angle of repose wet/dampened sand 2

[femarie]

I am presently involved with the verification of steel bulkheads in seagoing ships - so called hopper dredgers-designed for the carriage of dampened sand.What angle of repose can I use to calculate the sand pressures against these bulkheads ?Any advice welcome.
Femarie

 

[fattdad]

If I were to give you an angle of 35 degrees, how would you then use that number to calculate a pressure? I'm not being glib, I'm just not sure of your process. . .

If you are determining pressures acting on the side of a bulkhead, to me there may be a few other factors. Consider the following:

How wide is the bulkhead (i.e., is it 40 ft tall but only 20 ft wide)?
What is the unit density of the damp sand fill (i.e., 125 pcf)?
Will the top of the sand fill above the top of the bulkhead be horizontal or at some angle?

For horizontal backfill against a non-yielding wall (and for unlimited width), the pressure acting on the bulkhead would likely be calculated using an equivalent fluid density of 50 to 60 pcf. This of course would be greater if standing water were to accumulate in the base of the sand.

I always struggle with the concept "angle of repose" and that may be my shortcoming.

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

Hello, Femarie. There are never simple questions in geotechnical engineering. In many cases, the answer to a question like this one, and even the proper framework for considering the problem, can vary tremendously depending on the density, water content, and grain sizes of the soil. If we are to be helpful to you, we need to know more about the nature of the material, how it has been obtained and/or processed, how it is to be placed in the hoppers, whether drainage will be provided by wells or sumps in the hoppers, etc.

If this is freshly dredged material, you may need to treat it as a completely fluid slurry, with little or no shearing resistance of its own, because it is quite loose and wet. If there is much more than a trace of fines (silt and clay, generally defined as particles smaller than 0.075 mm), the dredged mixture will be very slow to give up its water.

On the other hand, if this dredged material has been drained, so that part of the voids contain air instead of water, it may be possible to heap it steeper than the angle of repose for dry material, because the capillary tension in the remaining water holds the sand together, making it stronger than dry sand - like a sand castle at the beach.

Best regards,
DRG

[I, personally, don't like to discuss lateral forces in terms of equivalent fluid density. It's convenient, but gets confusing when there is a water table or a sloped surface involved. I prefer to just talk about effective unit weight and coefficient of lateral earthpressure, and calculate the hydrostatic component separately.]

 

[civilperson]

The angle used for wet or damp sand would be much steeper than the dry conditions and thus less horizontal pressure. However, unless there is provision for maintaining the moisture a dry value should be used for worst case pressures.

 

[fattdad]

The angle used for wet or damp sand would be much steeper than the dry conditions and thus less horizontal pressure.

This is "correct" to a fault. Damp sand can hold a steeper angle, by virtue of the surface tension of water interacting with the individual sand grains. However, in earth pressure calculations we never allow for this "cohesion" in our engineering calculations. Damp sand weighs more than dry sand and would impart a greater horizontal stress. Friction angle is determined in the saturated state, so I'd not consider variation in friction angle to be a factor between damp sand and dry sand.

(+1 on DG's footnote. That said, equivalent fluid density is fairly safe to use if the soil mass is not saturated.)

f-d

¡papá gordo ain’t no madre flaca!

 

[concretemasonry]

The fact that you used the term "sand" is troublesome when it comes to a suggestion.

If it is a dredged material, it may contain a significant amount of fines the will affect the properties of the material. It can have dramatic effect of the rate of drainage, moisture content and amount of moisture retained by the physical ability of the material to retain moisture in the long term.

Dick