The technique is based on factor of safety method. The procedure of applying this method in slope stability analysis is studied.
Slope stability analysis - Wikipedia
The methods of calculating parameters standard deviation are summarized. Engineering application shows that Taylor series method is feasible and practical in slope stability analysis and it provides a new method for slope reliability analysis. Pan and Y. Request Permissions. Probabilistic slope stability analysis by Finite Element [J]. Journal of Geotechnical and Geoenvironmental Engineering. Evaluating the reliability of existing levees[R]. Vicksburg: U. Factors of safety and reliability in geotechnical engineering [J]. Journal of Geotechnical and Geoenvironmental Engineering, , 4 : Beijing: China WaterPower Press, All Rights Reserved.
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321.1 Design of Earth Slopes
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By Author A. Bishop Norbert Morgenstern. No search history Recently Viewed. Soil survey recommendations have ranged from 1V:2H through 1V:3H The 1V:3H has been based on the CH classification, which is a misinterpretation since it is a CH residual from carbonates and is cherty. The slope selected should be based on the predominant phase.
If it is mostly CL loess with only a few feet of residual soil, the 1V If it is almost all residual soil with only a few feet of windblown soil then 1V:2H should be adequate.
It should be emphasized that this chart is a guide. It is based on some theory and it is tempered by experience. It fits most situations, but there are exceptions. Some of the exceptions have been addressed in research reports. For the most part, this chart is based on stability considerations, but in one area it is been shaded a bit for erosion control purposes. This is for the ML loesses. When dealing with a very tight right of way situation, it may be practical to steepen slopes in this material to 1V:2H at the expense of some increased erosion problems or erosion control measures.
If in doubt, shear tests can be done. To repeat, this chart is a guide - it is not carved in stone. For grade separations, consideration should be given to selective grading of fill materials so that better materials are placed in fill spill slopes i. While such handling will likely increase costs for fill placement, some additional handling can be justified if it results in reducing bridge length. Selective fill placement is not generally practical on stream crossings -- only on grade separations.
Table This recognizes the fact that the effective height of the spill slope will be reduced by at least 6 to 8 ft.
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This concept becomes more complicated at stream channel crossings where it is necessary to consider the depth and condition of the stream channel and their effect on bridge end location. For example, one might have CL glacial soils and a height differential between grade and toe of slope of some 15 feet.
However, if the stream channel is entrenched in CL soil another 15 feet deep so that the total height differential is greater than 20 feet, the bridge ends should be stepped back to or beyond a point determined by projecting a 1V For typically steep channel banks, this will generally leave a substantial bench at natural ground level, which provides some room for bank sloughing without affecting the integrity of the spill slope. The spill slope would remain at 1V:2H but the bridge end would be located as if it were at least 1V Now things get even more complicated.
To this point, we have not really considered some of the possible complications to the stability of stream and channel slopes. There is a caution in the text beneath the slope selection chart that, "Factors such as foundations, seepage, susceptibility to inundation, etc.
Determining proper slopes in such circumstances involves consideration of a complex intermingling of factors such as flooding rate, height and duration, rate of recession, water velocity, scour potential, soil strength, weight, permeability, swell potential, and seepage rates, all further complicated by considerations of costs and the risks and consequences of failure. The following general comments and guidelines are offered, however, to supplement the Guide for Slope Recommendations. First, use the chart to determine the slope spill or side you would use if water were not a factor.
This is the slope to which you will make adjustments based on the following considerations. For moderate stream flows of average flood duration, about 1V For prolonged flooding followed by drawdown, flatter may be appropriate. For intermittent or low-flow streams subject only to flash flooding, no flattening may be needed. Always inspect stream slopes for evidence of slides and sloughs and inspect the condition of adjacent structures over the same stream. Consider the width of the embankment; a 4-lane roadway is more likely to fail into a stream channel than a narrow county road or railroad fill.
Consider also the consequences of failure; be more conservative for heavily traveled arterial roadways than for minor or rural supplemental roads for example. Keep in mind that many stream channel slopes are stable only because of mature tree growth along the banks and the reinforcement provided the banks by the root structure. Remember that trees will be destroyed by construction, the roots will rot, and maintenance will prevent their regrowth.
The net result will be less inherent stability where most needed. Channelization has led to much stream bank instability, particularly in the northwest part of the state. It is especially prevalent in Lafayette, Atchison and Holt Counties.
The invariable result is channel deepening, sometimes severe deepening. Careful examination of banks will often reveal massive slides, sometimes so massive as to resemble natural terraces. Always look at your county map; if the stream follows a straight line, it has been channelized. The streambed will have deepened and the banks, if not already failed, will be in precarious condition.
The procedure for design of earth slopes following LRFD is quite similar to procedures for conventional ASD analysis, with two important differences. The first difference occurs in No. The second difference occurs in No. In this respect, the LRFD procedure is indeed more straightforward than current procedures in that the analysis target or limit is consistent for all stability cases for the LRFD procedure whereas the analysis target for conventional ASD procedures varies from one application to another.
The result of these differences is simply that, for LRFD procedures, uncertainties in the analyses are accounted for through factoring of the input parameters whereas for ASD procedures the uncertainty is accounted for through a single factor of safety. By factoring individual input parameters, it is possible to more appropriately apply conservatism to the individual parameters involved in the analysis, and therefore to effect more consistent levels of safety across a broad range of cases.
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