TEXTILES.ORG
A personal sidebar by Bob Koerner
Introduction
By 1961, I was 27, married to Paula, and we had two young sons, Michael and George. Academically, I had received my BSCE degree from Drexel in 1956, with one course in soil mechanics, and half of a master’s degree at Columbia with two courses in soil mechanics.
The Drexel course was introductory and never got to the point of doing stability analysis but it did leave me a very good textbook written by Professor Donald Taylor of MIT. The Columbia courses were more in depth, but the professor apparently had a reluctance for teaching consolidation theory and effective stress concepts.
I also had four different jobs between 1956 and 1961: contracting, design/build, dredging, and now consulting. The latter saw me almost immediately transferred from New York City to being the resident inspector for the building of two very large molasses storage tanks in Wilmington, Del. This required the family to move from New York City to Delaware, thus ending my Columbia experience.
On the job
The project contractor at the Wilmington Marine Terminal was mobilizing equipment when I arrived on-site.
I knew a good deal about pile installation and design from prior jobs. Thus, the sand drain installation portion of the project was fully understandable. However, piezometers and their installation were completely unknown to me, as was their use in an effective stress analysis for soil stability calculations. At any rate, after the sand drains and piezometers were installed, a 3-ft-thick sand blanket was placed and then a soil fill (called a “surcharge”). The target height of the surcharge was 33ft (10m) at a rate of 20in. (0.5m) per week (which I thought, at the time, was very little).
My first inkling of discomfort was to observe the rise of water levels in all of the piezometers during ongoing placement of the surcharge fill. Even though the contractor was only placing about 4in. (102mm) of fill each day, the piezometer levels never decreased markedly, even over weekends when fill was not placed. I suspect that my thoughts at the time were of the order that “these things simply don’t work.” However, I diligently reported the information to my supervisor in New York City. It should be noted that communication in 1961 was via snail mail and I didn’t even have a telephone in the small job site trailer. (And, of course, cell phones and e-mail were both decades into the future.)
Trouble coming
During the next four weeks, things really started to happen.
I busied myself self-learning effective stress stability analysis from Taylor’s textbook using the ever-rising water levels within the piezometer tubes. These levels became significantly higher than the fill itself, to the point where I had to actually support the plastic tubes and had to use a ladder to read the water levels until I put gauges on the tubes.
There also were a few waterline breaks under the service road between the project site and a two-story brick warehouse that was founded on a pile foundation. I don’t recall worrying about these leaks since they were probably “none of my business.” (Little did I know at the time!)
The next three to four weeks were much more trying. Sequential events that occurred during this time period:
- Cracks occurred in the warehouse wall facing the site to the point where bricks became loose to the degree that some of them could be removed by hand.
- The asphalt warehouse floor closest to the job site started rising. It rose slightly initially but then the building superintendent tried to hold it down with two-ton lead ingots, but to no avail. It rose more than 1ft, with many fresh tension cracks becoming evident. (At this point, I began to sense that my project was the cause of these somewhat disconnected disturbances.)
A few days later a laborer called for me to come to the top of the surcharge fill, which was now approximately 18ft above the roadway surface, and he told me to watch as a crack in the soil was slowly, but surely, proceeding to open parallel to the road and building.
As the crack continued to grow, the north side of the surcharge (adjacent to the road and building) was dropping until it was about 6–9in. (150–230mm) lower than the south side.
Factor-of-safety graph
This was as much as I could take in arriving at my very tentative conclusion that the “sand drain-surcharge fill” system was simply not working.
My ongoing hand-calculated stability analysis was now indicating a factor-of-safety slightly less than 1 (see the original hand-written “Figure 3.7” from 1961), using minimum strength values and I clearly had had enough. That Friday afternoon (after I failed to reach my supervisor from a pay phone located off-site), I made the most monumental decision of my young technical life: I instructed the contractor to start removing the surcharge fill.
The contractor worked all night and throughout the following day (Saturday) until about two-thirds of the soil surcharge was removed. By that time the warehouse floor had stopped rising and the crack in the soil no longer appeared in the now greatly reduced surcharge fill.
By Monday morning, of course, I had many visitors representing both the client and the property owners. They were joined by my supervisor, his boss, and his boss, etc. In short (and for the first time), I now had lots of company at the job site, including every person who had even a causal interest in the situation.
Of small consolidation to me personally was that some important person said that I had done the right thing by removing the surcharge fill. At this point, it became apparent by “connecting the dots” that we had a massive shear failure, which is shown in Figure 4 (1–4).
