UniNews - Summer
1998 UniNews - Summer
1998
The question in the heading is a frequent one, and many are under the misconception that the answer is "Yes". The correct answer is a resounding No!
Downdrag is the when the settling soil drags the pile down and loads have nothing to do with this phenomenon. Dragload is the load appearing at the neutral plane due to prestressing of a pile by the interaction of negative skin friction above the neutral plane and positive resistance (shaft and toe) below the neutral plane. If the soil settles only little, there is no downdrag. In contrast, be the settlement large or small, there is always dragload.
Downdrag is always undesirable. Regardless of the magnitude of the dragload and the pile capacity, if the soil at the neutral plane settles, then, the pile will settle too - be dragged down. The dragload is of no concern whatsoever for the capacity of the pile. However, if the dragload becomes too large, the pile structural strength may be exceeded, which indeed is an undesirable situation. Now, please understand that just because a dragload always develops for a pile does not mean that an excessively large dragload always develops. It is rare for a pile shorter than about 30 m (100 ft) to experience an excessive dragload. In fact, the Canadian Bridge Design Code states that if the pile is shorter than 80 diameters, there is no need even to check the magnitude of the dragload (the design must always check for settlement, of course). For longer piles, how does one determine if the dragload is excessive? The answer: Use UniPile to determine first the location of the neutral plane, which is the equilibrium between the dead load and the negative skin friction acting downward and the positive shaft resistance and the toe resistance acting upward. Because of the very small movements necessary to fully engage the shaft shear, it is a reasonable approach to assume fully mobilized shaft shear above (negative skin friction) and below (positive shaft resistance) the neutral plane. The toe resistance is more difficult to estimate, however. The following diagram will provide some assistance.
This diagram shows the soil settlement around a pile and the settlement of the pile, Ss and Sp, respectively, at the ground surface, and the distribution with depth. The pile compresses due to the load, but in the scale shown, it is almost a vertical line. The most important note in the diagram is that of "Net toe movement". This is the movement that determines the magnitude of the pile toe resistance. Usually, if the net movement is smaller than about 10 % of the pile diameter, the toe resistance is not fully mobilized. If the soil settlement is mostly due to reconsolidation of the soil after the driving, the toe resistance will always be small. Notice, the magnitude of the toe resistance not only affects the location of the neutral plane, but also the magnitude of the dragload and the maximum load in the pile.
UniPile will let you quickly complete the design for the dragload and maximum load in the pile. [The diagram is copied from a recent ASCE seminar paper presented by Dr. Fellenius. The full paper is available for downloading at UniSoft's web site. See below under the heading "Technical Articles"].
Modern software is increasingly complicated. Unexpected bugs can pop up in the most extensively tested program; usually just when the final version has been released. No program developer can guarantee that a program is error free in every combination of input. However, UniSoft guarantees to fix any bug discovered, and for free. For example, all our main programs were recently discovered to share a calculation bug that would appear in certain rather rare combinations of data. The programs affected are UniPile Version 3.0, UniSettle Version 2.3, UniBear Version 1.0, and UniTest Version 3.1. Every Registered Users has been asked to have the original disks replaced for a bug free version (next higher number) and most have done so now. There are still a few who have not reacted yet. Please, do not procrastinate.
As this newsletter is being prepared, we notice that UniPile Version 3.1 (the "bug free" release) does not do the loading test simulation correctly with regard to the compression of the pile shaft. A minor bug, so we are not at this time instigating a complete replacement action. However, anyone who wants to replace UniPile Version 3.1 (and 3.0, of course) can do so by just sending us back the old disk and we will replace it at no charge with Version 3.2.
We have been asked why UniBear cannot calculate the earth pressure from a layered soil. Well, UniBear is intended for calculations of bearing capacity of footings, cantilevered walls, and bridge abutments, which structures usually have a homogeneous backfill. However, you can use UniPile to calculate earth pressure in a layered soil: In the soil profile (at the position of the beta-coefficient), input the earth pressure coefficient and use a square pile with a diameter of 250 mm (or 0.25 ft). The capacity calculation returns the earth pressure per unit of depth. The magnitude of the maximum load and the location of the neutral plane are the earth pressure resultant and its location, respectively. If the calculation is to be for a cohesive soil, input the 2c-value for the c' and UniPile will provide the answer. Line loads and surcharge loads, as well as tie backs and strut loads, can be handled by UniBear and added separately. While the earth pressure calculations can be done by a judicious combination of UniPile and UniBear, as suggested, it is a slow process. Is there an interest in a sheetpile wall program that can handle earth pressure calculations, and also provide moment distribution for the design of the wall and its bracings and anchors?
Talking about UniBear, As expected and intended, this program has become a favorite amongst highway and bridge designers. Our User in Alabama DOT, Hank Covington, sent us a letter of appreciation stating that "your program is just excellent", emphasizing that the program is easy to use and offering a "thorough endorsement". We appreciate the praise and promise to keep ensuring that our programs are will stay up-to-date with the advances in technology and yet be fully responsive to the needs of non-computer engineers.
This web site contains several technical articles of interest to foundation engineers which can be downloaded at no-charge. For example, a paper on the Critical Depth - How It Came into Being and Why It Does Not Exist. A recent addition is an article by Bengt H. Fellenius presented to an ASCE Seminar held in April in New York City. The article includes an exposé of several case histories on studies on dragloads and settlement in piles and presents the principles of the Unified Pile Design for Capacity, Negative Skin Friction (Dragload and Downdrag), and Pile Settlement and concludes with testing and analysis details on a case study on long piles.
We owe thanks to Wayne B. Janusson, of B. C. MTH who recently alerted us to an error in two equations in the "Red Book" by B. H. Fellenius: Eqs. 3.1a and 3.2a have the numerator squared once too many and Eq. 3.2a misses a "sin" in the denominator. Please, take note.
The UniSoft programs are truly finding a following internationally. The number of countries with Registered Users is now 50. We thought that the downturn in Asia would affect sales volume, but not so. One Asian customer explained that "in the current tight and competitive market, we can no longer afford to spend time on analyses. UniSettle (the program considered) does in a few minutes what our engineers needed a day to do in the past".
UniPile V. 3.2 can simulate a static loading test, which is particularly useful when having the analysis calibrated by results from a static loading test where the records include the pile toe movement (toe telltale), or for correlation to a CAPWAP simulation of a static loading test. The "matched" t-z functions will provide insight in the amount of movement of shaft and toe necessary to generate the pile resistance; a decisive value to a pile settlement analysis. One of the most useful applications is the Osterberg-Cell test which supplies both movements and loads as measured for the shaft and the toe of a pile or drilled shaft. UniPile can quickly match the test results from the two independent "up and down" O-Cell values and produce a "head-down" test that includes the E-modulus and stiffness of the pile shaft as well as effect of residual load in the pile.
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