Geotechnical Magic Stuff

Early in my career, I was startled to hear one of our best clients refer to our geotechnical work as “GMS” (Geotechnical Magical Stuff), implying our best advice was pulled out of a magician’s hat. I thought, “GMS?! This is technical stuff !”

But as the years have gone by, I’m increasingly seeing his point. We geo-professionals issue our geo-reports chock full of parameters with caveats that they should be applied for one set of conditions but not for another. And if the conditions do change, well then, the formulas and parameters are recanted and magically reconstituted in a supplemental report to be interpreted in a different way.

Take, for example, the support of floor slab systems. For most projects, floor slabs are not a real issue. We geo- professionals cut-and-paste a blurb in our reports replete with a capillary break recommendation and a mysterious “modulus of subgrade reaction (k)” selected from a correlation with an equally mysterious CBR test correlated in turn with our ubiquitous SPT N-values. When we consider heavily loaded warehouses with real (large) slab pressures, slab settlement becomes a real issue. We recall from Foundations 101 that “k” is defined as the ratio of applied pressure (a load divided by area) to resulting deflection. This subgrade reaction modulus is a function of dimensions of the loaded area, which makes it difficult to consider it a unique soil property value. And since “k” is a measurement of real deflections (or slab settlement) to real pressures, the value of “k” that might have been 200 pci elsewhere becomes only 20 pci beneath a warehouse slab! GMS for sure, or so it must appear.

Geo-professionals have several tried-and-true methods for dealing with predicted settlements beneath heavily- loaded slabs. We have been doing it for decades, so many of us default to removing/replacing the compressible soils – it’s safe and practical (and doesn’t require much design). Or, we might provide an expensive alternative of pile-supported slabs. Pile-supported slabs are even “safer,” except that now the slab is no longer a slab; it’s a two-way structural beam with steel reinforcement to transfer shears and bending moments to the piles. In between those two options, we can offer ground improvement technologies, like Rammed Aggregate Pier® elements, that are much less expensive than pile-supported slabs but require a unique geotechnical “magic.” The piers are coupled with the matrix soil to support the slab loads, and us geo-professionals must provide our clients with the appropriate subgrade reaction modulus for this composite mass.

It doesn’t matter what the “real” subgrade modulus value is, and it doesn’t matter that it is applied with a different set of guidelines for lightly loaded slabs-on-grade vs. heavily loaded slabs on soft ground. We know from theory and observation where the slab loads go and what the deflections might be in different spots below the semi-structural slab. It may appear to be GMS but the magic is really not magic at all – it is a fundamentally sound understanding of complicated geo-mechanics and when that is achieved, it gives magical results. I’ve come to believe that my friend, the client, was right. Our field is full of GMS – and that’s the beauty of it all.

Kord J. Wissmann, Ph.D., P.E., D.GE is the President and Chief Engineer for  Geopier Foundation Company. Kord has more than 25 years of geotechnical engineering experience, joining Geopier in 1998. As president and chief engineer, he has led the company to prove multiple innovative ground improvement technologies now deployed worldwide. Kord holds over 17 patents, has authored more than 35 papers, and served recently as President of the GeoInstitute. He earned his Bachelor of Science and Doctorate degrees in civil engineering from Virginia Tech, and a Master of Science degree in civil engineering from the University of California, Berkeley.

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