The entire project’s design focus quickly changed in that two large steel sheet-pile ringwalls were driven through the remaining surcharge fill and then through the soft foundation soil. The large steel tanks were then constructed within their respective circumferences. This was followed by a slow filling of the tanks with molasses, which caused a controlled consolidation of the foundation soil. The process of filling took some two to three years.
Lawsuits
The ensuing lawsuits lasted until the late 1960s, resulting in the following two judgments:
- The property owner sued the tank owner (our client) in Delaware court and won a $1.2 million lawsuit for building, street, and utility damages and repairs.
- The tank owner sued the designer (the consultant that I was employed by) in New York court and won a $6 million lawsuit, which included the prior judgment, the alternative sheetpile ringwalls, the rental of numerous tanker ships required to hold the molasses (called “demerage”), the loss of capacity of the storage tanks, and maintenance/repair of the storage tanks.
Interestingly, the reason the New York court found the entire judgment to go against the consultant was that he felt that the consultant did not convey to the tank owner that a certain degree of risk was involved. Indeed, sand drains were still relatively new in 1961, but the consultant felt confident that this was the correct approach.
Of course, throughout this period I was being deposed by numerous lawyers representing the various parties involved. Most damaging during this period is that the plaintiff’s attorneys gained access to the numerous reports I was sending to my company in NYC. Within the various reports was the following foundation stability graph (the infamous “Figure 3.7” is the actual hand-written graph from 1961) of impending failure of the foundation soil at the site. I had made factor-of-safety calculations at 3ft, 5ft, 8ft, 11ft, 15ft, and 18ft of surcharge fill height.
As seen in the graph, using minimum strength values the lower curve predicts a global shear failure at about 14ft of surcharge fill height, whereas the actual failure occurred at about 18ft. This is really a remarkable (lucky?) prediction of impending failure, especially considering that I was doing everything by hand using only a compass, graph paper, and calculator, and particularly since I had no experience or training in such calculations and was extremely unsure of myself. I can easily understand that this specific prediction curve could well have been at the heart of the ultimate decision against the defendant consulting company.
Of course, the essential technical question is: Why did the organic silty clay foundation soil not consolidate as designed?
Clearly, I do not know, but suspect that “smear” creating a much lower “ch-value” than expected was the major item. The issue of smear zone properties adjacent to the sand drain itself is unanswered today, nearly 50 years after this case history occurred.
I, of course, was devastated at the time and left the consultant company’s employment after the sheet pile ringwalls and storage tanks were constructed. My personal progress was to hurry and finish my master’s degree using this sand drain/surcharge project as the topic of my MSCE thesis. Its citation is: “Consolidation of Compressible Soils by Sand Drain Installation and Surcharging” by Robert M. Koerner, June 1963, published by Drexel Institute of Technology, Philadelphia, Pennsylvania, 120 pages.
I then went to Temple University Law School for a short while (this was an obvious reflective impulse), decided that I really didn’t like how the law and lawyers necessarily go about their business, and then went to Duke University for an doctoral degree in geotechnical engineering. This was completed in 1968, whereupon I returned to Drexel for a 40-year career in academia, teaching, advising, and conducting research. Of course, the research effort quickly led to geosynthetics and the writing of the now 30-year-old book that essentially compelled me to drop all other research and concentrate on geosynthetics (Construction and Geotechnical Engineering Using Synthetic Fabrics by Robert M. Koerner and Joseph P. Welsh, John Wiley & Sons, 1980, 267 pages).
Epilogue
As closure to this personally significant and emotional experience of being the sole inspector on a project that sustained a major failure, I submit several reflections.
- The most obvious commentary is that my handwritten reports (as basic and unsophisticated as they were) should have had oral communication and the attention of my immediate supervisor.
- In light of this not happening, I should have had the courage to force this issue and/or go to the next higher level in the company’s organizational structure. Of course, this I did not do and it was a personal failing on my part.
- Today, sand drains are passé and I wonder if and how much wick drains (i.e., PVDs) would have helped the situation. With the tensile strengths afforded by hundreds of wick drains, the surcharge could have gone higher, but by how much is unknown. I suspect that we would have had the failure anyway and it would have been perhaps even more dramatic than it actually was.
- I really needed a technical mentor to discuss the situation of both events at the site and my analysis of the stability situation. In this regard, I could have gone to my Drexel professor who first taught me soil mechanics, but I didn’t take the time to do so.
- Lastly, I also needed a second mentor who might have advised me on business and company protocol. I simply did not know what to do and, again, did not have the courage or foresight to ask for such a discussion or opinion from anyone else.
Today, I believe that every young engineer needs not one, but two, mentors: one technical and the other for business protocol and ethics. My hope, nearly 50 years after this failure occurred, is that others can learn from these five recommendations to help prevent such incidents from occurring to them.