Good Soil, Bad Dirt

In 1978 and 1979 I worked at the Syncrude oil sands mine at Fort McMurray,  Alberta. I was part of a Dames & Moore team mapping the characteristics of the overburden, the non- or poorly-bituminous near-surface soils above the economic oil sands deposits underlying the future expansion areas of the deep open pit. The team was lead by Ted R~ an Absentee Geologist. Cecil U~, a clever, resourceful and conscientious geotechnical engineer from New Zealand was the brains behind our work.  Mike E~ and Al F~ , both geological engineers, were also part of the team.    Me? – I was the legs.

When I worked at Syncrude, the pit was about 3 miles long and 1000 feet wide. The largest in the world at that time, enormous dragline excavators scooped the oil sands in long, high windrows. Bucket wheel excavators (or “reclaimers”), chomped into the piles and disgorged oil sands onto conveyor belts that ran the entire length of the mine delivering oil sands to the processing plant. Eventually the whole side of one side the mine was deepened and the draglines would have tp move to another part of the mine and start the process over.

When the draglines had reached as far as they could, they walked – like waddling ducks – into the next stationary position to continue scooping. The draglines were very heavy. (The English language does not have words enough to describe how heavy. So how about very very very heavy?) When they walked, the entire weight of the machines was concentrated on their “feet” creating great bearing loads on the underlying soils. The actual bearing pressure (bearing load divided by contact area) was about 25 psi when the 35 cubic yard bucket was full. 35 cubic yards is 945 cubic feet. (Imagine a cube about 10 feet in each direction.  A  tourist-type photograph would show a pick-up truck parked in the big bucket!) For pressure comparison – I exert about 5 psi, when standing on one foot. But that pressure extends only a short distance into the ground because my foundation footing is so small at about 4 inches by 9 inches. The duck footing of a dragline was 72 feet by 16 feet so the pressure extended much deeper. And that pressure acted on the foundation soils to greater depths.

Of concern was the strength and stiffness of the foundation soils, both below and for some distance around,  since failure or excessive settlement of the ground beneath would lead to tilting of the draglines, or possible slope failure at the top of the pit wall – both ground movements could lead to a dragline falling into the pit. The fear was justified: it happened at least once. There were similar concerns related to the behavior of the foundation soils beneath the very high windrows, the bucket-wheel excavators and the conveyor belts. Although further away from the pit highwalls, ground failures  under these structures would interrupt production.

The geology of the overburden soils was complex. The area had been subjected to glaciers, permafrost, river: and all kinds of other geological influences, which had created a variety of soils which ranged in thickness from a few inches to many feet. The soils were not continuously layered over great distances, but instead were heterogeneously mixed up. We identified well over a dozen soil types, ranging from brick hard glacial tills to boulder/cobble deposits, sands, weak soft clays and and peat soils. These varied soils had a vast variety of mechanical properties and would behave very differently under the same pressures. In that way soils are no different than people – some soils behave well under pressure and others behave badly when exposed to similar pressure and stresses: some are stiff and unyielding and others buckle. (BTW: some melanges, the geological materials I specialize in,  behave so badly that I consider them anti-social).

But the odd behaviors of soil are joyful tidings to geotechnical engineers, who specialize in the engineering properties of soil. To geotechnical engineers, soil is not to be confused with the soil in your back yard, which you might call dirt. That is OK: call it dirt if you want. Even the less-stuffy soils engineers call soil dirt, and at least one published university professor calls soils Dirt.  (There is also research on Soil Behavior if you are interested in getting the dirt on soil…)

The ultimate goal of our work was to create relatively level, stable working areas for the draglines, bucket wheels, windrows and conveyor belts. These areas were extensive: 1.5 to 2 miles long and about 600 feet t0 2000 feet wide.   In places the weak soils were many feet thick and would have to replaced with better soil. The only way cost effective way to do that was to excavate the poor soil, import better soil,  and then compact that new loose soil into strong fill layers. This kind of work is called earthwork construction, and requires bulldozers, scrapers and compactors. It also requires surveys to show where the low spots and high spots would have to be levelled, by the process known as cut-and-fill. It is always hoped that a cut and fill job can be neatly arranged such that all the soil in the high spots is good soil that can be used to fill in the low spots, which in the  case of the Syncrude preparation areas would also include where weak soils would have to be excavated and removed.

