is about 3 ft.

I feel like the geotech should provide the bearing capacity. Thoughts?

Also, any experience out there on whether to expect a lot of movement from freeze thaw with gravel?

Thanks

 

[dik]

Should... if the soil frost heave susceptable? Geotekkie should provide additional info on design for usage.

Dik

 

[aeoliantexan]

Yes, the geotechnical engineer should provide gradation and compaction requirements for the gravel and an allowable bearing capacity.

Silty gravel can be frost-susceptible. You need clean gravel. With a frost depth of 3 feet, you need to be more concerned about frost action in the subgrade. Surface water will penetrate the gravel. If the subgrade has a low permeability, the water will pond in the gravel at the top of the subgrade and be available to feed frost lens formation in the subgrade soil.

The risk may be low if the equipment is on a rigid frame and the connections to utilities and other equipment are flexible. If the equipment is touchy about movement, you need footings below frost depth. Alternatively, you might put rigid insulation below the gravel.

 

[BigH]

Further to the above, a number of points to consider.

1. We do not know what the underlying soil is. Is it a clayey silt? Silty Sand? Clean Sand? - presume the "compaction" is of the natural soil. Hence - is the underlying soil (below the gravel pad) frost susceptible? If not frost susceptible, may not be a problem.

2. Where is the water table? Deeper than 3 ft? if the underlying soil is non-frost susceptible, the capillary action will be so small that if the water table is low, one needed worry about frost heave. I had a job in Northern Ontario where the frost depth was about 9 ft. Water table was 12 ft deep; soil was a clean sand. We put the foundations about 2 ft down . . .

3. Agree that the gravel pad should be "specified" - just to minimize arguments in the field if the material differs on various deliveries - and "what is gravel?"

Anyway - my few rupiahs of thought.

 

[haemon17]

The underlying soil is reclaimed strip mine spoils. It's 28.5 to 50 ft deep. Spoils are shale and sandstone fragments in lean clay. N values range from 3 to 75.

Ground water was not encountered during drilling. They didn't do a monitoring well.
Split spoon samples were moist to very moist. Standard Proctor test showed moisture at 24%, optimum is 18.9%.

Turns out the gravel pad is 8" of #2 stone topped with 4" of 304.

bottom line: Sounds like the geotech needs to provide some more guidance on frost susceptibiltiy of the spoils, potential movement and a bearing capacity.

 

[fattdad]

"Bearing capacity" in the practical sense relates to ultimate capactiy and also to service capacity. In other words, who cares whether or not the soil actually fails if the settlement is so great that all the connections to the equipment are severed? Did the geotechnical engineer know the weight and size of the equipment? If not, s/he may not have the information necessary to determine ultimate bearing capacity or forecast settlement.

Optimum moisture content from a Proctor test and in-situ moisture content from the subgrade is not relavent to the bearing capacity question. Many properly compacted soils get saturated after placement and retain their strength (consider an embankment dam).

Dig a hole 3 ft deep, place some separation geotextile, fill it with open-graded aggregate and prepare the site. Once the loading conditions are known by the geotechnical engineer, s/he should be able to assess ultimate capacity and settlement, providing the scope of work was properly defined in the contract.

f-d



¡papá gordo ain’t no madre flaca!

 

[shobroco]

I don`t think there is nearly enough information in the OP. Is the 12`` thick pad 3`square or 33` square. Is the equipment a propane BBQ for week-end parties or a temporary refinery. Does it matter if frost heaves the pad.

Sorry, but there is something wacky with the site & punctuation does not come out right so I can`t use question marks or inches & feet.