The Geotechnical Engineer by Tom Witherspoon
Hi, I’m Tom Witherspoon. I am structural and geotechnical engineer. I’m actually a diplomate as a geotechnical engineer, recognized by the Geo-Institute. And by the Structural Engineering Certification Board, I am a structural engineer. I also owned a foundation drilling company and ran that for over 22 years, S&W Foundation, and did a lot of limited access drilling and things that were considered to be impossible. So I’m going to talk to you a bit about Geotechnical Engineering.
And geotechnical engineering, is basically, I call it from “the ground down”. It is evaluating everything in the subsurface to provide factors, design factors, for a structural engineer to design the foundation for that structure.
In any kind of geotechnical engineering study, or analysis, includes:
• establishing the water table,
• determining the swell potential for the underlying stratum, especially in expansive soil areas,
• developing the soil parameters that determine the depth and termination stratum for a drilled pier or an auger cast pile.
When available, soil borings will penetrate into suitable rock formation to provide the axial compressive capacity of the drilled piers or the auger cast piles. This analysis must include the bearing capacity at the base, the skin friction that improves the axial capacity of the underpinning, and expansive clay environments develop skin friction to analysis to prevent upheaval of the underpinning. These factors must also establish reinforcing steel requirements for the shafts and the geotechnical factors, such as skin friction, will provide the structural engineer with all of the the data that he needs to establish the foundation statistics.
When applicable, the geotech must establish a lateral soil and rock pressures on the underpinning so that the structural engineer will be able to establish shaft diameter and reinforcing requirements to resist the lateral movement. In some cases, there will be a necessity to intersect fault or fissures in the rock formation. And the geotech must provide the necessary lateral or sliding pressure to stabilize a hillside. And in practice, I have seen that on various situations.
In remedial situations, there may be a need to install haunches to distribute loading from the existing structure to the pier or pile, and the geotechnical factors will establish that. In the case of retaining walls, the geotechnical engineer must establish lateral loading factors which will determine the type of reinforcing retaining wall that is most appropriate for the site.
These walls can be gravity, which includes gabion, stone gravity or reinforced concrete. They can be cantilevered, reinforced concrete retaining walls, or an MSE which is a mechanically stabilized earth wall, or a drilled pier supported wall, such as exist along creeks or rivers. I’ve designed a lot of those because they are important to resist the lateral movement, and I’ve seen failure of a lot of these other types. This all ties back to the geotechnical report.
A Geotechnical Study Prevents Failures
In the words of Benjamin Franklin, “an ounce prevention, is worth a pound of cure”. A lot of times clients will not spend the money on a geotechnical report because they want to save this $1500 to $3000, and it ends up costing them a lot more in the end. Not having the proper geotechnical study was a Brick River Parking Garage and this this occurred back in 1985. The realized that the inverted T’s on these piers at that parking garage, right here in Dallas, that they were settling.
And they finally came in and did a geotechnical study, and the geotechnical study showed that there was trash and debris an everything down below the surface, that no one checked out. They just came in there and drilled some piers 15 foot deep and there was, you know, over a period of time, in that environment, because it was a trash dump in Dallas, they started settling because they didn’t have soil borings and geotechnical studies of that area. It ended up costing them $350,000 because of the remedial work that had to be done. There are several other examples that I have witnessed in my career, both as a contractor, and as a geotechnical engineer, and in most cases, they don’t want to spend the money for the geotechnical study and they end up paying for it in the end. The geotechnical study is very important! I mean, we’re not in a high earthquake area in the Dallas area, but in Oklahoma there’s some earthquakes that are there, and the geotechnical studies are so important to establish the cracks, the fissures and the faults in that rock because if they’re not intersected with the drilled peers, then that building or structure is going to move.
And so all of this is extremely important to establish the geotechnical parameters so that the structural engineer can properly design the foundation for that structure. A good, proper geotechnical study would have established how bad the soil conditions were so that it’s not just putting piers and isolating it, but establishing a treatment for the expansive soil.
And one thing that’s extremely effective, and Marshall Addison, PE did studies for his PhD dissertation at UTA, showing the potassium chloride in water, which is oilfield technology, Johnson and Johnson engineers, the husband and wife did all kinds of studies on that and found that potassium chloride in water would make an ion exchange with the clay and would stabilize it. Well, the way to do that is not only doing a study before you start, to know how you treat the soil, but doing one after you treat it to establish it’s correct.
