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Re: passive pressure for retaining wall[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc(--nospam--at)seaoc.org
- Subject: Re: passive pressure for retaining wall
- From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
- Date: Wed, 3 Sep 1997 14:19:24 +0500
As Stan Johnson pointed out, "I would think that a few limitations should be looked at" before calculating passive pressure using the methods in the example in REINFORCED MASONRY ENGINEERING HANDBOOK BY James Amrhein - I also suggest that most designs using a keyway to develop passive pressure are based on an invalid free body diagram. Regarding limitations of Amrhein's method, in my opinion Amrhein presents one such limitation on sliding resistance in the "Alternate Design of Shear Key" shown just after his special method being discussed (presented on page 256 of the fourth edition). Again as Stan pointed out, "His approach does seem to have some logic to it. Essentially he is saying that the pressure from the footing above is providing a confinement pressure for the soil". It may be true that the soil immediately behind the keyway has enhanced passive resistance due to confinement pressure, but the overall wall does not have such confinement beyond the toe of the footing - thus the "Alternate Design of Shear Key" which considers passive pressure at the toe (without confinement pressure) should actually be viewed as a "limitation" of passive resistance for the overall wall, rather than as an "alternate" design. It should also be noted that Amrhein's examples do not include a factor of safety against sliding (typically 1.5). The "alternate" design method presented by Amrhein is basically the method I typically use to analyze sliding resistance with a keyway - with at least one essential correction. Since the passive pressure is considered to the bottom of the keyway, the active pressure must also be extended to the bottom of the keyway. Although most design examples I've seen for sliding resistance with a keyway do not address this added active pressure, a proper free body diagram would include the soil pressure on both sides of the keyway. Consider for a moment if the entire footing were thickened 1 ft rather than just a keyway extended down 1 ft - there would be no question that the active pressure diagram should be extended to the bottom of the footing in an opposing direction to the resisting passive pressure. The fact that only a portion of the footing is extended down as a keyway should not alter this load effect. (Can anyone explain why this added force is neglected in many design examples??) And this is a significant effect - for example, with a 10 ft high retaining wall with 2 feet of passive soil resistance, the active soil pressure at the top of the keyway with 40 pcf EFP would be 400 plf - the same as the passive soil pressure at this point using 200 pcf EFP. Thus the active and passive soil pressures would approximately balance, eliminating the benefit of the keyway in developing passive soil resistance. Thus I rarely use keyways since they are not very effective in increasing passive soil resistance. The main benefit a keyway seems to provide is that the sliding plane is moved from the concrete-soil interface to a plane within the soil. Per Amrhein's example, a higher coefficient of friction may possibly be used at this location, although geotechnical recommendations rarely address this improved sliding coefficient below the base. Additionally, I would include the weight of the soil wedge mobilized between the face of the keyway and the toe of the footing in the vertical force used to determine frictional sliding resistance. Thus a keyway may provide enhanced sliding resistance due to added frictional resistance rather than due to any gain in passive resistance. Regarding the value used for passive soil pressure, it is generally recognized that greater wall movement is required to develop full passive pressure. Since I typically don't want much movement in the walls I design, I use a conservative value of passive pressure which can be developed with little wall movement. For example, I would use a lateral passive coefficient of 1.00 for sand in lieu of 3.00 (see Figure 1 in Chapter 3 of NAVFAC DM-7.2). This further limits any gain in passive pressure from keyways.
- Re: passive pressure for retaining wall
- From: Stan Johnson
- Re: passive pressure for retaining wall
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