We devised Soil Useability profiles for the several soils. On the basis of experience and laboratory testing, we assigned a range of engineering properties to the soil types. Geopractitioners call that process characterization –it is a bit like sketching a person’s temperament based on a few observations of how they interact with us. We get a general idea – which may be dead wrong if we are unskilled at character-readings, or the we catch the person having an unusually good/bad day. Generally though, once we knew the geometry of a certain type of soil deposit, and where the final site grade was going to be, we could estimate the behavior of that deposit under the expected loadings.

Fortunately the overburden soils at the site contained good soil aplenty – the trick was to discover where those good soils were and their areal and thickness extents.  That was largely my job. I was  lowered into trench excavations inside of excavator buckets, a neat way to get up close to soils in test pits. (One is not allowed to even think of doing that nowadays because of safety restrictions. I also spent months in the field, much of it walking. That is why I wrote that I was “the legs” of the team. I tramped through muskeg swamps, ponds, ditches and creeks mapping exposed surface soils and weak bedrock. By wandering around the  site, even in the midst of the northern Alberta winter, I was able to glean enough information from the field observations, boring data previous geological maps, air photos and the like, to be able to both map my interpretation of the soils.

I drew the distributions of the soils onto closely spaced cross-sections. Civil engineers then took those sections and calculated the volumes of the soil types and tried to balance the cut and fill and work out construction quantities and ultimately prepare documents to put out to bid from contractors. The earthwork work was worth many millions of dollars.

Grading is noisy and exciting. I’m not as excited about it as I used to, but in the 1970’s there something impressive about herd of enormous yellow bulldozers and scrapers ripping up ground, hauling it hither and thither, spreading it thither and hither and then rolling around on it. Boys grow up doing this kind of work in the back yard. Big yellow equipment is expensive and there are high costs associated with incorrect volumes of useable soils.   So it is also impressive to see  pissed-off contractor staff storming around yelling angry commentary on the “engineers” for getting the ground conditions wrong. And then calming down enough to devise construction claims for more money and time to complete the work.

The contractors had the most difficulty with the deposits of soils with poor engineering character. These were the ugly bad soils, as opposed to the lovely good soils. Furthermore, the maps that we produced, intended to be helpful to the contractors by showing the distribution of good and poor soils were not helpful.  Most earthwork contractors are not geologists- they are earthwork contractors. So, we devised very simple Useability maps showing where the Bad Dirt was: 0.5 m Bad Muskeg; 1.0 Good Sand and so on.   These worked very well indeed. I have never since seen the like of these maps, created for practical earthwork contractors. Perhaps that is because we professionals have a hard time demystifying dirt.

It was on this project that I learned one of the most important lessons in my career: lots of geological details are all very satisfying to geologists but you have to answer the BIG question: “So What ?” relative to the end result – construction.

There were many other challenging geotechnical, geological, geohydrological, geoconstruction, and geoeverything challenges at Syncrude. The operation benefited  by hiring a group of committed and talented Syncrude staff; as well as periodic guidance by a  Geotechnical Review Board composed of international-class consultants. It was an exciting place to work; so much so that I accepted a job offer from Syncrude to work directly with them rather than through Dames & Moore. That offer was sufficiently arresting to Dames & Moore that they quickly stopped their hourly-paid relationship with me and offered me a full-time position that I could not refuse (although Syncrude was rightly upset with me for changing my mind). My colleague Al F~ did accept Syncrude’s offer, and 32 years later he is now the very senior, respected Manager of Syncrude’s well-known  Research and Development group.

About Ed Medley

Ed Medley has been on a random walk for over 50 years. Many scribbles and snapshots at this site are from his vagabond transits; others are from his decades of international experience in geological and geotechnical engineering, academia, and mineral exploration prospecting.
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