Engineering and computer science students from UTC displayed 130 projects at the second Tech Symposium in the downtown library.
It’s been a while, but we hope it’s worth the wait: the monograph Effective Hyperbolic Strain-Softened Shear Modulus for Driven PIles in Clay is now available. It was presented at the Research Dialogues for the University of Tennessee at Chattanooga 9-10 April 2019. The abstract is as follows:
Abstract: Although it is widely understood that soils are non-linear materials, it is also common practice to treat them as elastic, elastic-plastic, or another combination of states that includes linear elasticity as part of their deformation. Assuming hyperbolic behavior, a common way of relating the two theories is the use of strain-softened hyperbolic shear moduli. Applying this concept, however, must be done with care, especially with geotechnical structures where large stress and strain gradients take place, as is the case with driven piles. In this paper a homogenized value for strain-softened shear moduli is investigated for both shaft and toe resistance in clays, and its performance in the STADYN static and dynamic analysis program documented. A preliminary value is proposed for this “average” value based upon the results of the program and other considerations.
The slide presentation for this follows:
On this day in 1898 the USS Maine was sunk in Havana harbour, which precipitated (after a great deal of “yellow journalism” on the part of the American press) the Spanish-American War. This topic is of interest, not only because of its place in American history, but also in the history of geotechnical engineering, as it was an early large-scale application of sheet piling and an early use of cellular cofferdams.
The cause of the Maine’s explosion is still a matter of debate, although the weight of the evidence leans toward some kind of coal explosion. The Maine used the same type of Scotch marine boilers that Vulcan preferred for its offshore hammers in the 1960’s and onward; coal was the usual fuel at the time. It was necessary, sooner or later, to get the wreckage off of the bottom of Havana harbour, and that involved a celluar cofferdam. The following description of the job comes from H.S. Jacoby and R.P. Davis, Foundations of Bridges and Buildings, New York, NY: McGraw-Hill Book Company, 1914:
The cofferdam for raising the “Maine” represents a special type of steel cofferdam, very large and strong. *”The problem was to surround the wreck of the vessel, lying in about 29 to 37 feet of water, with a cofferdam, which when unwatered would be tight enough to prevent leakage, strong enough to resist outside water and mud pressures, and a protection that would assure safety during the work. The cofferdam should be self-sustaining, if possible. Bracing by struts across its interior to resist the water and mud pressures might be difficult to install and would interfere with the operation of removal. The borings indicated bad conditions for foundations. The building of a cofferdam without internal bracing, which would withstand pressures from a head of 37 feet of water and practically 21 to 23 feet of mud, was an unprecedented task.
“The cofferdam should be not only self-sustaining and safe against the pressures to which it was to be exposed, but it should also be capable of complete removal after it had served its purpose. It should be able to support more or less superimposed loads, for working platforms had to be built upon it. The work of unwatering the area enclosed had to be carried on from the top of the cofferdam; and afterward, men and materials had to be transferred from there to the interior, for work upon the wreck…The cofferdam decided upon consisted of 20 equal cylinders, 50 feet in diameter, and composed of steel piling 75 feet long…” A plan is shown in Fig. 71e.
“The length of the major axis of the cofferdam was practically 399 feet, and of the minor axis 219 feet, leaving a 20-foot clearance at the submerged bow of the ship and a 14-foot clearance at the stern, with 45 feet at the side cylinders. Such clearance was necessary to avoid portions of the wreck which had been blown beyond the position occupied by the hull.
“The units of the cofferdam were made cylindrical for the reason that the extremely high pressures, which would be exerted by the mud filling, would act radially and uniformly on each pile, straining each joint to the same amount at equal depths, and in the entire cofferdam cylinders would deform least from play in the piling interlocks.”*
The cylinders were driven tangent to one another and to insure their stability and prevent leakage of water through them when the cofferdam was pumped out they were filled to the top with clayey material that was dredged from the bottom of the harbor. A curved diaphragm of steel-piling, as shown in Fig. 71f, was driven to connect the adjacent cylinders, and the space between this arc and the outer surfaces of the large cylinders was likewise filled with dredged material.
The piling used was the Lackawanna section, weighing 35 pounds per linear foot, and had a web 1/2 inch thick. The piles were driven so that their tops were 2 to 3 feet above normal water level (Fig. 71g) and the 75-foot length of piling, which penetrated the harbor bottom to a distance of approximately 35 feet, was made of two lengths spliced together with channels.
*Bulletin No. 102, Lackawanna Steel Co., Buffalo, N.Y.
Cellular cofferdams have gone on to become an important type of retaining wall structure. Probably the most significant change from this project is that cellular cofferdams are always built with permeable materials such as gravel and not clays such as were dredged from Havana harbour. More information on this project and related topics are here:
- Sheet Piling (which includes free documents on cellular cofferdams.)
- Sheet Pile Design by Pile Buck
- Pile Driving in Old Havana
- Information on historical sheeting sections and pile driving techniques
The wreckage of the Maine wasn’t the only result of the Spanish-American war. The United States virtually ended the Spanish empire, which had once covered much of the Western Hemisphere. Cuba became independent. The Philippines became an American possession (except for Japanese rule during World War II) until their independence in 1946.
Puerto Rico also became part of the United States by military invasion and annexation, and (through a long process) Puerto Ricans became full American citizens. That’s something I remind my students about every time I teach this subject; an American history lesson never hurt anyone. And the Puerto Ricans I go to church with (and I have in class) are grateful.
For this post I’m featuring an article by J. David Rogers, Professor and Karl F. Hasselmann Chair in Geological Engineering, Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science and Technology, which he entitled Disappearing Practice Opportunities: Why Are Owners And Engineers Taking Increased Risks? What Can Be Done To Counter This Threat? It’s been around a while but bears repeating, especially for one thing: why we need to restore engineering geology to the civil engineering curriculum.
Van der Merwe’s method, which was first introduced in the 1960’s, is a simple method for estimating the vertical movement of expansive soils. The method at its most basic is described in publications such as Foundations in Expansive Soils, and basically looks like this:
The problem with this presentation is that it is entirely in “Imperial” units, which were the standard in the South Africa of van der Merwe’s day. We need to convert this to make it usable in SI units as well. First we present the potential expansion chart, from Soil Mechanics:
Then we reconstruct van der Merwe’s equation to make it applicable for SI units as well as US (or Imperial) units:
Note that the swell potential is now in the denominator. That’s a convenience to make the PE values of a reasonable order of magnitude. But now it’s dimensionless. Also note that the depth reduction factor has a different exponent for SI units than it does for US units.