The Collin County Appraisal Office, they did not do a proper geotechnical study, they had an engineer that “thought” he knew the area, and he drilled piers down about 15 foot, and they were socked into Austin Chalk. But in that location, had they done a geotechnical study, it would have shown that the Austin Chalk had swell potential, so they ended up with heaving all under the Collin County Appraisal District.
And it was an interesting thing me getting involved, but because they had one engineering firm that said you had to tear the building down. They had another engineering firm, working for the builder, that said all of this is just normal movement… there’s no problem, you know, don’t look here! And they brought me in literally under the cover of night. The attorney that hired me, had me go to that appraisal office by myself with my instruments and everything, at 6:00 PM on a Friday evening. And I had to go in and completely draw up the building. Do an elevation survey, show all exactly what was wrong with it, and then as a geotech, doing proper soil borings and studies, establish what additional swell potential there was in that building.
And I established a program of going through drilling 5 foot on center in that entire building, and injecting potassium chloride in water, down 10 foot to stabilize the clays. But I didn’t just take what I thought it was doing. I came back afterward, we’d inject an area, and did soil borings to establish yes, it had reduced the swell potential to below 1%.
So it was two geotechnical studies. I think the remediation of that building ended up being like a half $1,000,000, but it was done properly, and I have done go back inspections and the building has settled out and everything is good. There were some areas that were so bad they had to be piered, excavated and lowered because of the swelling there and there were pockets that were extremely bad. I’m very blessed to have the experiences that I’ve had as an engineer and as a contractor, because a contractor you’ve got to make this work! As an engineer, you can come up with all these theories, but it has to be workable. And mine were workable.
As a geotech, you look at all the factors. There are certain tests and everything you do in your laboratory to establish this. You do a soil boring, you’re taking samples every 5 foot all the way down to the depth that you have to go. A lot of times, clients will want to save money by only doing a 15 foot deep soil boring. I think that they need to be deeper than that, because if you haven’t encountered rock, you probably should find out where the rock is to establish whether it’s going to be economical to develop here, or to sock it into rock. If you have rock reasonably close, then you want to sock it into rock.
But some of the swell potential: there is potential vertical rise, and there are tests for that. There’s soil suction tests that can be done that will establish the volatility of that soil and a lot of it is tied into the plasticity index. The plasticity index is a factor that tells you the swell potential of that soil.
And doing a geotechnical study, it varies from the time of year, because if you’re doing a geotechnical study during the rainy season, then you’re not going to have much swell potential in that upper 10 foot.
But if you’re doing it during a dry season, then you’re going to have enormous swell potential in an expansive soil area. So the geotech has to not only look at what is the swell potential now, but if this old soil dries, what is the swell potential when it dries? Because all of that is important.
Now there’s other areas, there’s expansive clay areas in Virginia. There’s expansive clay areas in various pockets around the country. There’s also in the Northeast, there were glaciers some time ago that came down from Canada, pushing all this rock formation down. And now they go in to build on top of it and you’ve got a varying condition that they have to do the geotechnical study to establish all those soil parameters. Because you have various pieces of igneous formation that have come down with those glaciers and it’s a whole different environment there.
And it’s something that has to be evaluated by a geotech that is competent to go through and established depths. And those are sometimes fairly deep, but cause of all the glacier deposits that were brought in.
A geotech in their geotechnical study will establish soil parameters, but they’ll also provide recommendations. They will say if you’re building a slab on grade, you have a potential vertical rise of so many inches, so that the builder can decide whether he needs to do some kind of soil treatment.
There’s also pockets in Nevada of karst, and karst when it receives water will just completely deteriorate and it causes a problem. So you have to also do the geotechnical studies. There’s different soil parameters around this nation.
And in other countries, you have to establish exactly what the soil parameters are, and the geotech will layout, “if a slab on grade, you need to expect this.” They will in some areas establish, “well there’s karst here, you’re going to have to deal with that.” But but they will establish certain specifications to give the structural engineer additional information to do his design.
And when you’re doing, when you’re establishing a foundation, the geotech normally will say, “this structure probably should receive piers.” But the final decision is going to be the structural engineer’s decision.
Whether it’s drilled shafts or auger cast piles, that’s considered underpinning, you’re underpinning that foundation, because you’re going to have a concrete foundation with grade beams and a slab, and in most cases it’s a good idea to put piers or piles under that foundation to support it. The piers will only prevent settlement. They cannot prevent upheaval, which is a reason the geotech has to establish whether soil treatments are needed for that site.
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